FLOOD07005 - Laserfiche WebLink

Home Browse Search

soil groups. The ground cover ranges from good to fair, with some poor areas, and Douglas fir and oakbrush are the dominant vegetation. The heavy snowmelt from this area runs off rapidly, causing some scattered flooding and much sediment production in the Plateau Creek basin. The valley areas are in the Zone Three soil group which include the Brown and Chestnut Zonal great soil groups. The dominant vegetation is pinon, junipe~ and sagebrush. The ground cover is poor, with some fair areas. Snowfall is light and remains only a short time. Short-duration and high-intensity summer thunderstorms are common in this area and cause heavy sediment production (Reference 3). The climate of Collbran is semiarid. The mesa areas surrounding Collbran are subject to moderately heavy precipitation. Elevation greatly influences the amount of precipitation. The annual precipi- tation at Collbran averages approximately 13 inches, and the higher elevations of the mesas receive from 20 to 40 inches. Occurrence of precipitation is fairly uniform in the Collbran area, and slightly less than one-half falls as snow from December to April. Most winter precipitation occurs in the higher elevations as snow, and a deep snowpack normally accumulates. Snowfall is generally dominated by a few large storms. Snowpack ordinarily begins in late October and snowmelt in late April. Snowmelt continues through early July. The mean annual temperature at Collbran is 46.40F. . Cooler temperatures prevail in the higher elevations (Reference 4). 2.3 Principal Flood Problems The principal cause of flooding on Plateau Creek and Buzzard Creek is a rapidly melting heavy snowpack during May, June, and July. Rainfall on melting snow may hasten the melting process and increase floodflows. On Grove Creek, thunderstorms are the principal cause of flooding. Beaver dams have been built on Grove Creek, and these may aggravate flood problems by blocking culverts or creating backwater. The Town of Collbran has no available records of flooding. According to information obtained from a stream gage on Plateau Creek located approximately 6 miles east of Collbran (Reference 5), a flood on Plateau Creek in 1922 had an estimated discharge of 3080 cubic feet per second (cfs). Based on the hydrologic analyses presented in this study, that flood had a frequency in excess of 100 years. 2.4 Flood Protection Measures Natural flow of Plateau Creek is regulated by Vega Dam, which is a zoned, rolled-earth and rock-fill dam completed by the U.S. Bureau of Reclamation in 1960. The dam has been operational since 1958 and is located on Plateau Creek approximately 11 miles east of Collbran. It has a drainage area of approximately 88 square miles. Vega Reservoir has a capacity of 34,000 acre-feet and 5