1982-03-18_REVISION - C1981013 (3)

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2.3 FLOW CHARTS ✓` The sequence of work in the hydrologic evaluation is shown in figure 2.1. The forms of maps, graphs, and tables are simplified represen- tations of the various standard forms used in the different States. The PI, which precedes the evaluation, is discussed in chapter 3. The design hydrology, which comes latest, is shown in figure 2.3; de- tails are given in chapter 21. After evaluation for the first condition is completed it is not neces- sary to repeat some of the early steps for the remaining evaluations. The second evaluation starts with hydrologic soil-cover complexes, the third with unit hydrographs for "with structures", then the evaluations proceed in the same way as the first except for obvious omissions. Of the basic data needed in the evaluation only the historic rainfall and streamflow data are likely to be unavailable; the rest are obtain- able from field surveys. Lacking rainfall and runoff data, the pro- cedure goes as shown in figure 2.2. The rainfall-frequency data shown in the figure are from U.S. Weather Bureau publications (chap. 4). Di- rect checks on runoff cannot be made, but indirect checks can be made if nearby watersheds are gaged (see table 5.2). Some steps in the procedures of figures 2.1 and 2.2 are taken in an entirely different way in the two methods for regional analysis and d concordant flow. REGIONAL ANALYSIS METHOD. This method is for estimating the magni- tudes and frequencies of peak discharges or runoff volumes for ungaged watersheds by use of relationships for nearby gaged watersheds. Some of the hydraulic work, construction of hydrographs, and flood routing are reduced or eliminated from the evaluation but not from the design hydrology. The method in its simplest form is as follows: e 1. Select nearby gaged watersheds that are climatically and physically similar to the ungaged watershed. These watersheds and nearby areas like them comprise the region that gives the method its name. 2. Construct frequency lines (chap. 18) for peak discharges or runoff volumes of the gaged watersheds. 3. Plot peak discharges or runoff volumes for selected frequen- cies (only the 2- and 100-year frequencies if the frequency lines are straight) of each gaged watershed against its drainage area size, using log paper for the plotting and making straight-line relationships for each frequency.