-ffj-
PB82-265489
Sediment Suspension and Resuspension from
Small-Craft Induced Turbulence
Anne Arundel Community Coll.
Arnold, MD
Prepared for
Environmental Protection Agency
Annapolis, MD
U.S. Environmsntal Protection Agency
Region III information Resourca
Center (3PM52)
84i Chestnut Street
Philadelphia, PA 19107
Sep 82
J
ILS. DipvtMiit of Conmecce
NitioMi Tcctaii • Wonrntion Swvict
-------
t
EPA 600/3-82-084 !
September 1982 »
PB82-265U89
SEDIMENT SUSPENSION AND RESUSPENSION FROM
SMALL-CRAFT INDUCED TURBULENCE
by
GUClnSki Regionat Centct for Environmental Information
Anne Arundel Community College us EPA Region
Environmental Center P
Arnold, Maryland 2J.012
EPA-78-D-X0426
William A. Cook
Chesapeake Bay Program
U.S. Environmental Protection Agency
2083 West Street, Suite 2E
Annapolis, Maryland 21401
CHESAPEAKE BAY PROGRAM
; OFFICE OF RESEARCH AND DEVELOPMENT
j AND
: MIDDLE ATLANTIC REGION III
f U.S. ENVIRONMENTAL PROTECTION AGENCY
i RrnouctD IT
s NATIONAL TECHNICAL
INFORMATION SERVICE
i IS OfM«l«(«l W CO«H(»Ct
I srtwcnuD. »• m«i
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TECHNICAL REPORT DATA
(Plcasf read Instructions on the rcrers? before completing)
1 RtPOHT NO 2
ITA 600/3-82-084
4 TITLE AND SUBTITLE
Sediment Suspension & '^suspension
Craft Induced Turbulence
from Small
7 AUTHORISl
Hermann Gucinski
9 PERFORMING ORGANIZATION NAMf AND ADDRESS
Anne Arundel Community College
Environmental Center
Arnold, Maryland 21012
12 SPONSORING AGENCY NAME AND ADDRESS
EPA, Chesapeake Bay Program
2083 West Street
Annapolis, Md. 21401
J RECIPIENT'S ACCESSION NO.
PB32 265489
6. REPORT DATE
^pntpuhpr QR?
6 PERFORMING ORGANIZATION CODE
8 PERFORMING ORGANIZATION REPORT NO.
CBP-TR-006S
10 PROGRAM ELEMENT NO.
11 CONTRACT/GRANT NO.
EPA-78-D-X0426
13. TYPE OF REPORT AND PERIOD COVERED
pj-o-jc»ni- Rppnrl-
14. SPONSORING ASENCY CODE
15 SUPPLEMENTARY NOTES
16 ABSTRACT
Th-3 objective of this study was to determine if small vessels, operatinc
in shallow waters, have any measurable effects in producing increased
turbidities by the resuspension of fine sediments which may affect sub-
merged aquatic vegetation (SAV) .
A two-phase approach was used, consisting of field tests in a suitable
sub-estuary of the Chesapeake Bay, and laboratory measurements of pro-
peller effects. During field trials, two different vessel types were
used to make passas at set speeds over known water deotlia. Before and
after measurements of light extinction , transmission, and garMimetric
suspended sediment determinations were used to identify effects. Labor-
atory experiments were conducted to delineate propeller contribution to
possible resuspension; this was done using laser-doppler anemometry
to map the turbulence field produced by propeller action.
17. KEY WORDS AND DOCUMENT ANALYSIS
3. DESCRIPTORS
•.
b IDENTIFIERS/OPEN ENDED TEF^.S JC. COSA'. 1 I .C.d/Group
f
Chesapeake Bay
SAV
18 DISTRIBUTION STATEMENT
19 bECURITf CLAhi
Unclassified
21. NO. O~
73
Unrestricted distribution
20 SEC'JRIT' C'.ASE •T'.K.r
nnclassified
22. PRICE
EPA Fnim 2220-1 ;R«». 4-77)
"BEV.OUS EDITION •; iasr:;.£-e
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DISCLAIMER
This report has been reviewed by the Chesapeake Bay Pro-
gram, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents neces-
sarily reflect the views and policies of the U.S. Environmental
Protection Agency nor does mention of trade names or coiwnercial
products constitute endorsement or recommendation for use.
ii
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PREFACE
The significant decline of submerged aquatic vegetation
(SAV) in the waters of the Chesapeake Bay has lad to a multi-
disciplinary approach in attempting to understand the underlying
causes. Many variables needed to be understood, and possible
interaction of several variables made it necessary to examine
factors that, taken alone, mignt well be of minor or no signifi-
cance. The resuspension of sediment by boat wakes and propeller
action is thought to be a minor factor; it is addressed in this
study because prior information is largely qualitative or stems
from studies in geographic regions that cannot be readily extra-
polated to waters of the Chesapeake Bay.
Many people contributed to this study in numerous ways,
and they deserve thanks. I would like to add unequivocally, that
any errors, misconceptions, or insufficiencies that may have
crept into the work or this report are solely my own. Dr.
Thomas Reif designed and conducted the laboratory experiments.
L. Chris Athanas, Westtech consultant, wrote a report, appended
to this document, assessing impacts of sediments on the sub-
merged aquatic vegetation (SAV). Technical assistants, students,
and volunteers drafted on short notice to help in the field
included Debbie Blades, Gus Coccolan, Tristina Dietz, Heidi and
Sally Gucinski, Michael Mallonee, Joey Phillips. Illustrating
and printing help was provided by Edward Sparks and William
Reem. The manuscript was typed by Paige Blick and Sandy Atwell.
Valuable comments and criticisms at various stages of the work
were provided by Bort t»run, Robert Cory, John Gerenia, Blair
Kinsman, Walter Valentine, Jerome Williams, and Don Wilson.
iv
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predict maximum depths to which effects may be noted. Using
this data, together with experimentally determined effects on
photosynthetic rates for SAV by other investigators, reduction
in carbon fixation can be estimated; however, the ecological sig-
nificance of changes in this range remains to be established.
This report was submitted in fulfillment of Grant No.
EPA-78-D-X0426 by Anne Arundel Community College and the U.S.
Naval Academy under the sponsorship of the U.S. Environmental
Protection Agency. This report covers the period of March 1,
1979 to March 1, 1980, and the work was completed as of February
5, 1980.
vi
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CONTENTS
Disclaimer ii
Foreword iii
Preface iv
Abstract v
Contents vii
Figures viii
Tables x
List of Symbols xi
1. Introduction 1
2. Conclusions 7
3. Methods and Materials 11
Field Observations 11
Laboratory Experiments .17
4. Results 21
Light Extinction Measurements 21
Transmissometer Measurements 29
Suspended Sediments 33
Laboratory Experiments 36
References 44
Appendices
A. The Effects of Suspended Sediments, Accumulated
Sediments, and Water Turbulence on the Growth of
Submerged Aquatic Vegetation 48
B. Estimating SAV Photosynthetic Rates From Light
Transmittance Values 65
C. Estimating the Decrease in SAV Photosynthesis
Resulting from Sediment Deposition on Leaves ... 66
vii
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FIGURES
Number
1 Sampling Sitas 12
2 Research Vesse1 Bottlenose 16 I
3 Research Vessel Csprey 16 i
4 Laboratory Test Geometry 18 ;
5 DISA Laser Doppler Anemometer and Test Flume 18 !
6 Propeller Turbulence Measurement Set-Up 20
7 Photometer readings from a single test (before and
5 min. after single tug pass at buoy 1) 20
8 Photometer readings from a single test (before and
23 mir.. after single tug pass at buoy 3) 22
9 Photometer readings from a single test (before and
35 min. after single tug pass at buoy 4) 22
10 Photometer readings from a single test (before and
53 min. after single tug pass at buoy 6) 23 '.
11 Plot of the arithmetic mean of extinction co-
efficients for tugboat runs (2000 rpm; at |
Fox Creek (1.8-2.Or. depths) 23 \
12 The effect of time after vessel passage on light :'
extinction 24 j
13 Plot of the arithmetic mean of extinction co-
efficients for tugboat runs (2000 rpm) at |
Sellman Creek (2.3-2.5m depths) 26
14 Photometer readings from a single test (before and
21 min. after single tug pass at buoy 3) 26
15 Photometer readings from a single test (before and
7 min. after single pass of the planing craft
R/V Osprey at Fox Creek) 27
16 Typical photor.iater readings for a single test at
2 buoys (before 8 and 11 min. after pass of
planing craft R/V Osprey at 200C rpm, vessel
off plane) 27
viii
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Number
17 Plot of the arithmetic mean of extinction
coefficients of the planing craft R/V
Osprey (4000 rpm) at Fox Creek .......... 28
18 Plot of the arithmetic mean of extinction
coefficients for the planing craft R/V
Osprey (2000 rpm) at Fox Creek .......... 28
- Representative transmissometer results for a
single test of the planing craft R/V Osprey
at Fox Creek ................... 30
20 Average change in light transmission for all
runs at Fox Creek (depths 1.8-2.0 m) ....... 30
21 Average velocity distribution in propeller wake
for low J value .................. 38
22 Turbulence intensity distribution for low J value. . 38
23 Average velocity distribution in propeller wake'
for high J value ................. 40
24 Turbulence intensity distribution for high J value . 40
25 Average velocity distribution in propeller wake
for intermediate J value ............. 41
26 Turbulence intensity distribution for intermediate
J value
ix
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TABLES
Number Paqe
1 Summary of Photometer Effects 29
2 Summary of Transmissometer Effects 32
3 Averaged Alpha Coefficients from Transmissometer
Measurement 32
4 Summary of Suspended Sediment Analysis 33
5 Comparison of Suspended Sediment Data for Three
Measurement Techniques 35
6 Summary of Suspended Sediment Based on All
Measurements 36
7 Representative J Values for Field Tests Conducted
in this Study 37
8 Estimated Maximum Depth for Propeller Effects
Scaled rrom Laboratory Tests 39
9 Maximum Depth of Propeller Effects as a Function of
J in Terms of Propeller Diameters 43
•s
4
ti
I
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LIST OF SYMBOLS
a Light transmission coefficient, defined by a = -~- In (y )
D Propeller diameter L o
A Average increase in suspended sediment concentration
6 Boundary layer thickness in test flume
5
7y Partial differential with respect to y
f Sediment particle drag coefficient
H Fixed vertical distance between reference plane and water
free surface
I Light intensity at depth of interest
I Reference light intensity of transmissometer
!„_- Light intensity just below the surface, the reference level
V
a Advance ratio = _f°'
TTDN
k Light extinction coefficient, defined by k =-i In =-—
LWL Vessel's waterline length ^ ref
AL Depth change for successive photometer measurements
N Propeller revolutions per minute
n Sample size
R Vertical distance from lab experiment fixed reference plane
p Density, kg m~3
—— •»!
SS Averaged suspended sediment concentration in mg 1
(SS~) Suspended sediment concentration inferred from transmis-
a someter data
(§o\ Suspended sediment concentration i; ferred from photometer
data . 3u 8v
T Reynold's stress, defined by T = -pu1 v1 4- U (-3—— +• -5—)
= -pu v - -grr- • -5 —
UAVG Velocity of water particle in lab experiment by integrating
vertically all measurements a fixed horizontal distance from
propeller
UINF Velocity of water particle remote from propeller in lab
experiment
um Orbital velocity of a fluid particle due to wave activity
u Normalized mean horizontal velocity of a fluid particle
u1 Instantaneous horizontal velocity of a. fluid particle
V Vessel velocity in m sec~l
Vo Fluid velocity in flume away from propeller incluence
v1 Instantaneous vertical velocity of a fluid particle
Y Vertical distance above reference plane, an experimental
variable
xi
*
'i
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SECTION 1
INTRODUCTION
When a boat or ship moves along the surface of the sea, a
disturbance is created in the water, part of which moves outward
and downward, carrying with it a considerable fraction of the
total energy expended to propel the vessel. If the vessel is
moving in shallow water, or if the outward travelling distur-
bance reaches shallow water, some of the available energy may
allow bottom interaction of sufficient magnitude to force sedi-
ments into motion and cause particle resuspension. The time
required for the sediment to settle out of the water column will
depend on the particle size, and the presence of background tur-
bulence and motion that may retard the settling rate and produce
significant lateral transport of the resuspended sediment.
The disturbance may have an effect on rooted aquatic vege-
tation if the erosive forces are of sufficient magnitude to
displace organic detritus and inorganic silts and muds normally
stabilized by the rooted plants; damage may result by subsequent
direct attack on root structures. If the resuspended particles
are small and hence have long settling times, they will contri-
bute to increased light extinction in the water column, with
possible impact on the photosynthetic rate of aquatic plants.
Moreover, the settling itself may not reduce the problem immedi-
ately, for settling could take place onto leaf structures when
appropriate conditions exist, contributing to the problem in
another way.
The widespread disappearance of submerged aquatic vegeta-
tion (SAV) in the Chesapeake Bay, beginning in the early
seventies, has been of intense concern to ecologists and resource
managers aliJce. Many mechanisms have been hypothesized to
explain negative or destructive effects on the vegatation. Among
them are increased sediment loads caused by hurricane-produced
runoff, the increasing reliance placed on herbicides in farm
operations, nonpoint source pollution resulting from heavy devel-
opment pressures in the coastal zone, as well as increased ;
turbidities from increased utilization of Bay waters and tribu- \
taries by recreational watercraft (Stevenson and Confer, 1978) . \
While it appears highly likely no single mechanism or process '
may ba the one causative factor that has lead to the type of |
stresses causing reduction in SAV growth, each of the modes of 3
action must be investigated and quantified, if possible, to <
-------
allow the isolation of the most significant, major impacts, or
to allow an understanding of the possible synergistic effects of
several interacting processes.
This study attempts to determine if small vessels, opera-
ting in shallow waters, have any measurable effects in producing
increased turbidities by the resuspension of sediments, particu-
larly the finer types of sediments. These have a tendency to
remain waterborne for relatively long periods and therefore are
capable of producing impacts away from \he vessel's operating
area if mechanisms for lateral sediment transport are present.
The study furthermore seeks to describe limiting conditions under
which impacts can be seen, so that some conclusions can be drawn
that allow these impacts to be put into perspective with respect
to impacts from other possible mechanisms. During field trials,
two different vessel types were used to make passes at set speeds
over known water depths. Before and after measurements of light
extinction, transmission, and gravimetric suspended sediment
determinations were used to identify vessel effects. Laboratory
experiments were conducted to understand and delineate propeller
contribution to possible resuspension. This was done using
laser-dcppler anemometry to map the turbulence field produced
by the propeller's action.
Systematic theoretical study cf waves produced by a dis-
turbance in the water, such as a ship, began with Lord Kelvin
(1887). It was continued by Taylor (1920) and many others when
it became clear that wave-making resistance is the dominant form
of resistance for a displacement type surface vessel. Ship
design considerations to minimize wave-making resistance are now
standard chapters in naval architecture texts (see for example,
Comstock 1967). Because our understanding is far from complete,
much work is continuing. For example, Peregrine (1971) examines
the interaction of the transverse waves astern of a vessel with
the bow and stern waves that move outward at an angle of nearly
20° of a vessel's centerline (see Tricker 1964, for an intro-
ductory discussion of ships' wakes, especially in shallow water).
Water motion by propellers has been difficult to charac-
terize and understand well. Early theories envisioned propeller
action to be like that of a screw advancing in a solid medium,
and it was not until advances in aerodynamics were applied to
propeller effects that a theory was developed that could account
for the most important forces and motions developed by a rotating
propeller in a fluid medium (see Comstock 1967 or Gilmer 19671
for basic principles). In essence, the rotating propeller blade
acts like an eirfoil, with lift generated by a pressure drop
across the convex, forward facing surface, which is oriented to
have an optimum angle of attack. Tho thrust developed by the
sum of the forces on all blades is transmitted via the shaft to
the entire vessel, while water is accelerated astern in a manner
that, some distance astern, resembles a jet-like flow. Propeller
-------
rotation imparts vorticity to the water, and the dissipation of
vorticity, the jet-like flow, and other eddy effects lead to
quasi-turbulent motion in the propeller wake.
While this disturbance does not propagate outward as does
the ship-generated surface wave system, locally high-particle
velocities can produce shear forces capable of considerable
sediment stirring if propeller action occurs in water suffi-
ciently shallow. Understandably, however, research in wave-
making resistance and propeller effects has focused on attaining
greater hull form and propulsion efficiency, with relatively
little attention given to environmental effects. Addressing
ships' wakes in particular, Sorensen (1973) notes that while
wave formation is relatively well understood for major hull
forms and speeds, data on the decrease of wave height with dis-
tance from a ship's line of travel is needed for specific
situations, while knowledge concerning the effects of some hull
forms is lacking. He does not explicitly state this, but his
analysis suggests this is particularly true for small craft.
The bulk of the literature reporting wave and turbulent
water m.ti effects comes from studies of coastal processes,
such as sediment transport by littoral drift. Nakato et al.
(1977), citing the lack of satisfactory theoretical models and
the useful insights from laboratory studies, report that resus-
pension from regular wave trains is proportional to wave ampli-
tude, with mean sediment displacement greatest nearest ripple
crests — when rippled sediment beds are found, a situation
likely in beach zones, but unlikely in estuarine tributaries —
and that sediment resuspension affects water motion in the
boundary layer. The exclusive use of quartz sands of fairly
uniform particle size, median about 140 y, may make applications
to the typical sediment regimes found in the Chesapeake Bay
difficult. Komar (.1977) also states that no satisfactory rela-
tionship between littoral sand transport set into motion by
breakers, but allows that the stress T which leads to resuspen-
sion of sandy sediments will depend on the density, a particle
drag coefficient f, and a term um that is a function of water
depth and wave energy as inferred from orbital particle motion.
A further complication is known when considering the nature
of sediments more typically associated for the problem under
investigation, namely clays and silts, which are cohesive sedi-
ments, i.e. sediments that can form highly cohesive aggregates
under some conditions. Gust and Walger C1976), making measure-
ments in tidal channels of the North Sea coastline and using
metalclad hot wire velocity sensors, found that the normally
assumed relationship for finding the friction velocity at the
sediment interface does not hold. Instead, suspended cohesive
sediments may have a turbulent drag reduction effect, inhibiting
erosive stresses to some degree. Finally, Suhayda et al. (1976)
add a last dimension of uncertainty by implying that measurement
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of pressure fluctuations at the sediment interface suggest that
the interface itself is accelerated in response to wave-irduced
oscillations, making the correct applications of the critical
velocity concept even more tenuous.
Studies that combine the analysis of ship induced water
motions with possiole environmental effects,such as are investi-
gated by coastal engineers,that have come to the attention to
this author are relatively few in number. It also comes as no
surprise that the majority take a strictly empirical approach.
The preponderance of studies have the rationale to determine the
effects of ship and barge traffic on channel bank erosion, such
as Johnson (1975) and Liou anC Herbich (1976). The latter study
appeared most useful to contributing information applicable to
small craft, as it was aimed at deriving a formula of sediment
motion induced by propeller effects. Several shortcomings of
the study limit its applicability. One is the use of the-disc
theory of propeller action; a theory that assumes one may replace
the propeller L>y a disc which simply imparts momentum to the !
water across its cross-section. From this and typical vessel
power and propeller data, a velocity distribution is derived
without recourse to velocit/ measurements. It might be noted ]
that it was the shortcomings of the disc theory that led to the i
presently used aerodynamic theory of propeller action. The dis- j
tribution of velocity some distance downstream from the propeller
is then analyzed using the theory of orifices, again without
recourse to field or laboratory studies. Finally, the sediment
interaction assumes noncohesive sediments, limiting interpreta- j
tions to sediments that have no significant clay fractions.
Of the studies the.t include effects of small vessels, that
of Collins and Noda (1971) addresses only erosive effects. Their
overall conclusions suggest that small craft effects will neces-
sarily be minor compared to other factors such as wind-wave
damage. The authors assert, without inclusion of good evidence,
that wind-induced circulation will be a substantial factor in
the lateral transport of resuspended sediments. By assuming
erosion rates proportional to the rate of energy dissipation, a
step made necessary by time and budgetary contraints, effects of
different particle-size distributions, and properties of cohesive
sediments are neglected. Thus the estimates of erosive effects
may differ from those including field measurements supplemented
by attempts to characterize the nature of the boat-induced water
motions. However, the approaches Collins and Noda take to arrive
at a ranking of boat-wake effects compared to effects by other
processes appear fruitful. They are done by looking at boat
traffic density statistics, modifying them for distributions of
differing hull types, and summarizing them in terms of energy
dissipated per foot of shoreline per year. While the choice of
units may not be the best possible, especially in terms of com-
parative analysis, an assessment of this nature is the input
required by the resource manager.
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Only one series of studies specifically addresses sediment
resuspension by small craft. Anderson (1974, 1975), also taking
an empirical approach, succeeded in partially plumbing a tidal
flat with numerous water intakes, both close to the sediment
interface and some distance off the bottom. This resultant sam-
pling network allows sufficient data acquisition to make compar-
isons of suspended sediment changes with other environmental
variables possible, something the author of the present report
found difficult to implement. Anderson shows the dramatic impact
of attendant environmental factors, particularly the state of
the tide, that affect measured boat-induced resuspension of sed-
iments. He concluded that horizontal bottom velocities as low
as 0.15 m sec" are sufficient for resuspension to take place.
Since the sediment particle size distribution includes cohesive
sediments, this measured value is likely to have applicability
to the problem of the study undertaken here. Anderson found the
average increase in suspended sediment, produced by the action
of two vessel passes during flood tide, to be 4 mg/1. He sug-
gested that differences in sampling results with the state of
the tide, even when sampled at identical tide stages (i.e.,
depths) may have in part been due to exchange of water masses
from shallower areas, complicating the interpretation of boat-
induced sediment resuspension.
As Anderson's work focused on wave effects, with the vessel
pass away from the sampling intake structures, no results for
either propeller effects alone or due to combined effects were
reported. His analysis of resuspension for differing boat types
deserves further discussion because of correspondence of some
results seen in this study. Low amplitude waves, as produced by
a small, low-powered boat had the effect of lowering the suspen-
ded sediment concentration near the bottom, which Anderson
attributes to dilution by vertical mixing. Variability between
sampling runs may result from lateral, downslope transport by
tidal currents. Higher waves, such as 0.10 m waves produced by
a 4.2 m (14 ft.) speedboat, produced an increase of suspended
sediments of greater than 4 mg/1, but followed by insignificant
increases on the second run. This suggests that dynamics of
sediment resuspension, sediment settling, and sediment transport
were not resolvable with the sampling scheme employed, as indi-
vidual effects could only rarely be factored out.
Contradictory fluctuations in suspended sediments remained
even when higher powered vessels, all planing types, were used.
Only when high-powered (300 Hp), large (10 m), displacement ves-
sels were used, or when waves exceeded 0.16 m was the observation
of resuspended sediments unequivocal.
Anderson's general conclusions are that resuspension from
planing power boats is minor, with resuspension greatest when
operated at maximum wave-making speeds, i.e., at nonplaning
speeds. Greatest resuspension takos place for displacement-type
I
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vessels. His findings, that sven at slow speeds (ca. 5 kn) the
effects are significant, are not surprising, for naval architects
generally concede that wave-making is greatest at hull-speeds
defined as approximately equal to the square root of the water-
line length multiplied by 1.4. (Here units must be feet and
knots; see Kerreshoff (1966) for discussion.) For the 10 m lob-
sterboat this is approximately 7.5, assuming a waterline length
of 9 m (30 feet).
Finally, the work of Yousef (1974) addresses the effect of
power boat operation on a number of water quality variables,
including the resuspension of sediments. Qualitative evidence
was gathered photographically, and showed that a 50 hp motor will
stir up sediments in water 1.2 to 2.4 m in depth. It is not
clear whether the motor was operated in a stationary mode, at
planing speeds, or in the displacement mode. These differences
are important because resuspension results from propeller turbu-
lence and Wave effects both. The wave effects will differ for
different speeds and boat types, while propeller effects will
also vary depending on the speed at which boat is moving.
Yousef reports that quantitative changes in turbidity
(reported in Jackson Turbidity Units) took place when a boat's
horsepower was increased from 28 to 50 and 115, in waters 1.2 to
3.3 m deep, but does not cite information on how boat speeds,
propeller immersion, and resulting wakes differed. As will be
discussed below, propeller immersion may be an important vari-
able that dictates the distribution of water-particle velocities
sufficient to resuspend sediments. Yousef obtains an empirical
relationship between effective mixing depth, which he defines as
lake depth where turbity changes are of such magnitude as to pro-
duce measurable changes in surface readings, and motor horse-
power. It appears that because the number of variables that
affect the changes of turbidity is quite large and the effect of
single variables is not understood or unreported, extrapolation
must be done with great care.
It is the lack of understanding of the contribution of
individual variables that led us to undertake a laboratory study
of a single aspect of the resuspension problem, namely propeller-
generated, quasi-turbulent water motion. While combining the
results from the laboratory study with the empirical field study
may not give a full understanding, it is hoped to contribute
information toward assessing ?. few major variables—such as the
depth to which effects can be felt, the relative magnitude of the
suspension, and possible impact of biological nature.
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SECTION 2
CONCLUSIONS
While the data trends for the laboratory and field measure-
ments show reasonably good agreement, the differences in pre-
dicted maximum water depth affected make synthesis of the two
sets of observation difficult. Measurements for at least one
more set of J values, including observation at a greater distance
downstream, might clarify the findings.
Nevertheless, the observations can be summarized as follows:
1. The resuspension of sediments in the path of a small
craft is influenced by these variables:
o water depth.
o depth of immersion and size of propeller.
VOD
o the advance ratio J, defined 33 J = —^
o wave-making tendency of the vessel.
2. The depth to which stirring is sufficient appears to be
quite limited. For example, a small displacement craft that
will produce significant stirring in waters 2.0 m deep will have
barely measurable effects when water depth increases to 2.3 m.
3. Applying the results of the laboratory tests to a size-
able yacht, 12 to 15 m in length, with a propeller 0.6 m in
diameter and immersed to a depth of 1 m, and assuming the worst
case for the determination of J, may result in v =5 m sec"1 at
N = 800 rpm. J will become 3.3 x 10~3, and the maximum depth of
stirring is 2.7 m based on propeller effects alone. Because the
empirical field tests su<~ »st resuspension from all variables is
likely at 1.5 times the a-^th predicted from laboratory results,
sediment resuspension to perhaps nearly 4 m can be expected.
4. The tugboat induced sediment resuspension up to 2.7 mg
I"1 along its line of travel in waters 2.0 m deep. The amounts
were reduced to 1.6 mg I"1 when the vessel's path crossed waters
2.3 m deep. Expressed in terms of reduction of light levels at
an arbitrary depth, say 1.0 m, this means the percentage of
available light at the depth changes from 18 percent to 13 per-
cent at a single vessel pass in waters 2 m deep. The corres-
i
!
t
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ponding reduction is from 23 percent to 21 percent in waters \
2.3 m deep. j
k
5. Applying the results expressed in (4) to the problem i
of biolcgical impact requires knowledge of quantitative rela- j
tionships becween suspended sediment effects, or reduction of
avciilable light on such factors as plant productivity or other j
indicators of healthy states for biota. Mr. C. Athanas (1979, ^
see appendix) reported that workers had applied Michaelis-Menton 5
kinetic? to measure effects of light reduction on primary pro- -
ductivity. Bayley (1970) determined values for SAV productivity
in the Rhode River. Using her approach, the reduction in pro-
ductivity was calculated from 0.71 mg C h-1 to 0.62 mg C h-1 for
a single vessel pass in 2 m of water, a drop of slightly over 12 i
percent for the duration of 1 hour. •
Similar1/, for a tugboat pass at the greater depth, the
change of the 1 m level is from 0.77 to 0.76 mg C h~l, only
about a 1 percent reduction.
Similar techniques can be employed for the effects of
accretion of suspended sediment on leaf structures, a process
that may take place when water motion slows and particles are
allowed to settle (Athanas 1979).
6. Is it possible to decide if the effects outlined in (5) •,
above are of sufficient magnitude to be a factor in SAV growth? ;
Is the postulated mechanism even valid? Frankly, we don't know.
It can only be hoped that some of the other studies of the SAV '
program allow better deductions concerning such effects to be j
made. Williams (1980) shows that other effects, such as sus- j
pended sediment increases during high runoff periods, changes I
due to tidal scour, etc. appear to outweigh effects due to boat- )
ing traffic. 4
These thoughts are offered on the problem: I
a. The depths to which boating effects allow sediment \
resuspension coincide to depths to which SAV growth is j
limited in Bay waters. |
b. No knowledge is available if multiple passes add line- '-
arly to the resuspended load (the work of Anderson 1974, |
suggests they do not, but the evidence is not conclu- j
sive). but it is not startling to envision that effects
in areas of heavy boating traffic can easily magnify
single vessel effects by an order of magnitude. The
study conducted by Roy Mann Associates (1976) pinpoints
areas of boating congestion. Comparison to SAV traps
(R. Anderson 1979) for upper Bay waters suggests that
areas of least SAV distribution, and slowest recovery
during the survey period, are also areas of greatest
-------
boating congestion. The same area suffers stresses
from other human activity, so that correlation here in
no way proves causation.
c. Knowledge of the sediment size distributions surround-
ing SAV beds would allow resource managers to decide
that some areas may merit protection due to the pres-
ence of fine sediments which have long settling times
(>1 hour), the capacity to allow accumulation on leaf
structure due to their cohesiveness, as well as by
maintaining higher light extinction in the water column.
Our attemps to make an analysis of a limited area using j
data gathered by the Maryland Geological Survey proved I
premature, as many of their recently surveyed sites |
await analysis (Halka, personal communication). Fur-
thermore, their present objectives and limited resources
have not allowed extensive sampling in the tributaries,
but are confined to Bay waters proper. »
j
7. The problem of sediment resuspension by the action of j
the outward travelling wake of a vessel could not bo adequately >
addressed given the resources of this study. It must be pointed •
out that this is a problem of considerable experimental diffi- ]
culty. The wake of a vessel consists of a set of transverse I
waves that extend astern and move in che direction of the vessel ]
itself, in deep water, the transverse waves are limited to a j
line extending outward and astern of the vessel, making an angle
of nearly 20° with the line of movement. Here constructive and
destructive interference reshapes the direction of ths waves.
Measurements can readily give wave height and amplitude for a
given vessel at a fixed speed. Orbital velocity calculations
then yield initial velocity for sediment resuspension. However,
when the wave begins to act as an intermediate water
depth wave upon interaction with the bottom, its speed, direc-
tion, and orbital particle velocities change in a complicated
way, making predictions for a given site difficult. This is
amplified when a multitude of boats, travelling at widely vary-
ing speeds, are to be considered. Our general conclusions,
that a fast moving, planing boat creates the lesser effect, do
hold in this case. Wake wavelengths are long, orbital velocity
reduced, bottom interaction commences at greater depth, and
energy dissipation (including that due to resuspension) is
spread over a greater area than would be the case for a set of
shorter, steeper waves for the same vessel travelling at its
maximum wave-making speed. A recent itudy by Byrne (1980)
points out that this "worst" speed is only a knot or two above
the common 6 mph linit posted in many state waters.
This raises the question if wake effects must be included
in the resorting of sediments in near shore waters of heavily
impacted areas. In a reconnaissance survey, Kerhin (1977) finds
sandy sediments predominant close to shore, in waters less than
-------
about 2.5m deep, at the mouth of the Severn River, while fine
clay-like silts and muds rre ubiquitous in deeper waters there.
Such distributions may have a protective effect for SAV
beds in high wave/wake energy environments, if only from the
view point of reduced fine suspended sediments.
8. We would cautiously advance these final statements:
Sediment resuspension can be initiated by boat traffic. It
appears effects will be measurable in shallow areas of less than
3 m for the most part, and are most likely minor until depths
are 2.2 m or less. The concept of the advance ratio allows pre-
diction of the maximum depth affected for known vessel speeds,
propeller diameters, and revolutions. Planing craft and other
vessels of low propeller immersion will have the least impact,
unless water depths become less than about 1.8 - 2.0 m. Effects
are increased for vessels moving slowly under a heavy load at
high engine rpms. Measured increases in light extinction for
specific, but not unusual, craft are statistically significant
and suggest decreases in photosynthetic rate for SAV may be pos-
sible. •
It is tentatively recommended that ecologically sensitive
areas be investigated for the presence of fine sediments (<60y) ,
in which case such areas be protected from excessive traffic,
particularly deep-draft, high-powered craft. Can the feasibil-
ity of a suspended sediment increase criterion be investigated?
Such criterion, if proven realistic from the point of biological ,
impact, would allow some deductions about optimum and maximum j
traffic for a given vessel type. Its greater benefit may well j
be in allowing comparison of impacts from many sources, among \
which vessel wakes may well be minor. j
-------
SECTION 3
METHODS AND MATERIALS
FIELD OBSERVATIONS
Site Selection
The objective of the field studies was to determine if
there were any measurable effects from sediment resuspension for
the methods chosen, to test the effects of different boat/motor
types, and to understand the effects for deeper waters and for
differing sediments. j
Field trials were made at three sites, with the greatest j
number of measurements at the site having the least depth and a '
small median particle size for the sediment. 1
*
Figure 1 shows the sampling areas, with the Fox Creek area ;
chosen as the prime site because it best met the following cri- j
teria: 1
j
a. reasonably uniform water depths I
b. minimum variation of bottom sediments with a high j
percentage small-sediment particles (less than 60y) s
c. availability of prior data on natural changes in j
suspended sediments j
d. availability of support facilities, i.e., docks and !
electrical power. 1
i
Site surveys at Fox Creek disclosed a mean water depth of nearly {
2 m, with variation across the site of less than 0.3 m. The j
site is bordered on the east by the locally steep bluffs of Big j
Island, on the west by the low shores of the Contees Wharf area. !
Pipette analysis (Carver 1971) showed median particle size to be
less than 30u, while Pierce (1973) reports the clay fraction to
be 78 percent by weight, with a dry organic content between 0.3
and 0.48 percent. Significant sand fractions exist in a narrow
strip along the beach area nearest the survey site, on Big
Island, and here strips of rooted aquatic vegetation also abound.
Pierce (1973) reports similar clay fractions, ranging from
72 to 77 percent by weight for the Sellman Creek site, chosen
for its similar sediment characteristic while averaging slightly
greater water depths, from 2.0 to 2.4 m.
11
-------
MAdOTHY RIVER
SITE
f |.|- Xt.HI
SELLMAN
CREEK_SITE
OX CREEK
SITE
Figure 1. Sampling Sites
12
-------
The Chest Neck site on the Magothy River was chosen because
of tne predominantly sandy bottom. It also had depths averaging
2.4 m, but these were minimum depths extending along the sampling
line that followed an extended but rather narrow ridge, with
water depths dropping off to 5 m on either side. Only a single
test was conducted at this latter site; the run served to verify
the expectation that no measurable result from the vessel pass
would be seen..
Measurements
Each site was marked with a series of buoys, from three to
six in number and spaced 25 m apart to mark a track for the test
boat to follow as well as to serve as sampling points. Prior to
the passage of the vessel a rowboat was used under oars as the
sampling vessel. Position during sampling was maintained by
tying the vessel to the buoys and avoiding any bottom stirring,
such as mignt occur with the ute of anchors. During initial
trials, measurements of light extinction and light transmission
were supplemented by measurements of conductivity, dissolved
oxygen, and tamperature using a multisensor Hydrolab Surveyor
unit.
At each buoy light extinction and light transmission was
determined at 0.5 m intervals initially, later increased to
0.25 m intervals. A set of 2 water samples were taken at the
1.0 m water depth. This was done in sequence for all buoys
before and after the vessel path.
Light extinction was measured using a Beckman Enviroeye
photometer. While the instrument may lack the sophistication of
inscruments such as the commonly used model manufactured by GM
Corp., careful comparison of the two units showed the latter to
have higher sensitivity, but the same degree of precision. As
the work was conducted in waters allowing light penetration
nearly to the bottom, maximum sensitivity was not deemed of
primary importance, and the smaller size and versatility of the
Beckman unit dictated its selection.
Light extinction (photometer) data was reduced by making
the assumption that extinction followed an exponential curve, as
given by Beer-Lambert's Law. Values obtained at a single buoy
were plotted on semi-log paper and a leasts-square fit was used
to obtain the straight-line segments whose slope represent
extinction coefficients. When analyzing for significant before
and after vessel pass differences, extinction coefficients
obtained from repeated tests were averaged arithmetically.
Light transmission was measured using a Beckman Envirotrans
transmissoroeter, having a folded light path of 0.1 m, but samp-
ling a real space of 0.05 r\. Again it was the compact size of
the instrument that lead to its choice, and during initial
13
-------
trials, when attempts were made to see if any changes in the
water column could be detected, it proved adequate. However,
for future studies the use of a transmissometer having a vari-
able path length would be advisable. It is our belief that it
is not any inherent irstrument characteristics that affect the
readings, but the volume sampled by the instrument. The pattern
of measurements allowed somewhat greater consistency in results
for readings that represent an average for sampling paths
approaching 1 m, rather than measurements where paths were of
the order of 0.1 m.
Calibrations
Field calibration of the transmi Soometer were mtide by set-
ting the instrument of 82% value in air, all surfaces dry before
each sampling sequence at any buoy. The 82% value in air cor-
responds to at 100% transmission in particle-free water.
During initial runs, readings were taken at the surface
and at 0.5 m intervals to a depth about 0.3 m short of the bot-
tom to avoid inadv^rtant stirring. With increasing experience,
the measurement frequency was increased to 0.25 m depth inter-
vals. Because possible detection of sunlight made surface
readings with the Beckman transmissometer suspect, readings were
not made at depths less than 0.5 m.
Because the relationship betv-een suspended matter and light
scattering or extinction is not completely known, water samples
were taken for gravimetric determination of suspended sediments,
following the method of Strickland and Parsons (1972). The
method had to be modified in regard to the desiccation of the
Millipore membrane filters, which curled and became brittle
under the standard heating process. Comparison showed that fil-
ters desiccated for 24 hr. at room temperature had similar weight
variability as those heated to 101°C, while weight changes of
previously desiccated filters did not exceed 0.1 mg after heat-
ing.
Samples for gravimetric determination of suspended sedi-
ments were taken with a Martek 1 liter sampling bottle, with
sampling held at a constant 1.0 m depth. The sample was decanted
into appropriately labelled polyethylene bottles, and a replicate
sample was taken for approximately one-third of the samples.
Samples were taken to the lab the same day, generally within 1
hour of the conclusion of the vessel tests, and filtered through
0.45 y Millipore filters under approximately 180 mPa of vacuum.
As filter clogging leads to inordinately long filtration times
for even 0.5 1 samples, 0.2 1 or 0.25 1 aliquots were filtered.
Care was taken to shake the sample bottle vigorously before
decanting the sample into the filtration funnel, and the funnel
walls were rinsed with de-ionized water during the last few
minutes of the filtration cycle.
14
-------
Replicate filters were prepared when the data variability
proved extremely high, thus of the totni of 134 gravimetric
determinations of suspended sediments, all samples beginning with
sample #59 have replicates, as do a number of prior samples. It
should be noted that t'lf: sampling bottle, which measures about
0.25 m in height exclusive of the triggering mechanisms and is
perhaps 0.08 m in diameter, also samples a relatively small vol-
ume of water. It microscale distribution of resuspe ir'°d sedi-
ment, whether it is done by boat activity or natura. hanisms,
produces large variation on a spacial scale of 0.1 m, i n the
gravimetric samples suffer from the same problem as the trans-
missometer measurements.
A sample blank determination was m>de for r>ach day's sam-
ple batch to be analyzed. Filters for blanks v~re treated as
filters for samples, they were desiccated, weighed, mounted on
filtration apparatus, removed, oven dried, and rewf-r'ghed. Only
Lhe actual sample filtration was omitted for blanks.
Completion of sampling prior to the vessel pass typically
took from a maximum of 60 min. during the early trials to as
little as 15 min. during later work, when the number of stations
sampled was reduced from 6 to 3. The temporal framework can
hence be called quasi-synoptic at best; however, since each buoy
location was sampled before and after the vessel pass, each loca-
tion has its own control, and temporal effects should at least
in theory be separable from spacial effects.
Vessel passes were made at set speeds, selected to be the
most likely speed for maximum wave making or maximum propeller
effect. In the case of the1 tugboat shown in Figure 2, the R/V
Bottlenose, a 9 m 5500 kg displacement-type vessel, powered by
an 85 hp diesel engine, this speed proved to be nearly 3.6 m
sec"1 (7 kn) at an engine rpm of 2000. The vessel swings a 19 x
22 three-bladed propeller, with its hub immersed at approximately
0.7 m. Only a single pass was made for each sampling test, as
it was desired to keep variables to a minimum. Two or more ves-
sel passes could conceiva>ly increase complexity because effects
may not be lirearly additive. If the time interval between pas-
ses is sufficient, limited settling and lateral transport my
take place, requiring a closely spaced, dense sampling grid for
complete evaluation.
Low-speed passes with the tugboat were made at 1500^rpm,
which corresponded to a speed of approximately 2.5 m sec'1
(4.8 kn). Wave making was slight, and propeller effects reduced
in proportion to the reduction in rpm.
Tests involving a planing speedboat were conducted with the
R/V Osprey, shown in Figure 3, a 6.7 m (22 ft.) runabout powared
by a 135 hp outboard engine. Its propeller dimensions were
15
-------
Figure 2. Research Vessel Bottlenose
*f*J^^&3ffi£&?S2i
-------
16x19 and the dapth of immersion of the hub was ca. 0.4 m.
The vessel's gross weight is estimated to be 800 kg. Tests were
conducted at near maximum planing speed, at 4000 rpm, when the
vessel's speed was nearly 13 m sec"-'- (25 kn) . Low-speed runs
were chosen as speeds when the boat was off the planing mode,
travelling at hull speed, i.e., maximum wave-making speed, in
this case 2000 rpm, estimated to be 2.8 m sec"1 (5.5 kn).
A few wave recordings of the tugboat at maximum wave-making
speed were made, using a Interstate Electronics Model 438-2000
wave staff attached to a Hewlett-Pachard Model 7071B strip chart
recorder. The wave staff operates via a conducting wire to
measure the changes in immersion during the passage of the waves.
Unfortunately^ similar recordings were not available for passes
of the planing runabout.
LABORATORY EXPERIMENTS
To determine the changes in water particle motion due to
the effect of a boat propeller, measurements were made under lab-
oratory conditions, which, by the use of appropriate scaling
laws, can be applied to field situations to predict the distri-
bution of stress sufficient for sediment resuspension.
An Armfield, Model 9097, open channel, rectangular flume
was used to circulate water at such speeds as to allow a station-
ary propeller to simulate a moving boat. The appropriate scaling
factor was the ratio of propeller advance velocity (i.e., vessel
speed) to propeller tip velocity, called the advance ratio J.
Measurements of velocity and turbulence intensity were made using
a DISA Type 55L Laser Doppler Anemciueter, electronically con-
trolled to allow signal averaging appropriate for the turbulence
intensities present, typice.lly 150 sec. Details cf the flume and
the laser doppler anemometer have been published elsewhere
(Reif, Metrakos and Hoyt, 1978). After initial experiments
showed that appropriate resolution was obtained, each run for a
given propeller rpm involved readings of velocity and turbulence
intensity at 0.0125 m depth intervals over the entire height of
the water column, approximately 0.21 m. Three such sets were
measured, at distances of 0.00, 0.135, and 0.270 m dowstream of
the propeller hub Complete experiments as described were con-
ducted for IkoO, 840, and 570 rpm of the model propeller. The
propeller rpm and its corresponding vessel speed were modelled
to represent the tugboat used under two different field condi-
tions.
The geometry of the test configuration is shown in Figure
4, while the lasei anemometer and the analyzer are shown in Fig-
ures 5 and 6, respectively.
17
-------
V*
H
Figure 4. Laboratory Test Geometry
Figure 5. DISA Laser Doppler Anemometer and Test Flume
18
-------
Methods
The advance ratio J is defined by the following equation
J TfDN~~ (1)
where V^, is the propeller advance velocity, or, in the laboratory
test, the velocity of che water in the flume at a location dis-
tant from the propeller. D is the propeller diameter, N is the
propeller rpm.
For a given advance ratio, model results may be translated
into dimensions of interest for full-scale application as fol-
lows: Changes along R, the distance from a reference plane to
the location of the propeller hul^, can be Jieasured in terms of
D, the propeller diameter which is the principal linear dimen-
sion.
Since the laboratory experiments were designed to simulate
the tugboat test at Fox Creek, R was chosen to correspond to the
propeller-Crv_ak bottom distance. The ratio D/R was uniform for
&.11 experiments as was D/H where H is the water depth.
The unitless ratio Y/R used in the resultant graphs can be
converted to represent any depth units if D and D/H are known.
In the tests D/H = 0.264 and since D for the tugboat was 0,48 m
H, the depth is D/0.264 = 1.82 m. Similarly, a unit along the
Y/R scale represents Y/R = 1.17 m for this test. For other
cases, but similar J values, the vertical dimension will vary
in direct proportion to the propeller diameter.
j
J
To ensure that boundary layer effects at the flume bottom J
were small enough to be remote from the propeller vicinity, the ^
boundary layer thickness was calculated from the experimental 'j
conditions, i.e., flume dimensions and velocity.
For a flow rate of 0.46 m^ min~l, and a water level of i
0.22 m, giving a velocity of a 0.40 m sec"^-, the boundary layer ^
thickness proved to be 0.04 m; small compared to the distance R i
of 0.14 m for these experiments. Test runs were conducted for ^
values of J x 10~3 of 1.6, 2.4, and 3.5. I
I
I
\
19
-------
Figure 6. Propeller Turbulence Measurement Set-Up
e
e.s
1 '
H
(1 1.5
a
2.S
TUGBOAT AT aeee RPM FOX ex T
^
^
^
4
s
'
f
s
,
'
0
^ I
/
/
^
RE 5 HIM
/
f
.'
*/
^
^
^
1
S 7 t,t 3
% TRANSMTTTED LIGHT
W^SSAOE AT BUOY
5 7 lta
Figure 7. Photometer readings from a single test {before and 5
min. after single tug pass at buoy i).
20
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SECTION 4
RESULTS
LIGHT EXTINCTION MEASUREMENTS
Light extinction measurements using the photometer gave the
most statistically reliable as well as most consistent results.
Moreover, the measurements show a reasonable correspondence to
the laboratory results, allowing the formulation of at least a
tentative hypothesis concerning the most significant variables
that affect sediment resuspension.
Figure 7 shows measurement results for a single pass of
the tugboat Bottlenose at Fox Creek in a water depth of 1.8 m.
The diagram may be taken as representative for test runs when
observed light extinction increases were at a maximum. Figures
8, 9, and 10 show that with the passage of time some recovery to
prior levels takes place.
The data were averaged for analysis of statistical signi-
ficance, and grouped into three time intervals, 2-9 min, 15-54
min, and 35-36 min after the passage of the tugboat. Figure 11
is the semilog plot constructed from the extinction coefficient,
k, obtained by averaging the extinction coefficients for each
sample run, measured at three or six buoys. The result repre-
sents 18 samples (for a total of 320 separate measurements). The
standard deviation for the 18 samples is shown. Student's t-test
for small samples shows the difference of before a^* after ves-
sel pass results to be significant at the 0.995 level.
Because tidal rise and f?ll varied the deptna at the samp-
ling site by approximately 0.2 m, samples taken when the depth
was 1.8 m, the mim-.um depth at which tests were conducted, were
analyzed ser>*:.*c.ely. While the difference in extinction coeffi-
cient 1:, small over that for all tugboat tests done at ths Fox
rioe.K site, 2.11 versus 2.01 m~l compared to a control value of
1.68, the statistical significance in the former case is yat
greater, being at the 0.9995 level.
Analysis of the influence of length of time after the ves-
sel pass does not yield unequivocal results statistically.
Results appear anomalous for the samples taken between 35 and 65
min after the vessel pass, as can be seen in Figure 12. Stu-
dent's t-test is significant only at the 0.75 level for samples
21
-------
e
S.5
H
« i.5
2
2.5
TUCI
^X"
X
/
OAT AT 80«e Rp'i FOX CK Tine» 2
^
'
/
X
x
X*
X
/
^
^
^
' /
Y/
/
X
_/
X
/
niN
x
^
x
X
ie«
3 S 7 1,1
% TRANSMrTTED LIGHT
« BEFOREl
5 7 wl
iAFTER
PASSAGE AT BUOY 3
Figure 8. Photometer readings from a single test (before and 23
min. after single tug pass at buoy 3).
H
n
• TUG
e.s
i
a
2.5
t«
^
X
^x
x
X
IOAT AT 8
x^
X
^
X
X
009 RPfl FOX CK Tinet 35 HIM
^
x
X
.-
^
Xx
x°
•BEFORE!
oAFTER J
1
PASS*
x
x
iGE
s*
AT
1
x
x*
Bua
I
X1
r
x x
x
4
l» 3 * 7. Ifi 3 B 7 !•
% TRANSMITTED LIGHT
Figure 9. Photometer readings from a single test (before and 35
min. after single tug pass at buoy 4).
22
-------
TuoBovr PT aeeo RPM rox CK ring s*
04
D e s
H
Ml a
1 6
g
18
e
3
/
\
/
\
S
/
Y
1
/
'
^
/
1
'
•
/
t/
1
X
3
/
X
^
5
/f
X?
7
i^
/
!•
% TRANSMITTED LIGHT
AT BUOY 6
Figure 10. Photometer readings from a single test (before and
53 min. after single tug pass at buoy 6) .
Before k= 169 solid n=!7
After k=2.0l dashed n=!6
After k=2.il dot -dashed (1.8 m depth only) n=17
I 10 100
1.0
1.5
o
E
P
T
H •
(m)
Figure 11. Plot of the arithmetic mean of extinction coefficients
for tugboat runs (2000 rpm) at Fox Creek (1.8-2.0 m depths).
23
-------
collected between 2 and 9 min after the vessel pass, but
increases to significance at the 0.9, and 0.995 level for the
12-34 and 35-65 min intervals.
TUG BOAT @ 2000 rpm, FOX CREEK
p
H
(M)
1.0
L5
Figure 12. The effect of time after vessel passage on light
extinction.
The displacement of the resuspended sediment due to currents
may be a factor here, though care was taken to have the majority
of observations taken within 1% hour of slack tide. The nearest
current measurement station, located at Fox Point, a creek area
having a cross-section approximately ;237 of the creek section
used in these tests, recorded currents averaging less than
.05 m sec"1 for time periods equally close to slack water (Cor-
rell, D., personal communication). As buoys were spaced in line
with the creek's longitudinal axis, transport could be expected
to have small lateral components. Maximum drift may have been
8-13 m during the first 9 min, the first interval tested, and as
much as 50-90 m during the maximum interval allowed between ves-
sel passage and sampling. Generally, the drift would have been
less, and as the buoy line was 150 m in length, and the vessel
tract generally extended beyond the buoy line at least 50 m to
avoid effects of vessel turns, little impact is believed to
result from currents.
Tugboat runs were repeated in slightly deeper water, at the
Sellman Creek site, where water depths average 2.2 to 2.3 m, and
sediment particle size distributions remain similar. The
24
-------
increases in light extinction were much less than those seen at
the Fox Creek site, and were only marginally significant when
analyzed statistically. The extinction coefficient prior to
vessel passes averaged 1.50 ir~l, and increased to 1.56 m~l after
vessel passes, as is shown in Figure 13.
Finally a single pass by the tugboat was made at the Ghost
Neck site in the Magothy River, where the water depth was
slightly deeper than Sellnan Creek, about 2.3 to 2.5 m, but the
sediment was primarily sand. As expected, no detectable impact
from the turbulent stirring of the boat pass was seen. It should
be added, however that this site, unlike the previous two, had
the typically weak tidal currents found in these waters running |
across the buoyed sampling track. It is conceivable that a |
stirred plume, if it persisted long enough to be detectable with- j
in the time frame of the test, might be swept out of the sampling
zona. Figure 14 shows the result of the deep water, coarse sub-
strate experiment.
Repeat tests were mad? at the Fox Creek site, where the
variables changed were the boat and engine type. In these sets
of vessel passes, the research vessel Osprey, an outboard power-
ed, planing runabout was used. In terms of the resulting water
motion dynamics, changing both boat and motor types represent a
fairly large nujiber of modifications, some of which are offset-
ting, others additive. The reduced displacement, the planing
attitude of the vessel at the speeds usp.d, as well as the re-
duced immersion of. the propeller and its smaller diameter would
tend to create a reduced volume of water set into quasi-turbulent
motion. The l"~ger power output, increased by over 60 percent
over that of the tugboat, and the higher engine and consequent
propeller rpm, and possibly the higher operating speed of the
vessel would tend to produce increased water motions, and hence ,.;
a greater likelihood of sediment resuspension. ";
i
Figure 15 shows a run by the R/V Osprey at 4000 rpm along *
the identical track used by the tugboat at the Fox Creek site. 4
In this case observed effects were maximum, while Figure 16 shows <
the results more typical of the majority of the runs. Figure 17 •
is a graph constructed from the arithmetically averaged extinc- \
tion coefficients. The net increase for a total of 13 samples, j
and about 90 measurements, show the extinction coefficient to go £
from 2.21 m~- to 2.30 m"1, statistically significant at the 0.75
level whun tested using student's t. These findings are summar- *
ized in Table 1 for comparison with transmissometer and suspended |
sediment results. i
25
-------
R/V BOTTLENOSE
0.?
n i.s
a.s
5 7 tel
% TRANSMITTED LIGHT
5 7 lea
Figure 13. Plot of the arithmetic mean of extinction coefficients
for tugboat runs (2000 rpm) at Sellman Creek (2.3-2.5 m depths).
TUGBOAT AT
RPfl
OTHV R. TltlCl 81 HIM
0,5
H
n i.s
a.s
/
V
^
/
'
',
/
/
<
/,
//
s
//
\/y '
^
y
/
^
/
,0 3 S 7 lei 3 S 7 le
% TRANSMITTED UOHT
OA^]*83*6**300*3
Figure 14. Photometer readings from a single test (before and 21
rain, after single tug pass at buoy 3) .
j
i
x
-<
26
-------
e
0.5
E 1
H
n i.s
a
a. 5
11
R/U OSPREV AT 4«99 RPM, CQX CK TIClEi? (1IN
^
^
/
\/
^
^
/
/
s"
/
^
^
/
^
^
/
-
-"
^
^1
*s~
^
^
^
/v
/-
• BEFa
^
IE
" aAFTER
1 1 1
/
(r
(n
^
• 1
•C
X
E)
S)
^
-
9 3 S 7 ui 3 6 7 It8
% TRANSMITTED LIGHT
Figure 15. Photometer readings from a single test (before and 7
min. after single pass of the planing craft R/V Osprey at Fox
Creek).
0.5
E 1
T
%
n l.s
a.s
R/U OSPREV «T aaca' RPH FOX
-------
e.s -
i r"
p
T
H
n i.s
1
T
2.5
<
^T1 J
.x^
5 7 lel
TRANSMITTED LIGHT
BEFORE (n = l2) AFTER (n=!3)
Figure 17. Plot of the arithmetic mean of extinction coefficients
of the planing craft R/V Osprey (4000 rpm) at Fox Creek.
% I/Io
P
H
(m)
Figure 18. Plot of the arithmetic mean of extinction coefficients
for the planing craft R/V Osprey (2000 rpm) at Fox Creek.
28
-------
TABLE 1. SUMMARY OF PHOTOMETER EFFECTS
Before
After
Location
Fox Creek
Fo.' Creek
Vessel
Tugboat
Out-
board
n
17
12
k
1.69
2.21
std.
dev.
0.2
0.33
n
17
13
k s*d.
dev.
2.11 0.44
2.30 0.30
Signifi-
cance
t-test
0.995
0.75
Sellman Tugboat 7 1.50 0-16
Creek
Magothy Tugboat 3 1.43 0.06
River
8 1.56 0.13 0.75
3 1.47 o.06
No field tests were conducted that could reliably and
demonstrably separate effects from propeller-induced motion to
wake effects, one of the reasons that lead to the laboratory
experiments. However, the result of a test of the R/V Osprey at
slower speed was instructive. In this case, summarized in Fig-
ure 18, the vessel's engine rpm was held to 2000, exactly halt
of that used in high speed, planing runs. At the lower engine
rpm the vessel did not plane, but moved through the water at the
maximum wave-making speed. This speed is taken to be the speed
when the stern wave actually begins to show its maximum ampli-
tude astern of the ship. The speed is empirically given for
nonplaning craft as
v (m sec'1) = 1.31 /LWL (2)
where LWL is the waterline length of the craft in m. Visually,
it appeared that confused wake and high stern waves should easily
produce greater effects that at the high planing speeds, however
the observations of extinction coefficients do not bear this out.
TRANSMISSOMETER MEASUREMENTS
For the majority of photometer measurements made, a trans-
missometer reading was taken concurrently. As was anticipated,
the variability of the transmissometer data was far greater than
the light extinction measurement, the general trend of the mea-
surement follows and perhaps thereby corroborates measurements
of light extinction coefficients. Figure 19 gives representative
results for particular runs using the planing runabout and the
R/V Osprey. Figure 20 shows the results summarizing all tests
29
-------
D
E
P
T
H
(m!
D
E
P
T
H
(m)
U
.5
1.0
1.5
. o
. o
O
Q
0
O
O
NO DOA
•c •
0
a
®
20 BUOY 1 (7min) BUOY 2 (ISmin.) BUOY 3 (2lmir
RUN AT 4000 rpm AT 1029 8/8/79
n20 30 40 50 60 70 20 30 40 50^.60 70 20 3O 4O ^ 60
U
.5
1.0
1.5
2.0
u •
o-
o
. o
(6mm.)
O •
O
o
(13 min. }
o
•o
• 0
(21 min.)
».)
7P
RUN AT 4000 rpm AT 1158
O BEFORE
. Ar fT*
Figure 19. Representative transmissometer results for a single
test of the planing craft R/V Osprey at Fox Creek.
TUG BOAT @ 2000 rpm ! R/V OSPREY @ 4000 rpm
0 30
.3
I.O
1.5
2.0
40
SO
60
70
0 20
30
40
SO
70
n«5l
O BEFORE
• AFTER
ONE STANDARD DEVIATION
n-30
Figure 20. Average change in light transmission for all runs at
Fox Creek (depths 1.8-2.0 m).
30
-------
conducted with the tugboat, the R/V Bottlenose, et Fox Creek,
as well as the tests with the R/V Osprey at the same location.
Tho trend, and the relatively lesser effects of the boat having
the least propeller immersion is apparent, and born o it by sta-
tisLxcal c.omudLiscn, 3>«-
-------
en
EH
U
H
b
Cf
U
05
u
EH
W
2
O
en
en
S
in
z
£
EH
b
O
£H
05
4-1
•rl (U -^
en rH
•0 >
4-> (1)
01 t3
dP •
• C
cu us
^ ^"!
(0 4-1
eu
rH
a CU
e= N
as -H
01 en
• •
•0 >
4-1 (!)
en "O
<*> •
to
• C
eu d
^ M
IT3 4-1
CU C
a cu
g N
(0 -H
01 01
i-H
CU
01
01
a>
c
o
-H
4->
OC
CM in CTI
• • •
1-1 vo m
i-H
m
CO CTi VO
ro CTI r~
*r n vo
00 O CTi
ro .-o
0
00 CO VO
vo CTI r^
in
• . .
o ro r-
m *r i£>
rH O VO
in ro
•O
^J lj j^j
A3 (1) (Q
Q 00
j^ »Q *Q
D^ ^J C^
333
EH O EH
.* J
rtj
•
M
U
m
<
E^
j^
0)
4J
0
MH
CU
CQ
IT)
CU
rH
a
•-I
rH
01
01
Q)
C
O
•H
4-1
m
u
S
VO O (N
CO rH CTl
00 CTl CO
O (N VO
oo ro CTI
vo oo ro
•O
4J M 4J
(0 nj n)
•Q >O Jj
Cn 4J Cn
3 3 3
EH O EH
,y
0)
CU
Ai X U
CU CU
Q) Q) C
CJ U 6
X X H
O O 0)
b b CO
32
-------
SUSPENDED SEDIMENTS
Carefully analyzed, the suspended .sediment measurements
must be judged a cautiously qualified failure. Their analyses
were easily the most time-consuming part of all analytical pro-
cedures employed, yet returned the least information per unit
effort. For initial measurements, there appeared to be a sig-
nificant trend corroborating the findings from photometric and
transmissometer measurements. When subsequent results began to
show unaccountable fluctuations, replicates were prepared for
each sample Laken. Blank runs were made for each day of field
wotk.
Thus, of a total of close tc 150 gravimetric determinations
of suspended sediments., 62 samples have replicates for assess-
ment of confidence levels, while 14 blank determinations were
made during the course of sampling. The average difference in
suspended sediments, expressed in mg I"1, for the replicates
-Droved to be 5.2, with a standard deviation of 4.3 mg I"1.
Similarly, the 14 blc.r>k determination had a mecin change in
weight resulting from identical treatment as the actual samples,
excepting the ac'.ual filtration, of 2.4 mg, with a standard devi-
ation of 2.6 mg.
The results were neverthelfjss tabulated, using the averaged
replicate pairs as individual results, and averaging these for
statistical comparison. Surprisingly, even these dubious results
follow the pattern established by the photometer results. Table
4 summarizes the data obtained from gravimetric determination of
suspended sediments.
TABLE 4. . SUMMARY OF SUSPENDED SEDIMENT ANALYSIS
Before
Location
Fox Creek
Fox Creek
Sellman
Creek
Magothy
Fox Creek
Fox Cieek
Vessel
Tugboat
Outboard
Tugboat
Tugboat
Tugboat
Outboard
engine
rpm
2000
4000
2000
2COO
1500
2000
ss*
(mg/1)
22.4
19.9
9.5
10.1
18.0
26.8
std.
dev.
11.7
9.44
4.3
7.8
6.7
6.0
After
~ss
(mg/1)
25.4
21.6
13.0
5.3
16.1
23.5
std.
dev.
9.7
7.1
5.4
3.3
2.8
3.7
sig.
level
0.900
0.750
0.975
none
none
none
n**
25
32
16
8
4
1
'SS = averaged suspended sediment concentration
**n = sample size
33
-------
The correspondence of results for the three measurement
techniques employed forces the question whether there is a defi-
nite relationship between the alpha coefficient determined by
trans!.;issometer and the suspended sediments determined gravi-
metrically. Theoretically derived relations often involve
assumptions about particle sphericity that may not hold for the
predominant sediments found suspended in Bay waters (Jerlov
1968) . Schubel (1968) reports an empirically derived relation-
ship specifically for suspended sediments found in the upper
Chesapeake Bay
a =» 0.42 SS (3)
where a is the transmission coefficient defined to be
o --J- In (i ) (4)
L ^-o
and SS is the suspended sediment concentration in mg 1~ . ^/*0
is the fraction of light received at the detector after passing
through length L of the water column. Shannon (1975) reports a
relationship between a and k, the extinction coefficient as fol-
lows:
k = 0.2 a + 0.4. (5)
The coefficient k is calculated using the relationship shown in
equation (4), with the understanding that L represents the depth
at which the measurement is taken, still a path length in a
sense. The physical significance of the measurement, of course,
differs. The equations are readily manipulated and combined to
allow an expression for SS both from a and k, the latter used as
follows:
SS = 11.9 k - 4.76. (6)
Using these relationships, we have calculated the averaged
results, as shown in the previous sections, and compared them to
the measured suspended sediments. Table 5 shows that in a sur-
prising number of instances, the agreement is rather good.
34
-------
TABLE 5. COMPARISON OF SUSPENDED SEDIMENT DATA FOR
THREE MEASUREMENT TECHNIQUES
Before After
Location Vessel Engine {§^ (§ff)^ -^ ^ (§^ gf
Fox Creek Tugboat 2000 17.9 16.2 27.-1 19.2 19.9 25.4
Fox Creek Outboard 4000 21.5 19.8 19.9 22.6 21.7 21.6
Seliman Tugboat 2000 13.1 9.4 95 13.8 9.3 13.0
Creek
Magothy Tugboat 2000 12.3 10.1 12.6 5.3
Fox Creek Tugboat 1500 13.9 12.9 18.0 12.6 15.7 16.1
Fox Creek Outboard 2000 27.4 24.3 26.8 27.6 24.2 23.5
*(SS). = average suspended sediment concentration from k, the
extinction coefficient.
** (SS) = suspended sediment concentration from a, the trans-
mission coefficient.
tSS - average suspended sediment concentration determined
gravimetrically.
Finally, the results of the three measurement approaches
have been averaged to arrive at a net increase in suspended
sediment due to the passage of a boat having characteristics
similar to the ones used in these tests, and operating in waters
of the same depths indicated. These results are shown in Table
6.
35
-------
TABLE 6. SUMMARY OF SUSPENDED SEDIMENT BASED ON
ALL MEASUREMENTS
Before After
Location
Fox Creek
Fox Creek
Sellman
Creek
Ma go thy
Fox Creek
Fox Creek
Vessel
Tuc'boat
Outboard
Tugboat
Tugboat
Tugboat
Outboard
Engine
rpm
2000
4000
2000
2000
1500
2000
Depth
(m)
2.0
2.0
2.3
2.5
2.0
2.0
SS
18.8
2C.4
10.7
11.2
14.9
26.2
SS
21.5
22.0
12.0
8.9
14.8
25.1
A
2.7
1.6
1.3
2.3*
0.1*
-1.1*
A = average increase in suspended sediment.
* = these increases statistically insignificant.
As can be seen, decreases in sediment load can be observed
at times, but such decreases were not observable when
data were averaged over many sets of observations. In the case
of the tugboat at 1500 rpm, and the outboard at 2000 rpm, only
the results of a single vessel pass are available, as for the
pass conducted in the Magothy River, hence the information in
the last three rows has very low reliability, and the interpre-
tations cited in the previous sections are believed more infor-
mative than the data in this table. On the other hand, the
averaged results shown in the top three rows of the table rep-
resent our findings for many tests. These findings will be
discussed in the light of the observations from the laboratory
tests of propeller stirring.
LABORATORY EXPERIMENTS
The laboratory measurements were designed to yield informa-
tion concerning the propeller effects alone. Using measured
values for the averaged velocity imparted to the water by the
propeller action, along with the turbulence intensity, it should
be possible to map the distribution of critical stresses, i.e.,
stresses sufficient to cause resuspension of sediment. Experi-
ments were done using three different advance ratios, J, to map
a range of applicable conditions typical of small craft found in
Bay waters. These J ratios were 1.61, 2.39, and 3.53 x 10~3
respectively. Table 7 gives the range of J values calculated
for the conditions used in the field tests for comparison.
36
-------
E 7. REPRESENTATIVE J VALUES FOR FIELD TESTS
CONDUCTED IN THIS STUDY
Vessel V (K se<:1) N (rpm) D (m) J x 10~3
Tugboat
Tugboat
Outboard
Outboard
3.6
2,5
13.0
3.1
600
500
2000
1000
0.48
0.48
0.41
0.41
4.0
3.3
5.1
2.4
Note that N, the propeller rpm, is not the sarr.s number shown in ]
the summary tables for the transmissorceter, photometer, and f>us- j
pended sediments data. In the latter cases the engine rpm w«is
listed. Engine rpm and propeller rpm are directly proportional
and a function of the reduction gear used witn a particular
engine. Thus, the tugboat has a 3:1 reduction gear, while the i
outboard uses a 2:1 reduction gear. i
All velocities are expressed as ratios to the propeller
advance velocity. In nearly all experiments the ratio of Va, to
the average velocity obtained by integrating the instantaneous
velocities over the height of the water column,shows small devi-
ations from urity, i.e., UAVG/UINF < 1, which shows that the S
water surface is raised slightly abaft the propeller due to the !
forces it creates. These changes are not believed to affact the j
resulting turoulence measurements. J
1
Figures 21 through 26 show the measurement results for |
increasing J values, paired so as to show -;he normalized mean j
velocity, u, and the corresponding turbulence intensity, -pu'v', j
in each case. The relationship of u and pu~rvr to shear stress jj
is given by the Reynolds stress equation
Txy = -Pu-v-- + V(ff- + H-) (7)
in its two dimensional form, in turn derived from the Nctvier-
Stokes equction. It uses the concept of the Reynold's decompo-
sition, whr.ch treats the velocity of a fluid particle as the sum
of a mean velocity plus a fluctuating term. A rigorous applica-
tion of ths equation is not attempted here; the mathematical
introduction is omitted. The reader is referred to standard
texts suet as Tennekes and Lumley (1972).
The figures show that the magnitude of propeller effects
is greatest for low J values and decreases for increasing J
values. This means that a vessel, moving through the water with
37
-------
UWG'UINF-e.988
3,2
2.4
1.6
0.8
A STANDARD
X X'R-8.5
D X'R«1.8
O X/R-1.5
0 X'R-2.8
8.4 8.8 1.2
Y/R
t.6
Figure 21. Average velocity distribution in propeller wake for
low J value.
J- l.6l *
UAUG/UINF-8.988
8.8
U 8.6-
8.4.
8.2
A STANDARD
X X/R-8.5
O x/R-i.e
O XxR-1.3
0 X/R-2.8
8.4
8.8
1.2
1.6
Figure 22. Turbulence intensity distribution for low J value.
33
-------
Tugboat 600
Outboard 2000
3.6
13.0
4.0
5.1
0.73
0.60
1.4
1.0
high speed while the engine is turning at a moderate number of
rpms, will have high J values and an effect that does? not extend
to great depth. Conversely, the same vessel under a load suf-
ficient to slow its speed through the water at the same engine |
rpm will have propeller-induced water motion extend proportion- j
ately deeper into the water column. \
Using the figures more quantitatively, and assuming that ,
sediment stirring will take place when propeller effects exceed j
background motions observed under laboratory conditions, the 1
following comparisons are estimated and shown in Table 8 for J j
values approximating those for the tugboat and outboard during j
high-speed field tests: j
TABLE 8. ESTIMATED MAXIMUM DEPTH FOR PROPELLER ]
EFFECTS SCALED FROM LABORATORY TESTS «
Vessel rpm v(m sec"^-) J x 10" Depth below Max. water
prop, hub (m) depth affec- j
ted* ]
— i
i
I
I
\
*This depth is found by adding the depth of propeller effects to
the immersion depth of the propeller hub.
These results tend to agree with the field measurements
that show greater measured sediment resuspension and greater
light attenuation for the tugboat tests than for the outboard
tests for water of the same depth. In the field tests the added
effects from wake also play a role, of course, and cannot be
neglected. These would tend to reinforce the pattern of find-
ings, as the wave height of the planing outboard's wake was
observed to be consistently lower than those produced by the tug-
boat travelling near its maximum wave-making speed.
A contradiction appears to arise when comparing propeller
effects from the same vessel at different speeds. Both for the
tugboat and the outboard, the J values increase when the vessel
is slowed, implying that propeller effects extend more deeply
into the water column. This increase was not observed for field
measurements conducted under appropriate conditions, suggesting
that other variables masked this effect. The effect does appear
to make physical sense, and a single field test conducted with
the tugboat stationary, anchored in Fox Creek, with the engine
turning 1800 rpm, showed suspended sediment increases between 15
to 30 mg 1~ , and a degrae of sediment stirring visible to the
eye as never observed to during any test where the vessel
remained in motion.
39
-------
J-3.53X I0'3
3.2
2.4
1.6
» e.e
Y
UAUG/UINF-8.879
^ STANDARD
E X/R-8.8
O X/R-1.9
0 XxR-2.9
8.4 8.8 1.2
Y/R
1.6
Figure 23. Average velocity distribution in propeller wake for
high J value.
J« 3.53 x 10'
UAUG'UINF-e.879
9.8
8.6
8.4
•.2
^ STANDARD
O x/R-e.e
O x/R>i.e
O X'R-2.e
e.4 e.e 1.2
Y/R
1.6
Figure 24. Turbulence intensity distribution for high J value.
40
-------
J- 2.39 x |0'3
UAUG'UINF«e.879
3.2
2.4
1.6
^ STANDARD
<> x/R-e.e
G X/R-1.8
O x/R«2.e
e.4
9.8 1.2
Y/R
1.6
Figure 25. Average velocity distribution in propeller wake for
intermediate J value.
J- 2.39 x 10'
UAUG/UINF-B.879
a.e
e.e
e.4
•.2
^ STAKDARD
O x/R«e.e
D x/R-i.e
O x/R«2.e
0.4 8.8 1.2
Y-'R
i
1.6
Figure 26. Turbulence intensity distribution for intermediate J
value.
41
-------
The laboratory measurements of average and fluctuating
(turbulent) velocity have been examined in an attempt to relate
them to a critical velocity sufficient to resuspend sediment.
The critical velocity is taken from the literature. It was nec-
essary to distinguish velocities measured above the water sedi-
ment boundary layer from the critical velocity at the sediment
interface. Drag effects, a function of sediment particle size,
cohesiveness, and sediment type, produce an exponential profile
for velocity gradients in the boundary layer. Gust and Walger
(1976) working with Nor-ch Sea sediments of similar particle size
as existed for this study, report critical velocities near
0.08 m sec"1. They cite the work of Einstein and Krone, who
report 0.07 m sec"1 for clay sediments (illite, montmorrilonite,
and kaolinite) in San Francisco Bay. In a previous paper, Gust
(1975) finds much higher critical velocities when such clay
sediments, known to be cohesive, are compacted as might be found
of areas of tidal scour. He reports critical velocities as high
as 0.36 m sec"1. Bagnold (1962) defines a critical stress 9t
for resuspension. However, his data, based on citation of
Shields, is for particles of 60y and up and must be extrapolated
to the 30 to 60y range of interest, for which values for 9t of
0.12 to 0.13 can be estimated. Rossman,Seibel (1977) cite crit-
ical velocities of 0.14 m sec"1 for 63y particles, the smallest
size they report, but make no reference to effects when sediments
are cohesive in particle ranges less than 60y.
We therefore consider the range of critical velocities
sufficient to resuspend sediments to be 0.08 to 0.14 m sec" as
representative for particle distributions found in Fox and Sel-
Iman Creek. These are velocities measured at the bottom, but
just above the boundary layer. If we examine the laboratory
result for J = 3.53 x 10~3 we find that a normalized velocity
unit, as shown in Figure 23, is determired as follows:
u/Voo = 0.879;_the velocity of water in the tank was Vm = 0.347 m
sec"1, hence u = (0.347) (0.879) = 0.305 m sec"1.
Using Table 7, we can interpolate this speed to be 2.7 m
sec"1 for the tugboat in field tests. Hence, if 1 normalized
unii (at J = 3.53 x 10~3) corresponds to 2.7 m sec"1, then the
critical velocities of 0.14 m sec"1 and 0.08 m sec"1 correspond
to 0.05 and 0.03 velocity units, respectively. By inspection
of Figure 23, the difference of 0.05 above background occurs at
Y/R = 0.48. Since D/H = .264 for the lab test (as defined in
Figure 4) and D = 0.48 m for the tugboat, H = D/.264 = .4S/.264
or H = 1.82. A unit Y/R represents 1.3 m. The distance below
the propeller hub at which Y/R is such that the critical veloc-
ity is 0.05 m sec above background is .52 Y/R units, or about
0.68 meters.
In the lab tests the propeller was held stationary while the
water flowed at a known velocity. In the field, of course, the
vessel is moving at a known velocity, and the water is considered
42
ri* '
31
'
-------
stationary. The "background" velocity must be taken as zero to
allow comparison. It is conceivable that propeller effects actu-
ally extend to greater depths than cur lab tests would indicate,
but such effects are not measurable by the approach feasible to
us in this study.
Velocity fluctuations, shown in Figure 24 as turbulence
intensity, are root mean square values by which the normalized
mean velocity could be exceeded. Turbulence intensities; high
enough to approach the critical velocity for resuspension, 6.14-
0.08 m see"1 correspond to 0.5 velocity units on the graph.
Such fluctuations extend to a depth of 0.5 Y/R units, or about
0.65 m below the propeller hub.
Taken together, and assuming that background (identified
as "standard" in figures) velocities represent zero velocities
which must be exceeded by 0.08-0.14 m sec"1 to obtain sediment
resuspension, we may conclude that the assumptions used to gen-
erate Table 8 are reasonable. In fact, the assumptions are
necessarily conscivative, as the background of flowing water may
partially mask propeller induced motion, although the shape of
the curves shown in Figures 22 through 26 suggest this to be a
minor factor.
Using this approach, we have attempted extrapolation of the
turbulence measurement data over wider J ranges, assuming an
exponential relationship, with a maximum value of J = 5 x 10~^
deemed as one extreme. This allows the compilation of the maxi-
mum depths affected for J values from 1.6 x 10~3 to 5 x 10~3
(with depths ir propeller diameters). With known diameters and
depths of hub immersion, the actual water depths in m can be
quickly estimated as shown in Table 9.
TABLE 9. MAXIMUM DEPTH OF PROPELLER EFFECTS AS A
FUNCTION OF J IN TERMS OF PROPELLER DIAMETERS
J X 10
-3
Depth
(Diameters)
Example for
Tugboat (m)
Example for
Outboard (m)
1.6
3.0
2.1
1.6
2
2.9
2.1
1.6
3
2.1
1.7
1.3
4
1.5
1.4
1.0
5
1.4
1.4
1.0
43
-------
SECTION 5
REFERENCES
Anderson, F. E. 1974. The effect of boat waves on the sedimen-
tary procosses of a New England tidal flat. Tech. Rpt 1.
Jackson Est. Lab., U of N. Hamp. 38 pp.
Anderson, F. E. 1975. The short term variation in suspended
sediment concentration caused by the passage of a boat wave
over a tidal flat environment. Tech. Rpt 2. Jackson Est.
Lab., U of N. Hamp. 40 pp.
Anderson, R. R. 1979, Distribution of submerged vascular plants
in the Chesapeake Bay. EPA Grant R805977. Ches. Bay Progr.
Athanas, L. Chris. 1979. The effects of suspended sediments, and
water turbulence on the growth of submerged aquatic vegeta-
tion. Western Eco-Systems Techn., Inc.
Bagnold, R. A. 1962. Mechanics of marine sedimentation in The
Sea Vol. 3. M. N. Hill, ed. Interscience.
Bayley, S. E. M. 1970. The ecology and disease of eurasian
watermilfoil in the Chesapeake Bay. Ph.D. Diss. Johns Hopk.
U., Baltimore, MD.
Byrne, R. 1980. Report on erosive effects of small boats given
before Environ. Matters Committee. MD House of Delegates.
Jan. 28, 1980.
Carver, R. E. 1971. Procedures in sedimentary petrology. Wiley
Interscience. N.Y., N.Y.
Collins, I. J. and Noda, E. K. 1971. Causes of levee damage in
the Sacramento-San Joaquin Delta. Rept P-218-1. Tetra Tech,
Pasadena, CA. 55 pp.
Comstock, J. P. 1967. Principles of naval architecture. Soc. N.
Ar. Mar. Engrs. N.Y., N.Y. 10006.
Gilmer, T. A. 1967. Modern ship design. U.S. Naval Inst., Anna-
polis, MD.
44
-------
Gust, G. 1975. Observations on turbulent drag reduction in
dilute suspensions of clay in sea-water. J. Fluid. Mech.
75(l):29-47.
Gust, G. and Walger, E. 1976. The influence of suspended, co-
hesive sediments on boundary-layer structure and erosive
activity of -rbulent seawater flow. Mar. Geol 22:189-206.
Herreshoff, L. F. 1966. The common sense of yacht design. Cara-
van Books. Jamaica, N.Y.
Jerlov, N. G. 1968. Optical Oceanography. Elsevie;r. Amsterdam,
Holland.
Johnson, J. H. 1975. Effects of tow traffic on resuspension of
sediment and dissolved oxygen concentration in the Illino:. •;
and upper Mississippi Rivers under normal poo] conditions.
Rpt. of Env. Eff. Lab. USA Engr. Wat. Exp. Sta. Vicksburg,
MI.
Kelvin, Lord. 1887. On the waves produced by a single impulse
in water of any depth, or in a dispersive medium. Mathemati-
cal and Physical Papers, Vol IV. London Cambridge University
Press, 1910. pp. 303-306.
Kerhin, R. T. 1977. Reconnaissance survey of tha Severn River
sediments. Info. Circ. 23. MD Geological Surv., Dept Nat.
Res., Annapolis, MD.
Komar, P. D. 1977. Beach sand transport: distribution and total
drift. Air Soc Cir Engr. Waterm., Harb. and Coast, Engr. Dir.
J. 101(WW):225-239.
Liou, Y. C. and Herbich, J. B. 1976. Sediment movement induced
ships in restricted waterways. SEA Grant Fub. TAMU-SG-76-209.
Nakato, T., Locher, F. A., Glover, J. R., and Kennedy, J. F. 1977.
Wave entrainment of sediment from rippled beds. J. Waterways,
Harb. and Coast. Engr. Dir. ASCE, 103 (WW 1). Proc Pap 12736.
Peregrine, D. H. 1971. A ship's waves and its wake. J. Fluid
Mech. 49(2):353-360.
Pierce, J. Vv. 1973. Discharge of suspended particulates from
Rhode Piver watersheds. In Proc. Workshop on Watershed Res.
in E. N. An. SI Press. Washington, D.C.
Reif, T. H. 1978. The effects of drag reducing pclymers on the
turbulence characteristics of the hydraulic jump. Rpt EW-11-
78 USNA, Annapolis, MD.
-------
i
Reif, T. H., Metrakos, A. P., and Hoyt, J. W. 1978. A laser dop- -
pier anemometer study of the classical hydraulic jump. Am j
Soc. M. Engr., San Francisco, CA. j
Rossmann, R. and Seibel, E. 1977. Surficial sediment redistribu-
tion by wave energy: element-grain size relationships. J. '
Great Lakes Res, Ir.t'l Assoc. Great L. Res. 3 (3-4) :258-262.
Rcy Mann Assoc. 1976. Recreational boating on the tidal waters :
of Maryland. Planning Study prep, for Energy a.id Coastal !
Zone Adminst. Dept Nat. Res. MD. j
Schubel, J. R. 1968. Suspended sediment of the Northern Chesa- \
peake Bay. Technical Report 35. Reference 68-2. CBI Johns <
Hopkins University. \
Shannon, J. D. 1975. Correlation of beam and attenuation coef-
ficient measured in '^Z^jted ocean waters. Proc. of SPIE
19th Ann. International Technical Symp., San Diego, CA.
i
Sorensen, R. M. 1973. Water waves produced by ships. J. Waterw.
Harb., and Coast. Engr. Div. ASCE. 99(WW2):245-256. Proc.
Pap. 9754.
Stevenson, J. C. and Confer, N. M. 1978. Summary of available ;
information on Chesapeake Bay submerged vegetation. Maryland
Department of Natural Resources, U.S. Environmental Protec-
tion Agency, Fish and Wildlife Service FWS/OBS/-78/66.
i
Strickland, J. D. H. and Parsons, T. R. 1572. A practical hand-
book of seawater analysis bulletin \~>r. (second edition) .
Fisheries Research Board of Canada, Ottawa, Canada.
Suhayda, J. N., Whelan, T., Coleman, K. M., Booth, J. S., and
Garrison, L. E. 1976. Marine sediment instability: inter-
action of hydrodynamic forces and bottom sediments.
Offshore Tech. Conf., Dallas, TX.
Taylor, D. H. 1920. The speed and power of ships. Wiley and
Sons. N.Y., N.Y.
Tennekes, H. and Lumley, J. L. 1972. A first course in turbu-
lence. MIT Press. Cambridge, Mass, and London, England.
; Tricker, R. A. R. 1964. Bores, breakers, waves and wakes.
i Elsevier. N.Y.
i
Williams, J. 1980. Effects of recreational boating on turbidity
and sedimentation rates in relationship to submerged aquatic
vegetation. EPA 78-D-X0426 Interagency Agreement, Ches. Bay
Progr., Annapolis, MD.
- - _ .
1 46
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Yousef, Y. H. 1974. Assessing effects on water quality by boat-
ing activity. EPA-6"/0/2-74-072.
47
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APPENDIX A
THE EFFECTS OF SUSPENDED SEDIMENTS, ACCUMULATED
SEDIMENTS, AND WATER TURBULENCE ON THE GROWTH
OF SUBMERGED AQUATIC VEGETATION
L. Chris
Western Eco-Sys terns Technology, Inc.
INTRODUCTION
Most submerged aquatic vegetation (SAV) is substrate bound
and is therefore limited, primarily by light , to coastal waters.
Although the depth of SAV occurrence can be considerable in clear
waters (Phillips 1974), SAV in the Chesapeake 3ay and its tribu-
taries is usually found in less than three meters of water
(Stevenson and Confer 1977) . Of the approximately 11 species of
vascular SAV in Chesapeake Bay (Kerwin, Munro and Peterson 1975)
only three, Zostera narina, Myriophyllum spicatum, and Ruppia
maritima have received much research attention, and only a part
of this has been directed toward light/productivity relationships.
Due to its widespread occurrence (Harrison and Mann 1975) and
ecosystem importance (Cottam and Munro 1954; Orth 1976) Zostera
has been the object of most of this research. Myriophyllum and
Ruppia have received less attention.
The productivity and abundance of submerged aquatic vegeta-
tion depends upon the intensity of light reaching the leaves
(Sand-Jensen 1977; Backman and Barilotti 1976). This, in turn,
depends upon the intensity of the light at the water surface, the
light reflected at the air-water interface, and the attenuation
of the light as it passes through the water column. The light
intensity at the surface of the water depends primarily upon the
latitude of the insolated site, the angle of incidence of the sun,
and local atmospheric effects, such as cloud cover (Reifsnyder
and Lull 1965) . The turbulence of the water surface will affect
the amount of light reflected from it (Jerlov 1968) , while the
attenuation of light as it travels through the water column is
influenced by both the length of the water column and its turbi-
dity. This report will discuss the effects, on SAV photosynthesis,
of light attenuation by suspended sediments ir: the water column,
and by sediments deposited on the leaves themselves. The
mechanical effeci.s of boat wakes on SAV will also be discussed.
Methods will be presented to estimate the possible effects of
these factors on SAV photosynthesis.
48
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** X I. • •"--"''
LITERATURE REVIEW AND CALCULATIONS
1. The effect of suspended sediments, on SAV photosynthesis.
A number of papers have been published concerning the rela-
tionship between light and the standing crop and productivity of |
SAV. These have included studies or the distribution and abun-
dance of SAV in relation to natural light intensities (teller and ]
Harris 1966; Peltier and Welch 1969; Orth 1977A), as well as those 4
where natural light levels were decreased mechanically (Backman J
and Barilotti 1967; Congdon and McCorab 1979). Rates of SAV pro-
duction in relation to light intensity have been measured both in
tne laboratory (Sand-Jensen 1977) and in the field (Penhale 1977;
McRoy 1974) . In this section of the report the relationship
between light and photosynthesis will be discussed, and a method
will be presented to calculate changes in photosynthesis result-
ing from changes in light intensity.
Keller and Harris (1966) noted an optimum range, in relation
to tidal depth, for the occurrence of Zostera marina at Humboldt
Bay, California. Plant density declined toward the deeper por-
tion of this range, probably because of decreasing light intensity.
Peltier and Welch (1969), studying Potamogeton perfoliatus in
Tennessee, noted a decrease in the net change of plant biomass
when the available light decreased by 24 percent. The decrease
occurred over a four week period. Orth (1977A) has suggested
that Zostera marina may eliminate Ruppia maritima by shading.
The two species seldom occur together except in shallower areas
where Zostera growth is decreased.
Backman and Barilotti (1976), working in California, con-
structed submerged canopies that reduced the light reaching
Zostera marina by about 63 percent. The experimental plants
showed a remarkably rapid change to the decreased light, as the
number of turions per area was reduced to an average of 27 percent
of the control density over 18 days. This is a decrease of about
1.5 percent per day. After nine months the experimental plants
had a density of 5 percent of the control plants. There were
also indications that flowering was suppressed in the shaded
plants, as well as vegetative regrowth in plants that had been
shaded for nine months.
A report by Congdon and McComb (1979) further emphasized the
long-term effects of shading on submerged aquatic vegetation.
Working with Ruppia maritima in Australia, their results demon-
strated a slower response to shading by Ruppia than Backman and
Barilotti (1976) showed for Zostera. After two months at a 60
percent reduction in light Ruppia standing crop was about 66 per-
cent of the control values. Ruppia however, showed a long-term
response to shading similar to Zostera. By a series of harvests
from 61 to 267 days after initiation of the experiment the authors
demonstrated that higher light intensities were required to
49
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maintain a. standing crop 50 percent of the control value. For
example, after 61 days the experimental standing crop equal to 50
percent of the control standing crop occurred at approximately 10
percent of the control light intensity, while at 164 days 50 per-
cent, of the experimental standing crop occurred at about 50 per-
cent of the control light intensity. At 267 days the 50 percent
level occurred at greater than 70 percent of the control light
value. Table 1 shows experimental yields of Ruppia (as a per-
centage of control yields) at various levels of light (as a per-
centage of control light) as found by Congdon and McComb (1979) .
TABLE 1. THE PERCENT YIELD OF RUPFIA MARITIMA AT SEVERAL LIGHT
INTENSITIES. (after Congdon and McComb 1974)
Percent of Control Light Percent of Control Yield
100 100
80 90
60 72
40 47
20 19
Although the preceding reports discussed standing crop, not pro-
ductivity, a decrease in standing crop during the height of the
growing season indicates decreases in productivity rates.
Sand-Jensen (1S77), in a laboratory study, investigated the
loss in photosynthetic rate in Zostera leaves due to epiphytic
growth. He compared the photosynthetic rate of leaves with
epiphytes and without epiphytes at increasing light intensities,
up to saturation at about nine mWcm~^. I have calculated a
regression equation from his Figure 4, which shows the decrease
in photosynthetic rate of a leaf without epiphytes, with decreas-
ing light intensity. The calculated equation (for less than sat-
uration light intensity) is:
Y=fi8.7-10.58X
where Y equals the percent reduction (from light saturated values)
of photosynthesis and X is equal to light intensity, in mWcm~2.
A 50 percent reduction in the maximum photosynthetic rate occurs
at about 37 percent of the saturating light intensity.
Two reports have been published which are important in con-
structing a general method of estimating SAV productivity. Pen-
hale (1977), working in North Carolina and McRoy (1974), working
in Alaska have done research on Zostera productivity using carbon-
14 uptake. I
-------
and the percentage of surface light: intensity as reported in
McRoy's paper. Below saturated light levels a hyperbolic curve
is generated; above saturation no further increase in photo-
synthesis occurs. In fact, above saturating light levels photo-
synthesis decreased.
McRoy (1977) described the curve below the 50 percent light
levo] as fitting the following equation:
V = V,
MAX S+KLT
wh«:re V is equal to the carbon uptake rate in mg C per gram dry
we:.ght of plant per hour; VMAX is equal to the maximum uptake
raze; S is equal to the in situ light intensity (as a percentage
of the surface intensityTT and KLT is equal to the percentage
light intensity where the photosynthetic rate equals % of
This equation is of the Michaelis-Menton form. Other bio-
logical phenomena, such as phytoplankton nutrient uptake, have
been described using this type of equation. Penhale (1977) ,
working with Zostera, used it to calculate maximum photosynthetic
rates from measured rates.
The abcve equation for estimating photosynthetic rates can
be used to calculate the decrease in photosynthesis* due to sus-
pended sediments. The equation requires throe values; a maximum
rate of photosynthesis, a percentage of the surface irradiance at
which the photosynthetic rate is *j of the maximum photosynthetic
rate, and a value for the in situ irradiance (as a percentage of
the surface irradiance) . fRV last value is site specific and
therefore should be a field Measurement. For the purposes of
this report the first two ar<5 values taken ::rom the literature.
Sand- Jensen (1977) reported a VMAX value of 1.25 mg Cg'^hr"1 in
his lab experiments on Zostera. Penhale (1977) calculated VMAX
of 1.19 mg Cg-ihr'1 from photc synthetic measurements on Zostera
~1~
and McRoy (1974) reported a VMAX of 1.18 mg Cg~hr~. An average
of the three values, 1.21 nvjC/ghr, is used in this report.
Values for KLT are more difficult to find. McRoy (1974)
j reported a value for KLx of 12.5 percent of the surface light,
i although this value might represent a physiological adaptation to
| far northern light patterns (Penhale 1977) . Another value for
I KLT ^s calculated from a figure from Sand-Jensen (1977) of 37
f percent of saturating light intensity. Burkholder and Doheny
I (1968; cited by Phillips 1974), reported a 50 percent reduction
- in photosynthesis at less than 15 percent of surface light.
Perhaps the best way to use these values is to calculate a
range of photosynthesis using the lowest and highest values. A
sample calculation, using light penetration data from the Rhode
River to estimate monthly changes in phctosynthetic rates, is
shown in appendix B.
51
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1.5
o»
o
o>
v>
'55
V
JC
(O
o
o
1.0 -
0.5 -
20 40 60 80
Surface Light Intensity 1%)
100
Figuire I. The relationship between Carbon
uptoko and the % of surface light reaching
the laaf. (After McRoy.1974).
52
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2. Sediment effects on SAV photosynthesis.
Sediment particles taken up into the water column can be
released when water conditions change. These particles can
accumulate on the leaves of SAV (Marsh 1973) . Epiphytic growth
on the leaves of certain SAV species commonly occurs (Marsh 1973;
Sieburth and Thomas 1973; Sand-Jensen 1977; Penhale 1977). This
epiphytic growth might produce light reduction effects similar to
those produced by sediment deposition.
Although work has been done on the effects of sediment depo-
sition on benthic invertebrates (Flemer et al, 1968; Pfitzen-
meyer 1973) , little information is available concerning sediment
1
deposition and SAV growth. This section of the report will dis-
cuss the effects of sediment deposition on SAV photosynthesis. j
The reduction of photosynthesis by epiphytic growth will also be
discussed. A method for estimating the reduction of SAV photo-
synthesis due; to sediment accumulation will be presented.
Marsh (1973) noted a bed of Zostera so heavily laden with
silt, epiphytes, and sessile organisms that the leaves were almost
prostrate. He noted that this particular bed was growing near
the lower limits of its light requirement. This accumulation
might have occurred because of a diminished leaf turnover time
resulting from reduced photosynthesis, since Sand- Jensen (1975)
reported a new leaf produced every 15-20 days in healthy Zostera ,
and a leaf growth rate of from 2-8g m~2d~1. Bourn (1932) noted
that a high sediment load could be detrimental to SAV. Odum
(1963) reported that a SAV bed in Texas was killed by a 30 cm
sediment deposition. Epiphytic growth on SAV, which might act
in a somewhat similar fashion as sediment accumulation in terms
of light attenuation, was shown by Sand-Jensen (1977) to reduce
Zostera photosynthesis by 31-58 percent. These epiphytic accumu-
lations can account for a significant proportion of combined leaf-
epiphyte weight (Penhale 1977) and, by their heterogeneous surface,
serve as a substrate for the accumulation of various biotic and
abiotic particles (Sieburth and Thomas 1973) . Zostera beds
reduce currents and turbulence in the water flowing through them
(Orth 19773) , thus decreasing the sediment carrying capacity of
the water. This might cause Zostera to be susceptable to
increased suspended sediments"! The relatively wide leaves of
Zostera probably facilitate the accumulation of material on them,
while other fine-leaved species, such as Potamogeton pectinatus,
apparently escape this problem (Stevenson and Confer 1977) .
There is little in the literature that addresses the problem
of sediment accumulation on SAV leaves. However, the equation
for estimating the rate of photosynthesis (see appendix B) can be
applied to calculate the percentage of light lost by sediment
deposicion. It can be done in the following manner.
53
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1. McRoy (1974) reported a mean Zostera productivity of
4 gC m~2d~l. This mean biomass in his study for June,
July, and August was 565 g m~2 (calculated here from
his monthly data).
2. Using the previously assigned value of VM&X (see
appendix B) of 1.21 mgC per gram of plant tissue per
hour, and the above biomass value, we calculate a
VMAx of 10 gc m~2 per 15 hour day.
3. The KL«P values will remain the same (12-15 percent).
4. The amount of suspended sediment settling out on a
surface of known area, and the resulting light atten-
uation, are calculated (these calculations are beyond
the scope of this report).
5. The photosynthetic equation is then applied using the
above Vj^x anc^ KLT values, field-measured light read-
ings, and the calculated light available after the
losses from sediment accumulation.
A sample calculation is shown in appendix C.
3. Physical damage to SAV.
Damage to SAV can result from the physical impact of dredg-
ing (Taylor and Saloman 1968) or from contact with boat propellers
(Zieman 1976). Damage resulting from less extreme effects, such
as boat wakes, is not documented in the literature. The follow-
ing discussion concerns known and possible effects of physical
disturbance on SAV.
SAV beds can be damaged when plant leaves and rhizomes are
injured or removed. Zieman (1976) reported long-term effects on
beds of Thalassia testudinum in south Florida. Shallow channels,
used frequently by boats,IKowed much of the damage. The damaged
Thalassia rhizomes grew back very slowly. Dredging over SAV beds
can also result in damage, since both leaves and rhizomes are
physically removed from the site (Taylor and Saloman 1968).
Damage might also result from less extreme factors. Wood
et al, (1969) noted that SAV beds (in this case Thalassia) require
a reducing sediment tor optimum growth. Zieman T1976) reported
that both pH and the n.dox potential were changed in mechanically
disturbed sediments. This sediment disturbance effect might occur
if boat wake turbulence was sufficiently strong. Boat wake tur-
bulence might also remove sediment from around SAV rhizomes.
This could leave the plant more vulnerable to uprooting by storms,
as well as exposing the rhizomes to grazing damage.
54
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At the present time, no methods seem to be available to
quantify these effects on SAV, and their importance remains
unknown.
SUMMARY AND CONCLUSIONS
A survey of the pertinent literature showed that decreases
in light intensity can result in a decrease in SAV productivity.
These changes in growth rate can occur rapidly. Long-term
decreases in light intensity can severely reduce SAV density.
Light reaching the plants can be reduced either by an increase
in water turbidity (suspended pediments), or by accumulation of
sediment on the SW leaves themselves.
Heavy sediment accumulation can destroy or severely damage
SAV beds. The effects of lighter sediment accumulation on SAV
leaves may increase the amount of sediment deposition on the
leaves.
The photosynthetic response of SAV to light is hyperbolic up
to saturation light levels. This hyperbolic curve can be
described by the Michaelis-Menton kinetic equation. The equation
can be used to estimate the changes in SAV photosynthetic rate
due to suspended sediments in the water column. The equation can
also be used to estimate the photosynthetic changes resulting
from sediment accumulation on SAV leaves. In each case, the
light intensity at the water surface, and the light intensity
reaching the leaf, must be known.
SAV beds can be damaged by dredging and by boat propeller
injury to leaves and rhizomes. The pH and the redox potential of
the substrate can be changed by churning of the sediment. The
water turbulence caused by boat wakes may also cause chemical
changes in the sediment. Turbulence may also remove sediment
from around the rhizomes, exposing them tc predators and decreas-
ing their ability to anchor the plant.
Submerged aquatic vegetation in the Chesapeake Bay is of
great ecological importance. The recent decline of SAV in the
Bay increases the importance of the remaining beds. The informa-
tion presented in this report indicates that SAV is vulnerable to
decreases in light intensity. Water turbulence might have an
effect on SAV rhizomes. With the decreasing coverage of SAV in
the Bay, and the increasing recreational and coisnercial boating
' use, localized effects on SAV assume greater importance. Several
; lines of investigation might prove useful in examining localized
; effects on SAV. A few examples are:
I 1. What is the light intensity and productivity of SAV beds
* in heavily utilized areas as compared to those in little
' used areas?
55
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2. What are the chemical and physical changes Jn th^ sedi-
ments of SAV beds in heavily utilized areas?
3. What percentage of SAV beds are subjected to heavy boat
traffic?
These questions, and others, should be asked if the
tance of local disturbance to £AV is to be deternu:ed
56
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LITERATURE CITED
Backman, T.W. and D.C. Barilotti. 1976. Irradiancp reduction:
effects on standing crops of the eclgrass Zostera marina in
a coastal lagoon. Mar. Biol. 34: 33-40.
Bayley, S.E.M. 1970. The ecology and disease of Eurasian
watermilfoil (Myriophyllum spicatum L.) in Chesapeake Bay.
Phd Dissertation. Johns Hopkins University, School of Hy-
giene and Public Health, Baltimore, Maryland. 192 pp.
Bourn, W.S. 1932. Ecological and physiological studies on
certain aquatic angiosperms. Cont. Boyce Thompson Inst.
4: 425-496.
Burkholder, P.R. and T.E. Doheny. 196&. The biology of eel-
grass. Lament Geol. Observ. No. 1227. 120 pp.
Congdon, R.A. and A.J. McC'Mtib. 1979. Productivity of Ruppia;
seasonal changes and dependence on light in an Australian
estuary. Aq. Botn. 6: 121-132.
Cottam, C. and D.A. Munro. 1954. Eelgrass status and envir-
onmental relations. J. Wildl. Vgt. 18: 449-460.
Flemer, D.A., W.L. Dovel, H.T. Pfitzenmeyer and D.E. Ritchie,
Jr. 1968. Biological effects of spoil disposal in Ches-
apeake Bay. Proc. Am. Soc. Civil Eng., J. San. Eng. Div.
August, 1968.
Harrison, P.G. and K.H. Mann. 1975. Chemical changes during
the seasonal cycle of growth and decay in eelgrass (Zostera
marina)on the Atlantic coast of Canada. J. Fish. Res. Bd.
Can. 32: 615-621.
Jerlov, N.G. 1968. Optical Oceanography. Elsevier, Amsterdam.
Keller, M. and S.W. Harris. 1966. The growth of eelgrass in
relation to tidal depth. J. Wildl. Mgt. 30: 280-285.
Kerwin, J.A. R.E. Munro, and W.W.A. Peterson. 1975. Distri-
bution and abundance of aquatic vegetation in the upper
Chesapeake Bay, 1971-1973. pp. 1-21. In; Impact of Tropical
Storm Agnes on Chesapeake Bay. Ches. Res. Consortium. 1975.
57
L
-------
Marsh, G.A. 1973. The Zostera epifaunal community in the York
river, Virginia. Ches. Sci. 14: 87-97.
McJ.oy, C.P. 1974. Seagrass productivity: carbon uptake ex-
periments in eelgrass, Zostera marina. Aquaculture 4:
131-137.
Odum, H.T. 1963. Productivity measurements in Texas turtle
grass and the effects of dredging an intracoastal channel.
Publ. Inst. Mar. Sci., Texas 9: 48-58.
Orth, R. 1976. The demise and recovery of eelgrass, Zostera
marina, in the Chesapeake Bay, Virginia. Aq. Botn. 2: 141-
159.
Orth, R. 1977A. Effect of nutrient enrichment on growth of
the eelgrass Zostera marina in the Chesapeake Bay, Virginia,
USA. Mar. Biol. 44: 187-194.
Orth, R. 1977B. The importance of sediment stability in sea-
grass communities. pp. 281-300. In: Ecology of Marine
Benthos. Univ. of S.C. Press, Columbia, S.C. 1977.
Penhale, P.A. 1977. Macrophyte-epiphyte biomass and prod-
uctivity in an eelgrass (Zostera marina L.) community. J.
Exp. Mar. Biol. Ecol. 26: 211-224.
Peltier, W.H. and E.B. Welch. 1969. Factors affecting growth
of rooted aquatics in a river. Weed Sci. 17: 412-416.
Pfitzenmeyer, H.T. 1973. Benthos of Maryland waters in and
near C and D canal. Hydrographic and ecological effects of
enlargement of the Chesapeake and Delaware Canal. Appendix
III. NRI ref. No. 73-113.
Phillips, R.C. 1974. Temperate grass flats, pp. 244-299.
In; Odum, H.T., B.J. Copeland, and E.A. McMahan, Coastal
Ecological Systems of the United States, Conservation Foun-
dation. 1974.
Reifsnyder, W.2. and Lull, H.W. 1965. Radiant energy in re-
lation to forests. Tech. Bull. No. 1344. U.S. Dept, Agri.,
Forest Service. Ill pp.
Sand-Jensen, D. 1975. Biomass, net production and growth
58
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dynamics in an eelgrass (Zostera marina L.) population in
Vellerup Vig, Denmark. Ophelia 14: 185-201.
Sand-Jensen, K. 1977. Effect of epiphytes on eelgrass photo-
synthesis. Aq. iotn. 3: 55-63.
Sieburth, J.M. and C.D. Thomas. 1973. Fouling on eelgrass
(Zostera marina L.). J. Phycol. 9: 46-49.
Stevenson, J.C. and N. Confer. 1978. Summary of available in-
formation on Chesapeake Bay submerged vegetation. U.S. Fish
t
and Wildl. Serv., Dept. of the Interior. 335 pp.
Taylor, J.L. and C.H. Saloman. 1968. Some affects of hydraulic
dredging and coastal development in Boca Ciega Bay, Florida.
Fish. Bull. 67: 213-241.
Wood, E.J.F., W.E. Odum, and J.C. Zieman. 1969. Influence of
sea grasses on the productivity of coastal lagoons. pp. 495-
502 In: Lagunas Costeras, un simposio. UNAM-JNESCO. Nov.
28-30. Mexico City.
Zieman, J.C. 1976. The ecological effects of physical damage
from motor boats on turtle grass beds in Southern Florida.
Aq. Botn. 2: 127-139.
59
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APPENDIX B
ESTIMATING SAV PHOTOSYNTHETIC RATES
FROM LIGHT TRANSMITTANCE VALUES
Monthly v.ilues of light penetration on the Rhode River,
Maryland in 1?61 (from Bayley 1970) are used to calculate monthly
rates of SAV photosynthesis, using the Michaelis-Menton equation.
Percent of Solar Radi- Calculated Percent Change in
ation Reaching a Depth Photosynthetic Photosynthesis
of 3 Feet Rates (mgC/ghr)
March
April
May
June
July
August
19.0
8.5
6.0
4.0
14.0
9.0
0.72 0
0.49 32
0.39 20
0.29 26
0.64 55
0.42 34
Sample Calculation
v =
s + KLT
= 1.21 mgC/g per hour o.l9°4-10.125
= 0.72 mgC/g per hour
V = Photosynthetic Rate = rogC/g Plant per Hour
VMAX = Maximum Photosynthetic Rate =1.21 mgC/g Plant per Hour
KT(T, = Percent Solar Radiation at which the Photosynthetic
LT Rate = 0.5VMav = 12.5%
S = Percent of Solar Radiation Reaching the Leaves
60
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1
1
APPENDIX C
ESTIMATING THE DECREASE IN SAV PHOTOSYNTHESIS RESULTING
FROM SEDIMENT DEPOSITION ON LEAVES
The percent light transmittance in the Rhode River in July,
1961 (from Bayley 1970) is used to calculate on SAV photosynthe-
tic rate. The percent light transmittance is then reduced to
simulate sediment accumulation on SAV leaves.
Sample Calculation
1. Without sediment accumulation
Si
V = V.
MAX Si + K
= lOgC per M2 per day
= 5.3 gC per M2 per day
2. With sediment accumulation
S2
V.
MAX S2 + KLT
= 10 gC per day —ft 07 + 0
= 3.6 gC per M2 per day
V = Photosynthetic Rate = ir.gC/g Plant per Hour
VMAX = Maximum Photosynthetic Rate = 1.21 mgC/g Plant per Hour
KTr, = Percent Irradiance at which the Photosynthetic
*"• Rate = 0.5VU.V = 12.5%
MAA
S^ = Percent of Solar Radiation Reaching SAV Leaves (no sediment)
82 = Percent of Solar Radiation Reaching SAV Leaves (with sedi-
ment)
61
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I ,
ABSTRACT 3
The objective of this study was to determine if small ves- }
sels, operating in shallow waters, have any measurable effects in j
producing increased turbidities by the resuspension of fine sedi- 1
ments which ™ay affect submerged aquatic vegetation (SAV). !
A two-phase approach was used, consisting of field tests in j
a suitable sub-estuary of the Chesapeake Bay, and laboratory t
measurements of propeller effects. During field trials, two dif- \
ferent vessel types were used to make passes at set speeds over ;
known water depths. Before and after measurements of light :
extinction, transmission, and gravimetric suspended sediment .:
determinations were used to identify effects. Laboratory experi-
ments were conducted to delineate propeller contribution to pos-
sible resuspension; this was done using laser-doppler anemometry
to map the turbulence field produced by propeller action. j
Results of field tests show that a 9000 kg displacement \
vessel with a 19x22 propeller, travelling at hull speed of 3.6 '
m sec~l in waters 2 m deep will produce statistically significant \
increase in light extinction and suspended sediment, averaging j
2.7 mg/1 when light data is converted to suspended sediment using
empirical formulations for Chesapeake Bay waters. In slightly \
deeper waters (2.5 m), of similar sediment size distribution
(median particle size 60y), the same vessel produced increases
averaging to 1.3 mg/1 but only marginally significant statisti-
cally. A fast, planing outboard boat with a 16x19 propeller,
produced significant increase in waters 2.0 m dco.p, but only when
travelling at high speed. At lower speeds, deemed to be speeds
inducing maximum wave making, no significant changes were detected.
Laboratory studies of propeller generated turbulence
show that the relevant variables are the advance ratio J, i.e.
the vessel velocity divided by the linear propeller tip velocity,
and the propeller diameter. Thn lower the J value, i.e. the
lower a vessels speed for a high propeller rpm, the greater the
vertical extent of turbulence., and hence the depth to which con-
sequent stirring can become possible. By modelling the propeller
effects of the tugboat and outboard used in field tests, even
when assuming that minimal turbulent stresses will resuspend
sediment, the depths to which effects extend were generally less
than those for which field tests nhowed impact. However, trends
indicated by laboratory measurements corroborate field tests and
allow limited extrapolation to cover a range of J values to
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