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2.3 Principal Flood Problems The normal low flow channel for Beaver Creek meanders through the study reach traversing approximately 7.4 miles through the 4-mile reach of the floodplain. The topography adjoining Beaver Creek is very flat to the west but quite steep to the east. Through the study reach, six major roads and railroads cross the creek. Two of these facilities, the Burlington Northern Railroad directly east of town, and County Road DLD, O.S mile south of town, significantly affect floodflows within the floodplain. In addition to the road and railroad crossings within the floodplain, the City of Brush 1S located in the floodplain area. The road and railroad embankments as well as the buildings and streets in the city serve as obstructions to the orderly passage of floodflows through this portion of Beaver Creek. Through the study reach, the main channel is small, inadequate, and restricted by the natural vegetation, thus limiting the effectiveness of the channel. The primary obstructions to floodflows through the study area are the brush and vegetation growing along the channel, roads, railroad embankments, and bridges in the floodplain. During periods of high water, these facilities tend to impede floodflows, thereby causing backwater conditions that increase flood heights upstream and flood velocities downstream. Debris that is washed downstream by floodwaters collects and obstructs downstream bridges and culverts, thus raising floodwaters in these areas. The accumulation of debris on the Burlington Northern Railroad bridge would greatly increase the threat and severity of flooding within the City of Brush (Reference 1). Flood events on Beaver Creek are the result of intense thunderstorms occurring in the basin. Being subject to heavy rain events of cloudburst intensity, flooding in the upper part of the basin will characteristically have high peak. discharges of short duration lasting only a few hours. As the flood event passes into the lower reaches of the creek near Brush, channel storage reduces the flood magnitude and the flood stages. The lOO-year flood is one with a peak flow magnitude that has a I-percent chance of being equalled or exceeded in any given year and a frequency of occurrence of about once in 100 years on the long-term average. Similarly, the IO-year, 50-year and SOD-year floods have a IO-percent, 2-percent, and O.2-percent chance, respectively, of being equalled Or exceeded in any given year, and an average frequency of occurrence of about once in the number of years indicated. These flows have been estimated based on recorded flood occurrences in the high plains area of eastern Colorado, western Kansas and southwestern Nebraska, and are typical of the Beaver Creek drainage area. 4