Estimating design-flood discharges for streams in Iowa using drainage-basin and channel-geometry characteristics

Abstract

Drainage-basin and channel-geometry multiple-regression equations are presented for estimating design-flood discharges having recurrence intervals of 2, 5, 10, 25, 50, and 100 years at stream sites on rural, unregulated streams in lowa. Design-flood discharge estimates determined by Pearson Type-Ill analyses using data collected through the 1990 water year are reported for the 188 streamflow-gaging stations used in either the drainage-basin or channel-geometry regression analyses. Ordinary least-squares multiple-regression techniques were used to identify selected drainaqe-basin and onsite and on topographic maps. Statewide and regional channel-geometry regression equations that are dependent on whether a stream has been channelized were developed on the basis of bankfull and active-channel characteristics. The significant channel-geometry characteristics identified for the statewide and regional regression equations included bankfull width and bankfull depth for natural channels unaffected by channelization, and 'active-channel width for stabilized channels affected by channelization. The average standard errors of prediction ranged from 41.0 to 68.4 percent for the statewide channel-geometry equations and from 30.3 to 70.0 percent for the regional channel-geometry equations. channel-geometry characteristics andto delineate two channel-geometry regions. Weighted leastProcedures provided for applying the squares multiple-regression techniques, which drainage-basin and channel-geometry regression account for differences in the variance of flows at depend on whether the design-flood different gaging stations and for variable lengths in estimate is a site On an ungaged station records, were used to estimate the Stream, an ungaged site on a gaged stream, or a regression parameters. gaged site. When both a drainage-basin and a channel-geometry regression-equation estimate Statewide drainage-basin equations were developed from analyses of 164 streamflowgaging stations. Drainage-basin characteristics were quantified using a geographic-informationsystem procedure to process topographic maps and digital cartographic data. The significant characteristics identified for the drainage-basin equations included contributing drainage area, relative relief, drainage frequency, and 2-year, 24-hour precipitation intensity. The average standard errors of prediction for the drainagebasin equations ranged from 38.6 to 50.2 percent. The geographic-information-system procedure expanded the capability to quantitatively relate drainage-basin characteristics to the magnitude and frequency of floods for stream sites in lowa and provides a flood-estimation method that is independent of hydrologic regionalization. Statewide and regional channel-geometry regression equations were developed from analyses of 157 streamflow-gaging stations. Channel-geometry characteristics were measured are available for a stream site, a procedure is presented for determining a weighted average of the two flood estimates. The drainage-basin regression equations are applicable to unregulated rural drainage areas less than 1,060 square miles, and the channel-geometry regression equations are applicable to unregulated rural streams in lowa with stabilized channels.