FLOOD10410 - Laserfiche WebLink

Home Browse Search

from the Rocky Mountain uplift some 60 million years ago. Exposed sedimentary rocks along the edge of the mountain front are tilted. Above the sedimentary foothills the pre-Cambrian mountain core is largely of pikes Peak granite. Soils of the region are largely gray desert soils with "islands" of dry, sandy soil on the plains, alluvial gravels capping foothill mesas, and arid-subhumid lithosoils in the mountains. Generally, the areas west of Monument and Fountain Creeks have high coefficients of runoff, steep stream gradients, and narrow flood plains. Areas east of the streams generally have lower co- efficients of runoff, gradual elevation changes, and relatively broad streambeds and flood plain areas. -. - Within the region, precipitation varies considerably because of elevations and major wind currents. Winter storms, typically from the northwest, tend to lose their moisture on the west side of the Rocky Mountains. The main precipitation in the front range comes through late spring snows influenced by southeasterly winds on upslopes and from summer thundershowers. Pikes Peak produces a "rain shadow" which causes a semiarid zone to the east. Annual precipitation varies from a low of 8 inches to a high of 20 inches at the higher elevations. Average annual precipitation in Colorado Springs is approximately 13.2 inches. Most of the flood-producing storms over the study area occur from May through August. The severest storms often Occur in the transi- tional periods of late spring or early fall when polar air intrusions are most intensive. Isolated summer thunderstorms are frequently severe, but limited in areal extent. Available records do not indicate that snowmelt has contributed significantly to flood occur- rences in the study area. Floods are characterized by high peak flows, moderate volumes, and short durations. Developments subject to flood damage consist of residential pro- perties and commercial and light industrial establishments. Some bridges and culverts across the streams are constrictive to flood- flows and contribute to flooding problems. Generally, the bridges across Fountain and Monument Creeks will pass at least the 100-year flood, with the most constrictive bridges and culverts being on the smaller streams. Other obstructions to flood flows include landfills and isolated levees that have been constructed to protect commercial and residental properties from flooding. Large floods are relatively unaffected by these levees. ~ 8