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<br />Again in August of 1951, a cloudburst over t3e Lyons area <br />created severe flooding conditions in Longnont 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 ~ong~o~t. <br /> <br />During the flood of ~ay 9, 1957, on St. Vrain Cree~, 3 <br />to 5 inches of rain fell over t3e entire basin causir-q <br />a peak discharge of between 3060 (cfs) and 9450 (c~s) <br />through Longmont. 7he resulting floodwaters destroyed <br />irrigation works and numerous bridge en route to the <br />So~th Platte River. <br /> <br />The flood of ~ay 7-8, 1969, on the St. Vrain Cree~ was <br />reported by the Longmont Daily TiLes-Call to be the worst <br />seen i~ 20 years. A combination of heavy rainfall ar.d <br />snownelt runoff produced high flows i~ the St. Vrain <br />Cree~ from 2,900 (cfsi at Lyons to 10,300 at the moutn <br />near Platteville, Colorado. The flooding conditior-s <br />were similar to those of the June 1921 =lood with flood <br />Plains on the averaae of a ~ile wide. Ex~ensive da~aae <br />J J <br />was done to bridges with severe erosion to roads r-ear <br />bridges and along the channel banks. Public and p~ivate <br />property da~age amounted to $50,000. <br /> <br />Le=t 5and Creek also ?roduced a large flood even~ on ~ay <br />7-8, 1969, with the p~i~ary damage being done to the <br />South Pratt Parkway b~idge, which was ultimately destroy- <br />ed by the floodwaters. There is little known regarding <br />floods of record other than wha~ was stated concerning <br />the gaged discharges. There are no existing qage data <br />for the floods on ~eft Hand Creek later than ~ay of 1957. <br />The largest flood on record was the one that occ~rred in <br />June of 1949. <br /> <br />Flood problems in the area have been the result 0= not <br />only rare stor~ events, but also i~proper flood ?lain <br />development. Vis~al accounts of floods ~ave noted the <br />debris which was picked up by the floodwaters. In~luded <br />were natural debris of trees, rock and soil, but ~ostly <br />items foreign to the flood plain such as houses, bridges, <br />automobiles, heavy equipment, lumber, house ~~aile~s, <br />butane storage tan~s, and other flotsa~. 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 ~uch da~age and clean-up cost rela~ed to mud and <br />silt deposition and property erosion. <br /> <br />8 <br />