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

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)

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

Major Rivers in South-East Arkansas

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

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

Levee Systems

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 

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

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 to RM COE Vicksburg District Provided Cross Section Geometry & Reach Lengths for HEC-2 Model from RM to RM No Peak Discharges or Manning’s ‘n’ Values Provided in Vicksburg District HEC-2 Model

Key Cross Sections

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

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

Memphis & Vicksburg HEC Models  Cross sections located using USACE MVD Navigation Book Memphis HEC-RAS Flows and Manning’s ‘n’ calibrated for 1% event Start – RM End – RM Covers Northern Desha County Vicksburg HEC-2 Cross Section Geometry Only Start- RM End- RM Chicot County Southern Limit at RM

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

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.

Splicing Cross Sections  Vicksburg COE HEC-2 Cross Section

Splicing Cross Sections  HEC-RAS channel data + USGS XS 590.4

Cross Section Layout

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

Hydrology 101  ≠ 3

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.

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.

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

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.

Resulting Multi-County Shaded X floodplain

DFIRM – Desha County Panel 0250 Red Fork Bayou Carter Bayou

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.

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)

Creating Value … … Delivering Solutions Questions?