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Detention/Retention Strategies and Techniques

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.

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Detention/Retention Strategies and Techniques

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  1. Detention/Retention Strategies and Techniques URBDP 598G – Floodplain Management December 8, 2008 Chris Scott Pat Keys

  2. 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.

  3. 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 (<2% slope) areas available for over land flow (detention) to increase friction. • Targeted areas for in-stream and stream-bank wier/LWD friction and diversion installations.

  4. Retention • Large 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

  5. Detention • Overland 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)

  6. Friction/ Retention • Targeted 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).

  7. GIS Process

  8. GIS Process – Site Selection

  9. GIS Process – Raw LiDAR

  10. GIS Process – Hillshade

  11. Slope Analysis

  12. GIS Process – Percent Slope

  13. GIS Process – Classed Slope

  14. GIS Process – Classed Slope (polygon)

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

  16. GIS Process – Hillshade w/ Slope

  17. GIS Process – Landuse w/ Slope & Channel Migration

  18. Hydro Analysis

  19. GIS Process – Flow Direction & Accumulation

  20. GIS Process – Reclassify Accumulation, to Polygon

  21. GIS Process – Soils with Very Good Infiltration

  22. GIS Process – Intersect Soils with Slope

  23. GIS Process – 37 Acres of infiltration

  24. GIS Process – Infiltration and Landuse

  25. 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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