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<br />upstream flood hydrograph and then routed through the ponding <br />area. The hydrograph after the routing process is called the <br />routed hydrograph. The hydrograph's peak flow is reduced and <br />the timing of the flow is delayed because of the impoundment <br />effect. However, the flood volume is not changed. <br /> <br />Figures 4-2 and 4-3 represent the routed hydrographs at street <br />crossings in Basin 6100 and Basin 6200, respectively. Table 4-5 <br />summarizes the routed peak flow at each location. The peak <br />flow rates obtained from the routed hydrograph were used as <br />the discharge input in the backwater curve analysis discussed <br />below. <br /> <br />HYDRAULICS <br /> <br />A modified backwater curve analysis program originally developed <br />by the U.S. Bureau of Reclamation was used to determine the <br />depth of floods with various frequencies. This analysis <br />determines the backwater surface level caused by flow restrictions <br />such as road crossings. By plotting the flood depths on plan <br />sheets with contours, the flood boundaries and flood plains <br />can be delineated. A flood plain is defined as the relatively <br />flat or lowland area adjoining a river or stream which has <br />been or may be covered temporarily by flood water. Volume II <br />of this report shows the flood plains or flood boundaries for <br />the 100-year projected peak under both existing channel conditions <br />and proposed channel improvements for future basin development. <br /> <br />The floodway for each drainage basin was also determined in <br />the study. A floodway is that portion of the adjoining flood <br />plain required to pass the 100-year projected flood with an <br />increase of 6 inches in flood stage above that of natural <br />conditions. Method 4 described ~n General Guidelines for <br />Development of Floodway Data, published by U.S. Army Corps of <br />Engineers, March 1972, was used to calculate this information. <br />The floodway data are tabulated On sheet 2 in Volume !I of <br />this report. <br /> <br />4-8 <br /> <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 />