In the previous chapter it was determined that runoff occurs whenever the rainfall intensity exceeds the infiltration rate. In many situations it is important to be able to determine the volume of water that will run off an area and/or the maximum rate of runoff. The volume of runoff is used to size drainage structures and water impoundments. One of the uses of the peak rate is to size drainage ways, culverts, and bridges. The peak rate can be easily visualized if the flow rate is plotted in the form of a hydrograph. A hydrograph plots the runoff rate for a watershed versus time. A watershed is a drainage basin where all of the water that runs off passes through one point, Figure 17.1.
This point may be the outlet into another stream or an impoundment like a pond or lake.
Figure 17.2 is a typical hydrograph and it shows that runoff gradually increases to a peak rate and then drops off after the rainfall stops and the watershed drains. Flow rates in streams are generally measured in units of cubic feet per second (cfs) or cubic meters per second (cms). This chapter will discuss the Rational method, which is one method that is used to calculate the peak rate of runoff for areas less than 20 acres.
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