FLOOD06431 - Laserfiche WebLink

Home Browse Search

AF QT(wgj AF~~. QT(cg) where QT(wgl QT(rg) tlTDA TDAg geometry regression-equation estimates QT(dbogl (equation 14); (2) only the drainage-basin regression-equation estimate QT(db); or (3) only the channel- geometry regression-equation estimate QTlcgj; is the adjustment factor for the gaged site and is calculated as (18) is the weight,ed design-flood discharge for the gaged site, in cubic feet per second, as defined by equation 16; is the regression design-flood discharge for the gaged site, in cubic feet per second, determined as one of the following: (1) the weighted average of both the drainage-basin and channel- geometry regression-equation estimates QT(dbog), as defined by equation 14; (2) only the drainage-basin regression- equation estimate QTldbl; or (3) only the channel-geometry regression-equation estimate QTlcgl; is the absolute value of the difference between the total drainage area of the gaged site (TDAg) and the total drainage area of the ungaged site; and is the total drainage area of the gaged site, in square miles, listed as the published drainage area in table 9. This procedure (I) adjusts the regression-equation estimate for the ungaged site QT(ruj by the ratio AF when the total drainage area of the ungaged site equals the total drainage area of the gaged site TDAg and (2) prorates the adjustment to 1.0 as the difference in total drainage area between the gaged site and the ungaged site approaches either 0.5 or 1.5 of the total drainage area of the gaged site. In other words, when the total drainage area of the ungaged site is 50 percent larger or 50 percent smaller than the total drainage area of the gaged site, no adjustment is applied to the regression-equation estimate for the ungaged site QTlrul' Example of Estimation Method--Example 8 Examole 8.--Determine the 50-year peak- discharge estimate for an ungaged site on Otter Creek, located on the Osceola and Lyon County line, at a bridge crossing on County Highway L26, 4.75 mi southwest of Ashton, in the SWl/4 sec. 31, T. 98 N., R. 42 W. Because a crest-stage gaging station is located on this stream, Otter Creek near Ashton (station number 06483460; map number 139, fig. 1), the 50-year recurrence interval regression-equation estimate calculated for the ungaged site can be adjusted by the weighted 50-year flood-discharge estimate calculated for the gaged site. Estimating the adjusted 50-year peak discharge for the ungaged site Q50(au) (equation 17) involves four steps. Step 1. A regression-equation estimate Q501ru) (equation 17) is calculated for the ungaged site. Both drainage-basin and channel- geometry flood-estimation methods could be used to calculate a weighted average estimate Q50ldbcg) (equation 14) for the regression estimate (Q50(ru)) or only one of these flood-estimation methods could be used to calculate the regression-equation estimate (Q50Iru))' For this example, only the statewide drainage-basin estimate (Q50Idb)) (table 2) will be used for the 50-year recurrence interval regression-equation estimate (Q50(ru)) at the ungaged site because channel-geometry measurements were not collected for calculating a channel-geometry estimate (Q50(cg))' (A). The characteristics used III the drainage-basin equation (table 2) are contributing drainage area (CDA), relative relief (RR), drainage frequency (DF), and 2-year, 24-hour precipitation intensity (TTF). The primary drainage-basin characteristics used in this equation are total drainage area (TDA), noncontributing drainage area (NCDA), basin relief (BR), basin perimeter (BP), number of first-order streams (FOS), and 2-year, 24-hour 38 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA