Urban Flood Risk Management

1. Urban Flood Risk Management

2. Objectives • Understand the Nature of Flooding & Flood Damage Alleviation • Learn how to quantify expected annual damages for existing and future without project conditions and benefits for future with project conditions • Describe inventory procedures • Learn application of depth-damage functions

3. Flood Control or Flood Damage Reduction or Flood Risk Management? • Floods have occurred throughout time, and are not necessarily damaging • Early legislation authorized “flood control” in response to devastating losses • We can’t really control floods, but we can modify water flows in space and time • Corps’ mission is reducing damages from floods, hence FDR

4. Flood Damages = fn (Water + Development)through space and time Flood damage analysis integrates the hydro- and the economic to evaluate damage reduction alternatives

5. Flood Risk Management Economic Analysis • Helps answer the question “which proposed flood damage reduction plan is the best from an economic standpoint?” • Evaluate the existing condition • Evaluate the future without project condition • Analyze alternative damage reductionplans • Compare plans using expected annual flood damage and flood damage reduction benefits • Since 1996, use risk analysis procedures

6. Hydrology and Hydraulics

7. Types of Flooding • River and stream overflow • Coastal • Alluvial • Ponding • Rising lakes

8. Stage Stage Discharge Damage Frequency Frequency Discharge Damage

9. Frequency Damage

11. Hydrologic Studies

12. Hydraulic Studies

13. Economic Analysis

14. Computing Expected Flood Damages • Three basic functions • Discharge-probability • Stage-discharge • Stage-aggregated damage • Derived functions • Stage-probability • Damage-probability • Integrate damage-probability to determine expected annual damage

15. Risk Procedures • Uncertainty in each function • Discharge-probability • Stage-discharge • Stage-damage • Monte-Carlo simulation • Sample the 3 basic functions

16. Risk Analysis • Monte Carlo simulation is a numerical integration technique used to integrate the three functions to obtain EAD with the consideration of uncertainty. • It does not add any information to the analysis but provides more information for decision makers.

17. Existing and Future Without Project Damages 1. Delineate Affected Area 2. Determine Floodplain Characteristics Identify damage reaches Inventory land uses: Residential, Commercial, Industrial, Public, Others Estimate first floor elevations - uncertainty 3. Estimate structure/content values 4. Apply depth-damage curves

18. Delineate Affected Area (Floodplain Outline)

19. Damage Reaches

20. Inventory Land Uses Structure Inventory Data • Structure ID • Location/address • Structure value • Content ratio • Damage category • Depth-Damage Function • First Floor Stage • Ground Stage • Coordinates • Stream Station

22. Apply Depth- Damage Functions

23. Categories of Depth-damage functions used by the Corps of Engineers Basement or no basement Masonry, frame or adobe Slab or block foundation Basement living or storage area Raised on piers One story One and half story Split level Two or more stories Townhouse/condo

24. Stage-Aggregated Damage Uncertainty • First floor elevation • Structure value • Content value (content/structure value ratio) • “Other” value • Damage in depth-percent damage function

25. Damage-Probability

26. HEC – FLOOD DAMAGE ANALYSIS PACKAGE • Developed by Hydrologic Engineering Center, Institute for Water Resources • Integrates hydrology/hydraulics/economics in a single model. • Multiple capabilities • Incorporates risk analysis • Documentation and training

27. Flood Risk Management Measures • Structural measures • Modify flood behavior • Dams and reservoirs, levees, walls, diversion channels, bridge modifications, channel alterations, pumping, and land treatment • Nonstructural measures • Modify damage susceptibility • Flood warning and preparedness; Evacuation and relocation; Land use regulations; Flood proofing; Area renewal policies

28. Evaluation of Without Project Condition

29. Channel Modification

30. Levee

31. Non-Structural Measures

32. Floodproofing • Raising : most cost effective • Closures : good to 3 ft. Above ground • Barriers: very expensive

33. Permanent Evacuation • Value of vacant land in new use - residual flood damage • Reduction of damages • Adjust costs to avoid double-counting

34. Identifying the NED Plan • Without-project annual damages • With project annual damages • Annual benefits are damages reduced (without minus with) • Annual net benefits are annual benefits minus annual costs • Compare across project scales and between alternatives to determine plan that yields greatest net annual NED benefits • Decision-makers always have the final say

35. OTHER BENEFITS • LOCATION BENEFITS • New activity moves into the protected floodplain • Benefits calculated as whichever is less of: • Increased potential damages in the changed land use assuming without project conditions; or • Cost of fill/flood proofing

36. OTHER BENEFITS • INTENSIFICATION BENEFITS • Existing activity is intensified • Benefits calculated as: • Increased income as a result of intensified activity • Increased land values • Computation of costs and revenues

37. ECONOMIC ANALYSIS

38. SUMMARY • The Corps’ mission in FRM is to contribute to reduce damages associated with floods. • Economic analysis of FRM alternatives helps identify the optimum plan (NED Plan) for implementation from a National economic perspective. • Decision-makers always have the final say • Center of Expertise: South Pacific Division