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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Sub-basins <br /> <br />The Niver Creek basin was divided into nine sub-basins, designated <br />N-l through N-9, shown on Plate 11-1; and also, each of the two tributaries <br />were divided into three sub-basins. The areas, lengths, distances to centroids, <br />percentage of pervious and impervious area, unit hydragraph coefficients and <br />assumed infiltration rates were computed for each sub-basin. It is nat usual <br />practice ta divide such a small drainage basin into so many sub-basins, but W'i <br />felt that it was the only way to make a reasonable estimate of the runoff at each <br />drainage structure and critical section. A tabulation of the sub-basin charac- <br />teristics, reprinted from the Phase A report, is shown on Table 11-1 at the end <br />of this chapter. <br /> <br />Unit Hydragraphs <br /> <br /> <br />Unit hydragraphs were prepared for each of the 15 sub-basins under <br /> <br />existing and under future conditions; a total of 30 unit hydragraphs were pre- <br /> <br />pared. Then, the design rainFalls were applied to the unit hydragraphs to <br /> <br />develop the storm hydragraphs. Appropriate values for losses, as recommended <br /> <br />in the Urban Drainage Criteria Manual, were used. An average infi Itratian <br /> <br />rate of one-half inch per hour, pervious storage of 0.3 inch, impervious stor- <br /> <br />age of O. 1 inch and 5 percent losses were used in all cases. <br /> <br />Storm Hydragraphs <br /> <br />The storm hydrographs of the 15 sub-basins were added tagether, taking <br />the different timing and conveyance characteristics of each reach into account. <br />The storage and retarding capacities of each bridge, culvert and conduit were <br />estimated, and the resulting combined flows were found. We selected 16 "design <br />points", or critical places along the watercourses where it WClJld be useful to <br />know the flow at each design frequency. <br /> <br />11-4 <br />