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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 />their lengths in terms of channel cross sect ion, side slopes, stream bed <br />(thalweg) slopes, roughness and channel material. Detailed mapping as <br />described el sewhere in this report has been prepared for the hydraul ic <br />analyses of these major waterways. <br /> <br />characterized by sand, gravel, silt and clay ranging in depths of 0 feet to <br />60 feet, with slopes of 0 to 15 percent. <br /> <br />Vegetation in Dad Clark Gulch basin can be characterized by four major <br />plant communities, grassland, shrubland, cropland and riparian woodland. <br />The predominant vegetation type is grassland. Shrubs and wooded areas are <br />located mainly along the larger drainage channels with low growing shrubs <br />sometimes overgrowing the channel bottoms. The cropland in Dad Clark Gulch <br />basin lies mainly in the upper reaches near the existing Highlands Ranch <br />head quarters. <br /> <br />The Highlands Ranch Development Plan delineates that major waterways and <br />significant surround ing area wi 11 remain in nonurban use; therefore, for <br />hydrologic analysis, the hydraulic lengths and slopes of the major basin <br />channels are assumed not to change between existing and future development <br />conditions. It should be noted here, however, that the presence of a <br />number of existing check dams within the Dad Clark drainage basin have been <br />ignored for the purposes of establishing hydraulic slopes since these check <br />dams are not considered permanent structures (i.e., these check dams could <br />be washed out during a future flood and it is unl ikely that they would be <br />repl aced). On the other hand, the presence of these check dams was con- <br />sidered in determining the maximum water surface profile and lOO-year <br />flooding limits since it is possible that these dams may not fail imme- <br />diately during heavy flooding. <br /> <br />C. CLIMATOLOGY AND FLOOD HISTORY <br /> <br />The climate over the Dad Clark Gulch drainage basin can be classified as <br />semi-arid. The average annual precipitation in the Dad Clark Gulch drain- <br />age basin is 17 to I9 inches, 75 percent of which occurs during the months <br />of April through September. The period of heaviest precipitation is during <br />these spring and summer months. Snowmelt is not usually a contribut ing <br />factor. Numerous thunderstorms during these months, while generally <br />limited in area, produce occasional periods of intense rainfall. <br /> <br />Existing culvert-type road crossings at County Line Road and the Highlands <br />Ranch access road, as well as the Highl ine Canal crossing have been in- <br />cluded in the hydraulic modeling of the Dad Clark Gulch flood plain. <br />Proposed future culvert-type road crossings have been located and have also <br />been analyzed for their potential backwater effects on the flood plain. <br /> <br />Most of the major floods from small watersheds in this area have resulted <br />from intense thunderstorm activity. The thunderstorms are generally the <br />result of convective act ivity occurring during the afternoon or early <br />evening. These storms are generally of short duration and moderate in- <br />tensity. The most severe thunderstorms are associated with frontal activi- <br />ty coupled with orographic effects. <br /> <br />The topography of the Dad Clark Gulch basin ranges from rolling rangeland <br />in the majority of the basin to steeply-eroded, northwest-trending ridges <br />in the upper one-quarter of the basin. Elevations range from approximately <br />5,458 feet at County Line Road to approximately 6,300 feet (above mean <br />sea level) at the uppermost drainage basin boundary. <br /> <br />No specific flood history is available for Dad Clark Gulch. In November <br />1961, hydrologic calculations were prepared for the then proposed Dad Clark <br />Gulch Dam (Mclellan Reservoir) wherein, with the drainage basin assumed to <br />be fully urbanized, the 1DO-year flood was estimated to be 8,200 cubic feet <br />per second at the dam, with a drainage area of 9.22 square miles (up to the <br /> <br />Dad Clark Gulch basin lies near the southwestern edge of the Denver <br />basin, an area extending from Colorado Springs north to Greeley, and from <br />the foothills eastward approximately 60 miles onto the plains. The Denver <br />and Dawson Formations underlie the Dad Clark Gulch drainage basin, and are <br /> <br />3 <br />