US Army Corps of Engineers

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Methods for Determining Maximum Flood Elevations Landward of Failed Levees: An Example from the Great Missouri River Flood of 2011

Presenter: Lowell Blankers, PE, CFMFlood Risk and Floodplain ManagementUSACE, Omaha District

Co Authors:Laurel Hamilton, EIRoger Kay, PETony Krause, PE, CFM

May 2012

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Presentation Overview

Background Modeling Techniques Used

►Steady HEC-RAS►Unsteady HEC-RAS

Water Surface Elevation Comparison to Gage Data

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L-575 Missouri

River Left Bank

21 Miles

4.8 Miles

Nebraska City

Hamburg

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Breach Development

3 Partial Levee Failure Breaches►June 4th

►June 5th

►June 9th

1 Full Levee Failure Breach ►June 13th

• Estimated Width: 300 feet • Estimated Depth: 11 feet

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June 4th 2011

L-575 1st Sign Potential Levee Failure

Levee Crest

Landward Toe

Seepage Berm

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Hamburg

Breach

Location

Minimum Levee Elevation

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June 5th 2011

L-575 2nd Sign of Levee Failure

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June 9th 2011

L-575 3rd Sign of Levee Failure

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June 13th 2011

L-575 Full Breach 10:00 AM

L-575 Full Breach 5:00 PM

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HEC-RAS Steady Flow Model

June 5th 2011 Hydraulic Modeling Begins Modeling Objectives

►Determine water surface elevations landward of the levee and to determine levee elevations for a secondary levee at Hamburg, IA

►Determine time available to construct temporary levees once a full breach occurs.

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Steady Flow Model Setup

0 50000 100000 150000 200000 250000 300000 350000895

900

905

910

915

920

925

Volume (acre- ft)

Ele

vatio

n

(ft)

Legend

Vol-Elev

Elevation-volume curve for storage area.

The curve was already available from a previous study.

Add Storage Area to HEC-RAS Model

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Steady Flow Model SetupAdd Lateral Weir at the Breach Location

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Steady Flow Model SetupAdd Lateral Weir at downstream return

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L-575 Return Flow 6/16/2011 at 11:00 AM

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Estimate the Elevation Landward of the Levee

• Estimate an elevation in for storage area.• Set in the steady flow file

• Run model and review the storage area inflow and outflow.

• Adjust the elevation and rerun to balance the inflow and outflow

Steady Flow Model Setup

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Inflow and Outflow from Storage Area should be equal.

Steady Flow Model SetupReview Storage Area Results

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Steady Flow Model Results

 Lower 575 Breach Analysis Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5 Scenario 6

Upstream into storage            

Breach Width 300 300 300 200 200 200

Breach Depth (To Grade) ~11 ~11 ~11 ~11 ~11 ~11

Downstream into channel            

Breach Width 0 300 300 0 100 300

Breach Depth 0 1 2 0 1 2

Storage Area Elevation NAVD 88 916.17 916.12 916.04 916.06 915.9 915.8

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Time to Fill Storage Area

Elevation-volume curve for storage area was used with weir equation to estimate time to fill the area behind the levee.

Manual calculation required since not computed in Steady Flow RAS Model

Elevation 916.17 NAVD88 at approximately 68 hours after breach

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Initial set up similar to the Steady Flow Modeling.

The breach data was not set in the geometry of the lateral weirs.

Levee breach parameters entered into the Levee Breach option under the Unsteady Plan File.

HEC-RAS Unsteady Flow Model Setup

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Unsteady Flow Model SetupBreach Parameters

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Unsteady Flow Model SetupBreach Progression Parameters

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Unsteady Flow Model SetupBreach Parameters for Downstream Notch

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Unsteady Flow Model Results

14 15 16 17 18 19 20 216/1/2011

902

904

906

908

910

912

914

916

918

Plan: 575_init Storage Area: Lower575

Time

Sta

ge (

ft)

Legend

Stage

918

916

914

912

910

904

908

906

90214 15 16 17 18 19 20 21

Ele

vatio

n N

AV

D88

Date (June)

Elevation 916.7 NAVD88 Time: 4 to 5 Days or 96 to 120 Hrs

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Gage Data

5 gage locations►USACE study gage►3 Staff Gages set during the flood►USGS Temporary Gage Set

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Gage Locations

Staff Gages

USGS Temp Gage

USACE Study Gage

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Time of Breach

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Secondary Levee at Hamburg, IA

June 18, 2011

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August 29, 2011

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Post FloodNovember 2011

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Questions