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~ variables for the entire reach. In the case of damage, several relation- ships are usually used, each representative of a particular damage category. The principal reason for computing flood damage is to determine the effectiveness of different flood plain management plans in reducing damage. This reduction is commonly referred to as an inundation reduction benefit and is measured as the difference in equivalent annual flood damage with and without a plan. Different management plans alter the stage, flow, frequency, and damage relationships in different ways (see Exhibit 1 for a detailed discussion). With a different relationship the damage is different--usua11y 10wer--than without the plan. Thus, for any plan which causes a change which can be quantified, damage with the plan can be computed. Without a plan, damage is still likely to change. Increases in damageable property in existing structures, residual damage to new structures, increases in stage from greater or more rapid runoff all can cause a change in future damage., 6. REACH SELECTION Frequency, flow, stage and damage data are used for each reach; thus, these data must be representative of the actual frequency of flood events, flow regime, and flood damage for that reach. Reach lengths should be selected to allow segregation of output as required for specific study needs. For example, for plan formulation output may be needed in sufficient detail that changes in stream and development characteristics are identified. Generally, hydraulic and hydrologic factors govern the selection of the index location. Frequency data input for one reach will be used for subse- quent reaches until another data set is input (FR cards). This does not hold for stage, flow or damage data (QF, SF, SQ, QS, SD, QD or DG cards). These data must be input for each reach. 7. FLOOD PLAIN MANAGEMENT PLANS The expected annual flood damage with any structural or nonstructura1 flood plain management plan can be computed provided the effect of such a plan can be expressed in terms of the stage, flow, frequency or damage data used by the program. Such plans can include nonstructura1 measures such as relocation, flood proofing, e1evation-on-fi11 or such structural measures as reservoirs, channelization and levees. A discussion of the effect of various measures on the stage, flow, frequency and damage relationships is presented in Exhibit 1. Up to nine flood plain management plans can be evaluated. Each plan's name and number are input (PN card) and the corresponding stage, flow and 5