US Army Corps of Engineers BUILDING STRONG ® 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
BUILDING STRONG ® USACE, Omaha District Presentation Overview Background Modeling Techniques Used ► Steady HEC-RAS ► Unsteady HEC-RAS Water Surface Elevation Comparison to Gage Data
BUILDING STRONG ® USACE, Omaha District L-575 Missouri River Left Bank 21 Miles 4.8 Miles
BUILDING STRONG ® USACE, Omaha District 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
BUILDING STRONG ® USACE, Omaha District June 4 th 2011 L st Sign Potential Levee Failure Levee Crest Landward Toe Seepage Berm
BUILDING STRONG ® USACE, Omaha District Breach Location Minimum Levee Elevation
BUILDING STRONG ® USACE, Omaha District June 5 th 2011 L nd Sign of Levee Failure
BUILDING STRONG ® USACE, Omaha District June 9 th 2011 L rd Sign of Levee Failure
BUILDING STRONG ® USACE, Omaha District June 13 th 2011 L-575 Full Breach 10:00 AM L-575 Full Breach 5:00 PM
BUILDING STRONG ® USACE, Omaha District 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.
BUILDING STRONG ® USACE, Omaha District Steady Flow Model Setup Elevation-volume curve for storage area. The curve was already available from a previous study. Add Storage Area to HEC-RAS Model
BUILDING STRONG ® USACE, Omaha District Steady Flow Model Setup Add Lateral Weir at the Breach Location
BUILDING STRONG ® USACE, Omaha District Steady Flow Model Setup Add Lateral Weir at downstream return
BUILDING STRONG ® USACE, Omaha District L-575 Return Flow 6/16/2011 at 11:00 AM
BUILDING STRONG ® USACE, Omaha District 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
BUILDING STRONG ® USACE, Omaha District Inflow and Outflow from Storage Area should be equal. Steady Flow Model Setup Review Storage Area Results
BUILDING STRONG ® USACE, Omaha District Steady Flow Model Results Lower 575 Breach AnalysisScenario 1Scenario 2Scenario 3Scenario 4Scenario 5Scenario 6 Upstream into storage Breach Width Breach Depth (To Grade)~11 Downstream into channel Breach Width Breach Depth Storage Area Elevation NAVD
BUILDING STRONG ® USACE, Omaha District 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 NAVD88 at approximately 68 hours after breach
BUILDING STRONG ® USACE, Omaha District 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
BUILDING STRONG ® USACE, Omaha District Unsteady Flow Model Setup Breach Parameters
BUILDING STRONG ® USACE, Omaha District Unsteady Flow Model Setup Breach Progression Parameters
BUILDING STRONG ® USACE, Omaha District Unsteady Flow Model Setup Breach Parameters for Downstream Notch
BUILDING STRONG ® USACE, Omaha District Unsteady Flow Model Results Elevation NAVD88 Date (June) Elevation NAVD88 Time: 4 to 5 Days or 96 to 120 Hrs
BUILDING STRONG ® USACE, Omaha District Gage Data 5 gage locations ► USACE study gage ► 3 Staff Gages set during the flood ► USGS Temporary Gage Set
BUILDING STRONG ® USACE, Omaha District Gage Locations Staff Gages USGS Temp Gage USACE Study Gage
BUILDING STRONG ® USACE, Omaha District Time of Breach
BUILDING STRONG ® USACE, Omaha District Secondary Levee at Hamburg, IA June 18, 2011
BUILDING STRONG ® USACE, Omaha District August 29, 2011
BUILDING STRONG ® USACE, Omaha District Post Flood November 2011
BUILDING STRONG ® USACE, Omaha District Questions