Detention/Retention Strategies and Techniques
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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
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.
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.
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 Site Selection
GIS Process Raw LiDAR
GIS Process Hillshade
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
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