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I I I I I I , I I I I I I I I I I I ,. I time of concentration, and "A" is the drainage area in acres, The areas (A) of the four basins were estimated by planimetering the delineated basins from the 1967 quad maps. The runoff coefficient (C) was estimated to be 0,41 for all basins assuming a 100-year design frequency and undeveloped pasture/range land with flat slopes (0 to 2 percent). In order to estimate the rainfall intensity (i), the times of concentration for the four basins were first estimated using SCS average velocity charts and normal depth calculations, A rainfall intensity-duration-frequency curve for Sioux City, Iowa (Iowa Department of Transportation) was used to obtain the 100-year rainfall intensities for the computed times of concentration. Application of the rational method with the above parameters yielded peak discharge values which are shown on plate 2. Culverts for the interior drainage were assumed to be cim1lar reinforced concrete pipe (RCP) and were sized with nomographs assuming inlet control. Due to the flat topography behind the levee, a maximum headwater to diameter of pipe (HWID) ratio of 1.0 was assumed so as to minimize the backwater ponding depth, The conceptual culvert sizes and locations are shown on plate 2, The culvert outlets would be fitted with flap gates to prevent floodwater in the Main Ditch from backing up through the pipes during high stage events. The ultimate culvert locations will have to be determined during final design. If possible, the inlets of the pipes should be located in sump conditions to maximize pipe capacities and to lTIinimi7e ponding depths behind the levees. The purpose of the cursory interior drainage analysis was to develop conceptual facilities and preliminary construction costs. The analysis includes assumptions that may not be suitable for final design purposes, Additional analyses for interior drainage facilities would be required for final design. Cost Estimates The preliminary construction costs for the proposed lOO-year and 500-year facilities are presented in table 2. The preliminary costs estimates include the major items required for construction of the proposed facilities. It was assumed that acceptable fill material for the levees could be obtained from the excavation of the channel improvements and the new diversion channel. For the 500-year levees, some fill material would be required from an unspecified borrow site. Miscellaneous items such as mobilization, clearing and grubbing, erosion protection, relocation of utilities, seeding, and restoration were not included. A 25 percent contingency was, however, included in the total estimated construction costs. 10