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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, CFM
Flood Risk and Floodplain Management USACE, Omaha District
Co Authors:
Laurel Hamilton, EI Roger Kay, PE Tony 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
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Breach Development
3 Partial Levee Failure Breaches ► June 4 th ► June 5 th ► June 9 th 1 Full Levee Failure Breach ► June 13 th • Estimated Width: 300 feet • Estimated Depth: 11 feet
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June 4
th
2011
Levee Crest Landward Toe Seepage Berm
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L-575 1 st Sign Potential Levee Failure
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Breach Location Minimum Levee Elevation
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June 5
th
2011
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L-575 2 nd Sign of Levee Failure
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June 9
th
2011
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L-575 3 rd Sign of Levee Failure
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June 13
th
2011
L-575 Full Breach 10:00 AM
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L-575 Full Breach 5:00 PM
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HEC-RAS Steady Flow Model
June 5 th 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 Add Storage Area to HEC-RAS Model 925 920 915
Legend
Vol-Elev 910 905 900 895 0 50000 100000 150000 200000 250000 300000 350000 Volume (acre- ft) Elevation-volume curve for storage area.
The curve was already available from a previous study.
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Steady Flow Model Setup Add Lateral Weir at the Breach Location
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Steady Flow Model Setup Add Lateral Weir at downstream return
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USACE, Omaha District
L-575 Return Flow 6/16/2011 at 11:00 AM
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Steady Flow Model Setup 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
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Steady Flow Model Setup Review Storage Area Results Inflow and Outflow from Storage Area should be equal.
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Steady Flow Model Results
Lower 575 Breach Analysis
Upstream into storage
Breach Width Breach Depth (To Grade)
Downstream into channel
Breach Width Breach Depth
Storage Area Elevation NAVD 88
Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5 Scenario 6 300 ~11 0 0 916.17
300 ~11 300 1 916.12
300 ~11 300 2 916.04
200 ~11 0 0 916.06
200 ~11 100 1 915.9
200 ~11 300 2 915.8
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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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HEC-RAS Unsteady Flow Model Setup 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.
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Unsteady Flow Model Setup Breach Parameters
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Unsteady Flow Model Setup Breach Progression Parameters
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Unsteady Flow Model Setup Breach Parameters for Downstream Notch
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Unsteady Flow Model Results Plan: 575_init Storage Area: Lower575
Legend
Stage Elevation 916.7 NAVD88 Time: 4 to 5 Days or 96 to 120 Hrs
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17 T ime Date (June)
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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
USGS Temp Gage Staff Gages 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 Flood November 2011
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USACE, Omaha District
Questions
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