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Example 3 Estimate the 10-year flood on Silver Creek at the crossing of County Highway 11 east of Rochester. 1. Inspection of figure 2 and table 5 reveals a gage on Silver Creek, station number 05372950, downstream from the selected site. Because the gaged site has only 15 years of record, a weighted estimate of the flood discharge should be made. The proper equation for this site, located in region D, is found in table 1. 2. The drainage area, outlined on the Chester, Minnesota, 7-1/2-minute topo- graphic map, is planimetered and found to be 9.87 mi2. 3. The main channel is extended to the drainage boundary and the length is measured at 6.28 mi. The 10- and 85-percent points are located on the map and the elevations interpolated. The difference in elevation is 1290-1150 _ 140 ft and contributing main channel length is 0.75x6.28 - 4.71 mi, thus: S - 140/4.71 - 29.7 ft/mi. 4. Total storage is determined by counting the grid squares of pond area within the drainage boundary and multiplying by the area per square. St - 2 squares x 0.00144 - 0.003 mi2 St - (0.003/9.87)xl00 - 0.03 percent. 5. Mean annual runoff is estimated to be 7.2 inches from figure 4. 6. The 10-year flood is estimated using the regression equations for Region D: QI0 12.3AO.728(St+1)-0.401S0.335RO.869 QI0 - 12.3(9.87)0.728(1.03)-0.401(29.7)0.335(7.2)0.869 QI0 - 1,114 ft3/s (31.5 m3/s) 7. The 10-year flood is estimated by transfer of data from the gaged site, using the basin characteristics of slope and drainage area, which change significantly between sites. Thus, the computed 10-year flood at the gaged site will be factored by the ratio of the areas raised to the 0.728 power times the ratio of slopes raised to the 0.335 power. Q10,u - QI0,g QlO,u - 1,990 (A /A )0.728 (S /S )0.335 ." g u g (9.87/17.3).728 (29.7/32.3).335 QI0,u - 1,285 ft3/s (36.4 m3/s) 15