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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />within this watershed (Figure 2), Louisville Reservoir is located at the headwaters of this <br />watershed; Erie Lake, Prince Lake No. 1 and No.2 are located in the center and Henderson Pit <br />sand and gravel quarry is located at the downstream end, Major east/west roads crossing this <br />study area are Baseline Road, Arapahoe Road, and Isabelle Road and the north/south roads are <br />North l09th Street, North 107th Street (US 287) and North 95th Street (SH 42). Other major <br />physical features include the Burlington Northern Railroad, the Union Pacific Railroad, Davidson <br />Ditch, the Highline Canal, Goodhue Ditch, South Boulder Canyon Ditch, Leyner Cottonwood <br />Ditch, Lower Boulder Ditch, and Boulder and Weld Ditch, Marlell Lakes, and Elmwood <br />Reservoir, <br /> <br />2.2 WATERSHED CONDITIONS <br /> <br />The climate of the study area is classified as semi-arid, which is characterized by low relative <br />humidity, low rainfall, moderate to high wind movement, and a large daily range in temperatures. <br />Wide annual temperature extremes exist in the area ranging from -26 degrees to 102 degrees <br />Fahrenheit, The average annual precipitation is 14 to IS inches. An average of 69 percent of <br />this precipitation falls during thunderstonns which occur during the months of April through <br />September. <br /> <br />This watershed is comprised of two distinct drainage sub-basins which combine in the lower part <br />of the watershed and discharge into Boulder Creek, These basins drain from south to north at <br />an average slope of approximately 2 percent, The main watershed is called Bullhead Gulch and <br />consists of approximately 4,070 acres. Only the uppennost part of the basin, located within <br />Louisville, is currently urbanized, with the remainder of the watershed being primarily rural, <br />agricultural land with scattered, sparse development, <br /> <br />The soils are primarily of the hydrologic soil group B, which have a medium infiltration rate, <br />About 10 percent of the basin is in the soil group C, which has a low to medium infiltration rate. <br />The B-soils are of the Ascalon and Manter soils series; the C-soils are of the Colby/Weld, <br />Hargrave, Loveland, Niwot, Nunn, Renohill and Valmont soil series. The existing soils <br />infonnation was obtained from the "Soil Survey of Boulder County Area, Colorado" (SCS, 1975). <br />Figure 3 indicates the hydrologic soil types found within the study area. The loss rate values <br />used in this study are listed in Table 2, where "fi" is the initial infiltration rate, "fo" is the final <br />infiltration rate and "x" is the Horton's exponential decay rate, <br /> <br />The main tributary to Bullhead Gulch, named Prince Tributary for purposes of this study, consists <br />of approximately 1,670 acres and is located east of Bullhead Gulch. The confluence is located <br />just south of the Lower Boulder Ditch and west of North l09th Street. This sub-basin is <br />primarily rural, agricultural land with some low density residential property at the north end. <br /> <br />Table 2 <br />Infiltration Loss Rate Data <br /> <br />Hydrologic <br />Soil GrouplJ <br /> <br />fi <br />in/hr <br /> <br />fo <br />i!!L!!!: <br /> <br />x <br />per second <br /> <br />A smaller tributary to Bullhead Gulch, named Indian Peaks Tributary for purposes of this study, <br />consists of approximately 435 acres, It is located on the west side of Bullhead Gulch, primarily <br />between Arapahoe Road and Baseline Road, This sub-basin is currently rural, agricultural land <br />with a small amount of low density residential land near the headwaters, <br /> <br />B <br />C <br /> <br />4.5 <br />3,0 <br /> <br />0,6 <br />0.5 <br /> <br />0,0018 <br />0.0018 <br /> <br />1) For sub-basins consisting of two soil groups, a composite loss <br />rate was estimated by areal weighing. <br /> <br />FLOOD HAZARD AREA DELtNEATION <br />BUUJlEAD GULCH AND lRIBUTARIES <br /> <br />FINAL <br />7131191 <br /> <br />4 <br />