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