Modeling 72-Miles of the Mississippi on a 2-Mile Budget - PowerPoint Presentation

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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

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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)

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Arkansas Counties

Southern ArkansasAshleyChicotDeshaDrew

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Major Rivers in South-East Arkansas

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Scope

1st time Countywide DFIRM StudiesNo New H & H studies on any streams

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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

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Levee Systems

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Levee Guidelines & Specifications for Levees

FEMA G &S Appendix HGuidance for Mapping of Areas Protected by Levee Systems (2003)5 Memos since: 34, 43, 45, 51, 532005 - 2009

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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

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However…

Limited Modeling Available along Mississippi River in SE Arkansas.

1

Complete Hydraulic Model Not Available for Study Area

2

COE Memphis District Provided a HEC-RAS Model of the Mississippi River 1% chance Flood from RM 956.97 to RM 594

3

COE Vicksburg District Provided Cross Section Geometry & Reach Lengths for HEC-2 Model from RM 621.03 to RM 321

4

No Peak Discharges or Manning’s ‘n’ Values Provided in Vicksburg District HEC-2 Model

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Key Cross Sections

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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

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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

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Memphis & Vicksburg HEC Models

Cross sections located using USACE MVD Navigation Book www.mvd.usace.army.mil/Gis/navbook/html/corps.htm

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Geometry

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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.

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Splicing Cross Sections

Vicksburg COE HEC-2 Cross Section 590.4.

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Splicing Cross Sections

HEC-RAS channel data + USGS DEM @ XS 590.4

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Cross Section Layout

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Chicot County Detailed Study

BFEs along Mississippi River in Chicot CountyCOE, Vicksburg StudyNo Peak DischargesNo Floodway Data1979 FIS report.

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Hydrology 101  1 + 1 + 1 ≠ 3

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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.

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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.

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Efficiencies: Reverse Engineering (Cont’d)

Chicot County (with 1979 BFEs)

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Bayou Bartholomew

Longest Bayou in the worldFlows ~375 milesParallel to Mississippi RiverConveys up to 7,500 cfs Drainage area: 1,110 sq. mi.Major conveyance systemShaded Zone X for Mississippi River West Bank levee will not extend west beyond this bayouLevee-like features (railroad & highway embankments) cannot be used as alternative.

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Resulting Multi-County Shaded X floodplain

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DFIRM – Desha County Panel 0250

Red Fork Bayou

Carter Bayou

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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.

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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)

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Questions?