Estimating Flood Limits in Large Riverine Systems Using Unsteady-State Hydraulic Modeling (HEC-RAS)

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Estimating Flood Limits in Large Riverine Systems
Using Unsteady-State Hydraulic Modeling (HEC-RAS)

• Brian Greathouse, P.E.
• Malcolm Pirnie, Inc.

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METHODOLOGIES

• US ARMY CORP OF ENGINEERS HEC-RAS RIVER ANALYSIS
  SYSTEM
• STEADY STATE ANALYSIS
• UNSTEADY STATE ANALYSIS

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HEC-RAS OVERVIEW

• MODEL COMPONENTS
• GEOMETRY DATA
• FLOW DATA
• BOUNDARY CONDITIONS

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GEOMETRY DATA

• CROSS SECTION DATA
• DEVELOPED FROM TOPOGRAPHIC DATA
• SURVEY DATA
• COMBINATION OF BOTH
• LAND COVER DATA MANINGS N VALUE
• DEFINED FOR THE CHANNEL AND OVERBANK PORTION OF
  THE CROSS SECTION
• STRUCTURE DATA
• BRIDGES, CULVERTS, WEIRS

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FLOW DATA

• STEADY STATE UNIFORM FLOW
• UNSTEADY STATE NON-UNIFORM FLOW FLOW HYDROGRAPH

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STEADY-STATE MODELING

• APPLICATION OF A UNIFORM FLOW THROUGHOUT THE
  RIVER SYSTEM
• TYPICALLY A PEAK FLOW TAKEN FROM THE FLOW
  HYDROGRAPH
• COMMON APPLICATION IS FEMA FLOOD MAPPING

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UNSTEADY STATE MODELING

• NON-UNIFORM FLOW
• APPLICATION OF A FLOW HYDROGRAPH
• BETTER REPRESENTATION OF THE HYDRAULIC
  CHARATERISTICS OF A RIVER SYSTEM

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CASE STUDY TALLAPOOSA RIVER MODEL

• MODELED FROM THURLOW DAM TO THE DEAD RIVER AREA

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FLOW DATA

• APPLIED ACTUAL DAM RELEASE RATES TO THE MODEL
• APPLIED HYPOTHETICAL RELEASE RATES WERE APPLIED

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MODEL CALIBRATION

• STREAM GAGE DATA USED FOR MODEL CALIBRATION
• COMPARE FLOW AND STAGE (MODELED vs GAGE DATA)
• APPLY A UNIFORM DISTRIBUTED FLOW
• CALIBRATION COMPENSATES FOR FLOW ENTERING SYSTEM
  DOWNSTREAM FROM DAM AND OFFLINE STORAGE

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Model Flow vs. Gage Flow
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Model Flow vs. Gage Height
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Model Flow vs. Gage Height
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FLOOD LIMITS

• MODEL PRODUCES WATER SURFACE ELEVATIONS AT EACH
  CROSS SECTION
• WATER SURFACE ELEVATIONS ARE APPLIED TO
  TOPOGRPAHICAL DATA AND THE FLOOD LIMITS ARE
  DETRMINED.

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QUESTIONS????????