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<br />Again in August of 1951, a cloudburst over the Lyons area <br />created severe flooding conditions in Longmont which <br />damaged railroad and highway bridges and destroyed <br />large amounts of harvested grain. The St. Vrain Creek <br />discharge was estimated to be 6200 (cfs) near Interstate <br />Highway 25 or about 7 miles east of Longmont. <br /> <br />During the flood of May 9, 1957, on St. Vrain Creek, 3 <br />to 5 inches of rain fell over the entire basin causing <br />a peak discharge of between 3060 (cfs) and 9450 (cfs) <br />through Longmont. The resulting floodwaters destroyed <br />irrigation works and numerous bridge en route to the <br />South Platte River. <br /> <br />~; <br /> <br />The flood of May 7-8, 1969, on the St. Vrain Creek was <br />reported by the Longmont Daily Times-Call to be the worst <br />seen in 20 years. A combination of heavy rainfall and <br />snowmelt runoff produced high flows in the St. Vrain <br />Creek from 2,900 (cfs) at Lyons to 10,300 at the mouth <br />near Platteville, Colorado. The flooding conditions <br />were similar to those of the June 1921 flood with flood <br />plains on the average of a mile wide. Extensive damage <br />was done to bridges with severe erosion to roads near <br />bridges and along the channel banks. Public and private <br />property damage amounted to $50,000. <br /> <br />Left Hand Creek also produced a large flood event on May <br />7-8, 1969, with the primary damage being done to the <br />South Pratt Parkway bridge, which was ultimately destroy- <br />ed by the floodwaters. There is little known regarding <br />floods of record other than what was stated concerning <br />the gaged discharges. There are no existing gage data <br />for the floods on Left Hand Creek later than May of 1957. <br />The largest flood on record was the one that occurred in <br />June of 1949. <br /> <br />Flood problems in the area have been the result of not <br />only rare storm events, but also improper flood plain <br />development. Visual accounts of floods have noted the <br />debris which was picked up by the floodwaters. Included <br />were natural debris of trees, rock and soil, but mostly <br />items foreign to the flood plain such as houses, bridges, <br />automobiles, heavy equipment, lumber, house trailers, <br />butane storage tanks, and other flotsam. With these items <br />plugging bridges and culverts, flood levels rise and <br />cause more extensive damage. Property which was not <br />structurally damaged by flood depths and velocities experi- <br />enced much damage and clean-up cost related to mud and <br />silt deposition and property erosion. <br /> <br />~ <br /> <br />~ <br /> <br />8 <br />