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ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA USING DRAINAGE-BASIN AND CHANNEL-GEOMETRY CHARACTERISTICS By David A. Eash 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 1 DO years at stream sites on rural, unregulated streams in Iowa. Design-flood discharge estimates determined by Pearson Type-III analyses using data collected through the 1990 water year are reported for the 188 streamflow-gaging stations used in either the drainage-basin Dr channel-geometry regression analyses. Ordinary least-squares multiple-regression techniques were used to identify selected drainage-basin and channel-geometry characteristics and to delineate two channel-geometry regions. Weighted least- squares multiple-regression techniques, which account for differences in the variance of flows at different gaging stations and for variable lengths in station records, were used to estimate the regression parameters. Statewide drainage-basin equations were developed from analyses of 164 streamflDw- gaging stations. Drainage-basin characteristics were quantified using a geDgraphic-infDrmatiDn- system 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 drainage- basin equations ranged from 38.6 to 50.2 percent. The geographic-information-system procedure expanded the capabiiity to quantitatively relate drainage-basin characteristics to the magnitude and frequency of floods for stream sites in Iowa 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 Dnsite 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 naiural channels unaffected by channel- ization, 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 3D.3 to 70.0 percent for the regional channel-geometry equations. Procedures provided for applying the drainage-basin and channel-geometry regression equations depend on whether the design-flood discharge estimate is for a site on an un gaged stream, an ungaged site on a gaged stream, or a gaged site. When both a drainage-basin and a channel-geometry regression-equation estimate 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 unregu- lated rural drainage areas less than 1,060 square miies, and the channel-geometry regression equations are applicable to unregulated rural streams in Iowa with stabilized channels. INTRODUCTION Knowledge of the magnitude and frequency of floods is essential for the effective manage- ment of flood plains and for the economical planning and safe design of bridges, culverts, levees, and other structures located along streams. Long-term flood data collected from a network of streamflow-gaging stations operated in Iowa are available for hydrologic analysis to compute design-flood discharge estimates for the gaged sites as well as for ungaged sites on the gaged streams. Techniques are needed to estimate design-flood discharges for sites on all INTRODUCTION f