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<br />SECTION 2.0 - WATERSHED DESCRIPTION
<br />
<br />2.1
<br />
<br />Location
<br />
<br />The study area includes the entire Ralston Creek watershed, including the Leyden Creek and Van Bibber
<br />Creek watersheds, This watershed is located northwest of Denver, Colorado, north of Interstate 70 and west
<br />of Sheridan Boulevard, The western edge of the watershed is located within the foothills and includes
<br />Golden Gate Canyon State Park, The watershed is located partly within the City of Arvada and partly
<br />within unincorporated Jefferson County, Figure 1 of Appendix A depicts the location and extents of the
<br />watershed,
<br />
<br />2.2
<br />
<br />Topography and Physiography
<br />
<br />The entire Ralston Creek watershed drains an area of approximately 92,3 square miles, Of this total, 11.9
<br />square miles are contributed by the Leyden Creek watershed, and 17,7 square miles are contributed by the
<br />Van Bibber Creek watershed,
<br />
<br />The basin originates to the west in unincorporated Gilpin County at an elevation of 10,390 feet. The creek
<br />flows east to its confluence with Clear Creek located in the City of Arvada near Sheridan Blvd, and
<br />Interstate 76 at an elevation of 5,250 feet, The western portion of the basin consists primarily of mountain
<br />and foothills terrain with rolling, grassy meadows surrounded by lodgepole pine forest, intermixed with
<br />smaller stands of aspen, fir, and spruce,
<br />
<br />The lower basin transitions from the foothills terrain to high plains, Development below Ralston Reservoir
<br />and Leyden Lake consists of both rural and urban zones, The area downstream of the confluence between
<br />Leyden and Ralston Creeks is densely developed with a combination of residential, commercial, and
<br />industrial uses, In many cases, development has encroached into the Ralston Creek floodplain,
<br />
<br />Soils in the basin are typically C and D hydrologic classifications as defined by the Natural Resource
<br />Conservation Service, These soils are generally characterized by low permeability and high runoff.
<br />
<br />The Leyden Creek portion of the basin exlends from Rocky Flats to the confluence with Ralston Creek near
<br />Simms Street and nnd A venue, The elevation of this basin ranges from 7,500 feet at the headwaters to
<br />5,400 feet at the confluence with Ralston Creek, The upper basin is generally undeveloped, with the
<br />extreme lower basin developed primarily with residential uses,
<br />
<br />The Van Bibber Creek basin begins at an elevation of 9,700 feet in the mountains and foothills just south of
<br />the Ralston Creek basin, The upper basin has similar characteristics to the upper Ralston Creek basin, Like
<br />the Leyden Creek basin, the Van Bibber Creek basin is generally undeveloped, except for the extreme
<br />downstream end, which is characterized by substantial commercial development. Some channel relocations
<br />have been made to the channel at the extreme downstream end, The confluence with Ralston Creek occurs
<br />near 58th A venue and Ralston Road at an elevation of 5,300 feet,
<br />
<br />Numerous inigation canals exist in the City of Arvada and many of the canals intercept stormwater flows
<br />from frequent storm events, Larger flows fill and overtop the canals, For modeling purposes, it was
<br />assumed that flows from all storm events bypass the canals with no interception,
<br />
<br />2.3
<br />
<br />Flood History
<br />
<br />In general, the streams within the City of Arvada are well defined with relatively narrow channels, Potential
<br />flooding problems along the streams can be attributed to the large developed areas that have encroached into
<br />the overbanks of the channels severely constricting the floodplain and diminishing the carrying capacity of
<br />large flood flows, Although the potential exists for severe flooding, there is little information regarding past
<br />floods on the study streams, Records of past flooding could not be identified during the reconnaissance and
<br />library searches, It is likely that severe flooding has occurred on the study streams which was not recorded
<br />since much of the development on the streams did not occur until recently,
<br />
<br />2.4
<br />
<br />File Naming Convention
<br />
<br />The models were prepared using the following naming convention:
<br />. CUHP files: rlxxxyy,chi CUHP Input File
<br />rlxxxyy,chd Data file for UDSWMM
<br />rlxxxyy,cho CUHP Output File
<br />
<br />where:
<br />
<br />xxx = frequency (i,e" 010, 050, 100, 500)
<br />yy = Model Description (see below)
<br />
<br />yy indicators:
<br />
<br />un = unadjusted model (used only for design points with tributaries less than 10 sq, mi.! otherwise,
<br />disregard)
<br />
<br />al = basinwide adjusted model (adjusted for 92 sq, mi,) - these values could be used anywhere, but they
<br />were compared to see if another appropriate model for a particular point resulted in higher values
<br />
<br />a2 = rainfall adjusted for western and central areas (adjusted for 49 sq, mi,) - used values from this model
<br />only for Ralston mainsfem above the outlet of ArvadaIBlunn Reservoir
<br />
<br />a3 = rainfall adjusted for eastern areas (adjusted for 40 sq, mi,) - used values from this model for Ralston
<br />mainstem below Ralston Reservoir
<br />
<br />a4 = rainfall adjusted for central and eastern areas (adjusted for 10-20 sq, mi,) - used these values for
<br />Leyden and Van Bibber only for design points with >10 sq, mi, tributary area (used unadjusted model for
<br />areas of these tributaries <10 sq, mi,)
<br />
<br />.
<br />
<br />UDSWMM files:
<br />
<br />rl2002,sin
<br />w02002,sin
<br />rlxxxyy,sot
<br />woxxxyy,sot
<br />
<br />UDSWMM Input File, with all reservoirs
<br />UDSWMM Input File, excluding Arvada Res/Leyden Lake
<br />UDSWMM Output File, with all reservoirs
<br />UDSWMM Output File, excluding Arvada Res/Leyden Lake
<br />
<br />3
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