FLOOD00269 - Laserfiche WebLink

Home Browse Search

During the winter. the moisture comes chiefly from the Pac~ic Ocean; tt is deflected upward by the western mountain chains and is precipttated in the form of snow, usually heaviest on the western slopes 01 the highest ranges and decrBasing eastward. Along the Front Range, some precipttation, even in winter is caused by moisture from the GuK. In general, however, the snowfall is heaviest 0 the western ranges, and its melting in June has a greater effect on the streams in that region than the metting of the snow over the eastern ranges has upon the stn~ams draining the Front Range. In late spring, precipttation not only the Front Range, but also farther west sometimes augments the metting snow sufficiently to cause floods. . . 3.3 Specific Basin Flooding Charactertstlcs- Flood characteristics specific to particular major dralnalle basins are described below. 3.3.1 South Platte River Basin Descriotion- The l1eadwaters of the South Platte River have their sources in the mountainous region surrounding the large basin near the center of the State and in the long eastern slopes of the high mountains forming the Continental Divide. The general course of the stream is eastward to Lake George, then through Platte Canyon northward to tts junction wtth the Cache la Poudre River near Greeley, and then east again to tts junction wtth the North Platte River at North Platte. Nebraskac Beyond Platte Canyon, the South Platte River emerges from the foothills and flows across the plains in a shallow valley for a distance of 190 miles to the Colorado-Nebraska State line. Through the canyon, the river has an average fall of 55 feet per mile, but across the plains, the river slope decreases from 15 to 7 feet per mile. The South Platte River is rarely subject to floods above the canyon section, atthough. the general storm of June 2-7, 1921, caused a flood that did serious damage. Floodina Characteristicsc Below the canyon, the river is subject to floods, caused chiefly by the tributary streams draining the Front Range, and rarely by the tributaries from the plains area. Floodwater.; on the South Platte River can resutt from intense localized rainfall or general rains, I~tther of which may be augmented by snowmett. The intense rainstorm floods resutt fmm intense rainfall over -areas of relatively small areal extent. These storms can prodUl~e extremely rapid concentrations of floodwaters and ensuant rapid rises in stream levels. Because of little warning time prior to streams attaining flood stages, this type of storm can produce floods which are extremely hazardous. This type of flood produces high peak discharge on tributaries in the general vicintty of the rainstorm. The discharge, however, attenuates rapidly on the South Platte River downstream from the contributing tributaries du,e to valley storage of the floodwater. ... . Colorado Flood Hydrology Manual 3.2 aw=r