C'/3
902R82001
F.INAL REPORT
BASELINE SEDIMENT STUDIES TO DETERMINE DISTRIBUTION, PHYSICAL
PROPERTIES, SEDIMENTATION BUDGETS AND RATES IN THE
, VIRGINIA PORTION OF THE CHESAPEAKE BAY
UNITED STATES
ENVIRONMENTAL PROTECTION AGENCY
GRANT NUMBER R806001010
Robert J. Byrne
Carl H. Hobbs, III
Michael J. Carron
Virginia Institute of Marine Science
College of William and Mary
Gloucester Point, Virginia 23062
Library
U. S. Environmental Protection Agency
Region 111
Central Regional Laboratory
839 Bestgate Koad
Annapolis, Maryland 21401
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DISCLAIMER
This report has been reviewed by "the Chesapeake Bay Program Office,
U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views
and policies of the U.S. Environmental Protection Agency, nor does mention
of trade names or commercial products constitute endorsement or
recommendation for use.
ii
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ABSTRACT
The distribution of the physical 'properties, patterns of deposition
and rates of accumulation of sediments provide an integrating framework for
investigations of the concentration and distribution of toxic substances.
Over 2,000 grab samples of surface-sediment (1.4 km grid) reveal that the
bottom of the Virginia portion of Chesapeake Bay is significantly sandier
than hitherto reported. About 65% of the area is sand. The number of
samples in this study is an order to magnitude greater (2,000 versus 200)
then previous studies allowing a significantly better delineation of
sedimentary characteristics.
Distribution of sediment size is, in large part, a function of
geomorphology, there being an apparently good correlation between depth and
sediment type; the finer grained sediments are usually confined to the
deeper channels. The exceptions to the depthisize relationship are the
presence of fines in the shallow, marginal embayments such as Mobjack Bay
and the absence of fines in the deep channel in the southeastern section of
the Bay. The occurrence of sands here is a function of infilling with
sands from the area of the Bay mouth and, perhaps, of scour into older
(Pliocene?) materials. Sediment distribution also reflects the local
source with the shallow-water, marginal sands derived from erosion of the
banks and relict features.
Several large geomorphic features are distinguishable on the maps of
sediment characteristics. These features include the deep channels, a
large sand-shield near Tangier Island, relict spits, and the zone of
influence of the Bay mouth.
Nine hundred samples, selected to avoid the coarser sands, were
analyzed for total carbon, organic carbon, and sulfur contents. There are
strong correlations between these characteristics and sediment type,
especially weight-percent clay. Additionally, there is a good relationship
between the organic carbon and sulfur contents. Although total carbon
content reached 10% in some samples; the average was 1.5%. Average organic
carbon and sulfur contents were 1.0 and 0.34%.
Comparisons of the bathymetry on boat sheets from the 1850"s and
1950's were used to delineate the patterns and volumes of cut and fill in
the Virginia portion of the Chesapeake Bay. The comparisons were adjusted
for relative sea level change and monthly variations in mean tide level.
In addition, propagation of the error was evaluated and the results
applied. Coupled with the data concerning the character of the surface
sediments and adjusted for water content, the volumes of. sediment shown by
the bathymetric changes to have been deposited were converted to the masses
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of sand, silt, and clay deposited in a 100-year period. The patterns of
deposition and erosion, particularly when coupled with grain-size
information, provide very important insights into the sedimentation
processes within the Bay. The main Bay-axial channel from Maryland and the
transition to the Virginia basin are the principal deposition sites for
clay. Silt-sized materials are somewhat more uniformly distributed
throughout the stem; however, about 33% of the silt accumulates in the
transition region, an area of relatively low tidal-current energy. The Bay
mouth may be a principal source. The -pattern of deposition of sand
suggests that the Bay mouth source is very significant as very fine sands
penetrate much further into the Bay than has heretofore been suspected.
The patterns are consistent with present understanding of circulation
within the Bay.
The project includes an attempt at constructing a sediment budget
using published values for silt and clay estuarine advection and
contributions from shore erosion measured against the bottom residual
accumulations. The residual accumulation of silt and clay is an order of
magnitude larger than previously estimated. No previous work has
considered the sand budget but the general assumption has been that the
contribution from shore erosion would be the principal source. This study
indicates the residue bottom accumulation of sand is greater than the shore
erosion contribution by a factor of 40. It is evident that additional
understanding of the flux of sediment through the Bay mouth and the mouths
flanking of the tributaries is required.
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CONTENTS
Page
' Abstract iii
, Contents v
List of Figures vii
List of Tables x
Acknowledgements xi
I. Introduction ..... 1
II. Conclusions 3
III. Recommendations 6
A. Management Recommendations 6
B. Research Needs 7
IV. Background of the Study 8
A. Geological History of the Chesapeake Bay 8
B. Recent Sedimentological Work 13
V. Methods 16
A. Characterization of Sediments 16
1. Sample Collection 16
2. Analysis of Sediments 19
a. water content 21
b. grain-size analysis 21
c. clay mineralogy 27
d. carbon and sulphur analysis 27
3. Quality Assurance 28
i a. shared and duplicate samples 28 i
b. calibration of the rapid sediment analyzer ... 28
| c. comparison of RSA's at VIMS and MGS 36 |
d. comparison of analyzer by pipette and Coulter i
Counter 40
e. Coulter Counter precision 42
f. carbon and sulfur analyses 45
B. Formulation of Sediment Budget 46
1. Bathymetric Comparison 49
2. Conversion of Sedimentation Rate to Mass
Accumulation Rate 57
3. Sources 64
a. shoreline erosion 64
b. biogenic sediment 67
C. Data Storage 67
VI. Results and Discussion 72
A. Sedimentology 72
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1. Description of Sediments 72
2. Comparison with earlier surveys of bottom
sediments 83
3. Clay Mineralogy 92
4. Carbon and Sulphur in the Sediments 95
a. previous work 95
b. results 98
c. discussion 104
d. conclusions ..." 107
B. Patterns and Rate of Sediment Accumulation 107
1. Patterns of Erosion and Deposition 107
a. the nearfield mouth belt 108
b. the central southern bay belt 108
i c. the upper southern bay belt 114
; d. the lower middle bay belt 115
2. Sediment Budget 116
References Cited 137
Appendices . 144
-1. Loran C Correction Data . .- - 144
2. Mass of Sediment Eroded from the Shoreline 149
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LIST OF FIGURES
*
Number Page
1. A plot of sea-level fluctuations during the Quaternary .... 11
2. Location of major paleochannels in the subbottom of the
Virginia portion of the Chesapeake Bay ............ 12
3. Rates of change in crustal elevation in the Chesapeake
Bay region ....................... ... 14
4. Map showing location of sample stations and subareas ..... 17
5. Photograph of sample scoops fabricated for this project ... 13
6. Photograph of the R/V Captain John Smith ........... 20
7. Flow chart for the analysis of sediments ........... 22
8. The Rapid Sediment Analyzer (RSA) as constructed at VIMS ... 25
9. Ternary plot of sand: silt relay ratios of samples analyzed
at both VIMS and the MGS ................... 31
10. Plots of cumulative a grain-size frequency distribution as
determined by both sieve and RSA techniques ......... 37
11. Comparison of frequency distributions as determined by
both pipette and Coulter Counter ............... 41
j 12. A plot of the envelope described by 20 replicate analyses
using a Coulter Counter ................... 43
13. A plot of the cumulative frequency percents at 1 0
intervals for 20 replicates of each of 2 aliquots of the
same sample ......................... 44
14. Location of 6-second cells with bathymetric comparisons
and Bay Sub segments
15. Flow chart for residual mass estimates ............ 5Q
' 16. Index map of bathymetric surveys for 1849-1856 ........ 51
vii
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17. Index map of bathymetric surveys for 1948-1961 ........ 52
18. Tidal variations used in the reduction of sea-level data ... 53
19. Schematic diagram of corrections applied to bathymetric
change data ......................... 55
20. Map showing locations of short cores ............. 59
*
21. Vertical profiles of water content .............. 60
22. Vertical profiles of water content, normalized to a common
origin ............................ 61
23. Nomogram for the determination of depth averaged water
content ........................... 62
24. A plot of the mass of dry sediment in one cubic meter of
bottom material as a function of water content ........ 65
25. Histogram of the mass of material eroded from the
shoreline .......................... 66
26. A sketch of the profile of an eroding shore depicting
material not included in the calculation ....... .... 68
27. Subareas used in estimating the biological contribution to
bottom sediment .......................
28. Isopleth map of weight-percent sand ............. 74
29. Isopleth map of mean grain-size ............... 75
30. Map of the distribution of sediment types .......... 76
31. Histogram of depths at sample sites ............. 77
32. Bathymetric Map ........ ............... 70
33. Isopleth map of weight-percent clay ............. 70
34. Isopleth map of graphic mean grain-size of the sand
fraction ........................... gQ
35. Ternary diagrams of sand:silt :clay ratios of sediments at
different depths ....................... 2
36. Scatter plot of mud versus water content
37. Isopleth map of mean grain-size of sands from Shideler
. (1975) ............................ 86
viii
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38. Location map of samples from earlier studies ......... 87
39 A,B,C Comparison of weight percent mud or profiles from
earlier studies ....................... 88
40. Histogram of values of total carbon, organic carbon, and
sulfur content ........................ 99
41. Isopleth map of total carbon content ............. 101
42. Isopleth map of organic carbon content ............ 102
43. Isopleth map of sulfur content ................ 103
44. Histogram of depths at sample sites for which there are
carbon analyses ....................... 105
45. Scatter plot of organic carbon versus sulfur content ..... 106
46. Bathymetric changes during the past 100 years in the
Virginia portion of the Chesapeake Bay ............ 109
47. Bathymetry and generalized bathymetric changes in the
Virginia portion of the Chesapeake Bay ............ HO
48. The mass of sand accumulated per square meter per century
in the Virginia portion of the Chesapeake Bay ........ HI
49. The mass of silt accumulated per square meter per century
in the Virginia portion of the Chesapeake Bay ........ H2
50. The mass of clay accumulated per square meter per century
in the Virginia portion of the Chesapeake Bay ........ H3
51. Cumulative mass accumulation as a function of latitude .... 125
52. Total sediment mass accumulation per unit area per century
as a function of depth interval ............... ^29
53a. Sand, mass accumulation per unit area per century as a
function of depth interval ..................
53b. Silt, mass accumulation per unit area per century as a
function of depth interval
53c. Clay, mass accumulation per unit area per century as a
function of depth interval .................. -j.32
ix
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LIST OF TABLES
Table * Page
1. Schedule of sampling 19
2. Grain-size nomenclature 23
3A&B. Grain-size characteristics of samples analyzed at both
VIMS and MGS 29
' Summary of RSA calibration data 33
5. Comparison of expected and observed diameters of
calibration spheres 34
6. Comparison of percent errors in fall velocity and mid
class diameter 35
7. Determination of average water content with depth 63
8. Calculation of the ash weight of biogenic sediment by
subareas 69
9. Distribution of sediment types 73
10. Approximate proportions of specific clay minerals 93
11. Table of R^ values for various regressions and subareas . . .
12. Suspended sediment budget after Schubel and Carter (1976). . .
13. Comparison between nearshore sand accumulation on the
western shore with sand from shore erosion 120
14. Bottom deposition of combined sand, silt and clay 122
15. Comparison of sediment budget terms 123
16A. Sand accumulation as a function of latitude zone 126
16B. Silt accumulation as a function of latitude zone 127
16C. Clay accumulation as a function of latitude zone
; ^ Sediment mass accumulation rate as a function of zone and
depth interval for ± 0.57 m level of estimation
18. Sediment mass accumulation rate per unit area as a
function of zone and depth interval for ± 0.57 m level
of estimation 134
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ACKNOWLEDGEMENTS
In a project as large as this, it" is impossible to credit even all the
major contributors as authors. We thank the following individuals and
acknowledge that the project could not have been completed without their
labors. J. Zeigler wrote the section on the Geologic History of the
Chesapeake Bay and was constantly available for discussion. P. Peebles
wrote the section on the statistical comparison of the Rapid Sediment
Analyzers. Additionally, she drafted most of the maps and figures and
participated in several other program areas. 0. Bricker, originally with
the Maryland Geological Survey, later our E.P.A. Project Monitor, and now
of the U.S. Geological Survey, provided much assistance and encouragement.
R. Kerhin, J. Halka, and J. Hill of the Maryland Geological Survey assisted
with technical development, inter-state coordination, and general
discussion. C. Sutton and D. Owen did much of the computer work. G.
Thomas, L. Morgan, D. Owen, S. Fenstermacher, K. Worrell, J. Cumbee, S.
Synder, M. Mitchell, A. Frisch, K. Farrell, and G. Chianakas collected the
samples. W. Athearn did much of the early logistical work. G. Pongonis
and D. Ward, Captain and Mate of the R/V Captain John Smith, provided
encouragement, assistance, and excellent service under often trying
circumstances. A. Haywood, E. Travelstead, C. Fischler, M. Mitchell, L.
Zellmer, R. Bowles, G. Chianakas, C. Lukin, M. Fedosh, B. Savage, H. Byrne,
and B. Comyns did laboratory analyses and data reduction. L. Kilch
assisted with the quality assurance program for carbon and sulfur analyses.
J. Boon provided guidance and discussion on error analysis. M. Nichols and
B. Nelson provided valuable assistance and discussion. B. Marshall, C.
Gaskins, and the staff of the V.I.M.S. Word Processing Center prepared the
various drafts. J. Gilley drafted some of the maps and figures. 0.
Bricker, C. Strobel, J. Klein, and D. Wilding, among others at E.P.A.,
assisted with technical coordination. The Chesapeake Bay Program, U.S.
Environmental Protection Agency funded the program.
XI
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I
INTRODUCTION
The basic reason for the study of the physical characteristics of the
bottom sediments of the Chesapeake Bay is that the sediment is the locus of
interaction between toxic substances that have been introduced into the Bay
system and the biological communities that use the same system. Whether
the biological elements make permanent use of the Bay by residing in it, or
temporary use through migration or seasonal habitation, they all are to
some extent dependent on the sediments and the sediment-formed strata which
form the physical structure over and through which the biota are
distributed. If, as often has been postulated, there are discrete
relationships between the substances of concern and specific types of
sediment, knowledge of the sediments is critical to the understanding of
the toxic substances problem. Thus, the first objective was to discern the
sedimentological characteristics of the bottom sediment at a sufficient
sample density so that reasonable interpolations may be made from a sample
subset which is analyzed for various toxic substances and other related
parameters.
Other tasks within the Environmental Protection Agency's Chesapeake
Bay Program are concerned with the transport and transformation of toxic
substances and upon the recent sedimentation history of the Bay. A very
important element in the analyses of these problems are detailed maps of
the characterization of the surface sediments. Thus, a second objective
was to supply maps of the characteristics of the bottom sediments to
support interpretations made in other phases of the EPA-Chesapeake Bay
Program, specifically those phases dealing with the transportation of
materials and with the history of recent sedimentation.
As one of the long term goals of the entire Chesapeake Bay Program is
to develop a system by which changes in the status of the Bay could be
monitored, a third objective of this project was to provide a comprehensive
statement of the "condition" of the bottom sediments of the Virginia
portion of the Chesapeake Bay against which future sediment samples and
characteristics could be compared.
Similarly, the study of sedimentation budgets and rates benefits both
the theoretical and practical understanding of the Bay's workings. Areas
of deposition might be expected to be reservoirs of existing or potential
pollutants which are related to the sediment. Areas of bottom erosion
would be very poor sites for disposal of dredged materials.
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Thus the objectives of Sediment Budgets and Rates subtask were
1. to identify the principal sites of deposition in the Virginia
portion of the Bay,
2. to establish sedimentation rate as a function of position in the
Virginia portion of the Bay, and
3. to formulate a sediment budget for the Virginia portion of the Bay
which incorporates the sedimentation rates from 2 and available
estimates of sediment supply from the tributaries of the Bay, the
ocean, and from shoreline eroeion.
The attainment of the objectives of both subtasks should be of benefit
in making management decisions concerning the fate of the entire Chesapeake
Bay. This study was integrated with a similar study in the Maryland
portion of the Bay conducted by the Maryland Geological Survey, thus
compatible Bay-wide data will be available to those persons making
interpretations and decisions affecting the region. An equally important
product of the project(s) is information leading to further basic research
problems which will augment the ultimate thrust of the EPA-Chesapeake Bay
Program and the general understanding of estuaries.
This report is organized with separate, generally complete discussions
of the sedimentology of the Virginia portion of the Bay and of the
volumetric cut and fill within that area. The discussion of the overall
sediment budget draws the sedimentology together with the volumetric
changes in the quantity of the surface sediment and additional information
in an attempt to determine and to rank the various sediment sources. The
several large sets of data which were created during the life of the
project and which were used in the intrepretations are included as separate
appendices or are available from Virginia's State Water Control Board and
its files in E.P.A.'s STORET data system.
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II
CONCLUSIONS
1.) The bottom sediments of the Virginia portion of the Chesapeake
Bay are significantly sandier than has been previously reported. The
difference is attributed to the much higher sampling density used in this
study rather than to gross changes in sediment characteristics through
time. However, there are locations where transitions may have occurred
during extreme events.
The high sampling density disclosed that grain-size gradients are very
steep in the transverse sections, generally a reflection of bathymetry.
The very fine grained sediments (mud) are generally restricted to the
deeper channels or the lower energy environments associated with shallow,
marginal embayments. The principal floor areas with mud sediments are the
axial channel and basin between the Potomac and Rappahannock tributary
estuaries. In addition the channels leading to the James and York
tributaries are mud as are the entrance channels and basins of the
embayments of Mobjack, and Pocomoke, and Tangier Sounds.
This study corroborates the strong correlations between total carbon,
organic carbon, and sulfur content of the sediment and the weight-percent
clay as previously has been found.
2.) The patterns of deposition and erosion, calculated by comparing
bathymetric data of the 1850's with that of the 1950's, provide very
important insights into the sedimentation processes within the Bay,
particularly when coupled with grain-size information. The deposition
patterns suggest that there is appreciable advection of fine sand from the
Bay-mouth region to at least thirty-five kilometers up the Bay. This is
represented by significant fine-sand and silt deposition in the Wolf Trap
Light region. This locus of deposition is argued to occur as a consequence
of net up-Bay estuarine circulation through the deep channel along the
Eastern Shore (Cape Charles Deep) mediated by the relatively strong tidal
current energies. As well, the contribution of fine and very fine sand to
the broad central basin north of this region is most reasonably
attributable to a Bay mouth source. Thus, the Bay mouth may contribute
very fine sand to as far north as the latitude of the Rappahannock River
entrance.
The immediate Bay mouth region, known to be a zone of active bedload
movement, is characterized by a pattern of "alternating" erosion and
deposition areas as expressed by slowly shifting shoals and intershoal
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deeps. This pattern is consistent with earlier studies of individual
components of the system.
The second region of particular interest is the transition area
between Potomac and Rappahannock tributaries. This region contains the
junction of the deep axial channel leading from Maryland waters which
flares into the central basin of the Virginia Bay, and the junction of the
channels leading to Tangier and Pocomoke Sounds. The main axial channel
and the channel system of the Sounds are separated by the sand shield
containing Tangier and Smith Islands. The western and southern fringes of
the sand shield exhibit appreciable deposition. This is attributed to sand
encroachment over the "edge" of the shield induced by net down-bay
circulation of the surface water layer augmented by wind drift currents and
wave driven resuspension accompanying strong north and northwest winds, a
. dominant component in fall and winter.
The main Bay axial channel from Maryland and the transition to the
Virginia basin is the principal deposition site for clay in the Virginia
portion of the Bay. The clay/silt deposition area is pronounced where the
axial channel flares in cross-section leading into the broad central basin.
Approximately 50% of the total clay accumulation in the Virginia Bay occurs
between the Potomac and Rappahannock tributaries. Silt-sized materials are
somewhat more uniformly distributed throughout the stem; however, about 33%
of the silt accumulates in this transition region, an area of relatively
low tidal-current energy.
The distribution of the fractional accumulations of sand, silt and
clay suggest that the principal clay sources are from the northern Bay
followed by the Bay mouth, that the principal sources of silt are the Bay
mouth followed by the northern Bay, and that the Bay mouth is the principal
sand source.
3.) The estimates of a sediment budget were constructed for the
] Virginia portion of the Bay using measured values for the contribution from
; shore erosion and residual bottom accumulation, and literature values for
i silt and clay importation from Maryland waters. The residual bottom
I accumulation of silt and clay exceeds the values from the estimated sources
by a factor of 12. The measured values of the silt and clay contribution
1 from shore erosion are an order of magnitude less than previously
estimated. Bottom accumulation of sand exceeds that contributed from shore
erosion by a factor of 40.
Previous attempts at constructing a sediment budget have dealt solely
with suspended sediments and with shore erosion as the sole contributor of
sand. The patterns of deposition and the magnitudes of the sand
accumulation clearly indicate that there is a strong advection of nearshore
sands into the Bay mouth and up the Bay stem. Previous estimates of the
sediment budget using only the suspended component have concluded, using
salt budget arguments, that the tributaries are sinks for materials
transported from the Bay. If indeed the tributaries are sinks for
materials transported from the Bay, then the apparent discrepancies between
-bottom accumulation and the previous estimates of source strength are
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enlarged. If the tributaries are sources rather than sinks, and if the Bay
mouth is a stronger source than hitherto estimated, then the order of
magnitude discrepancy for silt and clay accumulation would be reduced.
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Ill
RECOMMENDATIONS
A. MANAGEMENT RECOMMENDATIONS.
The parameters addressed in this report are not amenable to control by
environmental management agencies. Rather, this study has provided
baseline information on the character of the sediments of the Bay stem, an
identification of the patterns of deposition with an interpretation of the
transporting agents involved, and, finally, an attempt at balancing the
residual sediment accumulation on the Bay floor with potential sources.
However, this does not mean the information portrayed is without utility to
management agencies. Quite the contrary, these results form a foundation
from which many management decisions will be founded when used in
conjunction with other components of the EPA/Bay Program. In particular,
the integration of the results of grain-size patterns and bottom mass
accumulation patterns will allow determination of the spatial accumulation
of trace metals and toxic organic compounds and estimates of their mass
accumulation.
In addition, the results of this study, when coupled with other
components of the EPA/Bay Program, should appreciably improve the
management of the disposal of dredged material.
Heretofore, the selection of sites for the disposal of dredge material
within the Bay stem has been made on economic considerations and on very
scanty information concerning the environmental character and operative
processes. As a result of the EPA/Bay Program, several very important
elements may be integrated to choose disposal sites:
1.) The patterns of deposition may be used to identify areas of
natural accumulation and therein sites where the dredged materials are
likely to be relatively stable.
2.) The bottom-sediment grain-size information may be compared with
that expected for the dredged material and the sites may then be evaluated
as to whether the benthic community colonizing the disposal area have
greater or lesser resource values.
3.) The expected tidal currents and residual circulation at the
potential sites may be estimated from the finite element hydrodynamic model
generated in the Eutrophication Program. When the aforementioned elements
are combined with bottom slope, salinity, water depth and expected wave
energy, potential disposal sites may be ranked relative to expected short
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and long term stability, and whether the areas, when recolonized, are
likely to have an altered benthic community as well as their relative
resource value.
B. RESEARCH NEEDS
1.) It is evident that considerable additional study is needed to
gain improvements in construction of a sediment budget. In particular,
research must be focused on the question of the flux suspended sediment
through the mouths of the major tributary estuaries with emphasis on fate
of materials discharged from the Potomac and the Rappahannock. The results
reported herein suggest that the Bay mouth may be a strong source for silt
as well as fine sand; further study is needed to evaluate the strength of
this source.
2.) To date there have been three major studies on the grain size
characteristics of bottom sediments of the Virginia portion of the Bay
stem. This report provides a comparison of these studies with a resulting
interpretation that the gross patterns are generally invariant with time.
Since the previous studies utilized very sparse sampling densities,
additional periodic sampling at a subset of the grid sites sampled in this
study should be undertaken to test the hypothesis that the patterns are
stable. Since it is the reservoir characteristics of the fine-grained
sediment are of principal interest, the subset of stations should be
focused in the areas of primarily fine-grained sediments.
Particular attention is warranted on flood events as deposition of
fine-grained sediments may be more widespread during these events. If such
widespread deposition does occur, then follow-on sampling should be
conducted to investigate the fate of the "thin" layer. The object would be
to determine how much of the material is "folded" into the sediment column
by bioturbation and how much is simply resuspended and advected into the
principal regions of deposition of muddy sediments.
3.) The interpretation of patterns of deposition and associated
grain-size characteristics has been based upon heuristic arguments about
estuarine circulation and dominant pathways of flow. Such interpretation
is tenuous until tested with a comprehensive set of measurements of the
vertical distribution of currents and density. An important contribution
toward that goal will be met through the ongoing study of circulation in
Bay conducted by NOAA/NOS. Analysis of this data set will provide the
framework to test the interpretation of the gross patterns. As well, it
will provide the background necessary to design further specific studies of
particulate resuspension and transportation in the various subareas.
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IV
BACKGROUND OF THE STUDY
The main stem of the Bay and its tributary estuaries form a system;
the physical characteristics of the system have evolved through time with
the current circumstances representative of conditions only slowly changing
over the last two or three thousand years. As viewed today the system
represents a trap for sediments entering the system from the fluvial
drainage and, as well, for materials entering the mouth of the Bay by an
impressed estuarine circulation.
It is important that the management strategies formulated for the Bay
incorporate the realization of a naturally changing system. The purpose of
this chapter is to review the geologic history of the Bay region and to
review the status of understanding of its sedimentological characteristics
prior to this study.
A. GEOLOGIC HISTORY OF THE CHESAPEAKE BAY REGION
The geologic history of the Chesapeake Bay spans time scales that
differ by orders of magnitude. The region is related to happenings
hundreds of million years ago and to modern sedimentation that sometimes is
governed by man-induced changes that occur within a
decade. Nevertheless, the parts can be fitted into a single, internally
consistent narrative.
Five to six hundred million years ago North America consisted only of
a low continent centered around what is now Hudsons Bay. The shoreline ran
approximately through the present day Great Lakes and southern Ontario.
The region that was to become the Chesapeake Bay lay hundreds of miles
offshore near the edge of the continental shelf where thick sequences of
muds and sands were being deposited (Hallam, 1974).
While this was taking place the continents of North America, Africa,
and Europe were drifting together. By the end of Permian time,
approximately 225 million years ago, the continents had been forced
together. The sediments caught between these huge plates were folded,
faulted, and metamorphosed into the schists, gneisses, slates, and other
crystalline rocks which now form the Piedmont and on which the coastal
plain sediments have since been deposited.
The collision was to be temporary, for during the Triassic the
continents began to drift apart. Huge faults cracked the continental edges
dropping blocks of the continental downward to form enormous rift valleys,
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similar to those in east Africa today, where the same process is taking
place. Sediments worn from the mountains, lava flows and ash poured into
the basins and, in some cases such as the basin which had formed near what
is now Richmond, coal swamps formed. These deposits, stacked in their
ancient valleys, are the Triassic "red beds" which can be found in basins
from the Maritime Provinces and New England to the Carolinas.
Ultimately the cracks between continents widened and the sea invaded
to form the beginning of the Atlantic Ocean. Rivers from the Appalachian
regions spread fresh water deposits over the low area at the continental
edge. These are the non-marine sands and silts of the Potomac Group of
Cretaceous age.
The widening continued. Local and regional uplifting and downwarping
took place as the continent adjusted to new loads of sediment and to forces
associated with continental drift. The sea inundated the area of the
mid-Atlantic states with the result that the nonmarine sediments of the
Cretaceous grade upward into the younger marine late Cretaceous and early
Tertiary materials from about 65 million years to perhaps 25 million years
age.
As formations of early Miocene age are missing, we infer that the sea
must have withdrawn until about the middle Miocene, perhaps 15 to 18
million years ago, after which time the sea returned and layers of marine
sands, sandy clays, clays, and shell beds were deposited as the Chesapeake
Group (Calvert, Choptank, St. Marys, and Yorktown formations of
Miocene-Pliocene age). This continued until perhaps 2 million years before
the present.
It is during this middle Miocene-Pliocene episode that the broad
outlines of the Chesapeake Bay are thought to have formed. Stephenson,
Cooke, and Mansfield (1933) pointed out that uplift took place during
Calvert time beginning in, or north of, Maryland, and spread southward
until the seas of Yorktown time receded. Contemporaneously with the
tipping-off of the seas, sands and gravels poured from the Delaware,
Susquehanna, and Potomac Rivers over the newly emerged coastal plain,
forming the land mass of what is now New Jersey and the Eastern-Shore of
Maryland, and Delaware. The rivers themselves adjusted their courses
around the sands and gravels, the Delaware ultimately flowing between the
coarse outwash plains of New Jersey and Maryland-Delaware, and the
Susquehanna and Potomac into the basin between the Maryland Eastern Shore
and the eroded uplands of the western shore.
We have sketched a history which shows that the basin which was to
become the Chesapeake Bay had formed before the Pleistocene and extended at
least as far seaward as the last Pliocene sea. The Virginia's Eastern
Shore, the lower portion of the Delmarva Peninsula, had not yet formed.
This brings us to the most recent geologic acts which formed the Bay;
namely, the glacially induced sea-level changes of the Pleistocene. The
Chesapeake Bay fills a broad, shallow valley which was cut, or in our
opinion modified, by Pleistocene rivers during multiple lowered sea levels
-------
and subsequently flooded by the rise of the sea during the past ten
thousand years to produce the modern Bay. This clearly seems to have
happened. However, the complete story is more complicated.
In general, sea level remained reasonably close to present sea level
between 2 x 10 and 1.5 x 10° years ago when a very high stand
approximately 30 meters (100 feet) above the present occurred. This
elevation coincides with that of the Surry Scarp, a prominent geomorphic
feature in Virginia's coastal plain. Following that very high stand, the
sea was close to or slightly above the present level for the next 500,000
years. Belknap and Wehmiller (1980) believe the core of lower Delmarva
formed during this million years between 2 x 10° and 1 x 10" years B.P. If
this be true, then the basin enclosing the Chesapeake Bay was virtually
formed and was filled with marine water to approximately its modern shores
by about 1 million years ago. Figure 1 is a composite of sea-level changes
based upon the work of Shackelton an Opdyke (1973) and van Donk (1976), as
modified by Zellmer (1979).
It is not the purpose of this paper to attempt to unravel the details
of these multiple lowerings except in a general way. However it is clear
that when sea level was so low that the entire Bay was drained, the basin
must have been occupied by rivers in deep channels. Each time sea level
rose above these channel margins the rivers were essentially lifted out of
their channels. When sea level dropped the rivers did not necessarily
settle back into their old channels but to one side or the other, except
where the basin was narrow.
The evidence for this is widespread. Schubel and Zabawa (1972, 1973)
reported a major buried channel they took to be an old channel of the
Susquehanna which connects the lower reaches of the Chester, Miles, and
Choptank estuaries. Drilling and seismic studies connected with
construction of the Bay-mouth bridge-tunnel reported by Harrison et al.
(1965) showed three large buried channels. Later a cross-section of the
bridge-tunnel crossing was refined and presented by Meisburger (1972).
Carron (1979) presented a map showing his interpretation of the old
drainage lines (Figure 2). Inasmuch as our main interest is in the most
recent or Holocene blanket of sediment, we need not concern ourselves with
unravelling the drainage network of the Pleistocene unless in some way it
impacts modern sedimentation.
A knowledge of the most recent rise of sea level is an important tool
in the understanding of the Holocene sedimentation. Approximately 17,500
years ago the shoreline of the Atlantic Ocean was about 100 km (65 miles)
east of the Chesapeake Bay mouth along the break in slope of the
continental shelf. Sea level was approximately 100 meters (300 feet) below
the present level. Much of the shelf was dry land or swamp over which the
river systems draining the Bay area spread sand and gravels. The basin of
the Bay was traversed by rivers confined within their valleys.
If we accept the erosion-deposition model of river activity by Jordan
(1974), the height of the glacial advance would have been a time of river
stability with a tendency for deposition as the glacial age began to end.
10
-------
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-------
Sea level rose approximately 80 meters (250 feet) in the first 7500 years
of deglaciation (17,500-10,000 B.P.), or 1 meter (3.3 feet) per century
(Shackleton and Updyke, 1973). The Bay would have started to flood at a
sea level of minus 15 meters (50 feet) and sediments would have begun to
fill the old channels rapidly. Although the rate of rise of sea level
slowed during the next 6500 years, from 10,000 B.P. to 3500 years B.P., it
still rose approximately 12 meters (40 feet), or about 18 cm (0.6 feet) per
century. The Bay 3500 years ago must have been very nearly the size it is
today. Sea level at that time was aboat 3 meters (10 feet) below the
present. Sea level has risen at an average rate of only 10 cm (0.3 feet)
per century in the past 3500 years (Newman and Rusnack, 1965).
It is perhaps self-evident that the level of the sea with respect to
the land can change because the volume of the sea increases or decreases,
or because the land rises or falls, or because of some combination of the
two. On the other hand, it is usually impossible to know which is the
mechanism at any one place, unless of course eustatic or world wide
sea-level is unchanging. For purposes of this discussion it is not
critical that we know the cause but it is very important to know the rate.
Mariner (1949) reported that sea level, as reported on tide gauges, was
rising along the entire east coast. Hicks and Crosby (1974) reported that
sea level at Hampton Roads and Portsmouth, Virginia, had been rising at a
rate of approximately 30 cm (1 foot) per century, 3 mm (0.01 foot) per
year, since 1928. Further confirmation that sea level is rising over the
region of the Chesapeake Bay comes from precise re-levelling between first
order benchmarks in the Bay area coupled with tidegage data, Figure 3
(Hohldahl and Morrison, 1974). The authors attempted to remove the
eustatic effects by substracting an assumed world wide rise of 1.0 mm per
year from their measurements. If correct, the entire Bay area appears to
be sinking tectonically but not everywhere at the same rate.
B. RECENT SEDIMENTOLOGICAL WORK
The earliest survey of the bottom sediment characteristics was that of
Ryan (1953) wherein he obtained 209 samples along transverse sections of
the Bay and the river mouths. Eighty-six of those stations were in the
Virginia section of the Bay. From the skeleton framework Ryan inferred the
spatial patterns of texture on the basis of an implied depth-texture
association. An improved portrayal of the spatial patterns in the Virginia
part of the Bay resulted from the work of Shideler (1975) who occupied 200
stations, again along transverse sections. This work provided clearer
definition of the areas dominated by mud. He observed the sands to be
relatively coarse in shallow water and to be very fine in deeper water in
association with mud. The differentiation of the sand sizes was
interpreted to be the result of wave energy with the coarser, fringing
sands representing a lag deposit from shoreline erosion and the very fine
sand in deeper waters representing the wave-winnowed fraction transported
into deeper water. In the lower portion of the Bay, from the York River to
the Bay mouth, were stringers of medium-grained sands which did not appear
to be depth controlled. These deposits were interpreted as being partially
reworked relict materials.
13
-------
-------
There are no previous attempts at constructing a sediment budget which
includes the sand component for the Virginia portion of the Chesapeake Bay.
Schubel and Carter (1976) formulated a suspended sediment budget for the
entire Bay stem utilizing a salt budget argument and field measurements of
Bay axial suspended sediment distributions during 1969-1970. They argued
that in the lower Bay shore erosion may be the largest source of inorganic
sediment. In addition they calculated that the input of suspended sediment
through the Bay mouth was a strong source and that the tributary estuaries
were, in fact, sinks for sediment materials in the Bay.
15
-------
V
METH6DS
The methods used in this study matched, to the extent possible, those
of the Maryland Geological Survey's (MGS) parallel study of the Maryland
portion of the Bay. The two studies used essentially identical protocols
for the analyses of sediment characteristics and chemistry. However, the
treatment of the rates of deposition and the information derived therefrom
were somewhat different due to differences in the availability of data from
bathymetric surveys and in the formatting for automatic data processing.
A. CHARACTERIZATION OF SEDIMENTS
1. Sample Collection. Two basic considerations, providing sufficient
coverage to delineate gradients in grain size and efficient utilization of
the time available, drove the design of the sampling density and pattern.
Moreover, the Virginia grid was designed to be as compatible as with the 1
square kilometer grid that the MGS already had established for their
project. The resulting "diamond shaped" sampling pattern in Virginia is
based upon the Universal Transverse Mercator (UTM) 1,000 meter grid. The
sample sites were at the intersections of even numbered rows with even
numbered columns and odd numbered rows with odd numbered columns. This
plan resulted in a nominal, minimum spacing of 1.4 km. The total field
collection was 2,172 sample sets from 2,018 locations (Figure 4).
Bottom samples were acquired with a stainless steel Smith-MacIntyre
grab sampler which has a volume of approximately 0.01 m . When the sampler
was on deck at least two subsamples were taken from the sediment surface.
Surface samples were skimmed from the top centimeter for the carbon-sulfur
analysis. These were placed in a labeled plastic vial and promptly
refrigerated or iced. The second subsamples were several hundred grams of
material from the top 4 to 6 centimeters. These were placed in large
plastic envelopes with top fasteners and, although not refrigerated, care
was taken to avoid long exposure to environmental extremes. Special scoops
were designed and fabricated (Figure 5) for the two sample sets so that a
uniform rectangular cross-section was plugged from the larger sample.
Using these devices avoided the bias introduced by inserting a circular
cross-section sampler into a sediment with vertical gradients in any
parameter of interest.
As part of the Quality Assurance Program, discussed later, so as to
minimize the possibility of losing the sampling information, two logs were
maintained. A "sample log" contained the date, sample code number as
recorded on the sample container, and nominal site location. The "cruise
16
-------
Figure 4. A map of the Virginia portion of the Chesapeake Bay showing
the location of subareas and sample stations.
-------
Figure 5. Photograph of sample scoops that were fabricated for use in
this project. The larger device, shown with sample bag, was
used to collect the large, bulk sample of the top 6 cm of
sediment for grain-size and water-content analyses. The
smaller device was used to skim a sample from the top 1 cm
of the sample for carbon and sulfur analyses.
-------
log" contained, in addition to this information, the time of collection,
water depth from an echosounder, name of the sampler, and a description of
the sediment surface and of the materials recovered. All sampling was
conducted from the Research Vessel Captain John Smith (Figure 6).
Sample stations were located by navigating the research vessel to
points defined by LORAN C signals; the predetermined values for the LORAN C
signals being obtained from tables supplied by the Defense Mapping Agency
through EPA and by applying a "Bias Correction Factor" to the listed
values. The tabulated values are theoretical and do not consider
variations in electromagnetic signal propagation across land masses and
across the land-water interface. Bias correction values were determined by
comparing the observed LORAN C values for known points with the theoretical
values for the same locations. The correction data is given in Appendix 1.
Once on station, LORAN C readings were automatically printed on paper tape.
The tapes were attached to the sample log-sheets. Later the readings for
each station were averaged, "un-corrected", and entered into a computer
program which yielded the latitude and longitude of the actual station.
In order to check the ability to return to actual sample sites, at the
conclusion of the sampling program, nineteen sample sites were relocated to
the averaged LORAN coordinates. Comparison of the calculated latitudes and
longitudes for the nineteen pairs of stations yielded an average return to
within 30 meters of the original location. With the ability to return to
sample sites as located by the observed LORAN or by latitude and longitude
as determined by other means, future researchers should be able to very
closely approximate the locations of the sites sampled.
Samples were collected on 75 days spread over eight months in 1978 and
1979. Sampling was very slow during the winter months when days were
short, windy, and icy. Milder spring weather brought with it a great
increase in productivity; Table 1 details the specifics of sampling.
TABLE 1
Schedule of Sampling
Year Month Days Samples Per Month
1978
1979
November
December
January
February
March
April
May
June
13
5
10
5
12
13
15
2
160
104
238
112
462
525
547
26
2. Analysis of the Sediments. As mentioned previously, two sample
subsets were collected at each station (aside from replicates). The
-smaller subsample, for carbon and sulfur analyses, was taken from the
19
-------
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cruise on ice and then frozen and held until pretreatment for analysis.
The larger subsample, secured for water content and size analyses, upon
delivery to the laboratory was mixed and split into at least three
subsamples. One was labeled for the archive, another was stored pending
granulometric analysis, and the third was promptly analyzed for bulk water
content. Figure 7 is a flow chart of the analytical procedures.
a. Water Content. The water contents were determined by placing the
sample in a preweighed beaker, weighing it, drying it at 65°C and weighing
again; the weight difference being the weight of the water. The percent
water content was calculated with the formula
u
u = "w ,nn
wc 77- x 100,
WT
where Wc is percent water content, Ww weight of the water, and W>j the
weight of the sediment-water mixture. No attempt was made to compensate
for the weight of salts from evaporation.
b. Grain-Size Analysis. As the sediments range from granules to clays, it
is necessary to employ different analytical techniques on different
fractions of the sample. The sand fraction was analyzed in a Rapid
Sediment Analyzer (settling tube), the granules by conventional sieving,
and the fines by Coulter Counter. The different data sets from each sample
were then joined in a miscegnatious marriage by the computer.
All the samples received the same pretreatment, separate digestions
with HC1 and I^C^, washing, fluid removal through filter candles, dispersal
in an ultrasonic bath, and wet sieving through a 4 0 (63 ym) screen.
Table 2 is a listing of phi, 0, classes and the equivalent metric
sizes. Phi, a logarithmic transformation of the linear metric measurement,
is calculated with the formula 0 = -Iog2 (diameter in millimeters).
Because phi is logarithmic and is, in one sense, a measure of the interval
between classes (McManus, 1982) it is inappropriate to compare phi and
metric standard deviations.
The material passing the screen was transferred to a 1,000 ml cylinder
and processed by conventional pipette techniques, including dispersants,
for total weight of material, and percents silt and clay. An additional
withdrawal of the total sample was taken and kept for analysis on a Coulter
Counter Model TA or, later in the project, TA II. This analysis was
performed using standard 2-tube (140 ym and 30 ym apertures) techniques
(Coulter Electronics, 1975 and revisions). Shideler (1976) discusses the
differences between standard pipette data and Coulter Counter data.
Further discussion is provided in the section on Quality Assurance. It
should, however, be remembered that neither pipette nor Coulter methods
give direct measurement of grain size. The former yields grain by
equivalence in fall velocities of the natural particles and spheres of a
given specific weight. The latter converts electronically estimated
particle volumes to spheres of equal volume.
21
-------
FLOW CHART
SEDIMENT ANALYSIS
VIRGINIA CHESAPEAKE BAY BOTTOM SEDIMENTS
| GRAB SAMPLE |
LARGE PLUG OF TOP
4 -6 cm INTO LARGE
WHIRLPAK
I
STORE OUT OF
DIRECT SUN
1
TRANSFER TO LAB
SCRAPING OF TOP
Icm INTO VIAL
I
REFRIGERATE
I
TRANSFER TO LAB
REFRIGERATE
DISCARD OVERBOARD OR
USE FOR "DUPLICATE"
SAMPLE (QUALITY ASSURANCE)
HOMOGENIZE
I
SPLIT
INTO PREWEIGHED
BEAKER
ARCHIVE
WEIGH
I
DRY
I
WEIGH
CALCULATE
WATER CONTENT
DISCARD
POWDER
SPLIT
I
HCL
DIGESTION
WASH
I
LECO
PROCEDURES
ORGANIC CARBON
CONTENT
I
LECO
PROCEDURES
TOTAL CARBON
CONTENT
LECO
PROCEDURES
SULFUR CONTENT
REMOVE LARGE
SHELL FRAGMENTS
DIGEST WITH HCL
I
WASH
FILTER CANDLE
WEIGHTS OF GRANULE, SANDS, SILTS, AND
CLAYS, AS DETERMINED IN THE SEPARATE
ANALYSES ARE SUMMED TO YIELD THE TOTAL
WEIGHT OF THE DRY SAMPLE THIS TOTAL
WEIGHT IS USED IN THE CALCULATIONS OF
THE RATIOS OF THE SIZE GROUPS AND THE
WEIGHT PERCENT OF EACH SIZE CLASS.
IF MOSTLY SAND
I
DIGEST WITH
APPROXIMATELY 8% M,O,
IF MOSTLY FINE
DIGEST WITH 15% H20,
HOT PLATE
I
WASH
I
FILTER CANDLE
I
ADO OISPERSANT
I
ULTRASONIC DISPERSAL
I
WETSEIVE THROUGH
63 MICRON SCREEN
INTO lOOOml CYL,
SAND AND GRANULE
INTO FILTER PAPER
AND EVAPORATIN6 DISH
I
DRY
GRANULE
DRY SIEVE
2mm SCREEN
_J 1
SAND
I
WEIGH
I
CALCULATE WEIGHT
OF GRANULES
J;
IF FLOCCULATION
~\
HEAT
DO NOT BOIL
DRY SIEVE FOR
SIZE DISTRIBUTION
ARCHIVE
I
WEIGH
I
CALCULATE WEIGHT
OF SAND
J;
BRING UP TO 1000 ml
AGITATE
I
COVER CYLINDER
TO PREVENT EVAPORATION
1
LET STAND AND VISUALLY
CHECK FOR FLOCCULATION
ARCHIVE
RSA
PROCEDURES
PER GRAIN
SIZE ANALYSIS
PIPETTE ANALYSIS
FOR TOTAL WEIGHT
OF FINES AND SILT
COUNTER ANALYSIS
FOR GRAIN
SIZE DISTRIBUTION
Figure 7. The flow chart for the analysis of sediments.
-------
Table 2
Grain-Size
Nomenclature
m
-2
-1
f
0
1
1.5
2
2.5
3
3.5
4
4.5
5
6
7
8
9
10
Granule
Very coarse
Coarse sand
Medium sand
Fine sand
1/2
1/4
1/8
1/16
1/32
1/64
1/128
1/256
1/512
1/1024
sand
Very fine sand
Silt
Clay
4
2
1
0.5
0.35
0.25
0.177
0.125
0.088
0.0625
0.044
i
-20 - -10
-10 - 00
00 - 10
10 - 20
20 - 30
30 - 40
40 - 80
80
500
350
250
177
125
88
625
44
31
15.6
7.8
3.9
20
0.98
Some researchers use 90 as the silt-clay
boundary.
Sediments finer than 40, that is both silt
and clay, are muds.
23
-------
If the sample were less than 10% mud (silt + clay), the analysis by
Coulter Counter was omitted, leaving only the 4 0 and 8 0 pipette data to
describe the distribution of the fine tail of the distribution. The 10%
cutoff was used for the first thousand or so samples; 5% was used for the
remainder.
The sand-size portions of the samples, that is the material held on
the 4 0 wet sieve and passing a -1 0 dry-sieve, were weighed, and passed
through a microsplitter until a 0.5 to 1.0 gram subsample was obtained.
This subsample was for analysis on the" Rapid Sediment Analyzer (RSA)
(Figure 8). The remainder was packaged and stored to be available for
other researchers. The RSA has a 1.5 m fall distance and is similar in
design to that described by Gibbs (1974) and is essentially identical to
the device used by the Maryland Geological Survey. The data delivered by
the RSA is in the form of a strip chart depicting proportion of sediment
fallen versus time since introduction of the sample to the system. The
strip chart was then processed on a Numonics 1224 Graphics Calculator which
served to put the fall velocity data onto computer compatible magnetic
tape. This information was converted to size data by a computer
application of the Gibbs, Mathews, and Link (1971) formula
r = 0.055804 V2 pf + [0.003114 V4 pf2 + (g(ps - pf) (4.5 V + 0.008705 V2ps))]1/2
g (ps - pf)
where r is sphere radius in cm,
V is fall velocity in cm/sec.,
Pf is fluid density in g/cm ,
ps is density of the sphere in g/cnH, 2.65 gm/cm-* was assumed,
g is the acceleration of gravity in cm/sec.2, 980 cm/sec2 was
used, , ;
H is the dynamic viscosity of the fluid in poises. :
As the fluid density and dynamic viscosity vary with temperature,
appropriate values from tables for distilled water were used.
If the sample were more than 5% granule, the particles coarser than -1
0 were sieved at 1/4 0 intervals in the conventional manner.
All the procedures and methods were, in so far as possible,
standardized with those used in a parallel project conducted by the
Maryland Geological Survey.
The weights of each major size class, granule, sand, silt, clay, were
summed and the ratios were calculated. As each separate class analysis,
Coulter Counter, RSA, sieve, was referenced to 100%, it was necessary to
multiply each separate phi-class by the respective granule, sand, or mud
24
-------
Figure 8. The Rapid Sediment Analyzer (RSA) as constructed at V.I.M.S.
-------
ratio to obtain the proportion of each class relative to the entire sample.
Similarly, phi classes common to two modes of analysis were algebraically
summed in a form of splined fit.
The standard graphic measures of Folk or Folk and Ward (Folk, 1974)
and the first four moments plus moment skewness and moment kurtosis were
computer calculated for each sample.
Graphic Measures
<*
Median = 05Q
Graphic Mean
M = 016 + 050 + 084
L* -
Graphic Standard Deviation
G = 084 - 016
2
Inclusive Graphic Standard Deviation
I = 084 - 016 + 095 - 05
4 6.6 ;
Inclusive Graphic Skewness
SK = 016 + 084 - 2050 + 05 + 095 - 2050
2(084 - 016)2(095 - 05)
i I
i
Graphic Kurtosis
KO = 095 - 05 ;
2.44(075 - 025)
Moment Measures
1st Moment =
100
where f is frequency percent of each 0 class, m0 is midpoint of each
class. By definition the first moment equals the mean, X, of the
distribution.
2nd Moment = f(m0 - X)2
100
26
-------
The 2nd moment is the square of the standard deviation.
3rd Moment (mean cubed deviation) = f(m0 -
Too
4th Moment = f(m0 - X)4
100
Moment Skewness = 3rd Moment
standard deviation cubed
Moment Kurtosis = 4th Moment
standard deviation to the 4th power
c. Clay Mineralogy. The methods used for the semi-quantitative
determination of the clay mineral contents of the sediment were quite
simple. A subset of the 2 pm (9 0) and smaller sediments was obtained by
standard pipette methods based on Stokes" Law fall velocity. The particles
were concentrated by centrifuge. This material was pipetted onto glass
slides and dried at 60°C. Another set of slides was dried then heated over
glycol. Finally, a third set of some slides was heated to higher
temperatures. The slides were analyzed in a General Electric X-Ray
diffractometer at approximately 43 KV and 30 ma. The analyses used a 2"
aperture silt and a 2° 20 per minute scan speed. Each slide was scanned
from 2° 20 to approximately 28° 26. The data was recorded on 10-inch wide
chart paper at 1° 2 per inch. The relative proportions of the various
major clay minerals was estimated by the ratios of the heights of the
diffraction peaks for each mineral.
d. Carbon and Sulfur Analysis. Approximately 900 samples were selected
for carbon and sulfur analyses. As the primary interest was in the
chemical alliances with the finer grained sediments, the samples were
selected on the basis of an inferred minimum of 15% mud by weight. It was
necessary to use percent water content to infer mud content because while
the water content analyses were complete for most samples, the
granulometric work had just begun. On the basis of 500 samples there was a
strong correlation, r = 0.96, between mud and water contents. Also, as all
the carbon and sulfur analyses were to be in at least duplicate, it was
necessary to reduce the number of samples to a manageable level.
The frozen samples were thawed, oven dried at 50°C, powdered with a
mortar and pestle, and divided into several aliquots with a microsplitter.
The aliquots were weighed and placed in small vials. The subsamples for
sulfur and total carbon analysis received no additional pretreatment. The
samples to be analyzed for organic carbon were treated with 10% HC1 until
evidence of continuing reaction ceased, then they were washed with
distilled, deionized water, decanted, oven dried, and weighed. Final
analyses were done in at least duplicate on a LEGO Gasometric Carbon
Analyzer, Model 572-100 and a LEGO 532-000 Titrator for sulfur. Both
analytical instruments were operated in conjunction with a LEGO 523-300
-Induction Furnace. The averages of the duplicate analyses are the reported
27
-------
values. Inorganic carbon may be calculated as the difference between total
and organic carbon. Samples with differences between paired readings that
were outside acceptable limits were re-analyzed with the result of the
multiple values showing less variation than the initial pair.
The LEGO carbon analysis equipment is relatively common and has been
discussed elsewhere in the literature (Leventhal and Shaw, 1980). The
method for determination of sulfur, however, is not so common, and is
described as follows (LEGO, 1975).
"The sample is burned in a stream of oxygen at a sufficiently high
temperature to convert about 90 to 97 percent of the sulfur to sulfur
dioxide. A standardization factor is employed to obtain accurate
results. The combustion products are passed into a dilute acid
solution containing potassium iodate, potassium iodide and starch
indicator. The blue starch iodine complex thus formed is bleached by
the SC>2. As combustion proceeds, bleaching the blue color, more
iodate is added to return to the original blue color. The amount of
standard iodate consumed during the combustion is a measure of the
sulfur content of the sample."
3. Quality Assurance. All aspects of the laboratory work were
subjected to a quality assurance program. The following text describes
some of that program.
a. Shared and Duplicate Samples. Several samples were split so that
analyses could be performed in the sediment laboratories of both the
Maryland Geological Survey and the Virginia Institute of Marine Science.
The primary data, sand:silt:clay ratios agree quite closely (Table 3A&B and
Figure 9). In all cases except one, No. 7, VCB 1626, the samples fall in
the same class. In the single exception, the ratios are very close, but
fall along the sandrsilty-sand boundary. With the samples for which there
are graphic data from both labs, the measures generally agree quite
closely. The exceptions are the very fine grained samples, No. 2, VCB 784
and No. 3, VCB 785, where very minor differences in measurement, in both
cases 0.003 mm in the graphic mean, appear disproportionately large on the
0 scale. i
Additionally duplicate samples were collected at 135 stations. These
samples were packaged as ordinary samples and returned to the laboratory
for routine analysis; the laboratory staff not being aware which samples
were the check samples. Although the data on the duplicates have not been
subjected to a rigorous statistical examination, a qualitative review
indicates a very close and satisfactory agreement between duplicate
analysis.
b. Calibration of the Rapid Sediment Analyzer. The calibration of the RSA
constructed at VIMS followed procedures that were described by the Maryland
Geological Survey (Halka, et al.). The MGS provided VIMS with a set of
glass beads of known diameters and densities. Members of the staff at the
MGS had inspected the beads for sphericity, sieved them at 1/4 0 intervals
for size classification, and floated them in various heavy liquids to
28
-------
Table 3A
Samples from the Virginia Portion "Of the Bay Analyzed by Both VIMS and MGS
Plotted % % % % Graphic Mean Graphic Std Dev
Sample
783 M
; v
784 M
V
785 M
V
786 M
V
1624 M
V
1625 M
V
1626 M
V
' 1627 M
! v
as
1
2
3
4
5
6
7
8
Gran. Sand
39.3
33.5
0.5 4.0
1.5
92.2
87.9
97.5
97.3
88.2
7.3 82.3
74.0
0.4 72.9
67.2
1.2 57.0
Silt
31.5
36.5
42.3
44.9
41.4
42.0
3.3
4.8
2.0
1.3
6.3
6.2
14.2
17.8
2.3
31.6
Clay
29.2
29.9
57.7
50.6
58.6
56.5
4.5
7.2
0.6
1.3
5.5
4.2
11.8
8.9
9.8
10.1
0
6.3
5.7
8.7
7.5
8.9
7.5
2.2
2.3
1.9
1.9
1.3
1.2
4.3
3.9
4.2
4.1
nun.
0.013
0.019
0.0024
0.0055
0.0021
0.0055
0.22
0.20
0.27
0.27
0.41
0.44
0.051
0.067
0.054
0.058
0
3.3
2.3
2.7
1.8
2.8
1.8
1.1
1.3
0.6
0.5
1.9
1.7
2.0
1.3
2.0
1.5
mm.
0.10
0.20
0.15
0.29
0.14
0.29
0.47
0.41
0.66
0.71
0.27
0.31
0.25
0.41
0.25
0.35
29
-------
Table 3B
Samples from the Maryland Portion of the Bay Analyzed by Both VIMS and MGS
Sample
R G
1148.51
1162.52
1138.03
1175.85
; 1191.14
1164.09
1269.09
| 1180.58
i
; 1113.06
1092.45
1071.10
1253.96
1908.50 M
i V
1912.50 M
V
1922.02 M
V
1846.02 M
V
1853.56 M
V
1861.80 M
V
1831.49 M
V
1867.00 M
: V
1815.95 M
V
1845.50 M
V
1764.94 M
V
1825.15 M
V
Plotted
as
9
10
11
12
13
14
15
16
17
18
19
20
% %
Gran. Sand
2.1
4.7 1.8
0.5
0.4
0 6.1
1.0 1.7
15.5
11.0
7.8
6.6
5.3
4.2
78.9
83.1
83.7
83.4
89.9
90.4
94.0
93.5
95.8
90.7
99.2
97.9
%
Silt
23.2
24.8
24.2
24.6
23.7
24.6
29.4
39.5
39.7
44.5
31.6
37.3
15.9
9.8
5.8
6,4
- 10.
4.1
- 6.
2.1
- 4.
0.2
- 0.
0.7
%
Clay
74.7
68.7
75.3
75.0
70.2
72.7
55.1
49.5
52.5
48.9
63.1
58.5
5.2
7.1
10.5
10.1
0 -
5.6
0 -
4.3
2 -
3.2
8 -
1.3
30
-------
o
2
_
>
O
2
31
-------
determine their density. The expected fall velocity of the spheres, as
predicted by the equation published by Gibbs, Mathews, and Link (1971),
served as a standard to which the observed velocities could be compared.
Several aliquots of very nearly 0.5 gram of each separate size class of
spheres were processed through the RSA. (Because of a limited quantity,
the aliquots of the 4 0 spheres were roughly 0.45 and 0.28 gram.) The
elapsed fall time for 50% by weight of the sample to fall through the
column was used as the time for the calculation of the observed velocity of
the mid-class size. Table 4 is a summary of those comparisons. The data
from the test samples of the range 0.25 0 through 2.25 0 agree quite
satisfactorily with the expected values and appear to have good
repeatability. The very coarse and very fine particles, however, pose a
question as the differences between observed and expected values are
significant. The greatest errors occur with the 3.25 0 and 4.0 0 spheres.
Aside from operator and machine errors, there are three possible
sources of error associated with the spheres themselves: 1) that the size
distribution of the sphere within a size class is not as assumed, 2) that
the particle density is not as specified, and 3) that size distribution
errors were introduced in "splitting" the collection of spheres into
smaller samples for analysis. The following example demonstrates the
impact of small differences in size and density on fall velocity as
predicted by the equation referred to above.
In the 3.25 0 class, the radius of the mid-class particle is 3.125 0,
0.00578 cm, and the given density is 2.49 g/cm^. The expected fall
velocity is 0.88 cm/sec., the observed 1.127 cm/sec, for a 28 "percent
error." If the mid-class size were 1/8 0 different, 3.0 0 instead of 3.125
0, the expected velocity would be 1.01 cm/sec, and the error a more
respectable 11.6%. Or if the size assumptions were correct but the density
was 2.92 g/cva- instead of the given 2.49 g/cnr, the expected velocity would
be 1.09 cm/sec, and the error 2.9%. Similar examples of the roles played
by density and size assumptions can be demonstrated with the other size
classes. It should be noted that the size class with the greatest error,
4.00 0, is the one most subject to errors in density measurement due to
particle-surface air bubbles, and to size distribution assumptions. At
small particle sizes, a small diameter change yields a large relative
change in fall velocity.
As a partial test of the likelihood of a size error, optical
measurements were made of the diameters of 10 spheres from each size class.
Table 5 is a listing of the data. Although 10 observations probably is not
a sufficiently large sample to give a highly significant mean, the sign of
the difference most likely is correct. Table 6 is a comparison of the
percent errors, or differences, between expected and observed fall
velocities and sphere diameters. As can be seen, in most cases, the sense
of the errors are the same, perhaps explaining some of the discrepancies in
fall velocity.
Although machine and operator errors are very difficult to assess,
there are specific mechanical problems in introducing the larger test
spheres into the RSA. The size and extreme spherecity of the particles, as
32
-------
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Table 5
Comparison of Expected and Observed
Diameter of Calibration Spheres
0
-1.0
-0.75
-0.5
-0.25
0.25
0.5
1.25
1.75
2.25
3.25
4.0
Expected
Diameter (mm)
2.190
1.840
1.545
1.30
0.920
0.775
0.460
0.325
0.230
0.115
0.06825
Observer
Mean (mm)
2.1008
1.7248
1.5408
1.2224
0.9048
0.7956
0.4684
0.3192
0.2348
0.1148
0.0684
Standard
Deviation (mm)
0.0784
0.0544
0.0830
0.1026
0.0334
0.0262
0.0204
0.0110
0.0103
0.0103
0.0091
% Error
4.1
6.3
0.3
5.8
1.7
2.7
1.8
1.8
2.1
0.2
0.2
NOTES: Predicted diameter is taken as the mid-point for each 0
interval.
Percent errors was calculated as
(100(Vexpectecj - V0bservecj)/Vexpecte(j).
Column 3 is the mean of 10 measurements.
-1 0 through 1.75 0 45 x magnification.
2.25 0 through 4.0 0 90 x magnification.
34
-------
Table 6
Comparison of Percent Errors
In Fall Velocity and Mid-Class Diameter
0
Class
-1.00
-0.75 .
-0.50
-0.25
0.25
0.50
1.25
1.75
2.25
3.25
4.0
% Error
Fall Velocity % Error
MGS VIMS Mid-Class Diameter
1.9
4.9
1.8
1.5
3.7*
3.8*
0.9
5.2
2.2
3.2*
19.1*
13.4
12.2
~ 7.1
9.2
1.0
1.5*
3.8*
1.6
3.3*
28.1*
45.1*
4.1
6.3
0.3
5.8
1.7
2.7*
1.8*
1.8
2.1*
0.2
0.2*
* Observed greater than expected.
NOTES: 4 0 velocity error, VIMS, is average of
2 sets of observations.
MGS fall velocity erro data from draft
reports by Kerhin, Halka, and
others.
35
-------
compared to natural sediments, severely limits the retention of the spheres
on the wetted, convex plate used to introduce the sample in the analytical
procedure. Additionally, there are possible errors associated with the
values for fluid density and viscosity. The values used were published,
tabulated values for pure water, not values measured from the water in the
RSA. Thus, while maintaining a continuing check on the reproducibility of
the RSA data, we accept the RSA calibration and data as satisfactory.
Even though rapid sediment analyzers or settling tubes have been used
for over two decades (Zeigler, Whitney, and Hayes, 1960) to determine the
grain-size frequency distribution of sands, questions still arise
concerning the comparison of analyses done by RSA and by sieves. Because
the two measure different properties, one resulting from a set of variables
concerning both grain and fluid, and other resulting from various facets of
both grain and mesh sizes and shapes, it would be expected that the
distributions depicted by each would differ. This does not state that one
method if better than the other, just that they differ. Sanford and Swift
(1971) published a comparison of data from sieving and settling techniques
in which they found that the results of the two techniques were quite
similar. As a check of the Chesapeake Bay samples, cumulative frequency
plots were made of data from both sieve and RSA of splits of each of 20
sand samples. Figure 10 is an example of one of the comparative plots.
In general, the plot of the RSA data indicates a slightly better
sorted sample than does the sieve data. This difference primarily occurs
in the "tails" of the distributions, usually the first and last 5% or less,
and would note be reflected in the calculated graphic-measures. In the
Chesapeake Bay samples there is no consistent relationship of coarser or
finer as was found by Sanford and Swift. This probably is due to the
differences between the methods used to convert fall velocity, which is
what an RSA measures, to equivalent hydraulic diameter. Sanford and Swift
used Schlee's (1966) fall times as a basis for their work whereas the
present study used Gibbs, Mathews and Links' (1971) formula.
Although the plots do differ, they are quite similar, showing
inflections in slope at similar grain sizes when plotted on probability
paper. This indicates that each method discerns similar mixings of
sediment populations with only minor differences in interpreting the means
and modes of those distributions.
c. Comparison of RSA's at VIMS and MGS. In order to determine whether or
not the measurements made on the RSA at the Maryland Geological Survey, it
was necessary to develop a method for characterizing the precision of an
RSA. It should be remembered that precision is not the same as accuracy.
EPA (1979) defines precision as "the degree of mutual agreement among
individual measurements made under prescribed, like conditions," whereas
accuracy is a measure of the proximity of a measurement to a true value.
The problem at hand is compounded by the fact that there is a neither a
standard sediment nor a standard RSA agains which to gauge accuracy. The
individual calibration process for each RSA, described elsewhere, is an
approximation of an accuracy determination. This section presents a
36
-------
I00-i
99-
90-
80
70
60
50
40
30
20
10
5
'2
I
0.1-
COMPARISON OF SAND SIZE DISTRIBUTION
OF SAMPLE VCB 0288 BY BOTH
*
RSA AND SIEVE
-1/2
SIEVE /
1/2 I I 1/2 2 21/2 3 3 1/2 4
0
Figure 10. Plots of cumulative grain-size frequency distributions of
the sand portion of a sample as determined by both sieve /
and RSA techniques.
-------
characterization of the precision of the RSAs based upon the traditional
model of normal grain size frequency distributions.
To accomplish this characterization, 42 aliquots of the same sandy
sediment (Whitemarsh No. 2 quarry sand, supplied by the MGS) were analyzed
on each RSA. The resulting quarter-phi grain-size-frequency distribution
data were used to calculate the first and second moments of each of the 84
separate grain size distributions. The mean vectors of these two
parameters for each RSA were determined and compared. The method of
comparison was the Retelling T^ test which is a statistical test of the
equality of mean vectors.
The formulae for the calculation of the moment measures, where the
mean is equal to the first moment and the standard deviation equal to the
square root of the second moment, were taken from Friedman and Sanders
(1978). The first moment is calculated as
Hi = zfM0
100,
where f is the frequency percent in each 1/4 0 size class, and M0 is the
mid-point of the class. The second moment, M2, is
Zf(M0 --X)
< 100
where the mean, X, is the first moment. This then is a distribution with
two variables (bivariate) on each of many samples from the larger overall
sample.
For all practical purposes, the RSAs at VIMS and at the MGS are
mechanically identical. The methods of treating the data vary slightly.
At the MGS, the instrument is wired to a microprocessor and the conversion
: of cumulative weight percent and fall time to grain size is
: "instantaneous," whereas at VIMS it is a two step procedure. The data
I product of the RSA is a paper strip chart which has to be digitized in
: order to allow computer transformation of distance to time, to velocity, to
grain size.
Eighty-four subsamples, each of approximately 0.5 gram, were split
from a homogeneous "archive" sample, half were analyzed at each sediment
lab. The frequency distribution data for the quarter phi classes from -1.0
0 through 4.0 0 (very coarse through very fine sand) were used to calculate
moment means and moment standard deviations of each subsample.
The measurement error, precision, herein considered is that which
occurs within each variate among all the samples measured. Because there
are many samples, we can assume the measurement error follows
the central limit thereon, which states, "If random samples of fixed size
are drawn from a population whose theoretical distribution is of arbitrary
shape, but with a finite mean and variance, the distribution of. the sample
38
-------
mean tends more and more toward a normal frequency distribution as the size
of the sample increases" (Koch and Link, 1970). Thus, one can use the
Retelling T2 test as a statistical means of comparison between the
population of measurements generated by each RSA. This test is used to
judge the hypothesis that the two mean vectors are equal. The mean vectors
of each variate of the data from the RSA at VIMS is tested against the
corresponding mean vector of the MGS data.
The Hotelling T2 statistic is calculated as
- x2] [s2]-i [xx - x2]
nj + n2
with (nj + n2 ~ 2) degress of freedom. [Xj - X2] is the difference between
the two mean vectors. [Xj - X2]l ^s c^e transpose of that difference and
[S2,] ~1 is the inverse of the pooled estimate of the variance-covariance
matrix. The test is the multivariant equivalent of the univariate t test,
. - t = Xj - X2
bp
where Xj and X2 are sample means, Sp is the pooled estimate of the standard
deviation (based on both samples), and n^ and n2 are the number of
observations in each sample.
For the Hotelling test, the calculated T2 is compared to tables of T2
distribution for a particular level of significance, based upon the number
. of variates and the degrees of freedom. If the calculated value exceeds
the tabulated value, the test supports rejection of the hypothesis of
equivalence. If the calculated T2 is less than the listed value, the
Hotelling T2 test supports the acceptance of the hypothesis that the mean
vectors are equivalent at the specified level of significance. If this is
' the case, the information yielded by the two RSAs can be considered
I equivalent.
Computer programs in the Statistical Analysis System and SAS Users
Guide (1979) were used to determine the variance-covariance matrices and
the Hotelling T2. The difference of the two mean vectors was computed by
determining the mean of each of the two variates, and arranging those means
in a 1 x 2 matrix (a vector) for each data set, and finally subtracting the
VIMS mean vector from the MGS mean vector.
The Hotelling T2 statistic was computed to be 0.442. The value listed
in tables of T2 distribution (Kramer, 1972) at the 0.05 level of
significance with p = 2 and 82 degrees of freedom is 6.303. Therefore, as
the calculated value is less than the tabulated value, the Hotelling T2
test indicates equivalence between the precision of MGS and VIMS Rapid
Sediment Analyzers. In turn this encourages the acceptance of the
assumption that data from the two RSAs may be used interchangeably.
39
-------
d. Comparison of Analyses by Pipette and Coulter Counter. As most of the
grain size distribution analyses reported in the literature for silt and
clay particles utilize the pipette-Stokes Law procedures (Folk, 1974;
Galehouse, 1971), it is advantageous to this report to compare the results
obtained using a Coulter Counter with those of the more traditional method.
Swift, Schubel, and Sheldon (1972), Shideler (1976), and Behrens (1978) all
present dicussions bearing on the subject.
Results obtained by the two methods are not expected to be identical.
Each measures a different characteristic and uses that to infer the
diameter of a sphere having the same characteristic. The pipette method
indirectly measures fall velocity which, when with certain assumptions is
used in the equation for Stokes" Law, yields the diameter (or radius) of a
sphere that has the same fall velocity. Fall velocity is influenced by,
among other things, both the specific gravity and shape of the particle.
As fine particles are platelike, the shape factor is quite important as it
results in fall velocities much lower than that of a sphere of equivalent
volume. The Coulter Counter makes an indirect measurement of particle
volume and assuming that as the volume of a sphere, determines the
diameter. The other significant difference between the methods is the
nature of the distribution each observes. The pipette distribution is open
ended; that is, given proper laboratory conditions, it is possible to
analyze through the full range of silt and clay sizes. Pipette results
yield the cumulative percent-coarser-than, thus if the procedure is not
carried to completion, the results may terminate at appreciably less than
100 percent. The Coulter Counter uses only a limited size range
(approximately 4 0 through 10 0 in this study) and the results are given
relative to the range used. The contribution of the particles outside the
machine's range is omitted from consideration. Thus the distribution as
defined by the Coulter Counter always closes at 100 percent.
With these considerations, one would expect that the distribution
depicted by the Coulter Counter would be coarser and better sorted than
that shown by pipette analysis. Indeed this is noted by both Shideler and
Behrens and is apparent in comparisons made for this study (Figure 11). In
addition to the two reasons just discussed, Shideler offers a third factor
contributing to the Coulter-pipette difference. This is the error that
occurs when two or more particles simultaneously pass through the Coulter
Counter's aperture and are sensed as a single, larger particle. With
proper sediment concentrations, this "coincidence error" should be less
than 3 percent. Behrens maintains that the most significant contribution
to the difference in results between the two methods is the omission
artifact resulting from the arbitrary truncation of the distribution by the
Coulter Counter.
Analysis of 20, or approximately 1 percent, of the Chesapeake Bay
sediment samples by both methods yields results which display the expected
differences. Figure 11 depicts the grain size distributions as determined
by both methods of the mud portion of one of the samples. The figure also
shows a plot of the pipette data adjusted or normalized to the Coulter
Counter end point. The Coulter Counter and adjusted pipette data are
nearly identical, agreeing with Behren's statement. Although not all of
40
-------
DC.
UJ
O
o
UJ
o
DC.
UJ
Q.
99.99 -i
99.9
99.8
ADJUSTED
PIPETTE
0.01
10
Figure 11. Comparison of frequency distributions as determined by
pipette and Coulter Counter techniques. The distribution
as determined by pipette but adjusted or normalized to
the Coulter Counter's end point is also shown.
-------
the plots for the 20 pairs are as close as the example, none vary
significantly. Assumedly, the difference between the end of the unaltered
pipette data and 100 percent is the material finer than 10 0.
The reasons for using the Coulter Counter in preference to the more
traditional pipette method are several. Using the Coulter Counter,
individual analyses can be completed in a fraction of an hour, whereas the
pipette method is much more time-consuming. Although several pipette
analyses can be run concurrently, analysis to 10 0 takes upwards of 15
hours with dramatically greater times required for finer sizes. Galehouse
(1971) states that pipette analysis probably should be ended at 8 or 9 0
unless laboratory conditions can be maintained stable for long periods.
Swift, Schubel, and Sheldon (1972) speak of the advantage of the short time
required for complete analyses by Coulter Counter. It is possible to
interpolate the distribution from the end point of the pipette analysis to
100 percent at 14 0 (Folk; 1974), there is question as to the validity of
the data so obtained (Galehouse, 1971). Also, as the pipette analysis
requires a great number of finely timed, carefully executed steps, there
are a number of opportunities for error. Although the Coulter Counter also
is subject to operator error, there are few separate phases in the analysis
and possibly a lesser opportunity for operator caused errors. Shideler
makes reference to other studies (Interagency Committee on Water Resources,
1964; Allen, 1968) which "indicate that the accuracy of electronic sensing
is probably superior to that of any other commonly used technique suitable
for routine analysis." If the Coulter Counter were not available, it would
have been impossible to perform the number of analyses reported for this
study.
e. Coulter Counter Precision. The precision, or reproducibility, of the
Coulter Counter is, within limits, quite acceptable. Shideler (1976)
stated that comparison of the curves from triplicate analyses of two
samples each analyzed by both Coulter Counter and pipette suggested a
greater precision for the more modern technique. A similar procedure, 20
replicates of each of two aliquots of the same sample, also indicates the
relative precision of the technique. Figure 12 is a plot of the envelope
of one set. When superimposed on the similar plot for the other 20
replicates, the two are indistinguishable from one another, indicating a
high degree of reproducibility. The thickness of the envelopes, however,
detracts somewhat from the confidence one might have in the precision.
This aspect of the precision perhaps is explained by the data in
Figure 13. For the first 10 or so replicates, the reproducibility appears
very satisfactory with no apparent trends in the data. The later
observations, however, tend towards decreasing values, especially for the
coarser particles. This most likely is an artifact of the sample size and
not of the instrument or operator. As successive small quantities of the
dilute sediment-water mixture are withdrawn from the vial for analysis, the
remaining suspension is progressively less representative of the whole.
Also, as the coarser particles are relatively fewer in number, their
progressive depletion may have a pronounced impact on the overall remaining
particle-size distribution.
42
-------
SAMPLE VCB 937F
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*
Figure 12. Plot of the envelope described by the cumulative frequency
distribution of 20 replicate Coulter Counter analyses
and a table of the means and standard deviations of the
cumulative frequency at whole # intervals for 2 sets of
20 replicates of the same sample.
-------
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Therefore, it appears that the precision of the Coulter Counter
analyses is quite satisfactory as long as care is maintained to assure that
the sample analyzed is representative of the whole.
f. Quality Assurance - Carbon and Sulfur Analyses. There were four major
elements to the quality assurance program which was developed and
implemented for the analyses of the carbon and sulfur content of sediments.
These elements were 1) personnel and procedures, 2) reagents and other
supplies, 3) records, and 4) the statistical interpretation of the quality
assurance statistics. A brief discussion of each of these areas follows.
Personnel: The majority of the analytical work was performed by
technicians and/or graduate assistants. Prior to initiating analyses, all
the equipment operators reviewed both the manufacturer's operations manuals
and the laboratory's protocol and operating guides.
Then they followed a "hands on" learning process working directly with
experienced technicians.
Reagents and Supplies: All reagents, chemicals, gases, and standards
used were of analytical quality and were purchased from appropriate
manufacturers or supply houses.
Records: Several, sometimes redundant, formats were used to record
and track the hundreds of samples and analyses. A master log was used to
list each sample and record the various procedures or analyses that had
been performed on each sample. A second set of records was kept to record
the specific analytical data and results for each sample. Additionally,
another log was kept to record the results of instrument calibrations and
standards. Finally the statistical comparisons of sample pairs were
calculated and plotted to see if the duplicate analyses fall within
satisfactory limits.
Interpretation: Statistical interpretation of the quality assurance
information was accomplished in two forms. Accuracy was checked using a
percent recovery technique which utilized known standards. Precision, or
the ability to obtain consistent results, was monitored with an industrial
control statistic,
/A - B/
I = /A + B/,
where A and B are the values from duplicate analyses of aliquots of the
same sample. As the total carbon and organic carbon analyses are
identical, only total carbon and sulfur were tracked for accuracy. Both
procedures follows EPA (1979) formats.
The percent recovery was calculated as the ratio of the observed value
to the actual value of the commercially purchased standards. The results
of the first 50 analyses of standards were used to establish the general
control limits for future analyses. The mean and standard deviation of the
carbon analyses were 98.5% and 1.4% respectively. The EPA (1979)
procedures set the control limits as plus or minus 3 standard deviations
-about the mean; hence the accuracy control limits for the carbon analyses
45
-------
were 102.7 and 94.3 percent recovery. Less than 10% of the observations
were outside the control limits. The majority of these observations were
traced to procedural errors or instrument malfunction. All observations
were plotted promptly and there were no consistencies or trends apparent in
the out-of-control samples.
The average percent recovery of the first 50 sulfur standards was
90.2% with a standard deviation of 5.5%; thus the control limits were
106.7% and 73.7%. All observations fell within two standard deviations of
the mean, well within the control limits.
The control limit for the industrial statistics used in the evaluation
of precision is equal to the average range of the absolute value of the
differences between duplicate determinations multiplied by 3.27, the
Shubart factor calculated for duplicate analyses. If the difference
between duplicates of a particular sample exceed the control limit, the
analysis is repeated until duplicate determinations are within control
limits. The control limits, calculated on the basis of the first 25
duplicates, were 0.339 for total carbon, 0.268 for organic carbon, and
0.095 for sulfur. Due to the great number of analyses, several persons
assisted with the visual examinations of the graphs of the industrial
statistic for each set of analyses in order to assure that all
out-ofcontrol samples were noticed and corrected.
B. FORMULATION OF SEDIMENT BUDGET
The estimation of the sediment budget for the Virginia portion of the
main-stem of the Chesapeake Bay involves the comparison of the residual
sediment mass as determined for a 100-year period. This estimation treats
the Bay as a "sink" for sediments derived from various "sources." The
determination of the residual sediment mass utilizes the method of
comparing corrected bathymetric data from the 100-year interval of
(approximately) 1850 to 1950 to discern the patterns of sedimentation and
erosion. This information, coupled with the data obtained from the 2,000
samples of the surface sediment, enables one to estimate the mass of
sediment deposited in the Bay, both in total and in terms of the separate
masses of sand, silt, and clay. The calculations require the synthesis of
several sets of data and the acceptance of several assumptions. The most
important of these assumptions are:
1. That the surficial sediments sampled are representative of the
sediment column beneath. As such, we assume the sand:silt:clay ratios
observed at the surface are constant over the sedimentation lengths
calculated for the 100-year period. This is a necessary assumption for the
estimation of residual sediment mass.
2. That the sense of the depth difference reflected in the
bathymetric comparison are representative of conditions at the time of
sediment sampling (1979-1980). For example, at a given point the
bathymetric comparisons between 1850 and 1950 may indicate a net loss of
sediment (a net erosional condition). In such a circumstance we would
expect a reduced water content in a surface sediment sample due to
46
-------
compaction. In fact, however, we may find that the sample station
exhibited a high water content indicative of recent deposition. This is
ignored in our calculations as the conversion from sediment volume to mass
would use the "high" water content and thus underestimated the mass of
sediment "eroded". Again, we have utilized a necessary assumption. It is
particularly noteworthy that the sedimentation dynamics associated with the
passage of the high, fresh water discharge due to Hurricane Agnes in 1972
may have temporarily switched an area of erosion to one of deposition. Or,
even more dramatically, there may have-been erosion followed by deposition
at a given site during the respective onset and relaxation of the event.
In order to array the bathymetric and sedimentological data at a
common level, the information was smoothed to a 0.5 minute grid using a
pseudo-two-dimensional, bicubic, spline-fitting program. This program was
; used separately with the roughly 40,000 corrected bathymetric-comparisons
(Figure 14) and with the sediment information from the 2,000 or so sediment
samples.
Thus, at the center of each 0.5 minute grid cell there were
interpolated values of sedimentation rates (based upon corrected water
depth comparison), surface sediment water content, and percentages of sand,
gravel, silt, and clay. The surface water content was used to estimate the
average water content over the sedimentation distance. Then the average
water content was used to convert the sedimentation rate to total mass
accumulation rate which, in turn, could be partitioned to component values
of sand, silt, and clay. The depositional patterns of the sand, silt, clay
components might be expected to be depth dependent and, as well,
latitudinally variable. Accordingly the final tabulation was arrayed into
66 one-minute north-south intervals (38°00' to 37°59' = zone 1) with depth
stratification into eight depth zones (0-6 ft., 6-12 ft., 12-18 ft. 18-24
ft., 24-30 ft., 30-36 ft., 36-42 ft., and > 42 ft.).* Finally, as the
western part of the Bay may exhibit different behavior than the eastern
part, the array was further divided into six sub-segments: the western
; shore and eastern shore divided by the Bay thalweg, eastern and western
sides of Smith-Tangier Islands, Pocomoke Sound, and Mobjack Bay (Figure
Within a given one-minute, depth-bounded latitude slice:
a. the area was approximated as the surae of the elemental 0.5
minute cells with centerpoints within the depth limits.
*Because the original bathymetry is in feet, or fathoms, many of the
calculations were performed using feet. Thus some of the intermediate
results discussed in this report are presented in English units, 1 meter
equals 3.28 feet, 1 inch equals 2.54 centimeters, 1 metric ton equals 1.1
English tons.
47
-------
I
I
LOCATION OF SIX-SECOND CELLS
WITH COMPARISON BATHYMETRY
Figure 14. Location of 6-second cells with bathymetric
comparisons and bay sub-segments.
-------
b. the average sedimentation length (and rate) was computed by
dividing the cumulative 5.0 minute cell volumes, prorated to
100 years, by the total area.
c. the accumulated masses of sand, silt, and clay were calculated
from the sum of the contributions from each 0.5 minute call to
yield mass of sand (and gravel), silt, and clay per 100 years.
The process flow chart is shown in Figure 15 and the details of the
calculations of depth difference, and conversion of sediment volume to
sediment mass follow.
1. Bathymetric Comparisons. The elemental information available
consisted of the most recent bathymetric information (circa 1950) available
from NOAA (EDS) wherein the average depth within six-second (approximately
150 x 200 m) cells were listed on magnetic tape, and the bathymetric "boat
sheets" of circa 1850. The latter were partitioned into identical
six-second rectangles and the depths within were algebraically averaged.
Thus, the basic data set for bathymetric comparisons were those six-second
cells for which there were recorded depth data for both survey periods.
The boundaries of the respective surveys are shown in Figures 16 and 17.
Approximately 40,000 grid points (out of a possible 420,000) were obtained
(Figure 14). The time difference between surveys ranged from 85 to 110
years but the preponderance was between 95 and 100 years.
In order to rectify the two data sets to the same mean-low-water
datum, it was necessary to consider three corrections (Carron, 1979);
a. eustatic sea level change,
b. crustal changes, and
c. semi-annual and annual tidal variations.
Eustatic sea level change was assumed to be 1 mm per year. The
correction was applied to the number of years between the center of the
1950 tidal epoch (1950) and the survey date for the 1850 series survey.
Crustal changes which would appear to be bottom erosion (all changes
were downward), when in fact no mass balance change took place, were
accounted for by applying a fifth order trend surface equation to the data
of Holdahl and Morrison (1974) (Figure 3), giving vertical crustal movement
rates for the same period used in this sedimentation and erosion study.
Semi-annual and annual tidal variation (Figure 18) corrections were
applied to the 1850-series data to correct for seasonal deviations of
observed mean low water from long-term mean low-water.
However, it is important ot note that no corrections were attempted to
estimate the depth differences solely due to compaction of the bottom
sediments. While negligible for sandy sediments, the effect of compaction
in fine-grained sediments over one hundred years could be significant.
49
-------
FIGURE 15
FLOW CHART FOR RESIDUAL MASS ESTIMATES
(1)
"Corrected" Bathymetric
Comparisons
40,000-6 sec. grid points
with associated #t
between surveys
(2)
Sediment Parameter
Information
2,000 grid points at
1.4 km spacing
Surface Water Content
% Sand, Gravel
% Silt
% Clay
Array to 0,5 Minute Grid'
Using Pseudo-Two-Dimensional
Bi-Cubic Spline
For each grid point, calculate average
water content as a function of surface
water content and observed corrected
depth difference.
Calculate total mass accumulation rate
using average water content, grain
density, and sedimentation length, D,
over survey period, t and normalized
to 100 years.
Partition total mass accumulation rate
to component sand and gravel, silt,
and clay (m - tons/M^/100 years).
Map
Tabulate for one
minute latitude
slices, depth
stratified.
50
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In addition, no corrections were attempted to compensate for the fact
that lead-line methods were used in the surveys of the 1800s whereas echo
sounding techniques were employed in the 1950s surveys. Again the
difference would depend upon the grain-size characteristics of the bottom
sediments. In sandy materials and at shallow to moderate depths, the two
techniques would be comparable. In muddy sediments, significant
differences might exist (Watts, 1954), and these errors would be
particularly sensitive in areas of very high water content, such as where
fluid muds are encountered. Other studies have not disclosed such
conditions within the main stem of the Bay in Virginia (Nichols, personal
communication). Moreover, as the larger survey vessels calibrate the echo
sounder against lead-line determinations, the differences would be
partially rectified in sediments with intermediate water contents.
The corrections applied are shown schematically in Figure 19 and given
as follows:
a. We assume the 1850 surveys have not been rectified for monthly
variation in MIL (or MLW). This correction is applied from Figure 18 as
follows (Figure 19A):
DO = DO' - T
-------
The sedimentation rate, S, normalized to a 100-year period is then
S = D 100 = D0' - Dn + E At + C At - T (4)
The above noted "corrections" take into account depth difference biases
which may amount to about 0.6 meter.
As previously mentioned, the corrections do not include the effects of
sediment compaction as shown in Figure 19B for an otherwise rectified
situaiton. Layer 1, deposited in 1850, may have become thinner due to
expulsion of water through time and due to overburden of new sediment. The
bathymetric comparison would yield AD and thereby underestimate the total
sedimentation by AS, the component due to compaction. The thickness of the
layer A would depend upon the vertical, water-content profile and the rate
of sedimentation.
In addition to the corrections noted above, the propagation of error
in the sounding comparisons must be considered. Each of the separate
surveys contains error, and, as well, the comparison between surveys
embodies error. For a given survey, the principal errors are in accuracy
of locating a hypothetical site, and the variability of repetitive
soundings at a fixed site.
The surveyors were aware of the problem of accuracy and, as a check on
their data, ran crossing survey lines. If the differences of the crossing
vlues for particular water depths were within given limits (Sallenger et
a1., 1975), the bathymetry was acceptable. For example, criteria adopted
in 1955 quote a maximum allowable difference between depth measurements at
0.3 m for water depth less than 20 m.
As a means of quantifying absolute sounding error, we examined the
crossing differences from both the 1850s and 1950s data. The crossing
differences are the absolute values of the differences in depth from two
lines of bathymetry where the lines cross. Because soundings from separate
lines were seldom coincidental, crossing values were derived from linear
interpolation along the separate lines. In neither survey were crossing
differences related to depth.
For two soundings at the "same location" (i.e. a crossing) we have
a = d + Ea
b = d + Eb
where a and be are the soundings at a crossing, d is the true depth, and
Ea, Eb are the respective errors from d. The difference is
a - b = Ea - Eb and
, " (a - b)2 = Ea2 - 2EaEb + Eb2
55
-------
A
^'0
-------
For comparisons at a large number of locations we assume EaE^ is small
compared to the squared terms and that aEa2 = SE^2. Furthermore, if Ea and
Ejj are random deviations with zero mean and the same standard deviation
then the variance is approximated as:
*2=(1/2)(a-b)2
Calculations for the 1850's and 1950's survey series give,
respectively, cr. = 3.03 ft2 and a_ = 0.52 ft2 for sample sizes of
N£ = 351 and N2 = 691.
The pooled variance arising from the comparison of individual
soundings at a given location is:
i al,2 = ?l + a2 = 3'55 ft2' and
the standard deviation
Slj2 = ± 1.88 ft (± 0.57 m)
The 95% confidence interval is 1.96 Sij2 or ± 3-68 ft (± 1>12 m^-
Thus, for a comparison between co-located individual depths on separate
surveys, a depth difference greater ± 1.12 m has a 5% probability of being
to survey error.
t
While the above applies to the comparison of individual co-located
depths, our comparison procedures should reduce the error as the average
depths within co-located six-second sedon grid cells are compared.
Furthermore, this grid cell sampling density is further smoothed by
application of the bi-cubic spline. However, the degree of potential error
reduction has not been evaluated.
2. Conversion of Sedimentation Rate to Mass Accumulation Rate.
: Recall that each 0.5 minute grid point had interpolated values of the
| sedimentation length (E corrected comparative depth difference), surface
; water content, and the percentages of sand, silt, and clay. The
! sedimentation length (+ as accumulation, - as erosion) when applied to unit
1 area, a square meter, yields cubic meters of change in sediment volume per
square meter of surface. Once the volume of deposition or erosion per unit
area at a site has been calculated, the problem is to convert the volume, a
mix of solid (mineral) sediments, shell, organic sediments, and water to
the mass of dry, mineral sediment. In order to arrive at the number of
metric tons of dry, non-organic sediment deposited per square meter, it is
necessary to discount the volume of water and the mass of organic material
from the volume of material deposited.
The most significant problem in the procedure for conversion from wet
volume to dry mass is estimation of a value for the average water content
over the estimated sedimentation length when given only the surface water
content. Although an exponential-like decrease in water content with depth
may be expected for uniform sediment material, the exact form of the
-equation cannot generally be stated since the water content (or porosity)
57
-------
gradient is also dependent upon the uniformity and rate of sediment
accumulation.
In this study, the water content gradient with depth, as a function of
surface-sediment water-content, was estimated using approximately fifty
(50) short (1 meter) gravity cores obtained in 1978 and 1979 by the MGS for
study of interstitial water chemistry (Figure 20). Their analysis included
determination of the water content gradient and a log of sediment type.
The surface water content of these cores varied from over 80% to less than
20%. The water content profiles were then grouped into 10 percentum
surface water content classes. These class groupings are exemplified in
Figure 21 where it may be noticed that most of the profiles fall within an
envelope monotonically decreasing values of water content with depth.
However, some of the profiles depart from the envelope with either a
dramatically nonmonotonic behavior or otherwise wide departure from the
general grouping (i.e. VA 78).
The second step was to shift all of the profiles exhibiting "normal"
behavior within a class group to a common surface origin (Figure 22). An
"average" profile for the class group was then drawn. At this stage in the
process any profile with a surface water content within a particular 10
percentum class interval would be estimated by the averaged profile for
that class.
The depth averaged water content was then determined at 10 cm depth
increments for each "average" water content profile. For any
given depth in the core, the depth-averaged water content of the overlying
material could then be expressed as a deficit relative to the value of the
surface water content. A nomogram (Figure 23) was prepared for this
purpose. Values for water content at between 1 and 2 meters are
extrapolations. Observation indicated that when the surface water content
in a core was less than about 30% there was little variation with depth,
and have thus been treated as constant.
Again, recall that each 0.5 minute grid point had an associated value
for sedimentation length (+ or -), and a value for surface water content.
The nomogram (Figure 23) was applied in tabular form (Table 7) at each
point wherein the sedimentation length, AD, and surface water content, were
used to calculate an average water content for the pertinent AD, value.
If we assume zero gas content and ignore the salt evaporate, the dry
mass of sediment per unit volume of wet sediment may be expressed as:
M = ps (1 - Wr)
(pS - pf)(Wc + 1)
where ps = sediment grain density,
ps = water density,
Wc = average water content.
58
-------
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62
-------
Table 7
Determination of Average Water Content with Depth
Surface
Water Content x, Deficit in water content relative to surface water content.
AD in cm < 30 30-39.9 40-49.9 50-59.9 60-69.9 > 70
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200+
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2.5
4.0
4.5
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.5
5.5
5.5
4.5
6.5
8.0
8.5
9.0
9.5
9.5
9.5
9.5
9.5
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
6.0
9.0
11.0
12.5
13.0
13.5
14.0
14.0
14.0
14.0
14.0
14.5
14.5
14.5
14.5
14.5
15.0
15.0
15.0
15.0
7.0
11.0
14.0
16.0
17.0
17.5
17.0
17.5
18.0
18.0
18.0
18.5
18.5
18.5
18.5
19.0
19.0
19.0
19.5
19.5
8.0
13.0
17.0
19.0
20.0
21.0
21.5
21.5
22.0
22.0
22.5
22.5
23.0
23.0
23.5
24.0
24.0
24.0
24.5
24.5
For a given AD and surface water content, Wc, subtract the values taken from the
table_from the surface water-content to obtain the depth averaged water content,
Wc: Wc = Wc - x.
-------
It should be noted that this expression differs from the saturated unit
weight generally used in geotechnical studies (Bennett and Lambert, 1971).
Consistent with the assumption of zero salt evaporate, we have assumed the
density of pore water is 1.0 g per cm^. The range of salinities in the
pore water of cores from the Virginia portion of the Bay ranges from 12 to
32 °/oo (James Hill, Maryland Geologial Survey, personal communication).
At 0°C, these salinities yield densities of 1.0096 and 1.025 g per crn^
(Knudsen, 1959). Dry grain density was taken as 2.7 g per cnH following
the findings of Harrison, Lynch and Alfcschaeffle (1964). This assumption
is valid for the mineral component, and nearly so for shell, but invalid
for the small component of other organic materials. A sensitivity analysis
indicates about a 2% error in dry mass calculation for the extreme of pf =
1.02 g per cm^ and ps varying between 2.65 and 2.75 g per cnr*. Figure 24
displays the graph of dry mass (m-tons per nH) as a function average water
content.
The preceding paragraphs explained the procedures for determination of
the average water content, Wc, over the sedimentation length, and the
determination of dry mass of sediment per cubic meter given the average
water content. Multiplication of the latter value by the length of the
sedimentation column, AD, then gives the mass accumulation between the
survey periods, At. Further multiplication by 100/At then gives the final
result of total sediment mass accumulation normalized to a 100 year period.
3. Sources
a. Sediment Mass Derived From Shoreline Erosion. The estimation of the
mass of material supplied to the Virginia portion of the Chesapeake Bay
from erosion of the shoreline utilized published data concerning areal loss
(Byrne and Anderson, 1977), field observations of shoreline geology and
geomorphology, and sediment samples. Byrne and Anderson determined the
area of shoreline change by comparing shoreline positions for the period
1850-1950. These estimates of area were converted to volume by multiplying
them by observed shoreline heights. The volumes, in turn, were converted
to mass using data on the unit dry weight of different sediment types from
Terzaghi and Peck (1948). The most frequently used values were 90 lb/ft3
(1.43 gm/cra^) and 99 Ib/ft3 (1.99 gm/cm3) for uniform and mixed grained,
loose sands, respectively.
In addition to the general observation of sediment type, samples were
taken at approximately 1 mile intervals along the main shoreline of the
Bay. At each site, samples of the beach and and fastland sediments were
obtained. The fastland samples were collected so as to be representative
of the stratigraphy. Thus, the sandrsiltrelay ratios of the samples of the
(eroding) sediments, coupled with the calculated mass of the material
eroded for each shore segment yielded information on the separate masses of
sand, silt, and clay supplied to the Lower Bay. Appendix 2 is a tabulation
by minute-of-latitude of the mass of material attributable to shoreline
changes. Figure 25 depicts both the mass of sediment per
rainute-of-latitutde and the areas of the Bay shoreline that were included
in the calculation.
64
-------
03
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Figure 25. Histogram of the mass of material eroded from the shoreline
for each minute of latitude. The areas used in the calcula-
tions are shown on the map with bold lines.
-------
The sum, approximately 42.3 x 10° metric tons, 90% of which is sand,
is probably a conservative, or low, figure. The calculated areas and
volumes of shore change use only data from approximately mean high water
and above. Sediment eroded from below MHW as part of normal shoreline
retreat is not included in the calculations (Figure 26). The quantity of
this material is a function of both the rate of retreat and tide range. In
areas of low shore elevations and gently nearshore slopes, the quantity of
material not considered could approach the calculated supratidal amounts.
*
b. Calculation of the Mass of Biogenic Sediment. Jacobs (1978) presented
basic data on the distribution of zooplankton through a large portion of
the Lower Chesapeake Bay. His data stems from two years of monthly samples
from each of eight subareas. The samples were collected by careful tows of
202 ym mesh nets and were later concentrated through 110 ym strainers in
order to retain broken specimens. Hence the mass of material deposited
from zooplankton and phytoplankton less than 202 ym in life size is not
included. Because this data is to be integrated with data on sediments
that have been digested in HC1 and H2C>2, only the ash weights of the
plankton samples are used in the calculations as the ash weight is
representative of the mass that would remain after the digestion process.
Table 8 shows the path of the calculations from the determination of
the monthly average ash weight of the zooplankton in a cubic meter of water
in each of the eight subareas through the total ash weight for a 100-year
period. The calculations use the assumption that all the potential,
zooplankton derived, ash material settles to the bottom and is incorporated
in the sediment. Figure 27 depicts the subareas. It should be noted that
subareas G and H extend north only to 37°40' latitude, not to the state
line, approximately 38°00'.
The calculated total ash, from Table 8 is 81.4 x 10^ metric tons per
100 years. If the contribution of subareas G and H are doubled, to extend
the area to include all of the Virginia portion of the Bay, the total of
the ash weight of the fraction of the biogenic sediment in the study area
is 1.252 x 106 metric tons. ;
C. DATA STORAGE
Several very large data sets were constructed to provide for the
logical storage of the information derived as part of this project. These
data sets can be divided into two groups. The first is the data associated
with the bathymetric changes. The other is the information concerned with
the two thousand bottom samples.
The raw bathymetric data is available on magnetic tape at the Virginia
Institute of Marine Science. This data includes digitized sounding data
from both the 1850 and 1950 bathymetric survey series. The latter
informaiton was obtained from NOAA. Additionally, bathymetric changes,
adjusted to compensate for the parameters noted section also are available
on tape.
67
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The data on the individual sediment samples are comprehensive and are
available from several sources. The data include the latitude and
longitude, water depth, water content, weight percents total carbon,
organic carbon, and sulfur (not all samples), weight percents gravel, sand,
silt, and clay, tabulated, 1/4 0 interval grain-size-frequency
distribution, and the several calculated statistics. This information was
submitted on magnetic tape to the Virginia State Water Control Board for
transferral to the EPA STORET data system and should be available through
both agencies. Additionally, both magnetic tape and paper tabulations of
the data are filed at VIMS.
71
-------
VI
RESULTS AND DISCUSSION
A. SEDIMENTOLOGY
1. Description of Sedimentology. One of the most striking attributes
of the bottom sediments of the Virginia portion of the Chesapeake Bay is
the dominance of sand over mud. Over 65% of the samples are 75% or more
sand (Table 9). The mean of the graphic mean grain size for over 2,000
samples is 3.17 0 (0.11 mm). Table 9 displays sediment type from Shepard's
(1954) ternary classification. The data is grouped by 5 minute bands of
latitude for the main stem and by fringing subarea. On most maps of
sediment characteristics, such as weight percent sand, mean grain size, or
sediment type (Figures 28, 29, and 30), the great extent of the sands is
clear. With few exceptions, the finer-grained sediments are confined to
the deeper portions of the Bay. Indeed the patterns of the various
sediment maps echo the bathymetry. However so little of the Bay is deep,
approximately 80% of the study area is less than 42 feet (13 meters) deep
(Figures 31 and 32) that silts and clays in shallow areas such as Mobjack
Bay and the sands in the deep channel near the lower Eastern Shore destroy
any significant correlation between grain size and depth. R^ for 2,018
depth versus percent mud samples is 0.13. Figure 33 depicts clay content.
Although the visual correlation between depth and clay content is strong,
the R^ value is only 0.11 for the entire suite of samples. Again this poor
quantitative relationship is, in part, a function of the hypsometric
distribution.
On all the above sediment characteristic maps the channels into
Pocomoke and Tangier Sound, the channel in the main stem that runs north
from near the mouth of the Rappahannock, and the York and Rappahannock
channels are clearly shown. Only the deep channel in the southeastern Bay
is lost. The presence of coarser grained sediments in this deep portion
probably is due to erosion of a sand substrate, or, locally, to the
transport of sediment into the channel from the region of the Bay mouth.
The qualitative depth versus grain size relationship is very well
depicted in Figure 34 which is a map of the graphic mean of the sand
portion of the samples. The distribution of the very fine sands, 3 to 4 0
(0.125 to 0.0625 mm), marks all the deeper areas of the Lower Bay including
the channel near Cape Charles. This map, as well as the others, depicts a
band of finer-grained sediments running southeast from the mouth of Mobjack
Bay. Although this area is in slightly deeper water than the surrounding
region, the bathymetry is not nearly so pronounced as with the York River
channel which the Mobjack band parallels. It is possible that this band is
72
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WEIGHT PERCENT SAND
:ONTOUR INTERVAL 20%
Figure 28. Isopleth map of weight percent sand.
-------
GRAIN SIZE
CONTOUR INTERVAL I*
Figure 29. Isopleth map of mean grain size.
-------
VT.1^-4v.
r?
S|C SEDIMENT TYPES
Figure 30. Map of the distribution of sediment types.
-------
DISTRIBUTION OF DEPTHS
FOR THE ENTIRE BAY
60 H
50-
40-
u
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20-
10-
15 27 39 51 63 75 87 99
"DEPTH
T T T T T 1iiiIiiir
123 135 147
Figure 31. Histogram of depths at sample locations.
77
-------
Figure 32. Map showing the bathymetry of the Virginia portion of the
Chesapeake Bay, 10-foot contour interval.
-------
Figure 33. Isopleth map of weight percent clay.
CONTOUR INTERVAL 20%
-------
Figure 34. Isopleth map of the graphic mean grain size of the sand
portion of the sediment samples.
-------
the surface representation of a channel filling with material derived from
the Mobjack Bay upland drainage.
The explanation of the distribution of the medium and coarse sands is
complementary to that for the very fine sands. Whereas the very fine sands
are deposited in the deeper portions of the Bay, the medium and coarse
sands are found in the shallower areas that are subject to agitation by
waves. It is arguable from site to site whether the coarser sands are the
lag deposit left behind as the shallow sediments have been reworked and
winnowed of the fines by waves or are relict, palimpsest, materials.
Indeed the two explanations are not mutually exclusive and often may both
be true. Dramatic shoreline retreat has accompanied the Holocene rise or
sea level. Rosen (1976) cited evidence for the shallow, 3.6 meters (12
foot) and less, flat terraces being the erosional platform created during
the relatively slow sea level rise of the past 3,000 years. Using the work
of Brunn (1962), Rosen related the change in bottom slope to the change in
the rate of sea-level rise that occurred approximately thirty centuries
ago. Thus the roughly shore-parallel zones of coarser sediment adjacent to
Accoraack and Mathews Counties may be the lag deposits of Rosen's erosional
terraces. The southeast-northwest trending coarse zones adjacent to the
York River channel, extending from southern Mathews County, and just north
of Windmill Point at the mouth of the Rapphannock may be the sediment of
the eroded morphology and not exclusively a "lag" material. These features
are likely to be the remnants of fossil spits formed during an earlier,
. lower stand of sea level. The large sandy-shield around Tangier and Smith
Islands also probably is a relict feature as there are insufficient modern
sources for this material.
The classification of sediments according to Shepard's (1954) ternary
scheme provides additional data. Figure 30, clearly shows the dominance of
sands and silty sands. The finer sediments, particularly the sandy- and
clayey-silts, describe the major channels and, as previously noted, Mobjack
Bay. The thin band of fines extending southeast from Mobjack Bay also
appears on this map.
The sample data, when plotted on sand:silt relay ternary diagrams
i (Figure 35) also illustrate the depth-sediment relationship. The samples
all fall in a swath running from pure sand to clayer silt. Although the
width of the swath remains nearly constant from depth to depth, indicating
a mixture of the same general sediment types, the distribution within the
swaths changes with the proportion of fines increasing with depth. This
pattern is very similar to that shown by Kerhin, Halka, and Wells (1979) in
the Maryland portion of the Bay except that the trend of their swath is
somewhat finer, running from sands to silty clays and having a greater
percentage of the samples falling on the silt-clay axis with little or no
sand. They also show the general trend of an increasing percentage of
fines with depth.
The generally finer nature of the sediments upstream in the Bay is not
surprising. The proximity to the mouth of the Susquehanna and the presence
of the turbidity maximum in the upper Bay work toward the resultant
presence of an abundance of fines. Also the general phenomenon of
81
-------
CLAY
to 12 FEET
335 SAMPLES
SAND
SILT
SAND
131o 24 FEET
404 SAMPLES
SILT
CLAY
25 to 36 FEET
547 SAMPLES
CLAY
37 FEET AND
GREATER
674 SAMPLES
SAND
SILT
SAND
SILT
Figure 35. Ternary diagrams of sand:silt relay ratios of sediments at
different depths.
-------
down-estuary coarsening has been noted by Nichols (personal communication)
in the James River. Nelson (personal communication) has demonstrated it
with successive down stream sand:silt:clay plots of sediments from the
Rappahannock.
Water content is closely related to sediment type. For mud (x) versus
water content (y) the R^ for 2,018 samples is 0.91 with the equation y =
0.459x + 18.6. The R^ values for mean grain size versus water content and
weight percent clay versus water contetit are only slightly less significant
at 0.881 and 0.879. Figure 36 is a scatter plot of the mud-water data.
As will be discussed later, these descriptions are somewhat similar to
those of Ryan (1953) and Shideler (1975). The present study demonstrates
that the sands are more widespread than was thought by the earlier works.
, Although some of the difference may be real, reflecting a change in bottom
sediment through time, much of the difference is due to the better detail
that results from the near order-of-magnitude greater sampling density of
the present study. A comparison of isopleth maps of the mean grain size of
the sand fraction (Shideler's Figure 3, this study's Figure 37)
demonstrates the different levels of detail. As an example, where Shideler
depicts a small shield of very fine sand near the mouth of the York River,
the present study shows two discrete band, one from the York connecting
with the main, central Bay bands fines, the other running from Mobjack Bay
: toward, but not connected to, the mid-Bay band.
2. Comparison With Earlier Surveys of Bottom Sediments. As the work
done through the course of this study covers some of the same ground that
was touched by the work of Ryan (1953), Harrison and others (1964), and
Shideler (1975), it is appropriate to discuss the differences in observed
characteristics and among results. This is especially important as the
major physical events of the 1962 Ash Wednesday storm and the floods of
Hurricane Agnes in 1972 occurred during the interval between these studies
and it may be possible to discern changes in the bottom sediments results
from these events. Broadly speaking there are two distinct types of
| differences that could exist between studies. The first are real or actual
: differences that result from true changes in the bottom. The second group
i is due to such causes as dissimilar sampling and analytical techniques,
, including sample pretreatment, errors in site location, differences in
sample spacing, and differences in the subjective interpretation of the
data. Although it is only the changes in bottom sediment which should be
compared across studies, it is difficult to isolate these differences from
those that are artifacts of the methods and techniques of the individual
projects.
Analytical differences can be minimized by using comparisons of the
weight percent mud (or sand) as the wet-sieving technique used to separate
sand from mud are standard. The major remaining places for variation are
in obtaining the sample and pretreatment. In the present study, the top 6
cm of sediment were sampled. Shideler used the top 10 cm, Harrrison et al.
the top 20 cm, and Ryan the whole "snapper" sample or the top 15 to 20 cm
of each core. None of the earlier reports indicated any pretreatment of
the samples. The VCB samples were pretreated by digestion with HC1 and
83
-------
WATER VS PERCENT MUD
FOR THE ENTIRE BAY
c
o
80 H
70-
6CH
50-
40-
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100
'Figure 36. Scatter plot of weight percent mud versus water content.
84
-------
to remove carbonate and organics. Although not typical of the VCB
samples, a laboratory study of 22 samples from a separate project in the
Elizabeth River indicated that the weight percent mud averaged 1.1% greater
(weight percent sand 1.1% less) for the digested samples. This suggests
that the difference due to the pretreatment is not significant. (The
differences in weight percent clay were significant, though, with the
digested samples having an average of a 9.3% greater clay content (less
silt) than the undigested). The other possible source of variation is the
wet sieve itself. Although the 4 0 separation of sand and mud is 62.5 ym,
sieves with nominal openings of 60 to 64 ym are commonly used. This plus
variations due to the condition of the sieve contribute to minor
differences in the final result. Thus if one accepts that analytical
differences are minimal, the remaining differences are due either to sample
position, sample thickness or true changes.
Ryan had approximately 200 stations through the entire Chesapeake Bay.
Shideler also had about 200 samples, but just from the Virginia portion of
the Bay. The present study has over 2,000 samples from the Virginia
portion of Bay including Mobjack Bay and Pocomoke and Tangier Sounds.
Harrison and others studied only a small section of the area. Figure 38
shows the location of the samples from those three studies and Figure 4
shows the locations of the 2,000 samples analyzed for this study. The
profile lines A through G on Figure 38 are the transects along which
comparisons of weight percent mud were made. These profile comparisons are
shown in Figure 39A,B,C. As samples were seldom exactly on the line, any
falling within a half kilometer were used. If the sediment distributions
were "patchy", this would be a source of error in the comparison. The
profiles shown in the figure depict the west to east variations in weight
percent mud for the present study (VCB) and Ryan, Shideler, and Harrison
and others where applicable.
The VCB data and Shideler's generally are similar. Profiles E
adjacent to the mouth of the York and G near the Bay mouth are especially
interesting as they show the differences in gradient that can result from
different sample spacings. The VCB data has steeper gradients and more
variations than the "smoother" data from Shideler. It is probably a result
of this greater detail that the present interpretations show much less
fine-grained material than the earlier studies.
Harrison's samples appear only on Profile B and show reasonable
correspondence with both Ryan's and the VCB data.
On Profile F the VCB sample which is almost 90% mud has been
identified as being taken from a spoil area.
Although in some instances, Profiles A and D, Ryan's data indicate a
greater mud content than either the present or Shideler's works, it is
difficult to assess the variations as either being due to the "patchiness"
of the sediment types or due to an actual change in sediment type through
time. As strong gradients of sediment properties are apparent, it is
desirable to have a great number of closely spaced samples. However
85
-------
40'N
37 00 N
SAND MEAN SIZE
0'
76°00'W
Figure 37. Isopleth map of the mean grain size of the sand fraction.
Reprinted, with permission of the publisher, from
Shideler, 1975.
-------
v v * ~i v i v r r T :
Figure 38. Location of sample stations from Ryan (1953), Harrison and
others (1964), and Shideler (1975).
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factors such as time, funding level, and logistics often necessitate the
acquisition of fewer samples than might otherwise be desirable.
The concept of the nature of the bottom sediments changing with time
should not be discounted. In considering this possibility it is worth
while to look at the dates of sampling and their relationship .to specific
physical events:
Ryan (1953)
«\
Samples collected 1950, 1951
March 1962 Ash Wednesday Storm
Harrison et_ aJL (1964)
Samples collected June 1962, April 1963
June 1972 Agnes floods
Shideler (1975)
Samples collected April-September 1973
This study
Samples collected November 1978-June 1979
The Ash Wednesday 1962 storm lasted several days and was accompanied
by greatly elevated water levels and heavy wave agitation. Hurricane
Agnes, 1972, provided all the elements necessary for the maximum transport
of sediments from the upper Bay and tributary rivers into the lower Bay.
It should be noted that two studies, Harrison et al. and Shideler, follow
within a year of the significant storms while the other two studies fall
during periods of "normal" processes. This then presents the questions of
both the nature and duration of the storm impacts on the bottom sediment.
The profile comparison between the information from Shideler and the
present (VCB) studies are somewhat ambiguous. In the profiles and plots of
weight percent mud are generally parallel and exhibit the greatest
differences in places where sample location, or rather lack of co-location,
could account for the differences. That is the "spikes" in one plot occur
between samples on the other indicating that, perhaps, the second study
failed to sample the "spike" area. However, if in the laboratory, one
plots both complete sets of mud-content data on the same map, another
interpretation becomes possible. In some areas Shideler observed bottom
sediments that were 10 to 15% muddier than the bottom sediments of the most
recent study. The greatest differences are immediately south of Tangier
Island in Shideler"s northernmost line of samples and in the lines adjacent
to and below the Rappahannock. A possible explanation is that the muddier
sediments are material that entered the Bay as a result of the major floods
that followed Hurricane Agnes in 1972. The sediments coming from either
the Maryland portion of the Bay the Potomac, and the Rappahannock. During
~the years between the two studies, the "normal" processes would have
91
-------
altered the immediate post-storm surface sediments by both redistribution
and burial. Unfortunately a similar comparison using Harrison et al.
post-Ash Wednesday storm data is in-conclusive.
The map comparison of Shideler's with the present study's data
suggests that there may have been unusually high loads of fine-grained
sediment deposited within the Virginia portion of the Bay as a result of
the Agnes floods and that five years of "normal" processes are sufficient
to "mask" the event. The profile comparisons of the several data sets
demonstrate, or at least very strongly suggest, that the distribution of
the sediment is quite patchy and that significant gradients may be lost to
a wide sample spacing. Indeed if sediment types can vary across an area of
one or two hundred meters width the relatively "close" spacing of the
present study gives only a slightly better picture than the less close
spacing of Shideler's work.
3. Clay Mineralogy. Little work has been done with the details of
the clay mineralogy of the Chesapeake Bay. A quarter century ago Powers
(1954) studies the diagenesis of clays in the Bay and reported the
formation of a thermally stable, clorite-like mineral that increases with
salinity and depth of burial. Hathaway (1972) had fewer than a dozen
samples from the lower Bay in his study of east coast clay-mineral facies,
although he does indicate the Bay as a transition area between glaciated
and non-glaciated sources. Nelson (1960) discussed some of the aspects of
the clays of the Rappahannock. Nichols (1972) examined the sediment of a
portion of the lower James River. Harrison, Lynch, and Altschaeffl (1964)
briefly mention the clay mineralogy of sediments in the portion of the Bay
roughly between the Potomac and Rappahannock Rivers. More recently,
Feuillet and Fleischer (1980) studies the clay mineralogy of the lower
James River and the extreme southern portion of the Bay. There has been no
comprehensive investigation of the clay mineralogy of the Chesapeake Bay.
This study slightly increases the body of knowledge in two ways. First, it
defines the distribution of clays as a size class (Figure 33) and second it
reports the clay-mineral composition of approximately 20 samples (Table
10).
The original intent of the investigation was to test three hierarchial
or nested hypotheses. 1) That there are mappable variations in the
clay-mineral assemblages of the lower Chesapeake Bay, 2) that the major
cause of the variations are differences in the material supplied by the
several sources, and 3) that the mapped pattern of clay-mineral assemblages
can be explained by logical processes, e.g. currents. This set of
hypotheses can be recast as the following questions. Do the major sources
of sediment to the Bay, the several rivers, the ocean, and shoreline
erosion contribute different, definable suites of clay minerals to the Bay?
Do those clay-mineral assemblages remain as identifiable units coding
deposited sediments as to their source? What processes govern the
distribution and physical form of the deposits? The reconaissance work is
only semi-quantitative in nature using ratios of peak heights of both
untreated glycolated sediment from the 9 0 (2 pm) and finer fraction of the
sample. As with the previous researchers, chlorite, illite, kaolinite and
montmorillonite, and vermiculite are the major clay minerals.
92
-------
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However the relative paucity of sediments with even moderate clay
contents raises questions about the significance and validity of the data.
If the samples analyzed are chosen only from those with appreciable clay
contents, the spatial distribution is quite limited, hence greatly
restricting the inference of broad processes. If, on the other hand,
samples are chosen as a random subset of the large sample grid, the data
from samples with minimal clay contents might be sufficiently suspect as to
invalidate specific conclusions.
*
Using both the new and published data though, there is evidence to
suggest that the hypotheses may be valid. Much of the published evidence
is summarized by Hathaway (1972), but other sources include Powers (1954),
Meade (1969), Nelson (1960), Harrison and others (1964), and Feuillet and
Fleischer (1980). The Susquehanna River, the Bay's major tributary, is the
only one of the Bay's tributaries which drains a glaciated region. The
potential availability of rock flour in addition to the clay products of
more conventional weathered bedrock should give the Susquehanna borne clay
assemblage different and differentiable characteristics. Similarly the
rivers draining non-glaciated areas transport sediment eroded from more
deeply weathered bedrock of the piedmont. Although Hathaway"s (1972) map
of kaolinite distribution in the Chesapeake Bay shows a mid-Bay peak, this
would be expected as a result of the upstream estuarine bottom circulation.
Indeed, Hathaway indicates that the kaolinite concentration of the James,
York, and Rappahannock Rivers is 4 to 5 parts in 10, with far and away the
greatest quantity contributed by the James, whereas the Potomac and
Susquehanna have 2 to 3 and 1 part in 10 kaolinite respectively. The
greatest concentrations in the Bay's clays away from the immediate river
mouths, however, are north of the Rappahannock. It should be noted that
Hathaway depicts 10 stations in the lower Bay and fewer than a dozen and a
half in the entire Chesapeake Bay. The progressively deeper and more
intense weathering of the southern areas is perhaps an explanation of the
increased kaolinite content of the more southerly rivers.
Other patterns outlined by Hathaway include increasing montmorillonite
content in the rivers away from the Susquehanna, but a relatively uniform
distribution in the mid and lower Bay, a definite progressive down-Bay
increase in chlorite, even though the mineral was not detected in the
rivers south of the Potomac, and an extensive area of illite concentration
surrounding a small island of lesser concentration adjacent to the mouth of
the Potomac. The present work echos the down Bay chlorite trend. The Bay
mouth changes which he notes can be attributed to the contributions of
sediment from outside the Chesapeake. Indeed, in a separate figure (his
Figure 14), he depicts a definite bottom transport of clay into the
Chesapeake during the Holocene.
Powers (1954 and other dates) attributed the lower Bay increase in
chlorite to the diagenetic formation of chlorite in increasingly saline
waters. Nelson (I960) also reported a downstream increase of chlorinte in
the Rappahannock but was unwilling to ignore causes oter than diagenesis
and withheld judgment.
94
-------
Meade (1969) drawing on the works of many others presents a strong
case for the "landward transport of bottom sediments." And as noted above,
the patterns depicted by Hathaway (1972) infer the presence of such a
process.
Harrison et_ ajU (1964) state that most of the clays in this area were
primarily illite (50%), chlorite (30%), and a mixed layer clay (20%); the
later being illite-montmorillonite and chlorite-montmorillonite. They
reported only a trace of kaolinite.
>
Feuillet's and Fleischer's work in the James is primarily concerned
with along-river trends in varying concentrations of various clay minerals.
In the extreme lower Bay between Hampton Roads and Cape Henry, they record
illite (approximately 50%), montmorillonite (12%), chlorite (8%), and
lesser quantities of kaolinite, vermiculite, and dioctahedral vermiculite.
4. Carbon and Sulfur in the Sediments. The spatial distribution of
the carbon, organic carbon, and sulfur contents, as weight percents, of the
sediments of the bottom of the Virginia portion of the Chesapeake Bay are
the major concern of this section. The delineation of relationships of
these distributions with one another and with physical characteristics,
such as water depth or sediment type, makes a contribution toward the
understanding of the processes active within the Bay.
The gross chemical attributes of the modern sediments provide
information of potentially broad use. When compared to the analyses
necessary for the determination of the presence of various complex organic
pollutants, the cost of the laboratory analyses performed for this study is
relatively small. It may possible, when data from the more complex
analyses become available, to correlate indices derived from the relatively
simple analyses with the different levels or aspects of pollution. If such
were the case, index values could be used to screen areas in order to
identify zones of possible concentration of contaminants. Additionally,
index data developed from samples taken on a tight grid might be used to
extend maps of more difficulty obtained data from a coarser grid.
a. Previous Work. Shideler (1975) discussed the total organic content of
the sand-gravel fractions of the sediments of the Lower Bay. The weight
percent organic content as determined by heating the greater than 63 m
sediments at 400°C and acid digestion ranged from 0% to 43% with most
values less than 5%. Shell material was the dominant component. He mapped
areas of concentration adjacent to but offset to the south of the mouths of
the Rappahannock and York Rivers, perhaps reflecting areas of sediment
enriched in nutrients by material supplied from the rivers. Harrison et
al. (1964), reporting on an area between the Potomac and Rappahannock
Rivers noted organic and inorganic carbon contents ranging up to 1.9% and
0.42% respectively.
Biggs (1967) reported on the organic carbon content of 120 samples
taken from the mid-Bay in the vicinity of the mouth of the Patuxent River.
The values range from 0.95% to 3.4% (organic) carbon, with the lower values
from shallow-water silty-sands, the higher from the finer sediments of the
95
-------
deeper waters. Biggs, working with relatively short cores, also reported a
general decrease in organic matter with depth of burial, confirming
observations of earlier workers. He related the increase in organic carbon
of the surface sediments with increasing water depth to four phenomena: 1)
a higher rate of sedimentation in shallow water which would serve to dilute
the organic matter with detrital material; 2) the higher oxygen content of
the shallow waters and the greater grain size (thus permeability) of the
shallow water sediments; 3) the scavenging activity or organisms in shallow
areas; and 4) the greater physical energy of the shallow areas.
Folger (1972) related organic carbon content to sediment texture and
cited the organic carbon content as an index to the level of pollution. He
stated that the organic matter in estuarine sediments is derived from plant
detritus carried to the estuary by rivers, from the debris of estuarine
plants and animals, and from various anthropogenic effluents. Referring
back to the earlier work of Trask (1932), he states, "concentrations of
organic matter vary inversely with sediment grain size." As did Biggs
(1967), Folger (1972) also found the sediment-organic matter relationship
to be a complex one including the settling characteristics of the particles
and permeability of the sediment, in addition to water chemistry and
microbiology. In a brief discussion of the Chesapeake Bay, he found the
organic carbon content to be as would be expected for the grain size; that
is, generally below 1% for sands, below 5% for silts and clays. In
reviewing the organic carbon content of estuarine sediments in general,
citing Whitewater Bay, Florida, and Deep Inlet, Alaska, as examples, he
stated that areas where the bottom waters are anaerobic (swamps and fjords)
there may be higher concentrations of natural organic carbon.
Although the impact on his paper probably is not great, Folger's
(1972) work has a synthesis of the work of several researchers who used a
number of different analytical techniques. Leventhal and Shaw (1980),
working with a shale, showed that precision within individual techniques to
determine carbon content is good (± 3%), but there can be significant (24%
or more) variations among techniques. Obviously this necessitates that
researchers carefully document and describe analytical procedures.
Mencher et al. (1968) reported very high organic-carbon-contents in
the surficial sediments of Boston Harbor, Massachusetts. The enormously
high values, however, are explained by the high level of pollution in the
harbor. The sandier sediments contained wind-carried fragments of coal and
coke; raw sewage was the probable prime contributor to the organic content
of the less than 62 m sediments.
Rashid and Reinson (1979) noted the relationship of organic-carbon
content and fine-grained sediments with an average 4.8% organic-carbon
content in muds and 3.6% in sandy muds of the Miramichi Estuary of New
Brunswick. They further state that "The sediments of the Miramichi contain
a much higher quantity of organic carbon than would be expected to occur
naturally in a shallow, well-mixed estuary", and attribute this apparent
anomaly to the discharge of pulpmills.
96
-------
LORAN-C INTERVALS
Chart
27318.4
27314.2
27340.8
27283.0
27223.2
27230.4
27203.1
27237.2
27275.1
9960-X
Theoretical
27321.56
27316.99
27343.38
27285.93
27226.48
27233.39
27206.18
27239.91
27278.01
27228.83
27221.93
27306.60
27271.34
27192.80
27260.17
27244.46
27255.68
27296.79
27190.93
27208.75
27222.25
27272.41
27194.55
27250.33
Actual Chart
27318.88 41634.6
27314.67 41719.2
t
27341.30 41925.9
27283.53 41904.0
27223.69 41987.1
27230.80 41841.5
27203.63 41840.6
27237.49 41703.8
27275.77 41575.8
27226.20
27219.71
27303.63
27268.75
27464.99
. 27257.46
27241.72
27252.97
27293.64
27188.49
27206.29
27219.53
27269.18
27191.98
27247.41
9960-Y
Theoretical
41636.81
41720.41
41926.90
41905.88
41989.74
41843.29
41842.76
41705.67
41577.68
41595.64
41497.59
41449.50
41443.50
41405.09
41366.01
41307.17
41280.57
41256.82
41300.78
41287.70
41267.81
41242.21
41247.25
41212.49
i
Actual !
41636.34
41720.06
41926.74
41905.26
41988.46
41842.49
[
41842.14
41705.07
41577.32
41594.59
41497.79 '
41448.44
41442.95
41404.32
41365.84
41306.50
41280.05 ;
i
41255.73
41300.63 !
\
41287.45
i
41267.52
41241.54
i
41246.94
41212.14
-------
LORAN-C INTERVALS
Chart
53476.9
53388.4
53208.4
53172.1
53028.9
53181.8
53326.4
53492.4
53425.3
53616.0
53516.7
53649.1
53619.8
53579.5
53685.9
53729.1
53767.3
53681.9
53712.0
53746.2
53739.0
9930-Y
Theoretical
53477.72
53389.54
53209.45
53173.02
53029.71
53183.07
53156.40
53327.22
53493.30
53426.16
53616.54
53517.31
53650.06
53620.81
53580.68
53687.13
53730.27
53768.08
53832.94
53683.12
53714.02
53747.41
53823.17
53740.27
53830.81
Actual
53475.63
53387.80
53207.93
53171.55
53028.40
53181.62
53154.65
53325.74
53491.92
53424.96
53614.99
53515.25
53648.40
53619.17
53578.97
53684.97
53728.52
53766.31
53831.33
53681.14
53712.21
53745.24
53821.02
53738.25
53828.74
Chart
70565.2
70528.5
70409.2
70464.3
70467.4
70533.8
70597.6
70631.2
70661.5
70666.6
70716.9
70670.4
70704.2
70788.4
70753.0
70797.4
70801.5
70845.0
70837.0
70836.7
70870.6
9930-Z
Theoretical
70563.26
70526.59
70407.26
70462.35
70465.40
70532.14
70552.11
70595.61
70629.16
70659.70
70665.19
70715.41
70668.84
70702.52
70786.73
70751.88
70795.95
70800.23
70775.83
70843.61
70836.12
70835.49
70805.54
70869.36
70840.76
Actual
70564.51
70527.90
70408.58
70463.77
70466.70
70533.74
70553.33
70597.00
70630.49
70661.26
70666.52
70715.94
70670.05
70703.74
70788.00
70752.93
70797.23
70801.17
70777.05
70844.67
70837.22
70836.55
70806.80
70870.47
70841.83
-------
Jones and Jordan (1979) report concentrations of organic carbon in the
estuary of the River Liffey, Dublin, ranging from 1.7% at the mouth of the
estuary to 25.4%. The River Liffey flows through the urban environment of
Dublin and the greatest levels of organic carbon occur at the upper limits
of sea-water incursion (the zone of the turbidity maximum), perhaps
reflecting the deposition of particulate wastes from paper mills and other
sources.
In a discussion of the 1 cm surface sediments from Lakes Ontario,
Erie, and Huron, Kemp (1971) reported that carbonate carbon was usually
less than 1%. Higher values, up to 5%, were attributed to bottom materials
that were locally derived from carbonate sediments. Organic carbon ranged
upward to 5%. Kemp also noted the strong relationship between organic
carbon and clay contents and an apparently equally strong correlation
between organic carbon and nitrogen. As in Chesapeake Bay, basin
morphology in the lakes is a factor in the distribution of sediment sizes
and, hence, in the spatial distribution of organic carbon.
Thomas (1969) addressed the relationship of organic-carbon content and
grain size and suggested that, partially as a function of the great surface
area of clays, carbon is associated with clay particles. He further stated
that environmental conditions might also have an influence.
Young (1968) addressed the chemistry of the sediments of a portion of
the Lower Chesapeake Bay. His reported values or organic carbon, 0.15% to
2.01% agree with the ranges noted in the present study. Young's procedures
included using a LEGO carbon analyzer to determine carbon content and by
digesting the sample in 10% HC1 to remove carbonate ("inorganic" carbon).
As Young's samples were short cores he provided some information on the
vertical trends. At most sites, the highest organic carbon contents were
between the 10 and 20 cm depths. In only 1 of the 19 stations was there an
increase in organic carbon below 20 cm. He found no trend with depth of
burial in the inorganic carbon. The highest organic carbon contents were
found in the deep water stations. He found a good correlation (R = 0.82)
between total iron and organic carbon content and noted that although
clayey sediment contained larger amounts of organic carbon than sandier
sediments, the percent organic carbon to percent clay correlation was not
significant (R = 0.40).
The literature on sulfur in estuarine sediments in relatively scant
when compared to that on organic carbon. Dunham (1961, cited in Goldhaber
and Kaplan, 1974) lists river estuaries and tidal lagoons among a limited
set of environments wherein the sediments may contain authigenically formed
reduced sulfur compounds. There is, however, a reasonable body of
literature on sulfur in marine sediments including, among others, the
summary work of Goldhaber and Kaplan (1974) and Berner (1970, 1981, 1982).
Goldhaber and Kaplan (1974) in tabulating the work of several other marine
researchers show sulfur contents ranging from 0.02% to 2.0% in marine
sediments.
The incorporation of sulfur in the sediments depends upon the
-reduction of sulfate to H2S and HS by bacteria. This process requires
97
-------
anoxic conditions, which, even if not present in the water column may exist
below the sediment-water interface. Although the sediments may have been
deposited through oxygen-rich water, the dissolved oxygen of the
interestial water is removed as it is used by the organisms inhabiting the
sediment (Goldhaber and Kaplan, 1974). Berner (1981), in a discussion of
sulfidic sediments, states that the occurrance of sulfides in marine
sediments is common because of the sulfate available from seawater and
nearly universal presence of organic matter in the sediments. Berner
(1969), however, sites local differences in the content of organic matter
as causing local differences in the sulfur content of anaerobic sediments.
Berner (1970), using a field example from the Connecticut shore of Long
Island Sound, further states that the availability of organic matter that
can be metabolized by sulfate-reducing bacteria is a factor limiting the
formation of pyrite in marine sediment. This relationship is depicted in a
plot of weight percent organic carbon, a rough indicator of the content of
organic matter, versus weight percent sulfur. The data points show very
little scatter about a line of best fit. Goldhaber and Kaplan (1974),
using data from several sources, present a similar plot. They state that
the relationship is approximated by a line with a slope of 0.36 but admit
to "a good deal if scatter the data." The scatter is due to a number of
causes, including the great range of depths (0 to 500 cm) below the
sediment-water interface from which the samples were taken.
b. Results. The ranges of values of total carbon, organic carbon, and
sulfur contents (Figure 40) present no anomalies. The values of organic
carbon up to a maximum of 3.9% with a mean of 1.0% are consistent with
literature values (Shidler, 1975; Folger, 1972; Kemp, 1971). Total carbon
contents are slightly greater, with mean 1.3% and are generally under 4%
but with a few samples reaching 9 or 10%. These very high values are from
areas where shell fragments constitute a significant portion of the
sediment. Although little was known to predict the values of sulfur
content, the distribution and range are not unreasonable with a maximum of
approximately 2% and a mean of 0.35%.
Although the samples were chosen with a bias toward finer sediments,
this bias is not significantly represented in the statistical correlations
of the various parameters (Table 11). The strongest correlations of the
chemical contents with a sedimentological factor are of the organic carbon
and sulfur contents with the percentage of clay in the sediments. The
table is a listing of R^ values from the SAS, GLM calculations of
regressions of the chemical contents against mean and median grain sizes,
mud and clay contents and water depth, and each other. The table has
values for the entire sample set and for sets of samples within subareas
because study of the spatial distributions (Figures 41, 42 and 43) seemed
to indicate a general trend of relationships between carbon or sulfur
contents and depth or clay content except in shallow and/or restricted
areas such as Mobjack Bay or Pocomoke Sound. In these areas the patterns
differed from those in the Chesapeake Bay proper. The several subareas
represent different physical environments and deserve separate
consideration.
98
-------
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organic carbon, and sulfur contents of the sediments.
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-------
Figure 41. Isopleth map of the total carbon content of the sediments
of the Virginia portion of the Chesapeake Bay.
CONTOUR INTERVAL I %
-------
ORGANIC CARBON CONTENT
CONTOUR INTERVAL 1%
Figure 42.
Isopleth map of the organic carbon of the sediments of the
Virginia portion of the Chesapeake Bay.
-------
JS'OO1
-{ .; ^' : .tr\ >' c-
\ '*, 'I f^n-f* "^
-^^ .-.-. '^v1^ .. <2T
SULFUR CONTENT
CONTOUR INTERVAL 0.5%
Figure 43. Isopleth map of the sulfur of the sediments of the Virginia
portion of the Chesapeake Bay.
-------
c. Discussion. Because the visual tie between depth and the various other
parameters, both sedimentological and chemical, is so appealing, the very
poor statistical correlations, maximum R^ of 0.47 and common values of 0.1,
is surprising. It most probably is explained by the frequency distribution
of depths (Figure 44). The distribution is decidedly asymetrical, with
most depths being quite shallow. Hence there are very few locations of
greater depth with high clay contents to offset the locations of shallow
depth and high clay content which results from the very sheltered nature of
the location.
Of the several characteristics used to describe the sediment type, the
percentage of clay has the most significant relationship to the carbon and
sulfur contents. This agrees with the conclusion of Thomas (1969), based
on the work of others, that "(organic) carbon is predominantly absorbed by
clay particles (Bader, 1962), the quantity of which is related to clay
surface area...". The poor percent-clay with total carbon correlation is
probably a function of shell material found in sandy sediments. The close
parallel between the clay-organic carbon and clay-sulfur relationships is a
further expression of the good organic carbon-sulfur (Figure 45)
correlation. R^ for the entire system 0.76, range for subareas exclusive
of Hampton Roads, 0.63 to 0.84. Figure 45 is very similar to Goldhaber and
Kaplan's (1974) plot. The very low organic carbon-sulfur correlation in
Hampton Roads is most likely the result of several factors including the
vast quantities of coal moved through the port and the industrial and urban
nature of the surrounding lands. The measures of the central tendencies of
the grain size distributions, mean and median, and the mud content of the
sediment are less well correlated than the percentage of clay. This points
out that the governing processes are the great surface area and
(potentially) active nature of clay particles and not purely grain size.
Hence the quantity of clay in the sediment is more important than an
integrating measure of the grain-size distribution.
The interrelationship of the various characteristics within individual
: subareas is a function of the local, physical environment of the subareas.
' Relatively shallow, semi-enclosed and little populated Mobjack Bay, from
i which roughly 35 of a possible 55 samples were selected for chemical
i analysis is the site of the strongest interrelationships. R^ values for
percent clay versus total carbon, organic carbon, and sulfur contents are
0.97, 0.96, and 0.90 respectively. The quality of the interrelationships
reflects the relatively simple nature of the set of processes active within
Mobjack Bay. There is only a minimum of fresh water inflow; the mean tidal
range is low, approximately 0.75 m; and fetches are limited as the bay is
open only to the southeast.
The York and Rappahannock River mouths, with 13 and 21 samples, also
exhibit reasonably good correlations reflecting the uniform nature of the
processes active across the limited areas of the river mouths. The
information from the other subareas indicates some of the differences
between them.
Pocomoke Sound is shallow and has a small drainage basin and
-freshwater inflow when compared to the rivers of the western shore. This
104
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^ 5/
^^
63 75
DEPTH
l23
147
44
'
c.:r
105
-------
is reflected in the relatively poor relationship of organic to total
carbon. This is probably due both to the less polluted waters of Pocomoke
Sound and to the abundance of shell material in the clean sediments.
Tangier Sound, which has a great range of depths, also has an abundance of
shell material. Also there is no freshwater inflow, hence no indirect
mechanism for the inward transportation of organic materials derived from
terrestrial sources.
d. Conclusions. There is a strong relationship between the clay content
and the total carbon, organic carbon, and sulfur contents of the sediments
of the lower Chesapeake Bay. This relationship probably is due to the
great surface-area presented by a large volume of clay particles and to the
chemically active nature of the clays. The ratio of organic to total
carbon is most strongly controlled by the presence or absence of shell
material and must equal one in the absence of shell.
Several subareas of the Bay are affected by different processes which,
in turn, influence the chemical and sedimentological relationships. The
urban, industrial surroundings of Hampton Roads create an environment quite
different from that in Tangier Sound or Mobjack Bay. The volume of
fresh-water inflow, the nature of the drainage basin, and the local energy
regime, in the form of waves and currents, all contribute to local
variations.
B. PATTERNS AND RATES OF SEDIMENT ACCUMULATION
1. Patterns of Erosion and Deposition. The patterns of erosion and
deposition in the Bay fit, with minor modifications, the physiographic
segmentation offered by Ryan (1953) based upon the Bay geomorphology:
Northern Bay: Susquehanna River to south of the Patapsco
River mouth.
Middle Bay: Patapsco River to the mouth of the Rappahannock
River.
Southern Bay: Rappahannock River mouth to the Bay entrance.
Within the Virginia portion of the Bay stem, the patterns of erosion and
deposition suggest a further zonation:
Middle Bay:
Lower belt: 37°55' to 37*35'
Southern Bay;
Upper (transitional belt): 37°35' to 37°25'
Central (farfield Bay mouth) belt: 37°25' to 37°15'
Lower (nearfield Bay mouth belt: 37*15' to 36°55'
107
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The principal loci of deposition and erosion are shown in Figure 46.
The generalized pattern is superimposed on bathymetry in Figure 47. The
patterns of deposition/erosion are displayed in terms of the rate of mass
accumulation in Figures 48, 49 and 50 showing, respectively, the sand,
silt, and clay components.
a. The Nearfield Bay mouth belt is characterized by a pattern of
"alternating" erosion/deposition areas across the Bay mouth entrance, and
depositional zones on the flanks of the Tail of the Horseshoe Shoal and
Thimble Shoal Channel (Figure 4). Erosion is pronounced off of Cape Henry,
and in False Channel and North Channel separating Middle Ground, Inner
Middle Ground and Latimer Shoals on the northern part of the entrance. The
shoal areas themselves are characteristically depositional, while the
location of the alternating cut and fill pattern suggests a migration of
the Inner Middle Ground and Middle Ground Shoals to the southwest. The
migration is consistent with the results of Granat (1976) who studied
Middle Ground Shoal and presented bathymetric comparisons of five surveys
between 1852 and 1975. The principal surface sediment type (> 80% by
weight) within this zone is fine sand although there are fingers of medium
and very fine sand (Figure 48).
The Chesapeake Channel is indicated as depositional throughout the
reach from the entrance to the latitude of Cape Charles. The surface
sediments are predominately fine and very fine sand with up to 40% silt.
The lower part of Old Plantation Flats Channel is included in this
belt. The deeper areas of the channel are erosional as is the eastern
flank; the bottom sediments are very fine sand with up to 30% silt in the
deep area while the eastern flank is composed of fine to medium sand. The
western flank, indicated as depositional, is very fine sand (> 607, by
weight) and silt.
Horseshoe Shoal, which forms the western portion of the Bay mouth
nearfield belt, is indicated as an area of relatively low deposition with
areas of erosion in the nearshore zone. This sand shield (> 80% sand) is
characterized by fine sand on the southern part whereas the region flanking
York Spit Channel is medium sand with patches of coarse sand. The latter
areas fronting Poquoson Flats are indicated as non-deposition or erosional.
This is consistent with independent observations from a coring study which
show only a veneer of sand over Pliocene sediments.
The Bay bottom south of Thimble Shoals Channel is indicated as
depositional near the seaward part of the channel, and erosional within the
inner segment (dredging zone). The Crumps Bank area is expressed as non-
to slightly depositional with fine and very fine sand and local areas of
silt. The nearshore zone along the southern Bay shore and Willoughby Bank
is indicated as being a relatively weak depositional zone with erosional
spots in the nearshore. Surface sediments are fine to medium sands.
b. The Central Southern Bay belt is considered a portion of the Bay mouth
system because of the presence of a pronounced depositional area (Figures
46 and 47) to which we attribute a Bay mouth source. This depositional
108
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37-00'
E3 >l METER "FILL"
.5 METER "CUT"
BATHYMETRIC CHANGES
CONTOUR INTERVAL 0.5 METER
Figure 46. Bathymetric changes during the past 100 years in the Virginia
portion of the Chesapeake Bay.
-------
"rii i "
Q >l METER FILL
>Q5 METER "CUT"
BATHYMETRY, CONTOUR INTERVAL 10 FEET
Figure 47. Bathymetry and generalized bathymetric changes in the Virginia
portion of the Chesapeake Bay.
-------
Figure 48.
MASS ACCUMULATION OF SAND
CONTOUR ^NTERVAL I M-TON / M'/CENTURY
The mass of sand accumulation per square meter per century
in the Virginia portion of the Chesapeake Bay.
-------
Figure 49.
MASS ACCUMULATION OF SILT
CONTOUR (NTERVAL .2 M-TONS/M*/CENTURY
The mass of silt accumulation per square meter per century
in the Virginia portion of the Chesapeake Bay.
-------
MASS ACCUMULATION OF CLAY
CONTOUR INTERVAL .2 M-TONS /M1/CENTURY
Figure 50. The mass of clay accumulation per square meter per century
in the Virginia portion of the Chesapeake Bay.
-------
area, centered at 37"20', includes the northern ramp of the Old Plantation
Flats Channel. Our hypothesis is that this depositional lobe (> 60% sand;
the remainder silt) results from transport from the Bay entrance through
the channel and up onto the ramp as the channel cross-sectional area
increases. The driving force for the transport is argued to be the net
up-Bay bottom-water circulation which is strongest along the eastern side
of the Bay. The recovery of bottom drifters (Harrison, et al., 1967)
released in the vicinity of the Bay mouth fortifies the hypothesis; one of
the strongest recovery zones within the Bay, and incidentally the
northernmost zone, was the shoreline west of the deposition lobe.
The eastern flank of the channel and the adjacent shallow terrace have
erosional loci of fine and medium sands (total sand > 80% by weight).
There are two additional zones of deposition in the western part of
the belt. The westernmost of the two is the nearshore terrace (water
depths generally less than 6 meters (20 feet) fringing Mathews County. For
the most part, the bottom sediments are medium sands (Figure 30) which may
represent bedload migration from the more northerly nearshore terrace which
is indicated, in part, to be erosional. In these shallow waters, the net
bottom flows could be expected to be down-bay. During strong northeast
wind events, the southerly wind-drift currents and wind wave driven bottom
agitation could, as well, play a significant role. To the east of the
terrace depositional area is a narrow erosional zone which is coincident
with the western flank of the spit-like shoal upon which Wolf Trap Light is
situated.
The second locus of deposition on the western side is directly east of
the Wolf Trap Shoal. The bottom materials are predominately fine and very
fine sand with pockets containing more silt. This depositional lobe may
represent an admixture of materials from the adjacent "Bay mouth derived"
depositional area and the western terrace. According to Firek (1975) and
Firek et al. (1977) the eastern lobe has relatively high concentrations of
garnet in the heavy mineral suite compared to the western lobe which has a
higher concentration of tourmaline and zircon which he considered to be
representative of western shore source.
c. The Upper Southern Bay belt is indicated as being generally
depositional (0 to 0.5 m/cent) but no strong depositional loci are
expressed. Erosional loci are indicated in the channel trough on the
eastern side of the Bay. As well, the nearshore terrace along the Eastern
shore has patchy erosional areas. A strong erosional locus is indicated on
the western shore terrace below the Rappahannock River. When the
depositional patterns are viewed in conjunction with bottom sediment type
(Figure 30), this belt appears as a transitional region .between the
predominately sandy regions to the south and the finer grained sediments to
the north.
Within the channel along the Eastern Shore, indicated as erosional,
the bottom is composed of fine sands with up to 25% silt. The erosional
areas on the western shore terrace are composed of medium and coarse sands.
-Zircon is a strongly dominate heavy mineral in the fine fraction (Firek,
114
-------
1975, Firek et^ al^. , 1977), which in terms of hydraulic equivalence is
consistent with the relatively coarse sand population.
The Bay floor area between the fringing terraces, without strong
depositional loci, may be roughly divided longitudinally. The area in the
central part is characterized as about 50% very fine sand, less than 20%
clay, and the remainder silt. In contrast, the western area, a confluence
of the Rappahannock Tributary and the main channel leading to Maryland
waters, is characteized by increasing dilution of very fine sand with
increasing contributions of clay and silt (Figure 28 and 33).
d. The Lower Middle Bay belt, ranging in latitude from the mouth of the
Rappahannock River to the Virginia boundary at the mouth of the Potomac, is
the most complex segment in the Virginia portion of the Bay stem. The
clearly defined deep channel which extends through most of the Maryland
portion of the Bay terminates in this section. The channel is flanked on
the east by the large sand-shield containing Tangier and Smith Islands. As
well, the segment contains the junction of the channels leading into
Tangier and Pocomoke Sounds.
The channel floor, except in the deepest parts and associated flanks,
are indicated as depositional. The sand component, very fine sand, is a
relatively minor constituent of the channel and lower flank, and the clay
fraction exceeds 40%. The deepest parts of the channel, with essentially
the same sediment constituents, are erosional loci. These erosional sites
include parts of the eastern channel flank where the sediments rapidly
grade into sands.
At approximately 37°40' latitude, the channel begins to flare in width
and cross-sectional area. The flaring section is associated with sites of
pronounced deposition. In this section sand (very fine sand) is a very
minor constituent « 5%) and clay exceeds 40%. The western boundary of
this section follows closely the 30-foot (10 meter) bathymetric contour,
shoreward of which sand sized materials dominate (> 90%). The
Tangier-Smith Island sand shield terminates at 37°40' (following the 10-13
m contours) so the channel flare is not bathymetrically bounded on the
eastern side. However, between 37°40' and 37°35' the dilution with the
clay constituent rapidly decreases to the east such that the mud component
increases to 25-30% with silt generally dominating.
Most of the surface sediments with significant clay fractions
(> 20%) are contained within the latitude range 37°35' to 37°55'. This
band apparently represents the principal residual deposits of material
delivered to the Virginia Bay stem from Maryland waters including the
Potomac River and, perhaps, the Rappahannock River.
The sand shield containing Tangier and Smith Islands is indicated as
null to mild deposition in water depths less than 7 meters. However, there
are erosional loci in very shallow waters. The western and southern limits
of the shield have strong depositional loci. On the west, fringing the
main Bay channel, deposition extends to water depths of greater than 17
.meters (50 feet) while on the southern terminus the depositional lobes
115
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extend to depths greater than 12-14 meters (36 feet). The shield is
generally greater than 90% sand with the deposttional loci composed of fine
to medium sand. The depositional nature of the deeper fringes of the belt
is attributed to encroachment of sand over the edge of the shield induced
by the net down-bay circulation in "surface" waters, augmented (or perhaps
dominated) by wind drift current with wave driven resuspension accompanying
strong north and northwest winds, a dominant component in fall and winter.
2. Sediment Budget. Previous work addressing sediment budgets for
the Bay has dealt only with the suspended sediment component. Moreover,
with one notable exception (Schubel and Carter, 1976), attention has
previously focused on the Maryland portion of the Bay. Observations of
suspended sediment concentrations in the Virginia part of the Bay are
relatively scanty.
!
A complete sediment budget for the inorganic constituents of the
Virginia portion of the system would include several terms for sources and
sinks:
- - 1.) the Maryland portion of the system (Bay stem and the Potomac
estuary) (probably a source),
2.) the principal Virginia tributary estuaries (Rappahannock,
Piankatank, York, and James), and minor fringing creeks (could be
either sources or sinks),
3.) the Bay mouth and near continental shelf waters (probably
sources),
4.) Shore eros.ion of the margin of the Virginia Bay stem (source),
5.) skeletal remains from primary production (source),
i 6.) carbonates from shell (source), and
I
i 7.) accumulation of material on the Bay floor (treated as a sink).
; As ensuing discussion will indicate, current knowledge is
insufficiently developed to address all of the requisite contributions.
Our attempt at a sediment budget utilizes the reported values for estimates
of suspended sediment (clay plus silt) for items 1, 2, 3 (Schubel and
Carter, 1976; Biggs, 1970), and 5, and information derived from this study
for items 4, 6 and 7. No estimates of influx of sand from the Bay mouth
have been reported. The strategy is to ask whether the observed sediment
accumulation on the Bay floor is consistent with the existing best
estimates of the contributions from various sources.
The most comprehensive evaluation of the relative sources (inorganic
and organic) within the Maryland portion of the Bay is that of Biggs
(1970). Utilizing estimates for the annual yield of silt and clay from
shoreline erosion, of skeletal material derived from plankton, and a
comprehensive set of suspended sediment measurements taken over a one-year
116
-------
(1967) period, he concluded that 10% of the total input of inorganic
material was transported, by estuarine circulation, into the regions south
of the Patuxent River.
Schubel and Carter (1976) advanced a suspended sediment budget for the
entire main stem of the Chesapeake Bay. They formulated this budget from a
single-segment, two-layer model using the salt balance equations to infer
net movement of suspended sediment. Axial distributions of vertical
salinity and inorganic suspended sediments down the Bay stem during a
twelve month period, 1969-1970, provided the primary data for the model.
They concluded that there is a net movement of suspended sediment into the
Bay from the ocean, and that the tributary estuaries are sinks for
suspended sediment from the Bay (summarized in Table 12). The results in
Table 12 display several interesting points aside from the conclusion that
the tributaries act as sinks for sediments from the Bay. In order of rank,
the sources of suspended sediment are the Susquehanna River, silt and clay
derived from shore erosion, and, then the ocean. The tributaries are
relatively weak sinks and, by subtraction, the remainder is deposition onto
the Bay bottom.
Whether the Bay stem responds as a net source or sink for suspended
sediments to the principal estuaries remains a question demanding detailed
investigation. Nichols (1977), in a study of the response of the
Rappahannock estuary to the flooding of the 1972 tropical storm, Agnes
calculated that over the sixteen days encapsulating the event, 10% (11,000
tons) of the suspended load escaped into the Bay. Officer and Nichols
(1980) applied a two dimensional box-model formulation to four sets of
suspended sediment concentration in the James and Rappahannock estuaries,
one of which was the Rappahannock response to Agnes. In the latter, the
estuary was found to export sediment to the Bay. However, in another data
set (spring, 1978) the estuary was found to be a sink for Bay materials and
the magnitude was the same order of the sediment input from the river. For
the James data sets, the estuary was found to be an exporter of sediment
during very high discharge but an importer of sediments from the Bay during
moderate river discharge conditions. Ludwick (1981) argued that the
relatively high deposition rate of fine grained sediments in Thimble Shoals
Channel exiting the James estuary is due, in part, to the deposition of
sediments from the ebb plume of the James. This in itself does not
constitute sufficient evidence that the James is a net sediment exporter,
but it does suggest tha James River sediments are deposited in the Bay.
The strength of the oceanic source could however be stronger than the
amount deposited with a net importation resulting. Feuillet and Fleischer
(1980) documented the mixing of clay minerals from the ocean source within
the James estuary.
It is important to recognize that none of these studies were
specifically designed to address the question of net flux. Rather than
investigation of transverse variations in salinity, flow, and sediment
concentrations at a cross-section, these studies have emphasized the
vertical distribution along the estuary axis. For example, in the 1978
series of measurements at the Rappahannock estuary mouth (Nichols et al.,
1981) a single vertical array of current meters on the northern side of the
117
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Table 12
Suspended Sediment Budget
(after Schubel and Carter, 1976)
Sources
Susquehanna River
Shore Erosion, Md.
Va.
Ocean
TOTAL
Sinks
Tributaries
Potomac
Rappahanncok
York
James
All Others
TRIBUTARY TOTAL
Deposition
TOTAL
Mass/Year
1.07 X 106 tons
0.40 X 106 tons
0.20 X 106 tons
0.22 X 106 tons
1.89 X 106 tons
0.04 X 106 tons
0.02 X 106 tons
0.02 X 106 tons
0.05 X 106 tons
0.03 X 106 tons
0.16 X 106 tons
1.73 X 106 tons
1.89 X 106 tons
118
-------
channel indicated a net landward flow at all depths. Prior measurements
indicated a net bayward flow at all depths on the south side of the
channel. Estimates of the flux at a cross-section would require sufficient
sampling to capture these pronounced transverse variations.
While the evidence that the tributary estuaries are effective traps
for riverine sediment is incontestable, considerably more detailed
measurements will be required to settle the claim that the Bay stem acts as
a net source for sediments to the tributary estuaries. The availability of
sediments from tidal resuspension would be a controlling factor. The
suspended materials "leaking" into the Bay stem from the tributaries may,
upon settling, arrive in regions where tidal currents are not strong enough
for appreciable resuspension.
The present attempt at a sediment budget does not attempt to resolve
whether the tributaries are net sources or sinks. Before venturing into
the evaluation of the sources, it is necessary to consider the residual
sediment accumulation derived from the treatment of bathymetric comparisons
(see section V B for the details of calculation). Taking into
consideration the errors in bathymetric comparison, the data are presented
at three levels of estimation; for data cells (1 minute, depth bounded,
latitude slice) with changes greater than ± 1.10 m, ± 0.57 m, and finally
treating the data as if there were no error, that is ± 0.00 m. Application
of ± 0.57 m and ± 1.10 m cutoff criteria is very conservative as these
values were derived from the error associated with comparison of individual
depth measurements. Here they are applied to information which has
undergone three levels of smoothing, each of which would be expected to
reduce the propagated error of the bathymetric comparison.
There are no reliable independent ways to ascertain the reasonableness
of the three levels of estimation. The approach herein used is to make a
comparison between sand derived from erosion and the mass of sand
"accumulated" in the nearshore zone. The sediment distribution maps show
the nearshore to be dominated by coarse sand. The test is applied to the
western side of the Bay as it is reasonable to argue that the major
tributaries act as barriers to sand transport and that the Bay's western
shore is isolated from sand sources other than shore erosion. Table 13
displays the correspondence between the mass of sand derived from shore
erosion and the "accumulated" mass for eight relatively isolated coastal
segments along the western shore of the Bay. Comparisons are made for two
outer-depth limits, 12 feet (3.7 m) and 18 feet (5.5 m). For the wave
height and period conditions found in the Bay, the 12 foot (3.7 meter)
depth is a reasonable limit for normal active wave induced movement of the
bottom sediment. The 18 foot (5.5 meter) depth-limit is taken as the
maximum depth to which appreciable sand from shore erosi-on is likely to be
dispersed. Zone 4 displays relatively large sand accumulations out to the
18 foot (5.5 meter) depth and beyond. Previous discussion has noted that
this zone, between the Piankatank River and the entrance to Mobjack Bay, is
at the latitude of appreciable deposition argued to have a Bay-mouth
source. Furthermore, this shore zone was the recovery area for a number of
seabed drifters (Harrison, et_ al_. , 1967). Comparison within this zone may
thus be "contaminated" with sands derived from a source other than shore
119
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erosion. Thus, the total without Zone 4 is of final interest. Both the ±
0.00 m and ± 1.10 m estimation levels for accumulation show poor
correspondence with the shore-erosion source. The ± 0.57 m level, however,
displays a correspondence within a factor ranging between one-half and to
one for the two depth limits. The individual zones show a better
correspondence for the 12-foot depth limit. Based upon this comparison,
the estimation level using depth changes greater than ± 0.57 m appears to
be the most reasonable. Ludwick (1981) used, without explanation, ± 0.61 m
as the level of significant change in his study of bathymetric change in
the Thimble Shoals area.
The values of accumulated sediment mass of sand, silt, and clay are
shown in Table 14 for the three levels of estimation. Only the Bay stem
excluding fringing sounds, embayments, and tributary mouths is included in
this discussion. Sand-sized materials clearly dominate the sediment mass
accumulation. Given the dominance of sand-sized materials in the surficial
sediments, and the method of calculating mass accumulation, this result is
not surprising. However, the summary values of Table 14 do provide an
estimation of the magnitude of the dominance. Given the mass of residual
accumulation, we may draw the first comparison between the strength of the
known sources and the residual accumulation, a sink, shown in Table 15.
The bottom accumulation dramatically exceeds the source terms for both the
silt and clay and the sand comparisons. The contribution of silt and clay
from Maryland waters (0.147 X 10°" m-tons/100 years) is taken from Table 12
as the contribution of the Susquehanna and shore erosion reduced by 90% as
argued by Biggs (1970). This value is uncompensated for any deposition
between the Patuxent River and Smith Point or any source or sink value for
the Potomac. The value of 0.025 X 10° m-tons/100 years for silt and clay
from shore erosion in Virginia, obtained in this study, is an order of
magnitude less than that value estimated by Schubel and Carter (1976). The
inorganic constituent from zooplankton is relatively small. The value for
oceanic source, also taken from Table 12, appears as a stronger source for
silt and clay than the estuarine contribution from the Maryland portion of
the Bay.
' :
If the ^_ 0.57 m average depth change is accepted as the appropriate
basis for calculation of mass accumulation, the bottom accumulation of silt
and clay (4.9 X 10° m-tons/100 years) exceeds the value from the estimated
sources (0.4 X 10** m-tons/100 years) by a factor of 12. This formulation
does not include the major Virginia tributaries as either sources or sinks.
Bottom accumulation of sand (16.9 X 10° m-tons/100 years) exceeds shore
erosion source by a factor of 40. Inspection of the axial trends of sand,
silt, and clay bottom accumulation may offer some insight toward explaining
the discrepancy.
The patterns of accumulation (Figures 48, 49, and 50) indicate that
the principal locus of clay deposition is between the Potomac and
Rappahannock Rivers. The silt accumulation occurs throughout the central
basin between the York River and the confluence of the channels to Tangier
and Pocomoke Sounds as well as within the axial channel leading into the
Maryland portion of the Bay. Sand accumulations are most pronounced at the
-Bay mouth area with secondary loci at about 37°20" latitude, and on the
121
-------
Table 14
Bottom Deposition of Combined Sand, Silt and Clay;
Values are cumulative from north to south
SAND, SILT, AND CLAY
(X 108 m-tons/100 years)
Zone
7
10
15
20
25
30
35
40
45
50
55
60
64
Latitude
37°54'
37°51'
37°46'
37°41'
37°36'
37"31' >
H
37°21' o
37°16'
37°11'
37°06'
37°01'
36°57'
TOTALS
SAND
SILT
CLAY
±
1.
5.
7.
10.
11.
12.
15.
17.
20.
21.
26.
27.
22.
3.
2.
0
3544
5002
1092
6834
1408
2542
2022
4840
8967
2808
5884
2558
6047
1038
2958
2051
± 0.57 m
1
3
6
7
7
8
12
14
15
17
20
21
16
3
1
.3291
.2860
.6093
.0761
.3292
.7420
.9009
.6242
.1358
.1037
.6011
.8609
t
.0685
.9074
.0594
.8409
±
1
1
2
2
2
4
4
5
6
8
8
7
1
0
1.10 m Geographic Location
.0527 Smith Point
.2800
.1247
.9129
.0970 Rappahannock Spit
.1527 Cherry Point
.2496 Wolf Trap Light
.1704
.8951 Entrance Mob jack Bay
.2419 Poquoson River
.1397 Fisherman Island
.4071 Old Point Comfort ;
1
.9523 Cape Henry j
i
.1685
.1017
.6821
122
-------
Table 15
Comparison of Sediment Budget Terms
Source
Maryland
Shore Erosion, Va.
Zooplankton, Va.
ash
Ocean
TOTAL SOURCE
SILT PLUS CLAY
X 108 m-tons/100 years
0.147
0.025
0.008
0.220
0.400
SAND
X 108 m-tons/100 years
?
0.400
0.400
BOTTOM
±
±
±
RESIDUAL
0 m
0.57 m
1.10 m '
5
4
1
.500
.900
.784
22
16
7
.104
.907
.168
123
-------
'fringes of the Tangier-Smith Island sand-shield. These patterns are
rendered in quantitative form in Figure 51 and Table 16 which indicates
cumulative mass accumulation as a function of latitude.
The curves for sand accumulation indicate that about 25% of the total
accumulation occurs between Smith Point (37°51') and the fringes of the
Tangier Smith Islands shield (37°43'). Between 37°43' and 37°25' there is
relatively small additional accumulation of sand. For the intermediate
level of estimation (>_ 0.57 m) about 65% of the sand accumulation occurs
below latitude 37°25' and 38% occurs within the Bay mouth entrance
latitudes (37°11' - 36°55'). These results indicate that the Bay mouth
acts as a gateway through which very large quantities of sand are advected.
As well, the accumulation of sand on the Tangier-Smith sand-shield
indicates an advection of sand from the Maryland portion of that feature.
I The sand contributed from shore erosion is then a relatively weak source.
The calculation of mass accumulation did not compensate for shell
content, thus, the "sand" accumulation is an overestimate. If we take a
liberal estimate of 10% by weight for shell fragments (Shideler, 1975; also
this study, Figures 41 and 42), the results of this study indicate that the
Bay mouth acts as a source for sand of the order ranging from 16 X 10° to 5
X 10" m-tons/year distributed as far north as 37°25'. At the intermediate
level of estimation, we find 12 X 10 m-tons/year.
The previously postulated sources for silt and clay cannot be readily
rectified to explain the disparity between the bottom mass-accumulation and
the estimated source-strengths. The bathymetric comparisons are based on a
period prior to the sediment influx due to runoff from Hurricane Agnes so
we cannot appeal to the anomalously high Agnes input from the Susquenna (31
X 106 m-tons, a factor of 25 to 30 greater than the "normal" year; Schubel,
1975). The results suggest that the major Virginia tributaries act as
sources for sediment to the Bay. Both the Rappahannock and York
tributaries show clayey-silt stringers entering the Bay. As well, Ludwick
', (1981) presents evidence that there is deposition of muds from the James
[ River estuary in the Thimble Shoals Channel. The relative strength of
these potential sources cannot be determined from available information.
Mass accumulation rate as a function of depth interval is shown in
Figures 52 and 53a, b, c and Tables 17 and 18 for the >_ 0.57 m (±) level of
estimation. The five latitude slices reflect the principal depositional
segments dicussed in section VI B 1. In water depths less than 18 feet
(5.5 m), the accumulation rate is relatively low. Maximum accumulation
generally occurs between 18 feet (5.5 m) and 42 feet (12.8 m) with a
relative redution in greater depths. This trend was noted by Carron (1979)
in a plot of sedimentation rate (deposition thickness per unit area y time)
derived from the same bathymetric comparisons but with different smoothing
procedures.
Maximum total sediment-mass-accumulation per unit area (0.968
m-ton/m^/lOO years) occurs in Zone 3. This is particularly notable in the
sand and silt components, each exhibiting maximum in this zone. It is
-germane to contrast the sand, silt, and clay components in Zones 1,2 and 3
124
-------
37°51'
37°4I'-
37°3I'
37°2I'
37° II1-
*f*-'*'+r-'-*~, **
37°OI
SAND
37°5I'
37°4I'-
37°3I'-
37°2I'
37° 111
37°OI'
3705I'
37°4I'
37°3I'
37°2l'
37° I I'
37°OI'
ret -.1
LEVELS OF ESTIMATION
£ ±0m
£ ± 0.57m
- 2 ± 1.01 m
-v^PER 25,50, and 75%
OF ACCUMULATION
10 20
CUMULATIVE
"'{xlO8 TONS/100 years)
SILT
I 2
CUMULATIVE
SMITH POINT
RAPPAHANNOCK RIVER
WOLF TRAP LIGHT
-*. MOBJACK BAY
YORK RIVER
CAPE CHARLES
CAPE HENRY
~4 (xlO8 TONS/100 yeors)
I 2
CUMULATIVE
3 (xlO8 TONS/ 100 years)
Figure 51. Cumulative mass accumulation as a function of latitude.
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- .- . ... ,v'.-^ -
ZONE l< 37°54' to 37*36'
6-12 12-18 18-24 24-30 30-36 36-42 >42
ZONE 2= 37°36' to 37°25'
ZONE 3- 37°25' to 37°I6'
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
ZONE 4= 37°16' to 37°05'
ZONE 5^ 37°05' to 36°56'
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
DEPTH INTERVAL (feet)
Figure 52. Total sediment mass accumulation per unit area per century
129
-------
UJ
>
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-X.
CJ
ZONE 1= 37°54' to 37°36'
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
ZONE 2 37°36' to 37°25
ZONE 3: 37e25'to37°l6l
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
O
i
ZONE 4: 37°I6'to 37°05'
ZONES: 37°05'to36°56'
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
DEPTH INTERVAL (feet)
Figure 53a. Sand; mass accumulation per unit area per century as a
function of depth interval.
130
-------
cr
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x
CVI
ZONE l« 37°54' to 37°36'
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
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ZONE 3 37°25' to 37°I6'
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O
ZONE 4= 37°I6' to 37°05'
0
ZONE 5' 37°05' to 36°56'
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
DEPTH INTERVAL (feet)
Figure 53b. Silt; mass accumulation per unit area per century as a
function of depth interval.
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<
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ZONE h 37°54' to 37°36'
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ZONE 5' 37°05 to 36°564
0-6 6-12 12-18 18-24 24-30 30-36 36-42 >42
DEPTH INTERVAL (feet)
Figure 53c. Clay; mass accumulation per unit area per century as a
function of depth interval.
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-------
relative to the earlier discussion of the mapable patterns. Zones 1 and 3
contain substantial accumulations of sand; the accumulation in Zone 1 is
attributed to southerly advection of sediment from the Tangier-Smith
sand-shield, and the accumulation in Zone 3 is attributed to sources to the
south (including the Bay mouth), and to additional contributing from the
shallows to the north on the western side of the Bay. This interpretation
thus views Zone 2 as a relatively inactive area of sand accumulation
between opposing sources. The contribution from silt to the total mass
accumulation parallels that of sand: -the strongest contribution is in Zone
3 followed by Zone 1 with a relatively weak accumulation in Zone 2. This
may argue for a mixture of sources, particularly since the absolute rate of
mass-accumulation and mass-accumulation rate per unit area follow the same
patterns (Tables 17 and 18) for sand and silt. As there is little basis
for arguing that the Maryland portion of the Bay's stem is a principal
source of silt relative to clay, the Bay mouth may serve with the
"Maryland" Bay-stem, the Potomac, and the Virginia tributaries as secondary
sources along with shore erosion. Such interpretation is, of course,
conjectural and is based upon the intuitive notion that clay would be
expected as the principal component contributed by the tributaries and by
the "Maryland" Bay-stem whereas the nearshore wave energy could maintain
silt from the Eastern Shore oceanic shoreline and lagoonal system in
suspension for advection into the Bay where tidal resuspension could foster
net up-Bay drift. This argument is tantamount to the claim that silt-sized
particles are carried from the Bay mouth to points throughout the Virginia
portion of the Bay.
The zonal distribution of the accumulation of clay-sized particles
indicates the strongest depositional center occurs in Zone 1 followed by
Zone 3, and a relatively weak contribution in Zone 2 (Table 16). The
relatively low accumulation of clay (viewed either as mass or mass per
area) raises some interesting questions. What is the principal source of
the clay deposited in Zone 3? What is the fate of the suspended sediment
which escapes the Rappahannock River? Does the sediment exiting in surface
waters become, after settling, entrained in up-estuary bottom-flows by
steps in tidal resuspension with ultimate deposition in Zone 1 where tidal
energy is relatively small? Or does the material follow the net down-Bay
drift along the western side of the Bay? Resolution of these questions
await further research. .
The patterns of deposition and magnitude of the accumulations suggest
the following summary interpretation:
1.) Appreciable influx of fine sand occurs at the Bay mouth (on the
order of 10^ m-tons/100 years). The influx apparently results
from tidal transport coupled with a net up-estuary bottom-flow
associated with estuarine circulation. The advection zone of the
sands extends at least as far up estuary as 37°25' and perhaps as
far as 37°41'.
In addition to the sand accumulating at the mouth of the
Bay, there is significant quantity of sand moving from the north
along the fringes of the relict Tangier-Smith Islands
135
-------
sand-shield. In this case, the transport processes are probably
dominated traction and suspension driven by wind-waves generated
during north to westerly wind events coupled with "surface"
wind-driven and net down-Bay estuarine circulation.
By comparison to these sources, the sands derived by
shore erosion are a minor constituent. Previous work (Schubel
and Carter, 1976) has assumed shore erosion would be the
principal source of sand. >
2.) Silt-sized particles also are an important component of the
influx of sediment through the Bay mouth. Principal centers of
deposition of silt occur in the central Bay between 37°15' and
37°45'. The degrees of partitioning of the total accumulation
between the oceanic and estuarine sources is not clear, but as
about fifty percent of the total accumulation occurs south of
37a25", the Bay mouth likely is a strong source.
3.) Fifty percent of the total accumulation of clay occurs north of
Rappahannock River mouth (37°35') which suggests that the
principal source of clay is the water to the north. However, as
the area between 37°40" and 37°55' is a region of relatively low
tidal-energy relative to the influence of estuarine circulation,
the region may also represent a trap for sources to the south.
136
-------
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143
-------
APPENDIX 1
LORAN C
BIAS CORRECTION DATA
144
-------
LORAN-C BIAS CORRECTION DATA
Location
Gwynn Island
Windmill Pt. Light
Smith Pt. Light
Tangier Is. Channel
Saxis Boat Ramp
Onancock Ck. Ent. #1
Onancock Ck. Town Wharf
Occohannock Ck» Ent. #1
Wolf Trap Light
Hungars Ck. Ent., Mkr. 1PA
Swash Channel, York Spit, Mkr. 3
Cape Charles Harbor, 3 M "5" 5A
Tue Marshes Light
York Spit Light
Wise Pt. Channel Mkr "269"
Back River Ent., Mkr. "4"
Thimble Shoal Light
Fort Wool Pier
James River Bridge
Chesapeake Bay Ent. 2T Bell
Fish Pier, C.B. Bridge Tunnel
Little Ck., BW "LG"
Middle Ground Light
Lynnhaven In. Marina
Lafayette River Ent.
Latitude
37°29.31'
37°35.79'
37°52.88'
37°49. 78'
37°59.18'
37°43.47'
37°42.74'
37°22.55'
37°23.41'
37°23.40'
37°15.73'
37°15.32f
37°14.13'
37°12.57'
37°06.92'
37°06.07'
37°00.87'
36°59.15'
36°58.68'
36°58.45f
36058.05'
36°56.95'
36056.70'
36054.28'
36053.57'
Longitude
76°18.14'
76°14.17'
76°11.04'
75°59.66r
75°43. 89'
75°51.07'
75°45.37'
75°57.86'
76°11.38'
75°59.72'
76°19.97'
76°01.95'
76°23.17'
76°15.27'
75°58.87'
76°15.77'
76°14.42'
76°18.08'
76°28.78'
76°02.28'
76°06.87'
76°10.75'
76°23.55'
76°05.08'
76°19.47'
-------
TO CORRECT FROM THEORETICAL TO REAL
9930-Y
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
_
2
1
1
1
1
1
1
1
1
1
1
2
1
1
1
2
1
1
1
1
1
2
2
2
2
.09
.27
.52
.47
.31
.45
.75
.48
.38
.20
.55
.06
.66
.64
.71
.16
.75
.77
.61
.98
.81
.17
.15
.02 .
.07
9930-Z
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
+ 0
+ 1
+ 1
+ 1
+ 1
+ 1
+ 0
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
.31
.31
.32 "
.42
.30
.60
.22
.39
.33
.56
.33
.53
.21
.22
.27
.05
.28
.94
.22
.06
.10
.06
.26
.11
.08
9960-X
- 2
- 2
' - 2
- 2
- 2
- 2
- 2
- 2
- 2
- 2
- 2
- 2
- 2
- 2
- 2
- 2
- 3
- 2
- 2
- 2
- 3
- 2
- 2
.68
.32
.08
.40
.79
.59
.55
.42
.29
.63
-
.22
.97
.59
-
.71
.74
.71
.15
.44
.46
.72
.23
.57
.92
9960-Y
- 0
- 0
- 0
- 0
- 1
- 0
- 0
- 0
- 0
- 1
- 0
- 1
- 0
- 0
- 0
- 0
- 0
- 1
- 0
- 0
- 0
- 0
- 0
.47
.35
.16
.62
.28
.80
.62
.60
.55
.05
-
.20
.06
.55
.77
.17
.67
.52
.09
.15
.25
.29
.67
.31
- 0.35
-------
APPENDIX 2
THE MASS OF SEDIMENT ERODED FROM THE SHORELINE
149
-------
MASS OF SEDIMENT ERODED OR ACCRETED
ALONG THE SHORELINE BY MINUTE OF LONGITUDE
FOR THE SOUTHERN SHORE OF THE LOWER CHESAPEAKE BAY
Minute of
Longitude
760 oi1
02'
03'
04'
05'
06'
07'
08'
09'
10'
11'
12'
13'
14'
15'
16'
17'
Percent
Gravel
1,073
(1,779)
(20,590)
(21,525)
(12,167)
14,195
14,195
ND
429
4,929
4,983
375
0
0
346
252
182
(15,102)
(0.5)
Sand
2,143,675
157,719
(315,568)
(329,912)
(186,472)
364,329
364,329
ND
8,075
92,864
93,873
150,334
157,133
251,551
172,464
125,782
90,770
3,340,946
100.5
Metric Tons
Silt
0
0
0
0
0
0
0
ND
0
0
0
0
0
0
0
0
0
0
Clay
0
0
0
0
0
0
0
ND
0
0
0
0
0
0
0
0
0
0
Total
2,144,748
155,940
(336,158)
(351,437)
(198,639)
378,524
378,524
ND
8,504
97,793
98,856
150,709
157,133
251,551
172,810
126,034
90,952
3,325,844
NOTES: ND = No data.
() = Accretion.
150
-------
MASS OF SEDIMENT ERODED OR ACCRETED
ALONG THE SHORELINE BY MINUTE OF LATITUDE
FOR THE EASTERN SHORE OF THE LOWER CHESAPEAKE BAY
Minute of
Latitude
370 07'
08'
09'
10'
11'
12'
13'
14'
15'
16'
17'
18'
19'
20'
21'
22'
23'
24'
25'
26'
27'
28'
29'
30'
31'
32'
33'
34'
35'
36'
37'
38'
39'
40'
Gravel
(1,298)
(1,298)
5,390
4,684
7,840
9,904
7,482
(1,568)
(5,153)
ND
(1,344)
(832)
8,043
20,685
ND
ND
ND
0
0
(4,328)
5,413
23,368
35,009
100,006
7,886
0
43
811
55
1,040
469
ND
ND
ND
Metric Tons
Sand Silt
(20,844)
(20,844)
392,413
341,069
707,924
894,219
680,966
(167,725)
(536,944)
ND
(204,702)
153,316
998,288
1,697,597
ND
ND
ND
138,356
159,109
87,059
308,422
996,104
1,643,257
1,133,770
(23,568)
50,059
(68,008)
810,499
20,949
398,025
179,560
ND
ND
ND
(6)
(6)
28,564
24,436
36,561
46,182
37,268
18,224
0
ND
(724)
(236)
1,148
180,230
ND
ND
ND
85,826
98,700
72,942
167
10,807
21,435
132,469
(126,758)
4,789
0
0
49
932
421
ND
ND
ND
Clay
ND
ND
18,708
16,260
23,908
30,199
24,225
11,334
0
ND
0
0
0
82,009
ND
ND
ND
20,292
23,336
17,248
0
43,403
86,807
108,639
-11,416
0
0
0
0
0
0
ND
ND
ND
Total
(22,148)
(22,148)
445,075
386,449
776,233
980,504
749,941
(139,735)
(542,097)
ND
(206,770)
152,248
1,007,479
1,980,521
ND
ND
ND
244,474
281,145
172,921
314,002
1,073,682
1,786,508
1,474,884
(131,024)
54,848
(67,965)
811,310
21,053
399,997
180,450
ND
ND
ND
-------
EASTERN SHORE (cont'd.)
Minute of
Latitude
41'
42'
43'
44'
45'
46'
47'
48'
49'
50'
51'
52'
53'
54'
55'
56'
Gravel
ND
ND
3,949
11,846
5,430
3,620
5,093
4,658
3,025
ND
ND
ND
11,364
106,059
82,824
2,565
Metric Tons
Sand Silt
>
ND
ND
341,841
1,025,522
175,373
116,915
162,419
146,903
94,471
ND
ND
ND
56,057
523,200
505,822
17,771
ND
ND
416
1,247
0
0
0
0
0
ND
ND
ND
224
2,093
3,381
143
Clay
ND
ND
0
0
0
0
0
0
0
ND
ND
ND
272
2,537
815
0
Total
ND
ND
346,206
1,038,615
180,803
120,535
167,512
151,561
97,496
ND
ND
ND
67,917
633,889
592,842
20,479
462,740 13,914,620 680,924 521,408 15,579,692
Percent 3 89 4 3
NOTES: ND = No Data.
() = Accretion.
152
-------
MASS OF SEDIMENT ERODED OR ACCRETED
ALONG THE SHORELINE BY MINUTE OF LATITUDE
FOR THE WESTERN SHORE OF .THE LOWER CHESAPEAKE BAY
Minute of
Latitude
37° 01'
02'
03'
04'
05'
06'
07'
08'
09'
10'
11'
12'
13'
14'
15'
16'
17'
18'
19'
20'
21'
22'
23'
24'
25'
26'
27'
28'
29'
30'
31'
32
33'
34'
35'
Gravel
2,428
4,654
19,410
54,736
167,155
232,563
ND
ND
9,359
15,816
66,518
79,307
ND
ND
ND
ND
627
2,099
(4)
(14)
(1,517)
(3,466)
382
24,045
22,769
49,608
55,269
22,469
52,427
37,448
168
2,878
12,326
ND
ND
Metric Tons
Sand Silt
123,767
237,220
627,943
799,222
1,025,104
1,426,232
ND
ND
602,365
215,154
173,436
194,079
ND
ND
ND
ND
41,807
371,528
8,386
(47,910)
(94,028)
(30,542)
36,748
400,228
379,219
587,676
562,543
586,336
1,368,116
977,226
50,663
307,445
879,936
ND
ND
0
0
33,240
33,240
0
0
ND
ND
2,009
5,735
2,314
2,514
ND
ND
ND
ND
3,802
6,929
30
0
(224)
(329)
(340)
555
525
1,537
1,862
3,694
8,618
6,156
41
301
982
ND
ND
Clay
0
0
56,084
56,084
0
0
ND
ND
5,754
9,584
2,993
3,351
ND
ND
ND
ND
2,963
8,343
0
0
(59)
(249)
9
1,623
1,536
1,690
1,242
3,078
7,182
5,130
0
0
0
ND
ND
Total
126,195
241,874
736,677
943,282
1,192,259
1,658,795
ND
ND
619,487
246,289
245,261
279,251
ND
ND
ND
ND
49,199
388,899
8,412
(47,924)
(95,828)
(34,586)
36,799
426,451
404,049
640,511
620,916
615,577
1,436,343
1,025,960
50,872
310,624
893,244
ND
ND
-------
WESTERN SHORE (cont'd.)
Minute of
Latitude
36'
37'
38'
39'
40'
41'
42'
43'
44'
45'
46'
47'
48'
49'
50'
51'
52'
Percent
Gravel
3,383
52,430
63,890
17,876
ND
11,576
34,728
14,691
21,803
ND
ND
ND
2,146
32,872
43,843
47,265
30,250
1,306,213
6
Sand
*
110,620
1,714,612
1,887,668
160,385
ND
169,686
509,058
220,521
355,842
ND
ND
ND
244,848
954,752
942,271
1,024,207
655,493
20,759,862
89
Metric Tons
Silt
9,766
151,376
174,567
33,485
ND
145
436
161
114
. ND
ND
: ND
51,162
104,972
61,008
66,313
42,440
809,136
3
Clay
3,395
52,627
71,166
21,782
ND
0
0
0
0
ND
ND
ND
37,081
69,551
34,832
37,860
24,230
518,862
2
Total
127,164
1,971,045
2,197,291
233,528
ND
181,407
544,222
235,373
377,759
ND
ND
ND
335,237
1,162,147
1,081,954
1,175,645
752,413
23,394,073
NOTES: ND = No Data.
() = Accretion.
154
-------
MASS OF SEDIMENT ERODED OR ACCRETED
ALONG THE SHORELINE BY MINUTE OF LATITUDE/LONGITUDE
FOR THE EASTERN, WESTERN, AND SOUTHERN SHORES
OF THE LOWER CHESAPEAKE BAY
Southern
Shore
Eastern
Shore
Western
Shore
Gravel
(15,102)
462,740
1,306,213
Sand
3,340,946
13,914,620
20,759,862
Metric Tons
Silt
0
680,924
809,136
Clay Total
0 3,325,844
521,408 15,579,692
518,862 23,394,073
Percent Southern Shore - 8
Eastern Shore - 37
Western Shore - 55
Total
1,753,851 38,015,428 1,490,060 1,040,270 42,299,609
Percent
90
155
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