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SECTIOItFOUR 4.1 METHODOLOGY FOR FLOODPLAIN DELINEATION Floods of the same or larger magnitude than those that have occurred in the past could occur within the Cherry Creek Corridor in the future. To determine the flood potential of the study area., the 10%, 2%, 1%, and 0.2% annual probability (i.e., 1O-year,50-year, loo-year, and 5oo-year, respectively) floods were analyzed. The results of this analysis are presented in this report as a means of demonstrating the effects of large floods. 4.1.1 Hydrology Discharge magnitudes for floods analyzed in this report were the same as used in the most recent FEMA FIS (FEMA, 1996). These discharges were based upon an analysis of stream gauging data at the USGS stream gages located near Franktown and Parker by the U.S. Army Corps of Engineers (USACE, 1976). Information on these stream gages was presented in Table 2. Discharge- probability relationships for the upstream and downstream limits of the study reach were developed QSingdataJrom the Franktown.andParker stream, gages, respectively; through 1976. In the-atlalysls,'" the presence of the 32 floodwater retarding structures in the Cherry Creek watershed, constructed by the Soil Conservation Service, was taken into account. . The following table snmmarizes the FEMA PIS discharge data utilized in this report. The gage data was verified by a hydrologic model that was developed for the watershed and through a statistical analysis of gage data through 2001 (URS, 2002). Table 5 Summary of FEMA F1S Pertinent Discharges Used in Current Study Cherry Creek Watershed Upstream Limit 220 6,000 17,500 27,120 100,000 (Scott Road) Stroh Road 241 6,610 19,570 31,510 104,200 Main Street 287 7,730 23,040 37,180 118,100 Lincoln Ave 289 8,100 24,200 39,190 122,740 Cottonwood Drive 312 8,670 25,940 42,200 129,700 Arapahoe Road 333 9,880 29,698 48,738 144,790 Downstream Limit 360 10,300 31,000 51,000 150,000 4.1.2 Hydraulics Water surface profiles for the study reach in this report were developed using the River Analysis System HEC-RAS computer program, developed by the U.S. Army Corps of Engineers Hydrologic flTlBE ROODS Engineering Center (USACE, 2oo1a., 2oo1b). An auxiliary computer program, HEC-GEORAS, which was also developed by the U.S. Army Corp of Engineers (USACE, 2000), Was utilized to prepare the river cross sectional data for the HEC-RAS model. The GEORAS program was also used to develop the floodplain maps by importing results from the HEC-RAS hydraulic model. The GIS-based HEC-GEORAS program was first used to generate geometric data (cross sections, stream stations, channel and overbank lengths, etc) for the HEC-RAS hydraulic model. ThedigitaI topographic information for the HEC-GEORAS program was a Digital Terrain Model (DlM) generated from the topographic data collected from a 2002 survey conduCted by Aspen Surveying, Inc. The HEC-RAS hydraulic model was then developed by importing the geometric data from HEC-GEORAS, adding bridge/culvert data, and specifying design flow discharges and boundary conditions. The results from the HEC-RAS model were later exported to ArcView with the GeoRAS extension to generate flood inundation boundaries for the different design frequency storms. The flood inundation maps were used to identify structures or areas that could be flooded within the corridor. The July 2002 survey by TransVision, Inc. provided bridge data for the hydraulic model. The bridge survey data include bridge opening, deck elevations, deck thickness, 10wchordele~IIJigI!Jl,J!i~rn}.lIlJ,~ ;md_thickness. Since_ ,_ the roalIproillesexteiidirig uomthebndges were-not surveyed, the Digital Terrain Model was utilized to retrieve the road elevation data in order to model any possible overtopping flow over the bridges. The following bridges are included in the hydraulic model: Arapahoe Road bridge, Cottonwood Drive bridge, E- 470 bridge, Iincoln Avenue bridge, Main StreetlWest Parker Road bridge, Stroh Road bridge, and Scott Road bridge. The proposed Broncos Parkway Bridge, located just downstream of the confluence with Happy Canyon Gulch, has been approved for construction by regulatory agencies and thus was also included in the model. Several low-level pedestri;m bridges exist along the corridor but they are not included in the model as they are unlikely to impose noticeable impact to the flood elevations for the design flow discharges. For the existing condition, the existing FEMA PIS flow discharges for the 10-, 50-, 100-, and 5oo-year design storm events (FEMA, 1996; USACE, 1976) were used in the HEC-RAS model for this report. These flow discharges are summarized at several key locations in Table 5. Table 6 lists the flow discharges for all cross sections used in the current HEC-RAS model that was used to establish the current floodplain limits. The Manning's friction coefficient was assumed to vary between 0.035 and 0.045 for the main channel and between 0.060 and 0.10 for the overbanks (floodplain), which is consistent with the range of values used in the previous PIS (USACE, 1976). The downstream boundary condition for the hydraulic model was assumed to be normal flow control with a slope equal to the measured averaged channel slope at the downstream end of the model domain. Limits for two (2) separate loo-year floodways were developed, based on encroachment resulting in maximum increases in the hydraulic energy gradient of 0.5 feet ;md 1.0 feet. The floodways were developed by assuming equal reduction in conveyance to both sides of the floodplain. I I I I I I I I I I I I I I I I I I I 4.2 FREQUENCY OF FLOODS The 5oo-year flood is not the largest flood that can occur within the corridor but the probability of larger floods is remote. As can be seen from the gauging records for Cherry Creek, discharges smaller than either the loo-year or 5oo-year floods are much more common. Large floods, however, can happen; this was clearly demonstrated by the Denver Area floods of 1965, the July 1997 Flood at Fort Collins, Colorado, and the July 1976 flood in the Big Thompson Canyon near Loveland, Colorado. 4-1