Methods For Estimating Magnitude And Frequency Of Peak Flows For Small Watersheds In Utah

Determining discharge in a stream is important to the design of culverts, bridges, and other structures pertaining to transportation systems. Currently in Utah regression equations exist to estimate recurrence flood year discharges for rural watersheds greater than 30 mi2, and the rational method is used for areas smaller than 0.5 mi2, however, there are no good methods available to estimate discharges for rural watersheds that fall between the two approaches. To solve this issue, flood frequency analyses were conducted for small rural watersheds with streamflow gaging station data within the state of Utah to develop regression equations for estimating flood flows for mid-sized watersheds. The watersheds selected range from 0.5 mi2 to 30 mi2, and have at least 10 years of annual peak discharges recorded by the United States Geological Survey (USGS). Flood frequency analyses were performed in accordance with the guidelines of Bulletin 17B (Interagency Advisory Committee on Water Data), using the USGS computer program PeakFQ (Flynn et al 2006). Computed flood year streamflows were regressed against multiple parameters (watershed geometries, soil characteristics, precipitation data, land use data, etc.) to estimate different recurrence flood year flows (i.e. 2-, 5-, 10-, 25-, 50-, 100-, 200-, 500-year). Regression equations were developed for seven regions in the state of Utah delineated according to hydrologic regions or climatic properties. Regression equations were developed in the format of the rational method where the runoff coefficient was regressed against appropriate determined data: basin characteristics, such as drainage basin area, max flow distance, sinuosity, composite curve number, saturated hydraulic conductivity, and climatic characteristics including, the basin centroid 2-year 24-hour precipitation, and basin centroid mean annual precipitation. The regression equations are presented within the document including errors associated with the regression processes. This document also summarizes the procedures a user should follow to use these equations in practice. Cautions are presented for the user to understand the limitations of the equations and to facilitate more efficient design of channel crossings.