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Mesa County, Colorado Red Canyon Floodplain Information Report shallow Hydraulic Analysis A Limited Detailed floodplain analysis of the hydraulic characteristics of Red Canyon was performed to provide estimates of the water surface elevations in the drainageway during the 100-year storm event. Water-surface elevations were computed using the U.S. Army Corps of Engineers HEC-RAS computer modeling program, Version 3.1.3 4.0 with the equation: concentrated flow average velocity can be calculated for unpaved areas V = 16.1345 * SO V = 20.3282 * SO the equation F or paved areas The Cross section data for the backwater analysis of Red Canyon was obtained from the Mesa County GIS Department data with 2-foot contour intervals. The contour information was imported into GIS and cross-sections were cut electronically. These prepared cross-sections were imported directly into HEC- GeoRAS for use as the base model. Information regarding structures that cross Red Canyon was compiled from a combination of drawings, structural reports, conversations with local officials, and field measurements. Locations of the cross sections used in the hydraulic analyses are shown on the Flood Profiles and on the Floodplain Maps (Sheets 1 through 6). There are two main structures: South Broadway which is a concrete arch that has 1 OO-year capacity, and South Camp Road which is a corrugated metal arch pipe with slightly less than 100-year capacity. Additionally, there is a pedestrian trail immediately upstream of South Camp Road which will overtop regularly with large storm events. Slope (ft/ft) Average velocity was determined with the Haested Methods Flowmaster computer program. This program uses Manning's Equation to determine a normal flow depth and normal velocity. The assumptions that were made to determine the velocity were a trapezoidal channel with a 12-foot base width, 3: 1 side slopes and a Manning's roughness coefficient of 0.05. A flowrate of 300 cfs was used to determine an average velocity. Where S = Catchment's include the following: GIS software determined some of the subbasin parameters. These parameters were chosen by engineering The ranges of roughness The model was started with a normal depth calculation water surface elevation assuming a 1 % slope as it enters the Colorado River 1 OO-year floodplain. Overbank 0.03-0.08 Manning's "n" roughness coefficients used in the hydraulic computations judgment and based on field observations of the wash and floodplain areas. coefficient values for Red Canyon are as follows Channel 0.025-0.07 Stream Red Canyon mi.) Curve Number calculations were divided into two reaches: 1) within the Colorado National Monument, and 2) the urbanized basin downstream. For the areas within the Monument the guidance from the NRCS was followed. SCS soil classifications were imported into GIS and divided into "rock" and "non- rock" categories, and classified by SCS hydrologic soil type "A", "B", "c" or "D". The Curve Number for the Pinyon-Juniper areas in "D" soils is 80, and the Rock Outcrops in "D" soils is 88. Composite Curve Numbers were developed for the sub-basins and time of concentration calculations performed. For the urbanized reaches, the percentage of residential properties V. acre or less and residential properties 2 acres or more were assigned a CN based upon hydrologic soil type. Composite CN were again developed. See the attached calculation worksheets. Contributing Drainage Area (sq. Catchment Length (mi.) Catchment Slope (ft/ft) It was assumed that all culverts and channels were free of silt and debris. The hydraulic analysis for this study was based on unobstructed flow at all structures. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. The hydraulic analysis of the study area was based on subcritical depth computations and information obtained from available GIS topographic data and field reconnaissance. The model was prepared assuming that the flow is steady, gradually-varied, and can be modeled in one-dimension. The results of the HEC-RAS hydraulic computer model can be seen in Table 5 ~ Flood Frequency ~ Elevation and Discharge Data. 100-YR DISCHARGE cfs Applications The National Flood Insurance Program (NFIP), encourages State and local governments to adopt sound Floodplain Management Programs. Therefore, each Flood Insurance Study provides 1 OO-year flood elevations and delineation of the 1 OO-year floodplain boundary to assist communities in developing floodplain management measures 3 Floodplain Management 4.1 688 1,594 1,715 1,778 1,912 2,041 2,036 Valley Filing DRAINAGE AREA sq. mi 1.3427 2.9326 ""'3:'1'8 3.8785 4.2149 4.7295 4.7 455 Manument Headwaters Calarada Natianal Manument Colarada Natianal Manument baundar" Sauth Cam p Raad * Upstream af Galf Caurse Dawnstream af Galf Caurse Near West Scenic Drive Outfall to. Co.lo.rado. River * Reference LANDesign, Lid. Hydrolagic & Hydraulic Analysis af Red Canyan Creek No. 5, dated March 1994 ~ 859.3 1,876.9 2,035.2 2,482.3 2,697.6 3,026.9 3,037.1 of the hydrologic analysIs are as follows TABLE 1 HYDROLOGIC SUMMARY TABLE The results LOCATION ID I DESIGNATION Junctio.n 1 Junctio.n 2 Junctio.n 3 Junctio.n 4 Junctio.n 5 Sink 1 J1 J2 J3 J4 J5 S'i