United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-85/024 May 1985
Project Summary
Combined Sewer Overflow
Sediment Transport Model:
Documentation and Evaluation
Thomas N. Keefer and Eric S. Clyde
A modeling package was developed
to study the movement and fate of
combined sewer overflow (CSO) sedi-
ment in receiving waters. The
package contains a linear, implicit,
finite-difference flow model and an
explicit, finite-difference sediment
transport model. The sediment model
is coupled to the flow model by
means of a file containing velocity,
depth, and discharge at each model
cross-section at each time step. The
operation and utility of the model
package were tested using data from
a 20-km reach of the Scioto River
below the Whittier Street outfall in
Columbus, Ohio. A preliminary field
investigation of the study reach in
July 1980 collected sufficient data to
calibrate the flow model partially.
Data from a CSO event in September
1981 were used to further calibrate the
flow model and evaluate the sediment
transport model operation. The flow
model reproduced stages and
discharges with sufficient accuracy
for linkage with the sediment model.
The sediment model produced
smoothed estimates of sediment con-
centrations that fell within the scatter
of observed data in most instances.
CSO sediment sizes and the armored
nature of the Scioto River channel
were such that all solids discharged
from the CSO were convected
through the reach with no deposition,
even at low flow. Experiments with
the sediment model indicate that it
can be used for qualitative
assessments of the fate of various
sediment size fractions if properly
calibrated.
This Project Summary was
developed by EPA's Water Engineer-
ing Research Laboratory, Cincinnati,
OH, 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
A major portion of the pollutant load
from combined sewer overflow (CSO)
events consists of suspended particulate
matter. The water quality impacts of
these materials depend on their size
distribution and on the hydraulic and sedi-
ment transport properties of the stream
channel into which they are discharged.
Mathematical models of transient stream
flow and sediment transport are useful in
predicting the fate of CSO materials under
a wide variety of hydrological events and
CSO control options.
This report documents the development
and application of a computerized flow
routing and sediment transport model
package especially adapted to consider
CSO releases. The computer code is a
refinement of an earlier experimental ver-
sion. The report contains a description of
the theory behind the models and pro-
vides detailed instructions on using the
associated computer program. A com-
plete listing of the FORTRAN source code
is included. The report also contains a
description of attempts to apply the
model package to field data collected dur-
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ing CSO events on the Scioto River
downstream of Columbus, Ohio.
Model Theory
The time-dependent flow model
discussed in the report is based on the
conservation of mass and momentum
equations for a one-dimensional channel,
where u is the average flow velocity, A is
the cross-sectional area, x is the
longitudinal distance, t is time, q is lateral
inflow, g is the acceleration of gravity, y
is the depth of flow, Z is the elevation of
the channel bed, and S< is the friction
slope.
The solution technique for these equa-
tions employs a linear, implicit, four-point,
finite difference scheme. The following
features are built into the computer pro-
gram that implements the solution:
Up to 40 cross-sections and 20
tributaries can be handled.
Flow resistance at each cross-section
can be specified as a polynomial
function of depth.
An arbitrary spacing of cross-sections
can be used.
Initial conditions are automatically
computed based on a prescribed set
of upstream, lateral, and tributary
flows at time zero.
A variety of upstream and
downstream boundary conditions are
available.
The program output provides
estimates of velocity, depth,
discharge, and water surface eleva-
tion at each cross-section and each
time step of the simulated flow
period.
The program also stores velocity, depth,
and discharge results on a disk file that is
later accessed by the sediment transport
portion of the package.
The sediment transport model is based
on the conservation of mass equation,
dG.
3x
dPz_=
at
where Gs is the total sediment transport
rate by volume, c is the sediment concen-
tration by volume (equal to G8/(uA)l, z is
the net depth of loose soil, P is the wet-
ted perimeter of the cross-section, gs is
the lateral sediment inflow, and all other
terms are as defined earlier.
An equation of this form is written for
each size fraction of sediment particles.
The set of equations is solved with an ex-
plicit finite difference scheme. At each
time step, the sediment transport capacity
of each channel section is computed. The
bed load portion of this capacity is based
on the Meyer-Peter Muller equation, and
the suspended load portion is computed
using a modified Einstein procedure. The
resulting transport capacity is substituted
into the conservation of mass equation,
and the equation is solved for the poten-
tial change in loose soil. The depth of
loose soil is adjusted accordingly, and the
computations are repeated for the next
time step.
The computer program that implements
this solution accesses the data file created
by the flow model to obtain the necessary
hydraulic input. The sediment transport
computer code contains the following
features:
Up to 10 size fractions of sediment,
40 stream cross-sections, and 5
sources of sediment inflow can be
handled.
A variety of channel boundary condi-
tions and sediment inflow conditions
(including the use of rating curves)
are permitted.
A variable soil detachment coefficient
may be used.
Aggradation/degradation of the bed
is assumed not to affect cross-section
geometry.
The model is designed for non-
cohesive, biologically inert sediment
materials with constant specific gravi-
ty and sizes larger than 0.063 mm.
Program output consists of total
transport rate, bed aggrada-
tion/degradation, and sediment con-
centration by size class for each
stream section and time step of the
simulation.
Field Application
The model package was tested using
data from a 20-km reach of the Scioto
River below the Whittier Street combined
sewer outfall in Columbus, Ohio. Data
collected during a dry-weather period in
July 1980 provided a preliminary calibra-
tion of the flow model under steady-state
conditions. The resulting bed profile
elevations and cross-section locations ap-
pear in Figure 1. Two storm events were
then sampledone in November 1980,
and another in September 1981. Only the
latter event provided sufficient data to use
with the model.
The results of calibrating the flow
model to the September 1981 event are
shown in Figure 2. The model was
capable of reproducing the observed
variations in stage with a maximum error
of 1 ft and a mean error between 4 to 6
in. The ability of the sediment model to
match the observed suspended solids
concentrations was less successful, as
shown in Figure 3. The predicted concen-
trations qualitatively followed the
measured ones with errors of 20% to
50%. Part of the discrepancies can be at-
tributed to the difficulty and eratic nature
of obtaining sediment concentration
measurements.
In addition, most of the sediment from
the CSO outfall was smaller than 0.063
mm, the lower size limit of the current
state of the art in sediment transport
modeling. Because of this small particle
size and the armored nature of the chan-
nel, CSO sediments are flushed through
215 -r
Whittier St. CSO
Frank Road Bridge
1-270
Approximate Water Surface
Shadeville Br. (665)
762 Br.
Uj
195
5 10 15 20
Distance Below Greenlawn Ave. Bridge (km)
Figure 1. Bottom profile of Scioto River and model cross section locations.
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690-1-
680 -
0)
Uj
670
660
Upstream Boundary
Frank Road Bridge
Sutron Corporation under the sponsorship
of the U.S. Environmental Protection
Agency.
USGS Gage
1-270 Bridge
+ = Observed Value
- - Modeled Value
Route 665 IShadeville) Bridge
Route 765 Bridge
40
52
64
76
to
10
to
Time (hr)
Figure 2.
Scioto River stage hydrographs between 4 p.m. on September 14 and 8 a.m. on
September 16. 1981.
the Scioto River reach with no aggrada-
tion occurring, even at low flows.
Conclusions
The following conclusions were drawn
with respect to the model package and its
application:
The model package is a useful tool
for qualitative assessment of the
movement of nonporous, non-
cohesive, biologically inert sediments
in receiving waters.
Considerable knowledge of hydraulics
and hydrology may be required to set
up, run, and interpret the model.
The sediment transport in the Scioto
River is similar to that in a rigid boun-
dary channel.
All the sediment material from the
Whittier Street outfall is fine enough
to be transported by the Scioto
River, even at low flow.
Sufficient correlation exists between
variations in sediment transport and
variations in other water quality
parameters measured during the CSO
event to suggest a close connection
between the two.
The full report was submitted in fulfill-
ment of Contract No. 68-03-2869 by the
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,/OOOr
Green/awn Ave. Br
64
Hours
76
7000
I
o
CO
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to 500
250
3
to
1-270 Bridge
40
52
64
Hours
76
I
7000
750
to 500
|
o>
to
250
Route 665 Bridge
52
64
76
Hours
7000
i
. 750
500
\
\
! 250
to
Route 762 Bridge
40
52
64
Hours
76
+ = observed value
- = modeled value
Figure 3. Variation of suspended solids with time.
Thomas N. Keefer and Eric S. Clyde are with Sutron Corporation. Fairfax, VA
22030.
Lewis A. Rossman is the EPA Project Officer (see below).
The complete report, entitled "Combined Sewer Overflow Sediment Transport
Model: Documentation and Evaluation," (Order No. PB 85-180 859/AS; 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:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
U. S. GOVERNMENT PRINTING OFFICE: 1985/559 111/10829
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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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