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FLOOD03824
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Last modified
1/25/2010 6:44:27 PM
Creation date
10/5/2006 12:05:34 AM
Metadata
Fields
Template:
Floodplain Documents
County
Statewide
Stream Name
South Platte, Arkansas, Colorado
Basin
Statewide
Title
Annual Snowmelt and Rainfall Peak Flow Data on Selected Foothills Region Streams, South Platte River, Arkansas River, and Colorado River Basins Colorado
Date
1/1/1982
Prepared By
USGS
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />1. Stations were chosen from regions where streams have experienced high <br />peak flows of both snowmelt and rainfall origin. <br />2. Stations with natural streamflow or where peak flows were only minimally <br />affected by regulation or diversion were used in the data base. If diversion above <br />the station was used for irrigation of an area greater than 10 percent of the to- <br />tal drainage area, the station was excluded. <br /> <br />Although many stations met the selection criteria, the sample of stations <br />from the South Platte River and the Arkansas River basins were limited to those <br />stations with a period of record including at least 10 years. Most stations in the <br />sample from the Colorado River basin included at least 20 years of record and were <br />selected in order to cover a broad geographic distribution. <br /> <br />DIFFERENTIATION BETWEEN SNOWMElT- AND RAiNFAll-PRODUCED PEAK FLOWS <br /> <br />Peak flows in the foothills region of Colorado are caused primarily by one of <br />two meteorologic sources--snowmelt or rainfall. In a majority of the basins in <br />this region, severe floods are generated by intense rainfall. However, in many <br />watersheds at higher elevations, the yearly peak flow is produced from snowmelt. <br />Previously no distinction was made between snowmelt- and rainfall-produced peak <br />flows in the same year. <br /> <br />To establ ish the data base for this study, streamflow records were examined <br />for 69 gaged sites, as shown in table 1. Gage heights were determined from <br />continuous-stage recorders to calculate peak discharges from snowmelt and rainfall <br />runoff. Indirect measurements of peak flow, if available, were used to supplement <br />station records. For a given water year, the published annual runoff peak was <br />checked against the streamflow chart and weather records and identified as either <br />a snowmelt- or rainfall-produced event. Then the chart was examined for a second <br />peak--the highest peak of the other runoff category. <br /> <br />Because the results of the study are to be utilized in flood-plain manage- <br />ment, interest was focused on the frequency with which portions of a flood plain <br />were inundated. Therefore, instantaneous discharge was identified rather than the <br />total volume of an event, regardless of the duration of the event. For example, <br />when choosing between two rainfall-produced peaks, the peak exhibiting a higher <br />instantaneous discharge was selected rather than one which may have had a greater <br />volume during a given event but had a lower peak discharge. <br /> <br />Precipitation, ground cover, aspect, soils, and geomorphology vary from basin <br />to basin. Similarly, the hydrologic character of each watershed is unique, but <br />patterns of runoff exist that enable an investigator to differentiate between <br />snowmelt runoff and rainfall runoff. <br /> <br />Snowmelt-runoff peaks result from the seasonal ablation of the snowpack. in- <br />spection of streamflow charts revealed that typically the snowmelt hydrograph had <br />a diurnal pattern in which daily peaks rose and fell over a period of several days <br />or weeks, as shown in figure 5. At elevations above 6,000 to 7,000 ft, these peaks <br />had a regular, well defined appearance. Daily snowmelt-runoff peaks for a given <br />basin often occurred at the same time 'of day, and the annual snowmelt-runoff peak <br /> <br />7 <br />
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