FLOOD09863 - Laserfiche WebLink

Home Browse Search

, , " . . . SAINT VRAIN CREEK HYDROLOGY ~~2io Q@~~ci~tiQO Saint Vrain Creek, a left bank tributary to the South Platte River, has its source in the Rocky Mountains west of Longmont, Colorado. North Saint Vrain and South Saint Vrain Creeks, which drain approximately 211 square miles of mountain area, join in the foothills at Lyons, Colorado to form the Saint Vrain Creek. A gauging station is located 0.4 mile downstream from the confluence of North and South St. Vrain Creeks. From Lyons, St. Vrain Creek flows in a southeasterly direction to Longmont. Lefthand Creek, a 68 square mile right bank tributary, enters St. Vrain Creek south of Longmont. Flowing in a easterly direction, St. Vrain Creek is joined by Dry Creek and Spring Gulch 3 miles downstream from Longmont. Approximately 1.5 miles downstream from this point, the major tributary, Boulder Creek, enters St. Vrain Creek. The drainage basin area of Boulder Creek is approximately 440 square miles. From the mouth of Boulder Creek, St. Vrain ~reek flows 15 miles in a northeasterly direction to its confluence with the South Platte River. A stream gauge located 1.2 miles upstream from the mouth of St. Vrain has a drainage area of 976 miles. Elevations in the basin range from 4,740 feet to 14,256 feet above mean sea level. ~~i2tiQg ~tc~~t~C~2 Buttonrock, Barker and Gross Dams are the three major structures in the St. Vrain basin. Although none of the reservoirs has a specific flood control function, they provide some residual flood control effects downstream. Buttonrock Dam, located 6 miles west of Lyons on the North Saint Vrain Creek, was completed in 1969 by the City of Longmont, and is used for municipal water supply. Numerous small reservoirs having uses other than flood control are located throughout the St. Vrain basin. Some of these reservoirs also provide residual flood control effects, but to a lesser degree than the major reservoirs discussed above. E1QQQ ~b~c~~tgCi2ti~2 Floods in the study area are usually caused by general rainstorms and cloudburst storms during the period May through September. Floods resulting from prolonged heavy rainfall over the watershed are characterized by high pe~k flow of moderate volume and duration. The cloudburst type is a high intensity, short duration rainstorm which produces a flood of high peak flow, short duration, and small volume of runoff. Peak flooding at Lyons will usually occur within a few hours after a single rainfall event. Flood potential also exists from the rapid melting of heavy snow cover in the late spring. Flood flows resulting from snowmelt 1