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Gold Star Canyon Floodplain Information Report Mesa County, Colorado Hydraulic Analysis 4.0 with the equation The shallow concentrated flow average velocity can be calculated for unpaved areas V = 16. A Limited Detailed floodplain analysis of the hydraulic characteristics of Gold Star 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. 1345 * S05 V = 20.3282 * S05 IS For paved areas, the equation Where S = Catchment' Cross section data for the backwater analysis of Gold Star 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 Gold Star 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 on the Floodplain Maps (Sheets 1 through 5) and Flood Profiles (sheets 1 through 3) 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 of300 cfs was used to determine an average s velocity There are three main structures: South Broadway which is a 1951 concrete box culvert that has less than 100-year capacity as evidence by overtopping in 1996, 20 Road which is a 60" corrugated metal (CMP) that overtops in a 100-year event, and Broadway which is a 84" CMP also with less than year capacity. Additionally, there are three ditch crossings that obstruct the flow: Hinderlider Redlands Second Lift Canal, and Redlands First Lift Canal pIpe 100- Lift Canal These parameters include the following GIS software determined some of the subbasin parameters ) mI Contributing Drainage Area (sq Catchment Length (mi.) Catchment Slope (ft/ft) . . . In the upper reaches of the developed drainageway between the boundary of the Colorado Nationa Monument and the Redlands First Lift Canal the drainageway is not well defined. Development has filled in the channel and large storm events create shallow sheet flooding over a broad area of 100 to 600 feet wide. Downstream, the channel is more incised and well defined confining the floodplain to usually ess than 100-feet wide 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 89. Composite Curve Numbers were developed for the sub-basins and time of concentration calculations performed. For the urbanized reaches, the percentage of residential properties 1J4 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 Gold Star is an intermittent channel that is normally dry, except during storm events or when the irrigation ditches discharge water into the channel. Manning's "n" roughness coefficients used in the hydraulic computations were chosen by engineering judgment and based on field observations of the channel and overbank floodplain areas. The lower reaches of the channel, downstream of the Redlands First Lift Canal, receive water from irrigation turn-outs and therefore support a very dense growth of vegetation. The overbank is generally less dense with vegetation due to a lack of water and often has a lower roughness than the channel. The upstream reach near the Monument is normally dry and relatively clear of vegetation. In the middle reach along Cunningham Court road, the channel has been obliterated by development and therefore has a low roughness. The ranges of roughness coefficient values for Gold Star Canyon are as follows Overbank 0.04-0.07 Channel 0.025-0.100 Stream Gold Star Canyon 100-YR DISCHARGE (cfs) 470 650 754 775 504 577 597 The results of the hydrologic analysis are as follows TABLE 1 HYDROLOGIC SUMMARY TABLE DRAINAGE AREA (acres) (sq. mi.) 492.7 751.7 951.7 1,037.2 1,999.7 2,209.3 2.255.2 0.7698 1.1745 1.4870 1.6206 3.1246 3.4521 3.5237 LOCATION South Broadway culvert Between 20 Road & Redlands First Lift Canal Redlands First Lift Canal near 20 Road Upstream of US Highway 340 US Highway 340 Broadway near Terrace Drive Outfall to Colorado River DESIGNATION Design Point 2 Junction 8 Junction 7 Junction 9 Junction 5 Junction 6 Sink 1 D DP2 J8 J7 J9 J5 J6 S1 water surface elevation of the Colorado River. Since 100-year floodplain, this assumption has very little started downstream with the 100-year is steep as it enters the Colorado River Gold Star floodplain The model the channe impact on the It was assumed that all culverts and channels were generally free of silt and debris. The hydraulic analysis for this study was based on unobstructed flow at all structures, except the South Broadway box 3