Transportation and Accumulation
of Fine-Grained Sediments in the
Estuarine Environment:
Recommendations for Research
J.R. SCHUBEL
H.J. BOKUNIEWICZ
R.B. GORDON
SPECIAL REPORT 14
REFERENCE 78-2
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MARINE SCIENCES RESEARCH CENTER
STATE UNIVERSITY OF NEW YORK
STONY BROOK, NEW YORK 11794
Transportation and Accumulation of Fine-Grained
Sediments in The Estuarine Environment:
Recommendations for Research
J. R. Schubel, H. J. Bokuniewicz
and R. B. Gordon
July 1978
Report of a workshop sponsored by: United States Environmental Protection Agency,
United States Energy Research and Development Administration, Office of Naval
Research: Geography Branch, National Oceanic and Atmospheric Administration: MESA,
Sew York Bight Project, United States Fish and Wildlife Service, Office of Biological
Services, the Stony Brook Foundation, and the Rockefeller Foundation.
Special Report 14 Approved for Distribution
Reference 78-2
J. R. Schubel, Director
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PREFACE
Origin and Purposes of the Workshop on
Transport Processes in Estuaries
In the spring of 1976 a symposium At the conclusion of the symposium
to review our knowledge of physical the following statement, drafted by
transport processes in estuaries was j. R. Schubel, was endorsed by the
held at the Belle Baruch Institute. participants:
"On 20-22 May 1976 a group of estuarine oceanographers
from the United States, Canada, England, and South America
met at the Belle Baruch Institute for Marine Biology and
Coastal Research of the University of South Carolina to
review and critically assess our knowledge of estuarine
transport processes. It was the very strong consensus of
the group that recent data show many of our previous ideas
of estuarine transport processes to be overly simplistic
and that a greater level of sophistication of our under-
standing of these processes is required not only for a
significant scientific advancement, but also for effective
environmental protection and management.
"A knowledge of the physical oceanography is
fundamental to understanding the biological, chemical, and
geological processes that characterize an estuary. This
information is in turn necessary for the formulation of the
predictive tools needed by governmental agencies for
effective management and rehabilitation of the estuarine
environment. Reliable predictions can not be made of the
dispersion of pollutants, the resuspension and movement of
dredged spoil, or the assimilative capacity of an estuarine
system without a working knowledge of its characteristic
physical processes.
"While millions of dollars are being spent each year
on monitoring of the estuarine environment, the resulting
data are generally of little use to oceanographers
interested in processes, or in formulating, constructing,
and veryifying analytical, numerical, or physical models.
The data are also, unfortunately, frequently of little value
to regulatory agencies in attaining their long-term pervasive
goal—effective management of the coastal environment.
Through proper coordination and planning, experimental
programs can be designed that not only satisfy the short-term
needs of regulatory agencies, but also provide the oceanographers
and managers with the data they require for development of
predictive tools.
"A proposal will be submitted to appropriate Federal
agencies within a few weeks for support of a workshop to
identify the important problems of physical transport
processes in estuaries, and to explore the most effective
ways of attacking these problems. Efficient utilization
of existing manpower and facilities for an adequate field
study of the dynamics of any single estuary will probably
require collaborative efforts of scientists from several
academic institutions and from governmental and management
agencies."
Pursuant to the foregoing statement, 10 November to 14 November 1976. Thirty-
a Workshop on Transport Processes in one participants from some 18 institutions
Estuaries was held at the Marine Sciences and agencies focused their discussions on
Research Center, State University of transports of water, salt, and fine-
New York, Stony Brook, New York from grained suspended sediments.
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The primary goal of the Workshop was
to identify the important unresolved
problems of physical transport processes
in estuaries; problems that must be
solved, not only for their scientific
urgency, but also for effective manage-
ment and rehabilitation of estuaries.
The secondary goals of the Workshop
were:
(a) To assess the manpower and
material necessary for the field
experiments on which the solutions of
important unresolved problems must
depend.
(b) To explore the means for
interinstitutional cooperation and
collaboration which will be required if
the necessary large-scale, extended field
experiments anticipated are to be made
feasible.
(c) To explore ways in which current
monitoring programs which are relatively
expensive, can be made more useful both to
management and to science.
The present report was written with
due consideration for the discussions
which occurred during the Workshop and for
the written suggestions submitted by the
participants, but it should not be inter-
preted as a report which has been endorsed
in full by all participants. In this
report we have focused on the transporta-
tion and accumulation of fine-grained
sediments. An earlier report concentra-
ted on the transports of water and salt.
Kinsman, B., J.R. Schubel, M.J. Bowman,
H.H. Carter, A. Okubo, D.W. Pritchard,
and R.E. Wilson, 1977. Transport
Processes in Estuaries: Recommendations
for Research. Marine Sciences Research
Center, State University of Hew York,
Special Report 6, 21 pp.
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TABLE OF CONTENTS
Page
Preface: Origin and Purposes of the Workshop on Transport Processes in Estuaries i
Introduction 1
Sedimentary Deposits—Or What The Record Can Tell Us 3
Sources of Sediment 5
Pluvial Inputs 5
Shofe Erosion g
Internal Sources--Primary and Secondary Productivity g
Input from the Sea 7
Routes and Transport 7
Sediment Flux 7
Routes of Sediment Transport and Rates of Aoaumulation 7
Stability of the Bottom 7
Instrumentation 9
Conclusion 10
Appendix A: Organisations Supporting the Workshop on Transport Processes
in Estuaries 12
Appendix B: Participants in the Workshop on Transport Processes in Estuaries ... 12
Additional Contributors 13
iii
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INTRODUCTION
Particles are added to the estuarine
environment by rivers, by the atmosphere,
by shore erosion, by biological activity
within estuaries, by the sea, and by
municipal and industrial discharges. The
sources are thus external, internal, and
marginal. The particles are both organic,
living and dead; and inorganic, naturally-
occurring and anthropogenic.
Man has clearly modified the natural
flow of particles into the estuarine
environment by deforestation, agriculture,
and urbanization of drainage basins, by
construction of dams and reservoirs on
tributary rivers, by diversion of rivers,
and by engineering projects to control
shore erosion. His activities have also
introduced significant quantities of
anthropogenic particulate matter.
Nutrient levels have increased as a result
of sewage effluent and runoff from agri-
cultural areas and, as a result, the
local production of organic matter has
been accelerated.
Because particulate matter can affect
the estuarine environment in a variety of
ways, the ability to predict its behavior
is of the greatest scientific and
practical importance. This is particular-
ly true of the fine-grained fraction,
silt and clay, which pose the greatest
problems: economic, aesthetic, and
environmental.
In most estuaries the sediment, that
fills navigation channels must be removed
by dredging at a cost of millions of
dollars each year, is mud—silt and clay.
Fine particles are important in
determining water "quality" of the
coastal environment; they serve as sites
for adsorption of a variety of contami-
nants including: radioisotopes, petroleum
products, halogenated hydrocarbons,
pesticides, metals, and oils and greases.
Since these constituents are rapidly
scavenged from the water column, their
transportation and deposition are
controlled in large part by the fine-
grained sediment dispersal systems.
Filter-feeding organisms which ingest
these particles and associated contami-
nants agglomerate the smaller particles
into larger composite particles in their
feces and pseudofeces thereby accelerating
the accumulation of fine particles within
the estuarine environment and providing
the contaminants in a more concentrated
form to deposit-feeding animals. Fine
particles can also serve as sources and
sinks for nutrients.
Fine particles suspended in the water
column decrease the transparency of the
water and therefore the depth of the
euphotic zone which is important, not only
in determining the distribution and
primary productivity of phytoplankton, but
also of rooted aquatic plants. The
distribution and character of suspended
and particularly bottom sediments are
important factors in governing the
distributions of benthic animals,
including commercially important species
of shellfish, such as oysters and clams.
Fine-grained suspended sediment can
also decrease the amount of dissolved
oxygen in estuarine waters both directly
and indirectly. Resuspension of fine-
grained, organic-rich sediments with a
high oxygen demand may produce a sag in
the oxygen distribution. This decrease in
depth of the euphotic zone which
accompanies increases in suspended
sediment levels causes decreased
production of oxygen by phytoplankton and
by rooted aquatic plants. Areas of the
bottom formerly within the euphotic zone
can be removed from it as a result of
man's activities.
Increase in the level of suspended
particulate matter above some threshold
level is aesthetically displeasing and
inhibits recreational use. This level is
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a function not only of the total
concentration, but also of the size
distribution and the composition of the
suspended material. A concentration of
100 mg/fc of fine quartz and sand does not
have the same effect on water color and
transparency as does the same concentra-
tion of organic-rich silt and
clay.
In summary, fine particulate matter
is one of the major factors in determi-
ning the "quality" of most coastal
environments. Regardless of the
meterstick (yardstick) one selects to
measure environmental quality, influxes
of fine-grained particulate matter into
the estuarine environment have a
deleterious effect on many uses of the
coastal environment and a salutary effect
of few, if any. This is true whether the
particles are suspended in the water
column, or are deposited on the bottom.
We recognize two quite different
approaches to the study of estuarine
sedimentation. One deals with specific
processes such as: the physical mechanisms
that control the deposition and erosion
of mud, the formation of composite
particles (agglomerates) by biological
and physical-chemical processes, and the
reworking of sediments by benthic
organisms and the consequent changes in
the physical properties of the sediment.
The other approach deals with the
characterization of the estuary as a
sedimentary system.
Many studies of the first kind have
been completed successfully; others such
as the effects of organisms on the
physical characteristics of sediments are
only now being addressed. The prospects
for resolving questions at this level are
good, if scientists find them exciting
and if reasonable support is provided
through conventional funding mechanisms.
But, studies of the second kind—studies
of estuarine sedimentary systems—have
hardly been considered. Where estuarine
sedimentary systems have been
characterized it has usually been
in response to a crisis. One of the best
examples of this is the work that led to
the explanation of the formation of the
mud deposits in the Thames and their
relationship to maintenance dredging of
2
the shipping channels to the London docks.
One can learn a great deal about the
mechanics of some sediment transport
processes through laboratory flume
experiments and isolated, short-term field
studies; by experiments of the first kind.
These studies, however, provide little
insight into the long-term manifestations
of estuarine sedimentation and the identi-
fication of the processes that control the
sedimentation in different parts of an
estuary. Attainment of this level of
understanding—the level that is necessary
for development of effective management
strategies—requires a holistic approach,
an approach that combines specific, short-
term field and laboratory experiments
with system-wide studies.
Ideally, one would like to know for
each estuaryj the sources of sediment—
their locations and strengths—the
character of the sediment introduced—its
size distribution, composition, and
associated contaminants—the routes and
rates of sediment transport—including the
transient repositories—the sites of final
accumulation within the estuary and the
amount lost to the ocean. There are also
many important biological and geochemical
questions, but these are not the subject
of this report. The problem is already
too large. Our task is to identify a
strategy, or strategies, that have a
reasonable chance of success in developing
understanding of estuarine sedimentary
systems.
Sedimentation processes in estuaries
are extremely variable in time and space.
They not only undergo tidal and seasonal
cycles but are occasionally disturbed by
major storms or floods which may dominate
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the sedimentation of the system, or at
least segments of it. Because of these
vagaries, it is frequently more effective
to begin an investigation of estuarine
sediment systems by examining the end
products of these processes rather than
the processes themselves. Indeed, where
infrequent events dominate, there is no
alternative.
Inglis, C. C. and F. H. Allen, 1957.
The regimen of the Thames estuary as
affected by currents, salinities and
river flow. Proc. Inst. Civil Eng. 7:
827-863.
SEDIMENTARY DEPOSITS—
OR WHAT THE RECORD CAN TELL US
If one thinks of an estuary as a
machine for handling sediment, then the
products of this machine are the permanent,
sedimentary deposits that line the estuary
floor. The sedimentary deposits are the
integrated results of many and variable
processes—river flow, wave erosion and
biological production, for example. To
help design an efficient and effective
study of the physical process of fine-
grained sediment transport in the estuary,
it is useful to know first where the
estuarine mud deposits are found, the type
and amount of material in these deposits,
and the rates at which they are
accumulating.
On nautical charts the designation
of "soft" bottom is a fair indication of
where muddy sediments can be found, but,
for many estuaries textural maps of the
surficial sediments have also been com-
piled. These maps not only show the
location of fine-grained sediment
deposits—the materials that are trapped
within the system—but also help identify
the types of materials that pass through
the particular estuarine system.
Development of surveying techniques that
measure the acoustic reflectivity of the
sea floor to identify sediment types would
make rapid and more extensive
investigations possible. Only in a few
estuaries, however, has the thickness of
these mud deposits been measured or have
changes in composition of the sediment
with depth been chronicled over the life
of the estuary. As a result, there is
usually no indication of how representa-
tive the present conditions are of the
long-term behavior of the estuarine
system; nor is there a basis for deciding
which of the present features of the
surficial sediment distribution are
permanent characteristics of the
sedimentary system—characteristics that
will be incorporated into the Record—and
which may be transient features.
Once the areas of estuarine mud are
located, the thickness of the deposits
may be meausred either with a network of
cores or by high resolution seismic
reflection. Ideally, contour maps of the
thickness and composition of the
estuarine mud would be produced using a
combination of these methods. Acoustic
surveys should be used to choose those
locations where cores and bottom samples
could be taken to most effectively map the
sediment deposits and chronicle their
depositional histories. Changes in the
composition of the deposits with depth
show the long-term trends in sedimentation
and the relative importance of fluvial,
littoral, and biological sources of
material over the life of the estuary.
While the approach we have outlined
to this point is conceptually straight-
forward, effecting it is anything but. To
apply these methods, the estuarine muds
must be readily distinguishable from
relict deposits. Due to Pleistocene sea-
level changes, the present estuarine
environment was probably preceded by some
combination of fluvial, sub-aerial,
glacial, and lacustrine environments.
Relict sediment deposits which are
characteristic of earlier environments
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must be excluded from determination of
the volume of estuarine sediments.
Unfortunately, in many estuaries the
contact between estuarine sediments and
the underlying deposits is ill-defined
and therefore not readily identifiable.
To make matters worse, fine-grained
estuarine deposits are frequently so
highly charged with gas bubbles
(primarily methane) that they are
virtually opaque to the high frequency
sound energy characteristic of high
resolution seismic profiling systems.
Sedimentary processes in coastal
waters not only undergo tidal and
seasonal cycles but are also episodically
disturbed by major storms. Because of
these changes, direct observations of
the rate of sediment accumulation may
require observational periods of tens or
hundreds of years to obtain a meaningful
long-term average sedimentation rate. It
is useful, therefore, to estimate the
sedimentation rate by measuring the
amount of material that has accumulated
on a geological time scale and the time
required for this accumulation. Such a
procedure can only be applied if a
starting time for the process is
well-defined. If the thickness of
estuarine sediment has been measured,
one way to get the average sedimentation
rate is to date the onset of estuarine
conditions in the basin from the local
rate of sea-level rise. A map of this
long-term average sedimentation rate
would identify areas of maximum and
minimum accumulation and the regions
where the sedimentation rate is likely
to be uniform.
Useful information can also be
obtained on the average rates of
accumulation of sediment over
approximately the past century by a
careful comparison of old and recent
original bathymetric survey sheets.
The surveys are usually at scales of
1:10,000 or 1:20,000 and have a high
density of soundings along the original
survey lines. Index maps summarize the
surveys available for each region and
bromide prints of original survey sheets
are available from the National Ocean
Survey.3 In using this technique it is
very important to make appropriate datum
shifts for surveys conducted before 1927.
The chronology of the sediment
deposit should also be examined by
application of geochemical techniques to
selected cores. The accumulation rate
would be expected to vary not only from
place-to-place, but also in time, and
radiometric dating has the advantage of
being able to detect accumulation rates on
various time scales. For example, lead-210
has been used to investigate modern rates
of accumulation over a period of the last
150 years while carbon-14 dating can span
a range of about 40,000 years. Other
indicators of the rate of accumulation
include pollen and plankton assemblages
and the depth of occurrences of culturally
introduced tracers. When interpreting the
distribution of any of these indicators in
a core, care must be taken to distinguish
among those features that are due to the
sequential accumulation of material, those
features produced by vertical mixing of
sediment by burrowing animals, and those
that may be the result of slumping or other
local mass movement of sediment. Although
this distinction cannot always be made,
examination of sedimentary structures in
cores visually or by x-radiography is
sometimes useful in sorting out these
disturbances. Geochemical indicators,
such as Cesium-137, may also be useful in
assessing the intensity of bioturbation of
the sedimentary record.
Fine-grained sediment can be
incorporated into the "permanent" sedi-
mentary record by a variety of mechanisms.
Particle-by-particle settling may proceed
from the dilute suspension of sediment in
the water column. These particles can be
either individual mineral grains or
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mineral-organic agglomerates, and
deposition can occur quasi-continuously
or at periods of slack water. Deposition
from dilute suspensions has probably
received the most attention both in the
field and in the laboratory, but it may
be of minor importance in some systems.
In some estuaries, the rate of sediment
supply is so large that the flow cannot
maintain it all in suspension. If the
level of benthic biological activity is
low, a fluid mud layer may be formed on
the estuary floor. The rate of
accumulation is then controlled by the
rate of de-watering and consolidation at
the base of the fluid mud layer. If the
rate of biological processing is large,
however, the estuary floor may become
blanketed by a layer of fecal pellets
that is easily resuspended. The
transition of this material into the
permanent mud deposit occurs at the bottom
of the layer presumably by a combination
of physical and chemical disagglomeration.
For the two situations just mentioned,
the mechanism of accumulation depends on
a balance among the rates of sediment
supply, resuspension, and biological
processing. The transport and settling
of sediment in the water column may have
little direct bearing on the depositional
processes.
In summary, the place to start
studying an estuarine sediment system is
with the record. Attention may then be
more effectively focused on the most
important areas and on those processes
that are rate-limiting.
National Ocean Survey, Rockville,
Maryland 20852
SOURCES OF SEDIMENT
The principal sources of sediment to
most estuaries are: rivers, shore erosion,
primary productivity, and the ocean.
Fluvial Inputs
One of the most important sources of
sediment to estuaries are the river
(fluvial) inputs. The fluvial sources are
the best understood of all sediment
inputs, but even these are poorly known for
most estuaries. The locations of these
"sources"—the river mouths—are well
known, but the strengths of most are poorly
documented. The strength of the source
is determined primarily by the riverflow
and the sediment yields throughout the
drainage basin which can vary by orders of
magnitude depending on lithology, vegeta-
tive cover, and man's activities.
Few rivers are sampled at locations
close enough to their mouths and at
frequencies adequate to permit reliable
estimates of sediment inputs to their
estuaries. For less than half the
estuaries of the United States do we have
any kind of regular measurement of the
input of river sediment. The U. S.
Geological Survey should examine the
locations of their gauging stations on the
lower reaches of major rivers and streams
and, where necessary, add new stations or
adjust the positions of existing ones
close to the landward limit of tidal
action. Coverage should be adequate to
ensure reliable estimates of the fluvial
sediment discharges into estuaries, and of
the character of the material—its size
distribution; mineralogic and chemical
composition; organic matter content; and
associated contaminants of concern.
Measurements should be made of both
suspended and bedload.
These stations should be permanently
maintained so that relative importance of
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occasional floods may be assessed. Only
a few rivers have been monitored long
enough to have documented the
sedimentological importance of extreme
events which may dominate the
sedimentation in estuaries, or at least
segments of them. During the hurricane
flood of 1955 the Delaware River carried
more sediment past Trenton in two days
than it had in all five years combined
in the mid-1960's drought. On three days
in December 1964 the Eel River in
northern California transported more
sediment than in the preceding eight
years; and during the week following
Tropical Storm Agnes in June 1974 the
Susquehanna discharged 20-25 times as
much sediment into the upper Chesapeake
Bay as it had during the entire previous
year. Two events—Agnes in 1972 and the
Great Flood of 1936—account for more
than 50% of all sediment deposited in the
upper Chesapeake Bay estuary since 1900.
losses (and gains) determined in this way
can be combined with measurements of
coastal relief and stratigraphy to esti-
mate the added volumes and masses of
sediment of different texture. Copies of
original survey sheets, rather than
published maps and charts, should be used
to document changes in the shoreline
because of their larger scale. Great
care must be taken to ensure that appro-
priate datum shifts are made in charts
prepared before 1927. This method of
estimating shoreline recession rates and
associated inputs of sediment to the
estuary is relatively inexpensive and a
more reliable indicator of long-term
average conditions than short-term direct
measurements of shoreline recession.
Short-term direct observations can,
however, provide useful information on
relative importance of different weather
and sea conditions in determining erosion
rates.
Shore Erosion
In many estuaries, or at least
segments of them, shore erosion is a
major source of sediment. In the middle
and lower reaches of the Chesapeake Bay,
for example, it has been estimated that
shore erosion is the primary source of
sediment. Estimates of the inputs of
sediment from shore erosion have been
made for but a small number of estuaries,
and the factors that control shore
erosion are poorly evaluated.
Nevertheless, these data are prerequisites
to the understanding and modelling of
estuarine sedimentary systems. The
management value of this information is
great.
The most reliable way of estimating
the long-term average (over decades)
input of sediment to an estuary from
shore erosion is through a critical
comparison of shorelines on old and
recent topographic survey sheets. Areal
Internal Sources--Primary
and Secondary Productivity
In some estuaries internal sources
may account for a large fraction of the
total sediment input. In the Delaware,
for example, diatom frustules have been
estimated to account for up to 50% of the
total amount of material dredged from the
main shipping channels. In most
estuaries living and dead plankton can
account for a large fraction of the total
suspended matter throughout the year.
More effective and diagnostic tools
and techniques are required for
estimating the abundances and kinds of
organic matter present in the water
column and accumulating on the bottoms of
estuaries. Until such methods are
developed, loss of total mass of
particulate matter on combustion can be
used to estimate organic content of
suspended and deposited sediments.
Estimates of suspended particulate
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organic matter should be available on a
seasonal basis; for bottom sediments
documentation of seasonal variability is
not required except in areas dominated by
rooted aquatics.
Input from the Sea
The sea may be an important source of
sediment to the lower reaches of many
estuaries. Coarse-grained material is
transported into estuaries from the
adjunct continental shelf as bedload;
fine-grained material as suspended load.
While information is available on the
routes and rates of transport of marine
sands into some estuaries, there are few,
if any, reliable estimates of the flux of
fine-grained suspended sediment through
the mouths of estuaries. Even the
direction is rarely known.
ROUTES AND RATES OF TRANSPORT
Sediment Flux
There are few direct measurements of
the fluxes of suspended sediment in
streams and estuaries. Fluxes of suspend-
ed sediment in estuaries have been
obtained by calculation from measured
current velocities and concentrations of
suspended sediment determined either
gravimetrically or from optical measure-
ments. Calculated fluxes from
measurements made over short periods--a
few tidal cycles to a few days—may be
very poor indicators of long-term
conditions. While current meters are
capable of providing measurements over
extended periods of time, recording in
situ optical devices are crude. The
difficulty and expense associated with
making concomitant direct measurements of
the concentrations of suspended sediment
preclude extensive time series data.
Until better suspended sediment sensors
are developed, we must rely on other
methods to determine average routes and
rates of suspended sediment transport in
estuaries.
Routes of Sediment Transport
and Rates of Accumulation
There are a variety of natural and
artificial tracers that can be used to
infer routes of sediment transport and
rates of sediment accumulation. These
include distinctive naturally-occurring
minerals with known source areas such as
coal in upper Chesapeake Bay derived from
the Susquehanna and glauconite in the
lower bay of New York Harbor derived from
New Jersey. Talc and other distinctive
minerals can be intentionally introduced
into aquatic environments to trace the
routes and rates of fine-grained sediment
dispersal. Radionuclides from natural and
anthropogenic sources are also potentially
powerful tools that have not been fully
exploited by coastal marine scientists.
Radionuclides from nuclear power plants
can provide very useful tracers in many
coastal environments. Pollen is another
tracer that has rarely been exploited in
unraveling the sedimentary history of
estuaries. Natural and anthropogenic
tracers have greater diagnostic value in
assessing routes of fine sediment
dispersal and rates of sediment
accumulation than do calculated fluxes
based on short-term measurement programs.
Stability of the Bottom
Sediment that enters an estuary will
either be exported or deposited within
the estuary eventually. Before it
reaches either of these permanent
repositories, it will pass into the water
column many times as a result of repeated
resuspension. In most, if not all
estuaries, there is a supply of muddy
sediment that is available for
resuspension, sediment that is on the
bottom part of the time and in the water
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column part of the time. The greater is
the amount of this material, the less
stable is the bottom. Bottom instability
and resuspension are important in two
ways to the overall properties of an
estuary. First, resuspended sediment can
react more freely with the ambient water
and may have a controlling influence on
some important geochemical processes.
Second, an unstable bottom is a poor
benthic habitat—bottom stability is one
of the principal factors controlling the
makeup of the animal communities that
reside on and in the bottom.
There is reason to believe that
there are marked differences in the
bottom stability of different estuaries.
In an estuary like the Severn (UK), which
has powerful tides, deep, temporary, fluid
mud layers form on neap tides. These
deposits are mobilized on the next spring
tide. The bottom is so unstable that no
benthic communities can establish
themselves: large quantities of silt-clay
material periodically enter the water
column and are available for chemical
reaction. When the level of tidal energy
in an estuary is lower than in the Severn,
benthic animal communities may be able to
establish themselves on the muddy bottom.
Both deposit and filter feeding animals
process new sediment entering the estuary,
thereby altering its physical form and
its susceptibility to erosion. The result
is a completely different sedimentary
regime, different estuarine geochemistry,
and a different food supply for bottom-
feeding fishes.
To characterize bottom stability, it
is necessary to know the power that is
available to resuspend sediment—its
intensity and the way that it varies in
time and space—and the susceptibility of
the sediment to erosion, which depends on
its mineral content, the degree and
nature of benthic processing and the
presence of cohesive materials in it.
All of these questions need to be
addressed in a characterization of the
sedimentary system of an estuary.
The power required to move sediment
through an estuary may be derived from
the tide, from wind-driven circulation, or
from waves on the water surface. The
effect of waves is probably the best
understood of these. In the shallower
water around the margins of an estuary
waves set water at the bottom in motion
and disturb the sediment. This motion
has been studied in detail by students of
beach and continental shelf processes.
Because the fetch in most estuaries is
limited, waves of long wave length are
not formed and the bottom in the deeper
water remains undisturbed by wind waves
on the surface. The depth of the wave-
affected zone can be estimated fairly
easily. In some shallow estuaries, wind-
driven circulation is strong enough to
move appreciable amounts of sediment but
these are thought to be unusual. In most
estuaries, the tide is the principal
source of power causing movement of
sediment. Tidal action is regular and
predictable, and the resultant sediment
movements should be too. The actual
situation is not that simple. In Long
Island Sound, for example, the amount of
sediment resuspended by the tide is much
greater under stormy conditions than in
calm weather even though the water is
much too deep to be affected by waves on
the surface. Wind stress on the surface
has been shown to increase the rate of
dissipation of tidal power but the
mechanism by which this happens is
unknown. Similar effects are anticipated
in other estuaries with deep water. This
issue needs to be resolved before the
stability of the muddy bottoms of
estuaries can be adequately described.
The amount of resuspension that will
occur on the muddy bottom of an estuary
depends on the properties of the sediment
as well as on the power dissipated. When
dealing with sandy sediment, knowledge of
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the grain size usually suffices to
characterize the erosion resistance of
the bottom. This is not true for
muddy sediment. Silt and clay particles
are naturally cohesive because of
electrostatic forces and organic matter
secreted by bacteria may add significant-
ly to the cohesion. Silt and clay
particles are passed through deposit and
filter feeding animals, emerging in
altered form, usually as larger agglomer-
ates. The resultant fecal pellets
have a much greater settling speed
than the mineral grains of which they are
made. Processing of mineral material by
benthic animals can alter the sedimentary
characteristics of an estuary markedly.
Chemical processes within the sediment
will also be altered by the burrowing
activity of animals.
The individual processes described
above can be studied in detail in either
the field or the laboratory. Some are
being so studied now; others receive
occasional attention. What has received
almost no attention is the way in which
these processes interact to produce a
sedimentary regime within an estuary.
For example, benthic animals have an
important effect on the erosion
resistance of the bottom, but what
animals can inhabit any given area of the
bottom depends on its stability, which
depends on its erosion resistance and the
physical environment. There is a web of
interactions between these and between
the effects of animal populations and
bottom properties, between bottom
properties and sediment supply, and
between the power dissipation spectrum
and the nature of the sediments on the
bottom and in the water column. These
inter-relationships will only be
resolved by field studies on estuaries
displaying a range of physical and
biological characteristics. Very detailed
studies may not be necessary or
appropriate until those processes that are
rate-limiting are identified in different
types of estuaries. The types of
measurements that are required include
(1) The physical form of the
sediment subject to resuspension. This
can be determined by examination of
undisturbed samples of suspended material
collected near the bottom by underwater
photography.
(2) The amount of material available
for resuspension and its relation to the
sediment supply. This can be determined
from the sediment inputs and the measured
volume of the unconsolidated material.
(3) Time series of the total amounts
of resuspended sediment in the water
column under a wide range of seasonal and
weather conditions. These can be
estimated by regular programs of water
sampling and data from continuously
recording instruments.
INSTRUMENTATION
The study of estuarine sedimentary
systems described in this report contains
elements of geology, geochemistry,
biology and physical oceanography.
Although a discussion of the relevant
technical problems in each of these
disciplines is beyond the scope of this
report, three general classes of
observations may be identified in which
technical capabilities need to be
developed further.
There is growing recognition of the
importance of infrequent storms and floods
in the sediment budget of the coastal zone.
To document the effects of episodes, self-
contained sensing systems are needed which
are capable of measuring concentrations of
suspended solids during these disturbances.
Unmanned mechanical samplers, or self-
recording acoustic or optical sensors could
be designed to fill this need.
Many characteristic properties of
sediments, whether suspended or deposited,
are patchy. Point sampling in the water
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column or on the sea floor may miss
important features of the distribution
of particulate material. There is, thus,
the need to design instruments or methods
capable of taking "snapshots" of the
distribution of sediment along extended
transects or over large areas. For
example, continuous measurements of
acoustic reflectivity using signals in
the 100 kHz range may be useful in
distinguishing variations in texture,
composition, and water content of bottom
sediments and in selecting sampling
stations for maximum information. Part
of the lateral inhoraogeneity is due to
irregular, contagious distributions of
benthic animals. Pulse-echo sounding
with signals in the mega-hertz frequencies
may be capable of resolving macrofauna and
provide a method for assessing the
distribution of organisms. Expanded use
of remote sensors, aerial and submarine
photography should also be considered.
There is a gap in our resolution of
the temporal variability of sedimentary
processes. Geological and geochemical
methods are used to estimate mean
sedimentation rates over periods of
decades to thousands of years. The
technology is also available to make
direct measurements of sediment motions
on time scales of from a few seconds to,
perhaps, several weeks. The time
variability of sediment distributions
for intermediate periods, months to
decades, is poorly documented.
Instruments designed »for this purpose
would require some method of averaging
measurements in order to record
unattended for long periods.
CONCLUSION
Schemes for classifying the
hydraulic regimes of estuaries have been
developed and widely used. There is,
however, no comprehensive method for
comparing different estuarine sedimentary
regimes. Such a classification could be
based on the hypothesis that there are a
small number of parameters that character-
ize any estuarine sedimentary system.
Although these parameters have not yet
been identified, one possible quantitative
classification would include:
1. The rate of sediment supply.
2. The amounts of sediment in the
water column, in temporary
storage on the sea floor, and in
permanent deposits.
3. The rate at which transport
proceeds; this rate is expected
to be proportional to the power
available to- move sediment
particles.
4. The total amount of time that a
particle is in the water column.
5. The final partitioning of
material between the ocean and
the permanent estuarine deposits.
A classification based on these, or
other similar parameters, would be useful
not only for comparing different estuarine
sedimentary systems, but also for
recognizing the range of different
conditions controlling a particular
estuary's sedimentation. Emery and
1^
Uchupi are correct in pointing out that
far more effort has gone into making
detailed studies of the sediments of
individual estuaries than either into
comparing results from a variety of
estuaries with similar physical and
geological characteristics, or into
critical evaluations of processes.
The kinds of studies we are
recommending are designed to produce, not
only a better understanding of estuarine
transport mechanisms, but more
specifically to produce, a significant
improvement of our knowledge of
those sedimentary processes that
characterize estuarine systems. This
level of understanding is required for
effective management of estuaries; many
of the most serious and persistent
10
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environmental problems in estuaries are
associated directly, or indirectly, with
fine-grained particulate matter.
Development of an estuarine sediment
system will require sustained research
programs over periods of years, perhaps
decades. Scientists interested in
understanding systems can not, and should
not, rely entirely on agencies, such as
the National Science Foundation for the
uninterrupted support required for
effective research programs. To a large
extent, some of the more important
research components of an estuarine
sediment system may be of local, or
regional importance and should be
supported by appropriate State and
Federal agencies.
Regional funding mechanisms should
be established to ensure sustained
support for research and monitoring of
estuarine systems. This
should include contributions from the
state(s) bordering the estuary as
well as support from appropriate Federal
agencies. An appropriate administrative
structure would have to be developed to
ensure that dollars were spent
effectively. Inter-institutional
cooperation would be required, at
least for large systems.
Events, natural and man-induced, can
have dramatic and persistent impacts on
the coastal environment and its biota.
These include floods, hurricanes, extreme
climatic deviations, and large accidental
discharge of pollutants. Because of
their episodic character, events are
inherently unpredictable and at best are
difficult to study. Rapid response is
required for effective investigation, but
present funding mechanisms are not geared
to provide support or an appropriate time
scale through the normal research
proposal and review procedure.
Contingency funds should be established
to provide immediate, short-term support
to initiate studies of events. These
funds should be Binder the direction of
top administrators who have authority to
respond rapidly to research opportunities.
They should be able to commit funds to
initiate studies by phone and then send
a representative to the scene to evaluate
the scientific merit of the investigation
and determine the resources that should
be allocated to the study. Until this is
done, we will continue to fail to take
advantage of some of the more important
experiments that are occurring in our
coastal waters—those associated with
catastrophes.
Emery, K. 0. and E. Uchupi, 1972.
Western Atlantic Ocean: Topography,
Rocks, Structure, Water, Life, and
Sediments. Amer. Assoc. of Petroleum
Geologists, Memoir 17, 532 p.,
Tulsa, Oklahoma.
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APPENDIX A
Organizations Supporting the Workshop on
Transport Processes in Estuaries
United States Environmental Protection Agency
United States Energy Research and Development Administration
Office of Naval Research: Geography Branch
National Oceanic and Atmospheric Administration: MESA,
New York Bight Project
United States Fish and Wildlife Service: Office of
Biological Services
Rockefeller Foundation
Stony Brook Foundation
APPENDIX B
Participants in the Workshop on
Transport Processes in Estuaries
K. Allen
R. Baltzer
Malcolm Bowman
Burt Brunn
Harry H. Carter
S. Chanesman
Dennis M. Conlon
T. John Conomos
Bruno d'Anglejan
Keith R. Dyer
Alan Elliott
John Festa
Robert Gordon
R. G. Ingram
Blair Kinsman
Bjorn Kjerfve
Ray B. Krone
G. Mayer
Tavit Najarian
Maynard M. Nichols
Charles B. Officer
Akira Okubo
David H. Peterson
Donald W. Pritchard
U. S. Fish & Wildlife
U. S. Geological Survey
Marine Sciences Research
Center
U. S. Fish & Wildlife
Chesapeake Bay Institute
NOAA
Office of Naval Research
U. S. Geological Survey
McGill University
Institute of Oceanographic
Sciences
NATO SACLANT ASM Research
NOAA
Yale University
McGill University
Marine Sciences Research
Center
University of South Carolina
University of California
Davis
NOAA
Chesapeake Bay Institute
Virginia Institute of Marine
Science
Marine Environmental Services
Marine Sciences Research Center
U. S. Geological Survey
Centro de Investigacion Cientifica
y de Educacion Superior
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APPENDIX B
(continued)
Maurice Rattray University of Washington
William S. Reeburgh University of Alaska
J. R. Schubel Marine Sciences Research
Center
D. P. Wang Chesapeake Bay Institute
Robert Weisberg North Carolina State
University
Robert E. Wilson Marine Sciences Research
Center
K. K. Wu Environmental Protection Agency
Additional Contributors
.V:.ir
David J. Hirschberg Marine Sciences Research Center
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