Estimating magnitude and frequency of floods in Wisconsin

Abstract

Equations for estimating magnitude and frequency of floods for Wisconsin streams with drainage basins containing various amounts of existing or projected urban development were developed by flood-frequency and multiple-regression analyses. Multiple-regression techniques were used to develop equations for estimating flood frequencies at ungaged urban sites. The flood-frequency equations are based on data from 32 urban gaging stations, including 19 crest-stage gages and 13 rainfall-runoff gaging stations. Significant characteristics in the equations are drainage area and impervious area. Standard errors of estimate for the regression equations ranged from 32 to 39 percent. Separate equations were developed for Milwaukee County. The U.S. Geological Survey Distributed Routing Rainfall-Runoff ModelVersion II was used to extend records by synthesis for the 13 rainfall-runoff urban stations. INTRODUCTION Purpose and Scope Estimates of flood frequencies and magnitudes are useful for planning and designing culverts, bridges, drainage systems, and for effective flood-plain management. This report provides a method for estimating the frequencies and magnitudes of floods of ungaged urban streams in Wisconsin. Multiple-regression techniques were used to develop flood-frequency equations by relating flood frequency and magnitude characteristics for 32 sites to basin characteristics, such as drainage area and impervious area (fig. 1). The resulting equations can be used to estimate flood magnitudes of urban streams that lack significant diversion or regulation. The equations apply only to urban streams. Flood discharges for rural Wisconsin streams can be estimated using the techniques described by Conger (1981). Annual peak data from 19 urban crest-stage gages with at least 12 years of record and annual synthesized long-term peaks from 13 urban rainfall-runoff gaging stations were included in the analysis to develop regional floodfrequency equations for urban areas in Wisconsin. Flood-frequency characteristics were estimated for these sites by the log-Pearson type Ill method described in the U.S. Water Resources Council Bulletin 178 (1981). Acknowledgments This report was prepared by the U.S. Geological Survey in cooperation with the State of Wisconsin, Department of Transportation, Division of Highways; the Southeastern Wisconsin Regional Planning Commission; and the Milwaukee Metropolitan Sewerage District. The crest-stage-gage stations in Milwaukee are maintained by the Milwaukee Metropolitan Sewerage District, and stage-discharge relations were developed by the U.S. Geological Survey. Long-term daily precipitation data and storm rainfall at 5-minute intervals were obtained from the National Oceanic and Atmospheric Administration.