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CHAPTER 11. FLOOD <br />11-6 <br />runoff, and storage may not be available if two winter flood events occur in short succession. The three <br />reservoirs have a combined storage capacity of 833,700 acre-feet (157,800 acre-feet in Keechelus Lake; <br />239,000 acre-feet in Kachess Lake; and 436,900 acre-feet in Cle Elum Lake). These reservoirs were <br />constructed for irrigation purposes, but are also operated for flood control on the basis of runoff forecas ts. <br />Irrigation Facilities <br />Ellensburg and Kittitas are surrounded by a complex irrigation system consisting of the North Branch, <br />Town and Cascade canals; the Whipple Wasteway; and the Reecer, Currier, Whiskey, Mercer, Wilson, <br />Cooke and Caribou Creeks. Covering over 330 miles, this system distributes water for irrigation and was <br />designed to provide some flood control. However, the system has a decreasing capacity downstream and <br />can become overtaxed when used to route floodwaters. Significant floods have occur red in the past when <br />this system diverted floodwaters from one basin to another. <br />Urban Flooding <br />Kittitas County has experienced rapid change due to urban development in once rural areas. Drainage <br />facilities in recently urbanized areas are many series of pipes, roadside ditches and channels. Urban flooding <br />occurs when these conveyance systems lack the capacity to convey rainfall runoff to nearby creeks, streams <br />and rivers. As drainage facilities are overwhelmed, roads and transportation corridors become conveyance <br />facilities. The key factors that contribute to urban flooding are rainfall intensity and duration. Topography, <br />soil conditions, urbanization and groundcover also play an important role. <br />Urban floods can be a great disturbance of daily life in urban areas. Roads can be blocked and people may <br />be unable to go to work or school. Economic damage can be high, but casualties are usually limited because <br />of the nature of the flooding. On flat terrain, the flow speed is low and people can still drive through it. The <br />water rises relatively slowly and usually does not reach life endangering depths. <br />11.2.4 Past Events <br />Since 1862, approximately 38 major floods with streamflow over 10,000 cubic feet per second (cfs) have <br />occurred on the Yakima River and its tributaries. Five of the highest peak discharges were measured at <br />USGS Station 12484500 on the Yakima River at Umtanum, 10 miles south of Ellensburg, on the following <br />dates. <br />– November 1906 (41,000 cubic feet per second (cfs)) <br />– November 1909 (22,900 cfs) <br />– December 1933 (32,200 cfs) <br />– May 1948 (27,700 cfs) <br />– February 1996 (27,200 cfs) <br />Additionally, the National Weather Service Advanced Hydrologic Prediction Service records the <br />heights of historic crests. The Yakima River at Cle Elum gauge recorded the following historic crests: <br />– 8.68 ft on 02/09/1996 <br />– 8.69 ft on 11/29/1995 <br />– 9.70 ft on 11/25/1990 <br />– 8.45 ft on 05/31/1983 <br />– 9.84 ft on 12/02/1977 <br />– 11.27 ft on 11/23/1959 <br />– 11.68 ft on 05/29/1948 <br />– 11.27 ft on 12/22/1933 <br />– 12.07 ft on 12/13/1921 <br />– 12.12 ft on 12/30/1917