Dynamic Floodways: Accounting for Both Storage and Conveyance

1. Dynamic Floodways: Accounting for Both Storage and Conveyance José MaríaGuzmán, P.E. Gaston Cabanilla, P.E., CFM May 2010, National ASFPM Conference, Oklahoma City

2. 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, SandeepGulati, Seungho Song, Michael F. Schmidt, Zhida Song-James

3. 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. ”

4. 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

5. 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

6. 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

7. When Not Using HEC-RAS, the Options are Limited • In the particular case of SWMM, FEMA guidelines suggest using the HEC-RAS tools: • Convert the model to a steady flow model such as HEC-2 (or HEC-RAS) to compute encroachment stations using the equal conveyance method

8. When Not Using HEC-RAS, the Options are Limited (cont.) • In the particular case of SWMM, FEMA guidelines suggest using the HEC-RAS tools: • Convert the model to a steady flow model such as HEC-2 (or HEC-RAS) to compute encroachment stations using the equal conveyance method • Apply the encroached transects to the SWMM model to compute the water surface elevations to account for the loss of floodplain storage

9. When Not Using HEC-RAS, the Options are Limited (cont.) • In the particular case of SWMM, FEMA guidelines suggest using the HEC-RAS tools: • Convert the model to a steady flow model such as HEC-2 (or HEC-RAS) to compute encroachment stations using the equal conveyance method • Apply the encroached transects to the SWMM model to compute the water surface elevations to account for the loss of floodplain storage • 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

10. 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

11. Applying the criteria set by the FEMA standard, CDM used SWMM as follows:

12. The Channel Geometry is Modified by the Engineer by Introducing Channel Encroachments

13. The Engineer Computes the Conveyance Reduction on Both Sides, and Makes Sure That They are Equal K is a Function of the Wet Perimeter, Cross-sectional Area, and Channel Roughness

14. Cross-Section Example Proposed Existing

15. The Process Becomes a Series of Tests Where Comparing Two Conditions

16. The Model Accounts for the Loss of Storage Associated with the Encroachments because it evaluates all the model cross sections at once

17. Increased Complexity Took the Team From the Basic Concept to the Development of a Dynamic Floodway Utility (DFU)

18. 7. Determine conveyance reduction for the left and right side of the transect: KL, KR 1. Run the 1% annual chance flow with natural channel geometry 12. Compare each reach's surcharge (at us and ds nodes) to maximum surcharge - increase or decrease percentage encroachment as necessary (negative possible) 2. Observe the following results from the simulation period: - Peak flow Qn (natural channel) WSEn (natural channel) 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 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? 3. For each reach, move transect stations inward to the intersection with WSEn, set the encroachment percentage to 10% NO 10. Observe the following values for the simulation period: - Peak flow QF (floodway) - Peak WSE at each node WSEF (floodway) 4. Set N = 0 YES 14. Prepare floodway table 5. Iteration - N - modify model geometry based on a percentage encroachment for each side 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

19. DFU Convergence Criteria • One of the following criteria must be met for each cross section in the floodway mapping portion of the model: 1 2 3

20. The Following Data are Given in a Text File to the DFU

21. 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 goto E0 • :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 • :Done

22. The Engineer Reviews a Summary Table Generated by the DFU

23. Results are Then Presented in the Floodway Data Table

24. Once the User is Satisfied With the Floodway Outcome, the Team Translates It into a Graphical Delineation

25. 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 floodplainstorage • 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

26. 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

27. Floodway Review at bridges and culverts Wide FW Narrow FW

28. Floodway Review at Confluence of Multiple Streams

29. Rasplot Software Was Used to Generate Profiles Specific to SWMM That Display the Nodes Instead of Cross-Sections

30. FIRM panel format based on link-node models such as SWMM

31. Conclusions

32. Dynamic Floodways: Accounting for Both Storage and Conveyance José MaríaGuzmán, P.E. guzmanjm@cdm.com May 2010, National ASFPM Conference, Oklahoma City

33. 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