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<br />22 <br /> <br />the curves necessary to evaluate average annual damage reduction. The <br />method consists of computing the flood damages for particular storm <br />events with and without a proposed drainage facility. The computed <br />damages are then plotted against the return interval (or frequency) <br />of their respective storms. The two sets of damage frequency plots <br />(with and without the facility) are connected to yield two damage <br />frequency curves as shown in Figure 111-1. The area between these <br />two curves is the reduction in average annual damages. <br />This then is the total benefit of the proposed project. However, <br />it is not necessarily the total benefit that will accrue to new <br />developments. Two other damage frequency curves are needed to estimate <br />the proportionate benefit of the facility that accrues to new develop- <br />ment. These two curves are also to be constructed during the Initial <br />Drainage Study. They are the damage frequency curves for existing <br />conditions with and without the proposed drainage facility. The <br />damage frequency plotting positions are computed in exactly the same <br />manner as the previous two curves except the hydrology used and the <br />damages estimated are those for existing conditions with and without <br />the proposed drainage facility. These two curves are plotted with the <br />"ultimate" development curves as shown in Figure III-2. The special <br />and general portions of the project cost are then computed as follows: <br />Step 1 -- Calculate average annual damage reductioQ (AADR) of the <br />project. <br />The economic benefit of a drainage project is computed as the <br />average damage reduction expected each year (or AADR) after the project <br />is constructed and after the basin reaches ultimate development. It is <br />the area in Figure III-2 between the Ul and the U2 curves. <br /> <br />'> <br />