United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA-600/S3-83-045 Sept. 1983
SERA Project Summary
Modeling Fine Sediment
Transport in Estuaries
E J. Hayter and A. J. Mehta
A sediment transport model (SEDI-
MENT IMA) was developed to assist in
predicting the fate of chemical pollu-
tants sorbed to cohesive sediments in
rivers and estuaries. Laboratory ex-
periments were conducted to upgrade
an existing two-dimensional, depth-
averaged, finite element, cohesive sed-
iment transport model. The utility of
SEDIMENT MIA was demonstrated by
laboratory resuspension and deposition
tests and simulations of the sedimen-
tation processes in a hypothetical canal.
The effect of salinity in these simu-
lations also was examined.
The model should enhance capabili-
ties for predicting water quality im-
pacts and for analyzing sedimentation
management issues. The improved
transport descriptions should be use-
ful in making more reliable predictions
of the fate of dissolved and sorbed
pollutants discharged into an estuary
or harbor by stormwater runoff or in-
dustry releases, thus assisting in the
evaluation of water pollution control
options. The enhanced descriptions
should also be useful in predicting the
movement of dredged material released
in open marine waters, identifying
harbor sites in estuaries and bays
where shoaling is minimized, predict-
ing changes in sedimentation that may
occur as a result of proposed changes
or developments of an estuary or har-
bor, and estimating shoaling rates and
maintenance dredging requirements
in areas of low flow.
This Project Summary was developed
by EPA's Environmental Research Lab-
oratory, Athens, GA, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Modeling the movement of cohesive
sediments to predict the fate of pollutants
introduced into an estuary is important
because a significant fraction of many
pollutants (e.g. heavy metals, radioactive
elements, and organic substances) is typ-
ically transported sorbed to these sed-
iments ratherthan in the non-sorbed state.
Modeling the transport of cohesive sed-
iments requires a knowledge of the geom-
etry of an estuary; the flow and salinity
fields; the coagulation, settling and depo-
sitional characteristics of the sediment;
the structure (i.e. bed shear strength and
bed density profiles) of the sediment bed
at several different locations in the es-
tuary; and the erosional characteristics of
these beds when subjected to an excess
bed shear stress.
None of the existing fine sediment
transport models has the capability of
determining the effect of salinity variation
(e.g. in the mixing zone between fresh and
sea water m estuaries) on the processes of
erosion and deposition of cohesive sed-
iments in a turbulent flow field. Empirical
laws employed in existing models for
these transport processes were derived
with the use of empirical evidence from
limited laboratory experiments conducted
in natural or artificial sea water. In addition,
the empirical laws of erosion and deposi-
tion cannot be considered to be the state-
of-the-art even for sea water, because a
considerable number of laboratory exper-
imental tests have been conducted since
these laws were proposed. Based on
findings reported in the literature plus
those derived from experiments conducted
in this study, new evidence has been
revealed about the erosional and depo-
sitional behavior of cohesive sediments.
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This study had the following five pri-
mary objectives: (1) conduct a compre-
hensive review of the existing theory on
the erosion and deposition of fine, cohe-
sive sediments; (2) determine the effects
of salinity on the rates of erosion of flow-
deposited cohesive sediment beds under
turbulent flows; (3) determine the effects
of salinity on the deposition rates under
turbulent flows; (4) upgrade an existing
"state-of-the-art" two-dimensional, verti-
cally integrated mathematical model of
cohesive sediment transport by improving
upon the capability of this model to predict
the erosion and deposition of cohesive
sediments in estuaries; and (5) demon-
strate the utility of the improved transport
model in simulating the erosion and depo-
sition of cohesive sediments and the effect
of salinity on these transport processes.
Research was directed towards obtain-
ing an improved understanding and quan-
titative description of the erosion (resus-
pension) and deposition of fine sediments
under turbulent flows. This was achieved
by investigating the erosive behavior of
flow-deposited (i.e. stratified) cohesive
sediment beds and the effect of salinity on
the rates of erosion and deposition. The
results of these investigations, in the form
of empirical erosion and deposition func-
tions and an improved schematization of
flow-deposited beds, were incorporated
into the selected cohesive sediment trans-
port model. The fifth objective given above
was accomplished by using the modified
model to simulate laboratory erosion and
deposition experiments and to demon-
strate the effect of salinity on the rates of
erosion and deposition of cohesive sed-
iments in both laboratory experiments and
in a 10-km-long hypothetical canal.
Model Description
The fine sediment transport model se-
lected for modification was a time-varying,
two-dimensional finite element model that
is capable of predicting the horizontal and
temporal variations in the depth-averaged
suspended concentrations of cohesive
sediments and bed surface elevations in
an estuary, coastal waterway, or river. In
addition, it can be used to predict the
steady-state or unsteady transport of any
conservative substance (e.g. salt) or non-
conservative constituent, if the reaction
rates are known. The model describes the
advective and diffusive transport of sus-
pended or dissolved constituents, the
settling, deposition (i.e. sink) and erosion
(i.e. source) of cohesive sediments to and
from the bed, respectively, and the con-
solidation of the bed due to continuing
deposition. Figure 1 is a schematic of the
Suspension in Transport
Deposition
Redispersion
Resuspension
Resuspension
Stationary Suspension
Consolidating Bed
Settled Bed
Figure 1. Schematic of erosion-deposition phenomena handled in SEDIMENT IIIA.
erosion-deposition phenomena handled
by the model. According to this descrip-
tion, fine sediments can exist in four
different physical states in a tidal estuary—
a mobile suspension, a stationary sus-
pension, a consolidating bed, and a settled
bed.
The existing model was refined by incor-
porating a newly developed bed schema-
tization model, as well as mass erosion,
surface erosion, and deposition algorithms
and the effect of salinity on the rates of
surface erosion and deposition. The modi-
fied version, SEDIMENT IIIA, also includes
modifications to the equation used for
calculating kinematic viscosity, the use of
nodal salinity values for evaluating the
settling velocity and deposition rate, and
the equation for calculating the density of
the suspending fluid.
These refinements were based on recent
developments reported in the literature and
on the results of laboratory experiments
conducted in this study. Experiments
were carried out to determine: (1) the
resuspension characteristics of partially
consolidated, flow-deposited cohesive sed-
iment beds under turbulent flows, (2) the
effects of salinity on the rates of erosion of
these beds, and (3) the effects of salinity
on the rates of deposition and the settling
velocity of suspended cohesive sediments
under turbulent flows.
Resuspension tests were conducted in
a rotating annular flume and in'a straight
recirculatmg flume at the University of
Florida using kaolinite and a natural mud.
These tests revealed that flow-deposited
beds are stratified with respect to bed
shear strength and density and can consist
of unconsolidated stationary suspensions
partially consolidated beds and settlec
fully consolidated beds. Both the cohesiv
shear strength and the bed density ir
crease with consolidation. When subjecte
to an excess bed shear stress, stationar
suspensions erode almost instantly, whil
partially and fully consolidated beds ur
dergo surface (aggregate-by-aggregate
erosion. An empirical expression for th
rate of surface erosion of partially cor
solidated beds was derived that is ana
ogous to the rate expression which result
from a heuristic interpretation of the rat
process theory of chemical reactions. Thi
rate expression indicates that the rate c
erosion varies exponentially with the ex
cess bed shear stress. The rate of erosio
of settled beds is linearly proportional t
the excess shear stress.
A cohesive sediment bed algorithm an
an erosion algorithm were developec
Based on an interpretation of typical!
observed Eulerian time-concentration rec
ords in estuaries, erosion is considered t
occur during accelerating flows. Likewise
deposition is considered to occur durmi
decelerating flows. The erosion algorithr
simulates mass erosion of stationary sus
pensions and surface erosion of partiall
consolidated and settled beds. The bei
schematization includes these three bei
sections and divides each section int
discrete layers. The amount of sedimen
eroded from the bed or deposited onto th
bed in each element is determined, am
the thickness and the structure of the bei
in that element are adjusted accordmgl\
Consolidation of the bed due to over
burden is accounted for by first filling u|
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the top stationary suspension layers and
then the partially consolidated layers as
deposition occurs.
A refined deposition algorithm was also
developed. This algorithm integrates the
concepts proposed by various investiga-
tors and represents a unified model of this
process. Deposition is predicted to occur
when the bed shear stress is less than the
maximum depositional shear stress. The
rate of deposition depends, among other
factors, on the bed shear stress and the
settling velocity of the suspended sed-
iment. The bed shear strength was found
to vary with the salinity of the eroding fluid,
for salinities less than 2 parts per thousand.
The rates of deposition of the natural
mud were found to increase with increas-
ing salinity. A power relationship between
the settling velocity and salinity was deter-
mined from analysis of deposition tests
conducted at different salinities and under
varying bed shear stresses.
Model Performance Testing
Model simulations of erosion and depo-
sition were conducted in laboratory ex-
periments and sediment transport tests
were conducted using a hypothetical pro-
totype canal. The purpose was to show the
capability of SEDIMENT MIA in predicting
cohesive sediment transport processes by
comparing measured and predicted re-
sults and the effect of salinity on these
same processes.
Simulation of various sediment prob-
lems using the model showed that the
numerical scheme is stable for all con-
ditions. The accuracy of the solution is
affected when the Peclet number, which is
the ratio of convection to diffusion, be-
comes too large (greater than 102) or too
small (less than 10'3).
The model was used to simulate a
laboratory resuspension test and four lab-
oratory deposition tests. Reasonably good
agreement between the predicted and
measured results was obtained in each of
these cases (see Figure 2, for example).
The effect of salinity on both the resus-
pension test and one of the deposition
tests revealed that, as expected, the rates
of resuspension and deposition decreased
and increased, respectively, with increas-
ing salinity. Sedimentation processes in a
10-km hypothetical canal in which both
erosion and deposition of sediment oc-
curred also were simulated at three dif-
ferent salinities to show the effect of
salinity under typical prototype conditions.
The effect of salinity in the simulation was
less apparent than in the case of the
laboratory tests because of the simulta-
9.0
8.0
7.0
6.0
Time (Hrs)
5.0 6.0 7.0 8.0
9.0 10.0
| 5.0
u
c
o
"4.0
o
1
I 3'°
V)
2.0
1.0
0.0
• Model Simulation
Measurement
I
0.0 1.0 2.0 3.0 4.0 5.0
Time (Hrs)
Figure 2. Comparison of predicted and
measured suspension concentra-
tions versus time for a resuspen-
sion experiment.
neous occurrence of erosion, convection,
diffusion and deposition along the canal.
Limitations on Model Use and
Application
A two-dimensional, vertically integrated
model such as SEDIMENT IMA can strictly
be applied only to estuaries, harbors and
basins (such as marinas) where the hori-
zontal dimensions of the water body are at
least one order of magnitude greater than
the vertical dimension. Applications to
partially mixed water bodies or especially
to highly stratified water bodies should be
made only when rough estimates of some
sedimentation process (e.g. shoaling rate)
are required.
Currently the model has the capability of
simulating the movement of only one
constituent (e.g. cohesive sediment, water
temperature, or algae, provided that the
source/sink expressions for a nonconser-
vative constitutent are known). It is pos-
sible, however, to modify the model so
that any number of constituents may be
incorporated.
Probably the main "limitation" of a
model arises from three sources: insuf-
ficient data, poor quality of data and limita-
tions of hydrodynamic modeling. The first
two sources are attributable to the fact
that, owing mainly to time and cost con-
siderations, all the bathymetric, hydraulic
and sedimentary data required for use in
such a model are rarely if ever measured
and/or collected in the body of water
being modeled. In addition, the quality of
the data is often questionable. Data re-
quirements and the field collection and
laboratory testing programs required to
obtain these data are briefly described in
the report The third source is often the
result of the first two in addition to the
technical deficiencies in the state of science
in modeling estuarine hydrodynamics.
The sediment transport descriptions
developed here, when incorporated into
newly developed water quality models
(e.g., TOXIWASP), should enhance the
reliability of these models in making
exposure predictions.
E. J. Hayter and A. J. Mehta are with the University of Florida, Gainesville, FL
32611.
Ft. B. Ambrose, Jr. is the EPA Project Officer (see below).
The complete report, entitled "Modeling Fine Sediment Transport in Estuaries."
(Order No. PB 83-223 362; Cost: $20.50, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
College Station Road
Athens, GA 30613
•ftUS GOVERNMENT PRINTING OfFICE 1983-659-017/7182
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Agency
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