Laserfiche WebLink
<br />. <br /> <br />. <br /> <br />. <br /> <br />A comparison of flood bars, which are used as paleostage indicators (PSis), that formed during <br />record high flows during 1995, induding a flood in Fortification Creek, for 63 sites on Colorado streams <br />having a broad range of hydraulic and sedimentologic characteristics. Analysis of the data indicates that <br />the elevation of the top of flood bars (PSis) generally are maximum of +/-<l.5 ft of flood high-water mark <br />(HWM) elevations; most PSis were +/- 0,2 ft of HWMs. Therefore, use of the top of flood bars as PSis <br />for streams in the Elkhead Creek study provides a hydraulically reliable estimate of th~ <br />paleoflood depth and discharge. f"';.; I"td'" -\ <br /> <br />Since 1901, 3,512 station-years of record have been collected at 198 gages in northwestern <br />Colorado above 5,200 ft Unit discharge decreases to less than 50 ft3/sI mi2 for basins larger than <br />about 50 mi2, The maximum unit discharge 477 ft3/slmi2 from an intense rainstorm on a 1.1 mi2, sparsely <br />vegetated, steep basin at an elevation of 6,335 ft. Unit discharge is less than 100 ft3/slmi2 for <br />vegetated basins above an elevation of about 6,500 ft. The maximum unit discharge for Elkhead Creek <br />near Elkhead gage is 44 ft3/slmi2 occurred in 1984, which was a record snowmelt runoff year. Small (<5 <br />mi2 - 10 mi2), high elevation basins draining Buffalo and Rabbit Ears Passes, which is the snowiest area <br />of Colorado, have had unit discharge of about 200 ft3/slmi2 in record snowmelt runoff years. <br /> <br />Analysis of rainfall data for 5 long-term (up to about 100 years) precipitation gages and rainfall- <br />bucket survey data for northwestern Colorado indicates a maximum 24-hr rainfall of 3.25 in" which <br />suggests a lack of intense rainfall amounts in the region. The largest rainfall for all of western Colorado is <br />5 to 7 in. in a few hours that occurred in Sweetwater Creek Basin on July 12,1976, That rainstorm, <br />which occurred al an elevalion of about 6,250 ft, produced the largest known unit discharge of about <br />1,600 ft3/slmi2 in western Colorado. From an analysis of rainfall, streamflow, and paleoflOOd data, an <br />inferred maximum size of an out-of-bank flood-producing rainstorm in northwestern Colorado is about 30 <br />mi2. The maximum size of flood-producing rainstorms in western Colorado also is about 30 mi2. <br /> <br />An analysis of rainfall, streamflow, and paleoflood evidence was made for two of the largest <br />rainstorms in northwestern Colorado, including the 2,57 in. rainstorm of June 14, 1921, in Steamboat <br />Springs. The 1921 rainstorm has had a major influence in floodplain studies and extreme flood estimates <br />in the region because it is used as evidence that rainfall produces large floods in northwestern Colorado, <br />The analysis indicated the area of the 1921 rainstorm was about 20 to 30 mi2 and occurred coincident <br />with the annual peak snowmelt runoff. The peak rainfall runoff of 950 to 2,000 ft3/S was 15 to 30 percent <br />of the annual peak of 6,820 ft3/S at the Yampa River at Steamboat Springs, which is the largest flood <br />since the gage was installed in 1904. If the 1921 rainfall runoff is subtracted from the annual peak <br />discharges, peak flows from snowmelt still are the third to fifth largest for the long-term (-100 years) <br />gaged streams in northwestern Colorado. There is no evidence of flooding in Steamboat Springs or in <br />nearby streams. Historical accounts indicated that undersized culverts caused localized, minor <br />backwater flooding, The volume of 1921 rainfall runoff in the Yampa River was about the same at the <br />gages at Steamboat Springs and near Maybell, which provides additional support that the rainstorm <br />was limited in amount and areal extent <br /> <br />2 <br />