Dynamic Floodways: Accounting
for Both Storage and Conveyance
José María Guzmán, P.E.
Gaston Cabanilla, P.E., CFM
May 2010, National ASFPM Conference, Oklahoma City
Outline
 Standard FEMA methods to develop a floodway
 Accounting for storage using SWMM
 Application to Jacksonville, FL (Duval County)
 DFU: A computational tool
 Discussion of floodway results
Acknowledgments:
Tom Nye, Sandeep Gulati, Seungho Song, Michael F. Schmidt,
Zhida Song-James
Floodway Definition Based on FEMA
Regulations
 FEMA’s regulations (Section 59.1) state: “A floodway is defined
as the channel of a river or other watercourse and the adjacent
land areas that must be reserved in order to discharge the base
flood without cumulatively increasing the water-surface
elevation by more than a designated height. ”
Floodway Definition Based on FEMA
Regulations (cont.)
 In the State of Florida the “regulatory”
floodway is based on a maximum allowable
one foot rise in the BFE when the floodplain
is encroached through the flood fringe
The USACE Developed Several
Methods to Establish a Floodway
 Methods were developed with the use of HEC-2
 In general there are five methods available
1
• User enters right and left encroachment
stations
2
• User enters fixed top width
3
• User specifies the percent reduction in
conveyance
4
• User specifies a target water surface
increase
5
• User specifies a target water surface
increase and maximum change in energy
HEC-RAS Includes Tools to Run the
Floodway Development
 User can select from
among the five
methods developed
by the USACE
 The model generates
the floodway
encroachments
When Not Using HEC-RAS, the Options
are Limited
 In the particular case of SWMM, FEMA guidelines
suggest using the HEC-RAS tools:
1.
Convert the model to a steady flow model such as
HEC-2 (or HEC-RAS) to compute encroachment
stations using the equal conveyance method
When Not Using HEC-RAS, the Options
are Limited (cont.)
 In the particular case of SWMM, FEMA guidelines
suggest using the HEC-RAS tools:
1.
Convert the model to a steady flow model such as
HEC-2 (or HEC-RAS) to compute encroachment
stations using the equal conveyance method
2.
Apply the encroached transects to the SWMM model
to compute the water surface elevations to account for
the loss of floodplain storage
When Not Using HEC-RAS, the Options
are Limited (cont.)
 In the particular case of SWMM, FEMA guidelines
suggest using the HEC-RAS tools:
1.
Convert the model to a steady flow model such as
HEC-2 (or HEC-RAS) to compute encroachment
stations using the equal conveyance method
2.
Apply the encroached transects to the SWMM model
to compute the water surface elevations to account for
the loss of floodplain storage
3.
If there is greater than the allowable increase (one
foot) in any of the cross sections, go back to the HEC
model and get new encroachment stations
CDM was contracted to update the 1989
FIS in Duval County, FL using SWMM
The project was developed in SWMM for
several reasons:
•
•
•
•
Extensive previous models
Successful model field verifications
Flat terrain and tidal influence
Integrated model that solves the entire
system as once and not as individual
branches
• Upcoming water quality applications
related to concurrent TMDL studies
Applying the criteria set by the FEMA
standard, CDM used SWMM as follows:
Instead of iterating between HEC-RAS and SWMM, we developed
criteria to develop floodway in SWMM
Method 4 was selected (Specified Floodway Surcharge)
The original concept consisted of comparing two stream geometries
and determining the differences in water surface elevation
Main differences with HEC-RAS
• Both geometries are evaluated as part of the entire model (networked model)
• Both models are run for the 100-year/24-hour storm (dynamic model)
The Channel Geometry is Modified by
the Engineer by Introducing Channel
Encroachments
KL 
2/3
1.49
 AL  RL
nL
KR 
2/3
1.49
 AR  RR
nR
The Engineer Computes the
Conveyance Reduction on Both Sides,
and Makes Sure That They are Equal
2/3
1.49
KL 
 AL  RL
nL
KR 
2/3
1.49
 AR  RR
nR
K is a Function of the Wet Perimeter,
Cross-sectional Area, and Channel
Roughness
Cross-Section Example
Proposed
80
78
76
Elevation (ft)
74
72
70
68
Existing
Natural Channel
66
Floodway
64
1% Annual Chance WSE
62
Main Channel
60
200
300
400
500
600
Station (ft)
700
800
900
1000
The Process Becomes a Series of Tests
Where Comparing Two Conditions
• Run Model
• Observe Qn,
WSEn
Existing
Condition
Encroached
Condition
• Run Model
• Observe Qe,
WSEe
• Conveyance
• Surcharge
Verification of
Basic Criteria
The Model Accounts for the Loss of Storage
Associated with the Encroachments because it
evaluates all the model cross sections at once
Increased Complexity Took the Team From
the Basic Concept to the Development of a
Dynamic Floodway Utility (DFU)
RUN SWMM
• DOS Version EPA SWMM 5.12
DFU screens model output
•
•
•
•
•
•
Read SWMM INP file
Read SWMM RPT file
Compares peak stages to BFE
Sets encroachment percentages for next run
Edits transect data to encroach cross-section
Re-writes SWMM INP file with new encroachment
Iteratively in DOS batch file
1. Run the 1% annual
chance flow with natural
channel geometry
2. Observe the following
results from the simulation
period:
- Peak flow Qn (natural
channel) WSEn (natural
channel)
3. For each reach, move
transect stations inward to
the intersection with WSEn,
set the encroachment
percentage to 10%
4. Set N = 0
5. Iteration - N - modify
model geometry based on a
percentage encroachment
for each side
7. Determine conveyance
reduction for the left and
right side of the transect: KL,
KR
12. Compare each reach's
surcharge (at us and ds
nodes) to maximum
surcharge - increase or
decrease percentage
encroachment as necessary
(negative possible)
8. Select the minimum value
between KL and KR. Adjust
the other station to ensure
that KL = KR.
9. Run model again for the
1% annual chance storm
with adjusted transects
10. Observe the following
values for the simulation
period:
- Peak flow QF (floodway)
- Peak WSE at each node
WSEF (floodway)
NO
13. Is the
average surcharge for
all reaches > threshold*,
< =1 ft, or are both sides
encroached to main channel,
or is the encroachment
fixed by a
ds node?
YES
14. Prepare floodway table
11. Calculate the difference
in peak WSE (surcharge) at
every node
N = Iteration index
KL = Left conveyance
KR = Right conveyance
WSE = Water surface elevation
* Threshold may be defined by user: in this
project we used 0.6
6. If any of the new
proposed stations
encroaches beyond the
existing channel banks, set
the encroachment station to
the existing bank
13a. N = N + 1
DFU Convergence Criteria
 One of the following criteria must be met for each
cross section in the floodway mapping portion of
the model:
The cross section’s increase in peak stage (average of
upstream and downstream node stages) should be
between a threshold (0.6 ft) and 1.0 ft
Both sides of the cross section are encroached to the
bank of the main channel
The cross section encroachment has been set back by an
increase in a downstream node
The Following Data are Given in a Text File to the DFU
List of cross sections that
are part of the floodway
analysis, including:
• Priority level (main
branch and then
tributaries)
• Cross section
position within each
level, so the tool can
read what sections
are upstream from a
given position –
without having to
decipher SWMM
connectivity
• Main channel stream
bank positions –
limits encroachment
DFU – Batch File Structure
@echo off
set E1=0
set E0=0
:E0
echo Iteration %E1%%E0%
swmm5 CF_2006_100_%E1%%E0%.inp CF_2006_100_%E1%%E0%.rpt
floodwayv3 CF_2006_100_%E1%%E0%.inp CF_2006_100_%E1%%E0%.rpt
if %E0%==9 goto E1
if %E0%==8 set E0=9
if %E0%==7 set E0=8
if %E0%==6 set E0=7
if %E0%==5 set E0=6
if %E0%==4 set E0=5
if %E0%==3 set E0=4
if %E0%==2 set E0=3
if %E0%==1 set E0=2
if %E0%==0 set E0=1
:E1
set E0=0
if %E1%==9 goto Done
if %E1%==8 set E1=9
if %E1%==7 set E1=8
if %E1%==6 set E1=7
if %E1%==5 set E1=6
if %E1%==4 set E1=5
if %E1%==3 set E1=4
if %E1%==2 set E1=3
if %E1%==1 set E1=2
if %E1%==0 set E1=1
goto E0
goto E0
:Done
The Engineer Reviews a Summary
Table Generated by the DFU
Cross
Section
Level
maxDiff
aveDelta
Left encr
Rate
Right encr
Rate
SW40002
1
1.03
0.42
100.0
43.2
SW40005FP
1
1.03
0.71
0.0
0.0
YES
SW40006FP
1
1.03
0.68
100.0
100.0
YES
SW40007
1
1.03
0.81
100.0
38.5
YES
SW40011
1
1.03
0.80
100.0
78.2
YES
SW40012
1
1.03
0.89
0.0
0.0
YES
SW40013
1
1.03
0.88
67.7
83.7
YES
SW40014L1
1
1.03
0.93
100.0
57.8
YES
SW40014L2
1
1.03
0.89
98.2
65.2
YES
SW40015
1
1.03
0.91
58.3
100.0
YES
SW40017
1
1.03
0.92
82.6
100.0
YES
SW40025
1
1.03
0.59
100.0
100.0
NO
YES
SW40034
2
1.03
0.19
0.0
0.0
NO
NO
SW40035
2
1.03
0.16
0.0
0.0
NO
NO
SW40036
2
1.03
0.12
0.0
0.0
NO
NO
SW40037
2
1.03
0.07
0.0
0.0
NO
NO
>= 0.6
@Stream
Banks
Results are Then Presented in the
Floodway Data Table
Once the User is Satisfied With the
Floodway Outcome, the Team Translates It
into a Graphical Delineation
Cross-sections need to be
accurately identified in the map
The encroachment stations are
identified along the cross-section
In the future, SWMM should improve
the depiction of cross-sections to
allow easy identification of floodway
points in the map
FEMA Has Been Involved in the Process
Through a Series of Memoranda
 FEMA staff are interested in the results of the
dynamic methodology
 Main FEMA concerns

Added criteria for floodplain
storage

Many engineers have
expressed concerns with
the current steady
application in some states

Having a standard
methodology that other
engineers can use to
replicate results
Manual Corrections to Dynamic
Floodway
There are instances in which the dynamic floodway is too
narrow or too wide due to hydraulic considerations
In these cases, the user should overwrite the floodway using
engineering judgment
The user should understand the interdependence of adjacent
cross-sections
Floodway Review at bridges and culverts
Wide
FW
Narrow
FW
Floodway Review at Confluence of Multiple
Streams
Rasplot Software Was Used to Generate
Profiles Specific to SWMM That Display
the Nodes Instead of Cross-Sections
FIRM panel
format based on
link-node models
such as SWMM
Conclusions
SWMM has been successfully utilized to prepare floodways for
FEMA FISs without conversion to HEC-2 or HEC-RAS
A DFU tool has been developed to aid in the placement of
encroachment transects in the SWMM input file and to compare
model peak stages to the base condition
The tool runs iteratively in a DOS batch file. No modifications
were made to the standard SWMM engine.
FEMA has approved the methodology for the Duval County, FL
FIS
Future integration of the DFU into the SWMM GUI is desirable to
facilitate widespread use
Dynamic Floodways: Accounting
for Both Storage and Conveyance
José María Guzmán, P.E.
guzmanjm@cdm.com
May 2010, National ASFPM Conference, Oklahoma City
Additional Challenges for SWMM
Users in Mapping
 WSE not reported at cross-sections
 Cross-section locations are not included in
the model
 Meandering channels
 Viewing the WSE
in the cross-sections
80
75
70
Natural
Channel
65
60
200
400
600
800
1000