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<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
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