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<br />I <br />I <br />I <br />I <br />I <br />I <br />i 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 />design capacity. In order to reduce excavation quantities and construction costs, the traversable <br />channel concept was abandoned in lieu of a more narrow trapezoidal channel with 3H to I V side <br />slopes. Obviously, the proposed channel would not be suitable for equipment to cross from one <br />side to the other. If desired, the diversion channel alignment upstream of Hiawatha Avenue <br />could be shifted to the southeast to minimize the amount of isolated agricultural land. <br /> <br />500- Y ear Flood Protection <br />A cursory evaluation was performed to identify the construction costs for channel modifications <br />and levee improvements that would provide a 500-year level of protection. For this analysis, <br />500-year levees were sized along the entire reach of the Main Ditch from Hiawatha Avenue to <br />the upstream study limit at County Road 69. Water surface elevations from the proposed <br />conditions HEC-2 model were used to determine the crest elevations of the proposed 500-year <br />levees along this reach. The conceptual levee heights do not account for any freeboard, Top of <br />levee elevations for 500-year protection are shown on plate 2 for the reach upstream of 9th Street <br />NE, however the 500-year levee is not depicted for the downstream portion of the study for <br />clarity, The 100-year channel improvements and the IOO-year diversion channel described <br />previously would be part of the 500-year protection plan, Most of the additional material <br />required for the 500-year levees could be obtained from the channel excavtion, assuming that the <br />material would be suitable. A portion of the fill material would have to be hauled in from an off- <br />site source. <br /> <br />Interior Drainllie Analysis <br />A cursory analysis was performed to evaluate the necessity for interior drainage features through <br />the proposed levees along the Main Ditch. The need for interior drainage facilities arises when a <br />levee, constructed along a waterway, prevents local surface runoff from draining into the <br />waterway. This is the case for the proposed levees located along portions of the Main Ditch. <br />The interior drainage areas behind the proposed levees were delineated on the 1967 USGS quad <br />maps for Pipestone North and Pipestone South, Minnesota, Four separate interior drainage areas <br />were determined and their basins were delineated on the quad maps. Most of the interior <br />drainage areas included undeveloped land, and therefore existing storm sewers were not taken <br />into consideration, The rational method was used to estimate peak discharges that would result <br />from runoff within the interior drainage areas, and a lOO-year design frequency was arbitrarily <br />chosen to size the required facilities. <br /> <br />The formula for the rational method is Q=CiA, where "Q" is the peak discharge in cfs, "c" is the <br />dimensionless runoff coefficient, "i" is the rainfall intensity in inches per hour for the computed <br /> <br />9 <br />