FLOOD05777 - Laserfiche WebLink

Home Browse Search

~.l Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for each flooding source studied in detail affecting the community. ~ Discharges on the Roaring Fork River, Castle Creek, and Maroon Creek were determined using a regional approach based on gage analysis, as described in U.S. Water Resources Council Bulletin No. l7A (Reference 5). The streamflow data were separated into snowmelt and rainfall peaks and each type was analyzed separately. . The following U.S. Geological Survey gages were used for the analysis. Roaring Fork River gage Nos. 0734 and 0735, with 55 years of snowmelt data and 41 years of rainfall data; Castle Creek gage No. 0748, with 13 years of snowmelt data and 12 years of rain- fall data, and Maroon Creek gage No. 757, with 13 years of snowmelt data. Discharges were developed from these data for the 10-, 50-, 100-, and 500-year floodflows for snowmelt- and rainfall-caused flood events, which were then statistically combined. The hydrologic analysis used is fully documented in a separate report, Hydrology Report for City of Aspen and Pitkin County (Unincorporated) Flood Insurance Studies (Reference 4). ' Peak discharge-drainage area relationships for Roaring Fork River, Castle Creek, and Maroon Creek are shown in Table 1. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the eleva- tions of floods of the selected recurrence intervals. Water-surface elevations of floods of the selected recurrence inter- vals were computed through use of the U.S. Army Corps of Engineers HEC-2 step-backwater computer program (Reference 6). . . Cross section data for the backwater analysis of the Roaring Fork River and Castle Creek were obtained from topographic maps at scales of 1.600 and 1:1,200, with a contour interval of 2 feet (Reference 7). Cross sections on Maroon Creek were digitized from aerial photographs and obtained from a previous Flood Plain Information report (Refer- ence 8). All bridges were field surveyed to obtain elevation data and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2). 5