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<br />As is characteristic of the region, flooding OD these streams <br />generally occurs between May and September. Peak annual flows <br />usually occur in May and JUne, however, resulting fram a <br />combination of snowmelt runoff and spring rains. Floods having the <br />most damaging effect in the Longmont area, such as the flood of <br />JUne 2-7, 1921, occur when a long-duration rainsto~ forms over the <br />St. vrain Creek basin with the heaviest rainfall accumulation <br />downstream of the Lyons gaging station. Other floods, such as that <br />of JUne 22, to those of the JUne 1921 flood, with floodplain width <br />averaging 1.0 mile. Extensive damage was done to bridges, with <br />severe erosion nearby to roads and along the channel banks. Public <br />and private property damage amounted to $50,000. <br /> <br />Lefthand Creek also produced a large flood on May 7-8, 1969, with <br />the primary damage being done to the South Pratt Parkway bridge, <br />which was ultimately destroyed by the floodwater. There is little <br />known regarding floods of record other than what was stated <br />concerning the gaged discharges. There are no existing stage data <br />for the floods on Lefthand Creek later than May 1957. The largest <br />flood on record was the ODe that occurred in JUne 1949. <br /> <br />Plood problems in the area have been the result of not only rare <br />storm events, but also improper floodplain development. Visual <br />accounts of floods have noted the debris collected by the <br />floodwater, including natural debris such as trees, rocks, and <br />soill but consisting chiefly of items foreign to the floodplain, <br />such as houses, bridges, automobiles, heavy equipment, lumber, <br />house trailers, and butane storage tanks. with these items <br />obstructing bridges and culverts, flood levels rise and cause more <br />extensive damage. Property that was not structurally damaged by <br />flood depths and velocities experienced much damage and cleanup <br />expense resulting fram mud, silt deposition, and erosion. Scenes <br />of past flooding in Longmont are shown in Figures 9 through 16. <br /> <br />The Town of Lyons lies in a natural bowl, on gently sloping land <br />surrounded by sandstone hills on all sides. The steep stream <br />slopes create swift currents during a flood, which produce <br />additional damage. Debris carried by the fast-moving water not <br />only threatens bridges, but may batter houses or other structures <br />on the floodplain. The bridge crossings are often the points of <br />channel restriction, thus raising the water-surface profile. <br />Erosion undercuts and destroys structures which would otherwise <br />receive little damage fram inundation. Large quantities of rocks <br />have been deposited in portions of the channel, leaving little <br />capacity for future floods. <br /> <br />Early records of floods in the study reach are fragmented and <br />lacking in detail. Flooding occurred on St. Vrain Creek in 1864, <br />1876, 1894, 1919, 1941, 1949, 1951, 1957, and 1969. The floods of <br /> <br />21 <br />