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) 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.

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