Where there are no or insufficient stream-gauging records available, peak-flow methods such as the “rational method” and the Natural Resources Conservation Service (NRCS) method may be used. The rational method is the most common procedure for determining the quantity of flow for the design of minor hydraulic structures. Its use in the United States dates back to the late 1800s. One of the basic design assumptions for its use is that
the rainfall intensity is uniform throughout the watershed. This assumption limits its use to relatively small watershed areas (in the neighborhood of 200 to 300 acres, or 1 km2).
The rational method is based on the simple intensity-runoff equation
Q = KCiA (5.2)
where Q = design discharge, ft3/s (m3/s)
C = runoff coefficient
i = average rainfall intensity, in/h (mm/h) for selected frequency and duration equal to time of concentration A = drainage area, acres (km2)
K = 1.0 for U. S. Customary units (0.278 for SI units)
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After precipitation falls to the earth, it either is infiltrated into the earth, is evaporated back into the atmosphere, is subjected to depression or detention storage, or becomes runoff. The runoff coefficient C in Eq. (5.2) depicts the percent of precipitation that will run off the ground from the storm. Representative values of C for undeveloped and developed areas, respectively, are given in Tables 5.1 and 5.2. Also, if the watershed is made up of various surfaces, a weighted average should be used for C. This may be determined for surfaces with coefficients C1, C2, etc., and areas A1, A2, etc., as follows:
The intensity value i in Eq. (5.2) is dependent upon the time of concentration of the storm and the frequency of the design storm selected. Once these two parameters are selected, the rainfall intensity may be determined from an intensity-duration-frequency (IDF) curve. Such curves, which are derived from an accumulation of rainfall data recorded over the years, are available from both local and regional public agencies. (See, for example, U. S. Weather Bureau Technical Paper No. 25, “Rainfall Intensity — Duration-Frequency Curves for Selected Stations in the U. S.”) A method for developing rainfall intensity curves and equations is shown in FHWA publication HEC 12, “Drainage of Highway Pavements.” A typical IDF curve is shown in Fig. 5.1.
