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<br />extensive gullying along drainageways and <br />considerable agricultural significance. <br />dissection by gullies severely limits use of <br />gullies increases highway construction costs. <br /> <br />narrow valleys is of <br />In many locations, <br />the land, and bridging <br /> <br />Climate in the Rangely area is classified as semiarid, with <br />relatively warm summers and cold winters. The mean annual <br />temperature is 460F, with extremes of -370F and 1060F. The lower <br />valley of the White River near Rangely has a frost-free period with <br />an average length of 124 days per year. Average annual <br />precipitation varies from 50 inches in the headwater regions of the <br />White River Basin to 9.3 inches at Rangely. Normally, <br />approximately 40 percent of the annual precipitation occurs during <br />August, September, and October. Wintertime precipitation occurs as <br />snow, and a deep snowpack usually accumulates in the high <br />elevations. The average annual snowfall is 22.3 inches <br />(Reference 2). Summer rainstorms cause short-duration runoff that <br />is characterized by high peak, low-volume flows. <br /> <br />The floodplains of the streams studied are presently not <br />extensively developed. There are various transportation facilities <br />in the floodplains. There are some industrial and light <br />residential land uses. However, continued and probably accelerated <br />development due to increasing emphasis on mineral and energy <br />extracting and processing is expected, and pressure leading to <br />intensified floodplain use will undoubtedly accompany such <br />development. <br /> <br />2.3 Principal Flood Problems <br /> <br />Because of its particular geographic location, Rangely is subject <br />to five different types of events that cause flooding: ice jams, <br />rapid snowmelt, rain on snow, general raIn, and local <br />thunderstorms. The low permeability of the soils in and around <br />Rangely is also a factor in producing flooding in the study area. <br />Historically, ice jams, rain on snow, and local thunderstorms have <br />caused the greatest flood damage in and around Rangely. Ice jams <br />are formed by ice flows piling up at an obstacle, such as an <br />existing ice accumulation, bridge, or streambed irregularity. The <br />ice blocks the entire channel section completely down to the river <br />bottom, and water then has to move through the ice plug by <br />infiltration. When infiltration is inadequate, the water-surface <br />elevation increases until spill occurs and adjacent land is <br />flooded. However, ice jams are unstable and often break up when <br />the upstream water level increases. Ice jams are usually caused by <br />Chinook winds and temperature inversions. During a Chinook, air <br />temperature may increase very rapidly (increases of more than 500F <br />in a few hours have been recorded in some areas subject to these <br />winds), and it is not unusual for a 2-foot layer of snow to melt in <br />1 day. Temperature inversions cause ice jams by melting upper <br />elevation snow and ice, while freezing conditions remain in the <br />lower elevations. The rising water heaves and breaks up river ice, <br />which floats into jams. <br /> <br />5 <br />