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 2005 - 2009

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 956.97 to RM 594

33. 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

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 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

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 590.4.

0 10000 20000 30000 40000 5000060

80

100

120

140

160

180

ShadedZoneX-hec2 Plan: 1) NoLevee 5/22/2009

Station (ft)

Ele

vatio

n (ft

)

Legend

WS PF 1

Ground

Bank Sta

.14 .032 .14

Splicing Cross Sections

HEC-RAS channel data + USGS DEM @ XS 590.4

0 10000 20000 30000 40000 50000 60000

100

120

140

160

180

ShadedZoneX-hec2 Plan: Plan-NoLevee 5/22/2009

Station (f t)

Ele

vatio

n (ft

)

Legend

EG PF 1

WS PF 1

Crit PF 1

Ground

Ineff

Bank Sta

.15 .03 .15

0 20000 40000 60000 80000 100000 120000 140000 160000 18000060

80

100

120

140

160

180

ShadedZoneX-hec2 Plan: 1) Shaded X 2) Plan-Levee

Station (f t)

Ele

vatio

n (ft

)

Legend

WS PF 1 - Plan-Levee

WS PF 1 - Shaded X

- Plan-Levee

Ground - Plan-Levee

Levee - Plan-Levee

Bank Sta - Plan-Levee

- Shaded X

- Shaded X

Ground - Shaded X

Ineff - Shaded X

Bank Sta - Shaded X

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 1 + 1 + 1 ≠ 3

Efficiencies: Reverse Engineering

Sensitivity Analyses 2 variables- Manning’s ‘n’ and Q100

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)

1000000 1100000 1200000 1300000 1400000

-100

-50

0

50

100

150

ShadedZoneX-hec2 Plan: Mississippi shaded zone X 5/29/2009 XS 594 - XS 507

Main Channel Distance (ft)

Ele

vatio

n (ft

)

Legend

WS PF 1

WS PF 2

WS PF 3

WS PF 4

WS PF 5

WS PF 6

WS PF 7

WS PF 8

WS PF 9

WS PF 10

WS PF 11

Ground

507.

29

509.

2

511.

01

512.

8

515.

03

516.

6

518.

21

521.

19

522.

76

524.

4

526

527.

8

529.

2

531

531.

7

533.

13

535.

17

537.

25

538.

56

540.

38

543.

21

544.

72

546.

83

549

551

553

554.

59

557.

21

560

562.

4

565.

19

567

569.

03

573.

15

574.

8

576.

21

578.

8

584.

86

589.

07

590.

4

RIVER-1 Reach-1

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?