detention/retention strategies and techniques
DESCRIPTION
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 PresentationTRANSCRIPT
Detention/Retention Strategies and
TechniquesURBDP 598G – Floodplain Management
December 8, 2008
Chris ScottPat Keys
Goal – Flatten hydrograph and limit peak discharge during flood events
Objective – 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 InvestigatedTypical conditions must include appropriate land use (i.e. vacant lands, resource lands, or privately held lands set aside for mitigation)
A)Large depressions over permeable soils (retention)
B)Large, flat (<2% slope) areas available for over land flow (detention) to increase friction.
C)Targeted areas for in-stream and stream-bank wier/LWD friction and diversion installations.
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