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OJ157~ PROJECT TITLE: Flood Hydrology of Foothill Streams in Colorado (fig. 8) COOPERATING AGENCIES: Colorado Department of Natural Resources, Colorado Water Conservation Board; Urban Drainage and Flood Control Dis- trict; U.S. Army Corps of Engineers; and the U.S. Vater and Power Resources Service PROJECT CHIEF: Robert D. Jarrett, District Office, Lakewood PERIOD OF PROJECT: October 1977 to September 1983 ProbZem.--More than three-fourths of the people in Colorado I ive along or near the base of high mountains. Streams along the foothills are subject to flood- ing from both snowmelt and rainfall, but by far the most destructive type of flood results from 1~loudburst-typ~' rainfall associated with severe thunderstorms dur- ing summer months. Because data on these floods belong to mixed statistical popu- lations, standard techniques of flood-frequency analysis are inadequate. In addi- tion, the data available are insufficient. Objectives.--Develop methods for determining flood frequencies from records of mixed-population floods and for estimating flood characteristics at ungaged sites on streams where mixed-population floods occur. Approach.--Tabulate and evaluate existing flood and precipitation data. De- velop methods for identifying and analyzing mixed-population floods using histor- ical flood records. Develop techniques for estimating flood characteristics at ungaged sites using physical and climatical characteristics of foothill basins. Design and test hydrologic model for application in foothill basins. Determine what additional hydrologic data are needed and develop a network to collect these data. Progress.--A literature search and review has been completed. Studies using geomorphic and botanic data and channel-geometry data to develop techniques for estimatin9 flood characteristics at ungaged sites are in progress. Twenty crest- stage gages have been installed and data are being collected. Methods are being developed for identifying and analyzing mixed-population floods. Records from an additional 30 gaging stations were used to separate snowmelt from rainfall floods, bringing the total number of separations to 60. A preliminary statistical analy- sis of mixed-population flood records indicates that rainfall-produced floods are less prevalent at higher elevations than expected. A paper on debris flows was submitted to the Association of Engineering Geologists for publ ication in their bulletin. A project-planning report was distributed to the cooperators and others interested in the study. PZans.--Vork will continue on identifying annual snowmelt and rainfall flood peaks and on studying the relation of the geomorphology and geometry of channels to the flood characteristics of basins in the Colorado River drainage. Work will be completed on techniques for analyzing mixed-population flood records and on techniques which will permit transferring flood information from gaged to ungaged sites. Both basic data and interpretive reports will be prepared. 23