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I I I I I I I I I, I I I I I I I I I' I SECTION 2.0 - DRAINAGE BASIN CHARACTERISTICS 2.1 Location Ralston Creek originates near SH 119 in unincorporated Gilpin County, Colorado and flows easterly to its confluence with Clear Creek in Arvada, Colorado. This watershed is located northwest of Denver, Colorado, north of Interstate 70 and west of Sheridan Boulevard. The western edge of the watershed is located within the foothills and includes Golden Gate Canyon State Park. This FHAD update only focuses on Ralston Creek from the Arvada/Blunn Reservoir to its confluence with Clear Creek, and Leyden Creek from Leyden Lake to its confluence with Ralston Creek. This portion of the watershed is located partly within the City of Arvada and partly within unincorporated Jefferson County. Figure 1 depicts the location and extents of the watershed. 2.2 Topography and Physiography The entire Ralston Creek watershed drains an area of approximately 92.3 square miles. Of this total, 11.9 square miles are contributed by the Leyden Creek watershed, and 17.7 square miles are contributed by the Van Bibber Creek watershed. See Figure 1 for the Ralston Creek Watershed Delineation. The basin originates to the west in unincorporated Gilpin County at an elevation of 10,390 feet. The creek flows east to its confluence with Clear Creek located in the City of Arvada near Sheridan Blvd. and Interstate 76 at an elevation of 5,250 feet. The western portion of the basin consists primarily of mountain and foothills terrain with rolling, grassy meadows surrounded by lodgepole pine forest, intermixed with smaller stands of aspen, fir, and spruce. The average ground slope of the upper basin varies from approximately 20 to 50 percent. The lower basin transitions from the foothills terrain to high plains. Development below Ralston Reservoir and Leyden Lake consists of both rural and urban zones. The area downstream of the confluence between Leyden and Ralston Creeks is densely developed with a combination of residential, commercial, and industrial uses. In many cases, development has encroached into the Ralston Creek floodplain. The average ground slope of the lower basin varies from 2 to 18 percent. Soils in the basin are typically C and D hydrologic classifications as defined by the Soil Conservation Service. These soils are generally characterized by low permeability and high runoff. The Leyden Creek portion of the basin extends from Leyden Lake to the confluence with Ralston Creek near Simms Street and nnd Avenue. The elevation of this basin ranges from 7,500 feet at the headwaters to 5,400 feet at the confluence with Ralston Creek. The upper basin is sparsely developed, with the extreme lower basin developed primarily with residential uses. The Van Bibber Creek basin begins at an elevation of 9,700 feet in the mountains and foothills just south of the Ralston Creek basin. The upper basin has similar characteristics to the upper Ralston Creek basin. Like the Leyden Creek basin, the Van Bibber Creek basin is generally undeveloped, except for the extreme downstream end, which is characterized by substantial commercial development. Some channel relocations have been made to the channel at the extreme downstream end. The confluence with Ralston Creek occurs near 58th Avenue and Ralston Road at an elevation of 5,300 feet. The Van Bibber Creek subbasin is not included as part of this FHAD study. 2.3 Flood History In general, the streams within the City of Arvada are well defined with relatively narrow channels. Potential flooding problems along the streams can be attributed to the large developed areas that have encroached into the overbanks of the channels severely constricting the floodplain and diminishing the carrying capacity of large flood flows. Although the potential exists for severe flooding, there is little information regarding past floods on the study streams. Records of past flooding could not be identified during the reconnaissance and library searches. It is likely that severe flooding has occurred on the study streams, which were not recorded since much of the development on the streams has occurred. 2.4 Flood Related Studies 1976 Ralston Creek and Leyden Creek, Phase A, Development of Alternate Plans, by Wright-McLaughlin Engineers, dated May 1976 1977 Major Drainageway Planning Ralston/Leyden Creek by Wright - McLaughlin Engineers, for Urban Drainage and Flood Control District, City of Arvada, and Jefferson County, dated September 1977. 1983 Modification to the Ralston Creek Floodplain, by Hydro-Triad Ud., dated March 1983 1984 Major Drainageway Planning, Lower Ralston/Van Bibber!Leyden Creeks, Phase B Report by Wright Water Engineers, Inc., for Urban Drainage and Flood Control District, the City of Arvada, and Jefferson County, dated March 1984 1986 Major Drainageway Planning Lower RalstonlVan Bibber/Leyden Creeks Phase A Report by Wright Water Engineers, Inc., for Urban Drainage and Flood Control District, the City of Arvada, and Jefferson County, dated February 1986. 3