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I I I I I I I I I I I I I I I I I I I their lengths in terms of channel cross section, side slopes, stream bed (thalweg) slopes, roughness and channel material. Detailed mapping as described el sewhere in this report has been prepared for the hydraul ic analyses of these major waterways. characterized by sand, gravel, silt and clay ranging in depths of 0 feet to 60 feet, with slopes of 0 to 15 percent. The Highl ands Ranch Development Pl an del ineates that major waterways and significant surrounding area will remain in nonurban use; therefore, for hydrologi<: analysis, the hydraul ic lengths and slopes of the major basin channels are assumed not to change between existing and future development cond it ions. It shoul d be noted here, however, that the presence of a number of existing check dams within the Dad Clark drainage basin have been i9nored for the purposes of establishing hydraulic slopes since these check dams are not considered permanent structures (i.e., these check dams could be washed out during a future flood and it is unl ikely that they would be repl aced). On the other hand, the presence of these check dams was con- sidered in determining the maximum water surface profile and 100-year flooding 1 imits since it is possible that these dams may not fail imme- diately during heavy flooding. Vegetation In Dad Clark Gulch basin can be characterized by four major plant communities, grassland, shrubland, cropland and riparian woodland. The predominant vegetation type is grassl and. Shrubs and wooded areas are located mainly along the larger drainage channels with low growing shrubs sometimes overgrowing the channel bottoms. The cropland in Dad Clark Gulch basin 1 ies mainly in the upper reaches near the existing Highlands Ranch headquarters. C. CLIMATOLOGY AND FLOOD HISTORY Existing culvert-type road crossings at County Line Road and the Highlands Ranch access road, as well as the Highl ine Canal crossing have been in- cluded in the hydraulic modeling of the Dad Clark Gulch flood plain. Proposed future culvert-type road crossings have been located and have al so been analyzed for their potential backwater effects on the flood plain. The cl imate over the Dad Clark Gulch drainage basin can be classified as semi-arid. The average annual precipitation in the Dad Clark Gulch drain- age basin is 17 to 19 inches, 75 percent of which occurs during the months of April through September. The period of heaviest precipitation is during these spring and summer months. Sno\\!l1elt is not usually a contributing factor. Numerous thunderstorms during these months, while generally limited in area, produce occasional periods of intense rainfall. The topography of the Dad Clark Gulch basin ranges from rolling rangeland in the majority of the basin to steeply-eroded, northwest-trending ridges in the upper one-quarter of the basin. Elevations range from approximately 5,458 feet at County Line Road to approximately 6,300 feet (above mean sea level) at the uppermost drainage basin boundary. Most of the major floods from small watersheds in this area have resulted from intense thunderstorm act iv ity. The thunderstorms are generally the result of convective activity occurring during the afternoon or early evening. These storms are generally of short duration and moderate in- tensity. The most severe thunderstorms are associated with frontal activi- ty coupl ed with orographic effects. Dad Clark Gulch basin 1 ies near the southwestern edge of the Denver basin, an area extending from Colorado Springs north to Greeley, and from the foothills eastward approximately 60 miles onto the plains. The Denver and Dawson Formations underlie the Dad Clark Gulch drainage basin, and are No specific flood history is available for Dad Clark Gulch. In November 1961, hydrologic calculations were prepared for the then proposed Dad Clark Gulch Dam (McLellan Reservoir) wherein, with the drainage basin assumed to be fully urbanized, the lOO-year flood was estimated to be 8,200 cubic feet per second at the dam, with a drainage area of 9.22 square miles (up to the 3