The peak discharge from a 20-acre watershed is required for a new culvert installation. The hilly watershed (average slope 7%) is composed of pastureland with dense grass and clay soils. The hydraulic length of the watershed is 1,200 feet. The watershed is slated for commercial development in the next two years. Using the IDF curve in Figure 11.26, determine the 10-yr peak discharge before and after development.

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25-yr average frequency 10-yr average frequency 2 4 -yr avcrage frequency 2-yr average frequency 2 0 10 20 30 40 50 60 70 80 9 100 110 120 Duration (min) Figure 11.26 Typical intensity-duration-frequency (IDF) curve