Peak-flow characteristics of small urban drainages along the Wasatch Front, Utah

Abstract

Designers and planners for local, State, and Federal agencies need up-todate methods for determining peak-flow frequency relations for urban drainages along the Wasatch Front, Utah. This report summarizes methods used to develop equations that estimate peak flows for small urban drainages along the Wasatch Front. Rainfall and runoff data collected from eight urban drainages along the Wasatch Front during 1984-86 were used to calibrate a Distributed Routing Rainfall-Runoff model called DR3M-II for each drainage. Long-term rainfall data collected during 1948-83 at the National Weather Service station at the Salt Lake City Airport were used with the calibrated models to estimate peakflow data for 1948-83 for each of the eight drainages. Log-Pearson fits were made to the peak-flow data and were used to estimate peak-flow frequency relations for each drainage. Mathematical equations were developed that relate peak flows for recurrence intervals of 2, 5, 10, 25, 50, and 100 years for small urban drainages, to basin characteristics. Data entry to the equations requires determination of basin slope, drainage area, and percentage of impervious area. Paired stations on Little Cottonwood Creek near Salt Lake City were used to help determine the effects of intervening urban drainage on peaks of larger streams that originate in the mountains. In general, peaks on larger streams caused by snowmelt and peaks caused by rainfall (where urban areas may have a significant effect) did not occur simultaneously. INTRODUCTION Population increases and urban expansion have increased concern about adequate design of highway and street drainage structures within the urban environment. About two-thirds of Utah's population resides along the Wasatch Front, which extends from Brigham City on the north to Nephi on the south (fig. 1). The Wasatch Front includes the western flank of the Wasatch Range and the densely populated eastern part of adjoining valleys at the base of the range. Population along the Wasatch Front has increased considerably since 1960. In Salt Lake County (fig. 2), which includes a large part of the population, there was a 61-percent increase from 1960 to 1980 (U.S. Bureau of the Census, 1963; 1980). Population increased 13 percent from 1980 to 1985, and a similar future increase is anticipated (Utah Office of Planning and Budget, Data Resources Section, 1987).