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<br />DESCRIPTION OF STUDY AREA <br /> <br />About So percent of the city of Northglenn is drained by Grange Hall Creek, <br />an easterly flowing tributary of the South Platte River. The drainage area of the <br />Upper Grange Hall Creek drainage basin is about 1,950 acres or 3.05 m~ On the <br />basis of urban development, the drainage basin was divided into two subbasins <br />(fig. 2). The larger subbasin (about 1,600 acres or 2.50 mi2) is referred to as <br />the downstream subbasin and the smaller subbasin (about 350 acres or 0.55 mi2) is <br />referred to as the upstream subbasin. During July 1979, sewer construction along <br />I04th Avenue west of Interstate Highway 25 altered both the total area of the <br />Upper Grange Hall Creek basin and the areas of the subbasins. The total area was <br />increased by 12.6 acres, with the area of the downstream subbasin being increased <br />by 22.2 acres and the area of the upstream subbasin being decreased by 9.6 acres. <br />Noncontributing areas that drain into irrigation canals occur in both subbasins-- <br />19 acres in the downstream subbasin and 3.2 acres in the upstream subbasin <br />(fig. 2). <br /> <br />About 66 percent of the Upper Grange Hall Creek basin is pervious and about <br />34 percent is impervious (24-percent effective impervious and 10-percent noneffec- <br />tive impervious). Land use consists of about 49-percent single-family residential, <br />2-percent multifam~residential, 14-percent commercial, 35-percent undeveloped, <br />and less than I-percent industrial. Other than Northglenn Mall, the largest com- <br />mercial or industrial activities in the basin are a cemetery (considered as un- <br />developed space) and a premixed-cement plant, both located in the downstream sub- <br />basin (fig. 2). <br /> <br />About 73 percent of the downstream subbasin is pervious and about 27 percent <br />is impervious (IS-percent effective impervious and 9-percent noneffective impervi- <br />ous). Land use consists of about 50-percent single-family residential, 2-percent <br />multifamily residential, 6-percent commercial, 42-percent undeveloped, and less <br />than 1-percent industrial. <br /> <br />About 66 percent of the upstream subbasin is impervious (55-percent effective <br />impervious and II-percent noneffective impervious) and about 34 percent is pervi- <br />ous. Land use consists of about 42-percent single-family residential, 2-percent <br />multifamily residential, 53-percent commercial (principally, Northglenn Mall), and <br />3-percent undeveloped. <br /> <br />There are eight reservoirs in the downstream subbasin (fig. 2). Seven reser- <br />voirs are located along the unnamed southern tributary of Grange Hall Creek, ar.d <br />one, a large flood-detention reservoir that is usually dry, is located on Grange <br />Hall Creek immediately upstream from Washington Street. The only reservoir in the <br />upstream subbasin is a small pond on Grange Hall Creek downstream from Northglenn <br />Mall (fig. 2). <br /> <br />4 <br />