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2.3 Principal Flood Problems storms producing floods occur during May through September. During this period, warm, moist air masses from the Gulf of Mexico combine with cold and comparatively dry air from the polar regions, causing thunderstorms. These are usually characterized by high rainfall intensities of short duration, producing high peak flows and moderate water volumes. Frequently, thunderstorms are severe and occur over rather limited areas. Floods are especially severe if the thunder- storms follow a period of either high snowmelt or pro10nged rainy weather. Historical documentation of flood damage is meager. This is pro- bably due to the small stream size and lack of extensive urbaniza- tion. Major flood damage in the watershed is to roads, bridges, irrigation structures, and agricultural land. Duration of flooding is brief, due to steep slopes and small drainage area. In general, peak flows last from 1 to 4 hours, during a flooding period of ~ to 24 hours. Floods occurring in Superior were reported in 1876 and 1891; June 3, 1921; and on May 26, 1935, These floods were caused by rapid snow- melt in the mountains and heavy rainfall in the area of Superior. A flood occurred in June 1896, which was reported as the maximum flood of record in the part of the valley near Louisville. The flood of September 2, 1938, was caused by cloudbursts in the mountains and foothills, resulting in a flood slightly higher than that of 1935, Using slope-area measurements, the u.s. Army Corps of Engineers estimated the flood discharges on Coal Creek near Erie to be 13,200, 7,800, and 3,500 cubic feet per second (cfs) in 1876, 1921, and 1938, respectively (Reference 2), The 1876 flood is the largest reported at Erie and approximates the lOO-year recurrence interval, as determined by the U.S. Soil Con- servation Service (Reference 2) using synthetic methods for flood routing. The mos~ recent floods in the watershed occurred on June 9, 1949; on May 9, 1957; and, in.May 1969. The following are recorded accounts of flood d~nage on Coal Creek: On September 2, 1938, parts of the residential area in Louisville were under several inches of water, but the business district was not damaged. One person was killed in an automobile accident on the flooded highway east of Louisville. The bridge at Empire Drive and state Highway 42 was on the verqe of failure, but was savpn. On June 9, 1949, the bridge at the junction of Coal Creek and Rock Creek, 3 miles north and 0.25 mile west of Erie, was threatened and closed to traffic. Thp bridge over Coal Creek, 0,75 mile east of Superior, went out. 4 ~ I . . ,.~: ~ I......'.. .. .~ :' I ~