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<br />8 <br /> <br />Runoff Vo 1 umes <br />~-- <br /> <br />Runoff volumes were computed using standard Soil Conservation Service <br />methods (SCS, 1980). Precipitation amounts used in determining these <br />volumes were obtained from Figure 4. Table 3 summarizes weighted runoff <br />curve numbers (CN) and runoff volumes for drainage areas above each node. <br />Runoff volumes were computed using the areas summarized in Table 1 and <br />the 10-year, 6-hour precipitation (2.5 inches) as determined from Figure 4. <br />The values in parentheses include the proposed street improvements in the <br />reach between nodes 2 and 3. These improvements will increase the impervious <br />area by approximately 33 acres. Ten-year, 6-hour runoff volumes will be <br />increased by 2.5 acre-feet with no appreciable increase in peak discharge <br />at node 4. <br /> <br />Proposed Development <br /> <br />Table 2 shows how badly the Pioneer Drainage Ditch falls short of <br />controlling 10-year local storm runoff. Overflows occur at Nodes 1 and 2. <br />Between Nodes 4 and 5, flow depthS are such that severe backwater flooding <br />occurs in the Whitcomb Addition. The Ditch is not even capable of <br />controlling 5-year runoff events. Leaf and RCI (1983) contains maps <br />which show the extent of flooding from 5-year storm runoff. <br />Before developing near-term mitigation alternatives, consideration <br />was given to the possibility of the Ditch receiving even higher peaks and <br />runoff volumes from future development. In making this evaluation, it was <br />assumed that as a minimum, the Ditch would be required to absorb impacts <br />from future development in the non-stippled areas of Figure 2. The <br />stippled areas were assumed to contribute runoff at existing levels. <br />