Detention/Retention Strategies and Techniques

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Detention/Retention Strategies and Techniques. URBDP 598G – Floodplain Management December 8, 2008 Chris Scott Pat Keys. Goal – Flatten hydrograph and limit peak discharge during flood events. - PowerPoint PPT Presentation

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  • Detention/Retention Strategies and TechniquesURBDP 598G Floodplain Management

    December 8, 2008

    Chris ScottPat Keys

  • Goal Flatten hydrograph and limit peak discharge during flood eventsObjective Develop techniques to define and locate areas for potential detention/retention upstream of vulnerable areas

    Objective Document GIS procedure for site location.

  • Definition of Sites Investigated

    Typical conditions must include appropriate land use (i.e. vacant lands, resource lands, or privately held lands set aside for mitigation)

    Large depressions over permeable soils (retention)

    Large, flat (

  • RetentionLarge depressions over permeable soils

    Requirements:

    Areas accessible, with minimum construction costs, to river at bank full state

    Permeable soils to facilitate aquifer re-charge and further improve downstream discharge rates

  • DetentionOverland flow area to increase friction.

    Requirements:

    Large area (>5 acres) accessible with minimum construction to river in bank-full condition.

    Good vegetation cover to facilitate frictional retention of river flow.

    Permeable soils to facilitate aquifer re-charge and further improve downstream discharge rates (less important for this type)

  • Friction/ RetentionTargeted areas for in-stream and stream-bank wier/ LWD friction and diversion installations.

    Requirements:

    Large non-vulnerable upstream area for increased water height and flooding.

    Natural bank width with a shallow drop in river elevation (areas with significant braiding may be ideal).

  • GIS Process

  • GIS Process Site Selection

  • GIS Process Raw LiDAR

  • GIS Process Hillshade

  • Slope Analysis

  • GIS Process Percent Slope

  • GIS Process Classed Slope

  • GIS Process Classed Slope (polygon)

  • GIS Process Aerial w/ Slope (clipped to channel mig. zone)

  • GIS Process Hillshade w/ Slope

  • GIS Process Landuse w/ Slope & Channel Migration

  • Hydro Analysis

  • GIS Process Flow Direction & Accumulation

  • GIS Process Reclassify Accumulation, to Polygon

  • GIS Process Soils with Very Good Infiltration

  • GIS Process Intersect Soils with Slope

  • GIS Process 37 Acres of infiltration

  • GIS Process Infiltration and Landuse

  • Conclusions & Recommendations

    Process is relatively simple

    In study area, ideal detention and retention areas were rare; may be characteristic of region

    Significant amount of 37 acres of suitable infiltration is likely currently streambed

    Instream frictional structures with upstream safe-fail floodable areas

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