and for each alternative floodplain management 

 plan. 



(a) Basic factors. Base the allocation on a com- 

 parison of the floodplain characteristics, the charac- 

 teristics sought by potential occupants, and the 

 availability of sought-after characteristics in the 

 non-floodplain portions of the affected area. 



(b) Criteria. The floodplain should not be used 

 unless it has characteristics that give it a significant 

 economic advantage to the potential user over all 

 other available sites within the affected area. If 

 such advantages exist, determine whether they 

 overcome potential flood losses, potential flood 

 proofing costs, and the costs of other related haz- 

 ards. Flood losses and costs should be specific to 

 the zone of the floodplain being considered. 



2.4.10 Evaluation procedure: Step 6— 

 Determine existing flood damages. 



Existing flood damages are the potential average 

 annual dollar damages to activities affected by 

 flooding at the time of the study. Existing damages 

 are those expressed for a given magnitude of flood- 

 ing or computed in the damage frequency process. 

 No projection is involved. The basis for the determi- 

 nation of existing damages is losses actually sus- 

 tained in historical floods; therefore, specify the 

 year and month of all significant recorded dis- 

 charges above zero point of damage and indicate 

 the damages actually sustained by reach or zone 

 and type of property and activity. Historical data are 

 often incomplete; urbanization and other changes 

 will have occurred over the years. Many streams 

 and reaches do not have gaging stations. There- 

 fore, data on historical flood losses should be care- 

 fully scrutinized and supplemented by appraisals, 

 use of area depth-damage curves, and an inventory 

 of capital investment within the floodplain. Further, 

 estimates of damages under existing conditions 

 should be computed for floods of magnitude that 

 have not historically occurred. Estimate average 

 annual losses by using standard damage-frequency 

 integration techniques and computer programs that 

 relate hydrologic flood variables such as discharge 

 and stage to damages and to the probability of oc- 

 currence of such variables. Annual hydrologic data 

 are normally sufficient for urban drainage estimates. 

 Access flood da.mages by activity-type and by 

 whether they are borne by the owner or by the 

 public at large. 



2.4.11 Evaluation procedure: Step 7— Project 

 future flood damages. 



Future flood damages are the dollar damages to 

 economic activities identified in step 3 that might 



use the floodplain in the future in the absence of a 

 plan. Use this step in combination with step 5 (land 

 use) to determine land use and associated dam- 

 ages for each future with-project and without-pro- 

 ject condition. "Future" is any time period after the 

 year in which the study is completed; in order to 

 relate costs ultimately to benefits, however, future 

 damages must be discounted to the base year. De- 

 termine future flood damages on the basis of 

 losses sustained both by the floodplain occupant 

 and by others through insurance subsidies, tax de- 

 ductions for casualty losses, disaster relief, etc. 



(a) Hydrologic changes. Changes in basin land 

 use may result in major alteration of drainage char- 

 acteristics, particularly surface runoff; project such 

 hydrologic changes for the planning period. Aver- 

 age future hydrologic conditions should not be 

 used, since they obscure situations in which the 

 level of protection afforded by a project may be sig- 

 nificantly different from average conditions by the 

 end of the planning period. 



(b) Economic changes. Economic changes can 

 be expected to result in a change in the level of 

 future flood losses. A benefit-cost ratio for the ex- 

 isting condition should always be shown. If the ratio 

 is greater than 1:1, the projection of future benefits 

 may be accomplished in abbreviated form unless it 

 would distort the comparison of alternative projects 

 or the cost allocation and cost sharing in multiple- 

 purpose projects. In the latter situation, the detail 

 and accuracy of the estimates of flood control 

 benefits should be comparable to the estimates of 

 benefits for other water resources purposes. 



(c) Projection of physical damages. Base mea- 

 surement and projection of flood damages on the 

 establishment of actual, observed relationships be- 

 tween damages, flood characteristics, and those in- 

 dicators used for measurement and projection. 

 These relationships should be modified as appropri- 

 ate by consideration of constraints that change the 

 historically derived relationship between flood dam- 

 ages and a given indicator. The relationships 

 should be made explicit in the report and their ac- 

 curacy and representativeness supported, to the 

 extent possible, by empirical evidence. Use three 

 steps in measuring flood damages for a future year: 

 estimate the number and size of physical units; es- 

 timate the future value of units; and determine the 

 damage susceptibility of units. 



(1) Physical units. The first step in measuring 

 flood damages for a future year is to determine 

 from step 2 (2.4.6) the number and size of physical 

 units with potential to use the floodplain by hazard 

 zones for each activity type. Care must be taken to 

 determine whether existing structures will continue 

 to occupy the floodplain over the period of analysis 



36 



