FLOOD02774 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I Several modifications were made to the existing-conditions HEC-2 input file in order to model the proposed improvements. All of the bridges within the model were removed so that channels and levees could be sized assuming no backwater effects. In addition, cross-section infonnation was modified in order to model the excavated channel reaches and the levees, Manning's "n" values were adjusted for the main channel to acCOlDlt for the excavated and maintained conditions of the Main Ditch, Nonnal depth calculations were also perfonned in order to size the proposed overflow diversion channel near the downstream end of the project. The proposed culvert crossing for the diversion at Hiawatha A venue was sized using an inlet control nomograph for concrete box culverts. DESCRIPTION OF EV ALUA TIONS AND CONCEPTUAL PLANS General For this Technical Assistance Study, proposed improvements were evaluated for the alternatives that were previously described in this report, The sub-sections which follow include discussions regarding the mapping, proposed improvements, interior drainage, cost estimates, floodplain issues, and structure acquisitions. Map'pin~ Available mapping which was used for this study includes topographic maps, a Flood Insurance Rate Map (FIRM), and one aerial photo, A description of each map is outlined below. TopoWlij)hic Mlij) of Pipestone. Minnesota . Mapping of Main Ditch area from Hiawatha Avenue to 9th Street NE . Developed from aerials taken June 26, 1986. . Developed for Banner Associates by Horizons, Inc. . Map Date: Unknown . Contour interval: 2 feet . Scale: I inch = 100 feet . Datum: None listed Flood Insurance Stuqy Work Mlij) . Base sheet: USGS Quadrangle map (1967) . Contour interval: 10 feet . Scale: 1 inch = 1000 feet . Datum: 1927 North American Datum . Work map developed by Omaha Corps (date unlrnown) . 100-year and 500-year flood boundaries and floodway plotted on map 5