1. Creating Value … … Delivering Solutions Modeling 72-Miles of the Mississippi on a 2-Mile Budget Mohamed A. Bagha, P.E., CFM Dong Nguyen, P.E., CFM Pradeepa Venigalla, EIT

2. Background  CF3R - Comprehensive Flood Risk Resources and Responses - ID/IQ JV for FEMA Region VI.  Baker - Managing Partner  Multiple County DFIRM Task Order (2008)  Texas (7 Counties)  Louisiana (3 counties)  Arkansas (9 counties)

3. Arkansas Counties  Southern Arkansas  Ashley  Chicot  Desha  Drew

4. Major Rivers in South-East Arkansas

5. Scope  1st time Countywide DFIRM Studies  No New H & H studies on any streams

6. Areas of Moderate Flood Risk, Subject to 0.2% Annual Chance Flood Includes Areas Subject to Shallow Flooding Areas of Residual Risk Behind Accredited and Provisionally Accredited Levees No BFEs or Flooding Depths Shown on FIRMs What Does a Shaded Zone X Area Mean? Purchase of Flood Insurance Not Mandatory Statistically, a 0.2% Chance Flood has a 6% Chance of Occurring During a 30-Year Period

7. Levee Systems

8. Levee Guidelines & Specifications for Levees  FEMA G &S Appendix H  Guidance for Mapping of Areas Protected by Levee Systems (2003)  5 Memos since:  34, 43, 45, 51, 53  2005 - 2009

9. Create / Obtain Hydraulic Modeling along Levee’d Stream Fail Entire Levee, not only localized breach Delete entire embankment and levee identifiers from geometry Extend Cross sections on overbanks Ideal procedure – Mapping Residual Risk Re-run model in without-levee mode Map resulting residual risk floodplain

10. However…  Limited Modeling Available along Mississippi River in SE Arkansas. 1 1.Complete Hydraulic Model Not Available for Study Area 2 2.COE Memphis District Provided a HEC- RAS Model of the Mississippi River 1% chance Flood from RM 956.97 to RM 594 3 3.COE Vicksburg District Provided Cross Section Geometry & Reach Lengths for HEC-2 Model from RM 621.03 to RM 321 4 4.No Peak Discharges or Manning’s ‘n’ Values Provided in Vicksburg District HEC-2 Model

11. Key Cross Sections

12. Maintain Quality of Product Challenges Limited FEMA Funding Per Countywide Study Compressed Schedule Delayed by Certification Process Follow Relevant FEMA Procedure Guidelines Create Model for Shaded X Floodplains for all Levees

13. Conceptualization Alternative Modeling Defendable & Reasonable Consistent with and ties into previous Studies Economical - a Detailed Study of the Mississippi Along this Reach May Prove Very Expensive Concurrence on Approach from RMC 6 and FEMA Region VI Vital Compressed Schedule - with delays due to PAL Requests, little time to complete task

14. Memphis & Vicksburg HEC Models  Cross sections located using USACE MVD Navigation Book www.mvd.usace.army.mil/Gis/navbook/html/corps.htm Memphis HEC-RAS Flows and Manning’s ‘n’ calibrated for 1% event Start – RM 956.67 End – RM 592.00 Covers Northern Desha County Vicksburg HEC-2 Cross Section Geometry Only Start- RM 594.21 End- RM 321.00 Chicot County Southern Limit at RM 504.66

15. Geometry HEC-2 cross section geometry HEC-RAS Model Upstream Peak Discharges Integrate 1:40,000 USACE MVD Navigation Maps Cross Section Stationing. GIS Base Mapping Locate USGS Topographic Mapping. 13 composite HEC- RAS Sections 42 composite HEC- 2 Sections Extend

16. HEC Geo-RAS Data Extracted from USGS DEM and Cross Sections Overlay with Cross Section from HEC-RAS and HEC-2 Models Use Surveyed Elevation Data in channels and on overbanks where Available Supplement cross section overbanks with USGS Contour DEM Data Geometry- Splicing Cross Sections Extend Cross Sections across county boundaries Determine appropriate Manning’s ‘n’ for overbanks.

17. Splicing Cross Sections  Vicksburg COE HEC-2 Cross Section 590.4.

18. Splicing Cross Sections  HEC-RAS channel data + USGS DEM @ XS 590.4

19. Cross Section Layout

20. Chicot County Detailed Study  BFEs along Mississippi River in Chicot County  COE, Vicksburg Study  No Peak Discharges  No Floodway Data  1979 FIS report.

21. Hydrology 101  1 + 1 + 1 ≠ 3

22. Efficiencies: Reverse Engineering  Sensitivity Analyses  2 variables- Manning’s ‘n’ and Q 100  While holding the Channel ‘n’ values steady:  Determine the % of flow addition that results in computed W.S. Elevations at Chicot County limits matching 1979 BFEs.  Multiple Profiles set up, each profile with Incremental Discharges.  40% of Arkansas River & White River peak discharges added yield best match with 1979 BFEs on Mississippi River in Chicot Co.

23. Efficiencies: Reverse Engineering (Cont’d)  While holding peak discharges constant:  Vary Manning’s ‘n’ values - ± 25%.  Upstream cross sections in Memphis COE HEC- RAS model show minimal WS Elevation change.  Results: Variation in ‘n’ values did not cause significant impact to W.S. Elevations along the Mississippi River.  Conclude that Model is sensitive to Q variation rather than ‘n’ variation.

24. Efficiencies: Reverse Engineering (Cont’d) Chicot County (with 1979 BFEs)

25. Bayou Bartholomew  Longest Bayou in the world  Flows ~375 miles  Parallel to Mississippi River  Conveys up to 7,500 cfs  Drainage area: 1,110 sq. mi.  Major conveyance system  Shaded Zone X for Mississippi River West Bank levee will not extend west beyond this bayou  Levee-like features (railroad & highway embankments) cannot be used as alternative.

26. Resulting Multi-County Shaded X floodplain

27. DFIRM – Desha County Panel 0250 Red Fork Bayou Carter Bayou

28. Summary  HEC-RAS along Mississippi across two counties.  Seamlessly integrated geometry (Vicksburg COE) with Memphis COE HEC-RAS model.  Cross sections extended on right overbank using USGS Topographic data.  Peak discharges estimated.  Manning’s ‘n’ values determined.  Shaded Zone X extents mapped.  Future studies in Northern Louisiana impacted.

29. Conclusion  Creative Approach to resolve missing data issue.  Quality End-Product - FEMA requirements.  Residual Risk Mapping has solid technical basis.  Short Schedule  2 week turnaround.  Limited Budget  3 member team to conceptualize, coordinate, process and report on results  Effective Coordination  Between Study contractor, RMC6, FEMA and USACOE Districts (Memphis and Vicksburg)

30. Creating Value … … Delivering Solutions Questions?