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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