Floodway Analysis with SWMM

Floodway Analysis for SWMM Models

This document explains the procedures to follow in performing a floodway
analysis in the U.S. Environmental Protection Agency's SWMM model.

Floodway analyses in unsteady flow models, such as SWMM, can consider
the loss of floodplain storage and the loss of conveyance, while steady
flow models can consider only the loss of conveyance in computing
floodways.  However, unsteady flow programs do not have an option to
determine floodways automatically to account for the loss of floodplain
storage and conveyance.  Therefore, the encroachment stations for the
floodway must be determined first from a steady flow model, such as
HEC-2, using the equal conveyance reduction method.  The computed
encroachment stations will then be inserted into the unsteady flow model
to compute the water-surface elevations for the floodway analysis to
account for the loss of floodplain storage.

The following procedures should be followed in computing floodways for
the SWMM model.  This procedure assumes that the floodplain areas beyond
the encroachment stations will be completely filled.

Data for natural cross sections

The data for the natural cross sections are specified by C2, C3, and C4
records (SWMM format) or NC, X1, and GR records (HEC-2 format).  The
natural (irregular) cross section is coded as 8 as the type of conduit
shape on the C1 record of the SWMM model.  The HEC-2 format is
recommended when specifying data for the natural cross sections in the
SWMM model.

The NC record specifies left overbank n-value, right overbank n-value,
and channel n-value.  The X1 record specifies cross section
identification number (same as the conduit number on the C1 record),
total number of ground stations on the following GR data lines, left
bank station, right bank station, length of channel reach represented by
this cross section, factor to modify the ground stations, and constant
to be added (+ or -) to ground elevation data.  The GR record specifies
the elevation and station of the ground points.  The user should refer
to the SWMM user's manuals for the detail explanation of the variables
on these records.

Maximum computed flow (discharge) and corresponding water-surface
elevation

The maximum computed flow at the cross sections (conduits) and the
corresponding time of occurrence is obtained from the Conduit Summary
Statistics Table for the 1% annual chance flood.  The SWMM output data
also include water-surface elevations at the junctions and flows at the
conduits for user-specified time intervals.  The corresponding
water-surface elevation at the time of occurrence of maximum computed
discharge at a junction is obtained from the output data.  The exact
time of occurrence of the maximum computed flow at a conduit may not
match the user-defined time interval.  In that case the water-surface
elevation at a junction should be obtained from the user-defined time
intervals before and after the time of maximum discharge occurrence.  A
conduit in SWMM model is bounded by two junctions.

The water-surface elevation at the upstream junction and the flow at the
conduit (cross section) should be used for the floodway analysis.  The
maximum computed flow at a conduit obtained from the Conduit Summary
Statistics Table should be verified with the flow at the same conduit in
the output data for the user-specified time interval.

Creation of steady flow file

The HEC-2 cross section data from the SWMM model are then copied to
create a separate HEC-2 file.  The user should refer to the HEC-2 user's
manual for a detailed explanation of each record.  The combination of
QT, ET, and X5 records provide encroachment stations at each cross
section based on the equal conveyance reduction principle for different
target surcharge values.  The maximum computed discharges from SWMM
output data are specified on QT records for each cross section.  The
equal conveyance reduction method is specified on ET record with
different target surcharge values.  The X5 record specifies the 1%
annual chance water-surface elevation for the unencroached profile and
the floodway water-surface elevations for the encroached profiles.  The
floodway water-surface elevation is obtained by adding the target
surcharge value to the 1% annual chance water-surface elevation.  The
HEC-2 program will not compute the backwater analysis if the
water-surface elevations are specified on the X5 record.  The HEC-2
program will determine the floodway encroachment stations for the
specified floodway water-surface elevations.

The following steps should be followed to modify the HEC-2 file thus
created to compute encroachment stations based on the equal conveyance
reduction method.

Insert T1, T2, T3, J1, J2, and J3 records.
T1, T2, T3 are the title records.
Specify 2 in field 2 of the J1 record.  Specify 0 in field 5 of the J1
record.  In field 9 of the J1 record, specify the 1% annual chance
water-surface elevation at the upstream junction of the first cross
section (conduit) obtained from the SWMM output data.  The number of the
first cross section (conduit) is specified in the first field of the
first X1 record.
Specify 1 in field 1 of the J2 record.  Specify -1 in field 3 of the J2
record.
Specify 110, 115, and 200 in fields 1, 2, and 3 of the J3 record to
obtain encroachment data in summary output tables 110, 115 and 200.

Insert QT record before every X1 record.
Specify the number of profiles to be analyzed in field 1 of the QT
record.  The number of profiles is equal to the number of floodway
profiles for different target surcharge values to be considered plus one
for the unencroached profile of the 1% annual chance flood.  For
example, if 5 different target surcharge values are to be considered,
the number of profiles will be equal to 6 (1 unencroached profile plus 5
target surcharge profiles).
Specify the maximum computed flow (discharge) obtained from the SWMM
output data at the conduit for the unencroached and floodway profiles.
For this example, the maximum computed discharge will be specified on
fields 2 through 7 of the QT record.

Insert ET record before the first X1 record.
Only one ET record is required before the first X1 record for the equal
conveyance reduction method.  Encroachment Method 4 is the equal
conveyance reduction method in HEC-2.  Specify different target
surcharge values and the encroachment method starting from field 3 of
the ET record.  For example, for a target surcharge value of 1 foot, the
number 10.4 is specified in field 3 of the ET record.  If additional
target surcharge values of 0.8, 0.6, 0.4, and 0.2 are to be considered,
the numbers 8.4, 6.4, 4.4, and 2.4 are specified in fields 4, 5, 6, and
7 of the ET record, respectively.

Insert X5 record after every X1 record.
The X5 record is used in HEC-2 to specify the known water-surface
elevation.  HEC-2 will use this elevation to compute the floodway
encroachment stations, and the backwater analysis will not be performed
at this cross section.  Specify the number of profiles to be analyzed in
field 1 of the X5 record.  Specify the 1% annual chance water-surface
elevation at the junction upstream of the conduit (specified in field 1
of the X1 record) in field 2 of the X5 record.  Specify the floodway
water-surface elevations starting from field 3.  The floodway
water-surface elevation is obtained by adding the target surcharge value
on the ET record to the 1% annual chance water-surface elevation in
field 2 of the X5 record.

Insert EJ record after the last GR record.

Insert the group of T1, T2, T3, J1, and J2 records for each target
surcharge value specified on the ET record.  For example, if five
different target surcharge values are specified, five groups of T1, T2,
T3, J1, and J2 records must be inserted.

Insert ER record.

Save the newly created HEC-2 file and run the HEC-2 program.

Obtain one set of encroachment stations for a selected target surcharge
value at each cross section from Table 110 of the HEC-2 output file.

Floodway analysis

The following procedure should be followed to analyze the floodway in
the SWMM model.

Modify the SWMM file by deleting the ground stations beyond the selected
encroachment stations and inserting the selected encroachment stations
at each cross section.  The weir length for the road profiles and weirs
must be shortened based on the width between the selected encroachment
stations at the cross sections bounding the road or the weir.  The other
conduit shapes that are considered between the natural cross sections
should also be modified based on the encroachment stations at the
natural cross sections, if necessary.  Save the SWMM file for the
floodway analysis and run the SWMM (EXTRAN) program.

The maximum computed flow at the cross sections (conduits) and the
corresponding time of occurrence for the floodway analysis is obtained
from the Conduit Summary Statistics Table for
the floodway run.  The corresponding water-surface elevation at the same
time of occurrence of maximum computed discharge at the upstream
junction for each conduit (cross section) is obtained from the output
data.

Create a table for junctions and the water-surface elevations for the
unencroached run and the floodway run.  The difference in water-surface
elevations will provide the surcharge value.  If the surcharge value is
more than the allowable value at any cross section, a new set of
encroachment stations from the HEC-2 run must be used to modify the
cross section in the SWMM input file and the new floodway run must be
conducted.  This procedure should be followed until all the surcharge
values are no more than the allowable value.