The Rational Method

In this test, you will apply the Rational Method for estimating peak discharges at the outlet of a small watershed. This is a simple means of estimating peak discharge expected from storm events, and is most often applied in small urban watershed problems, e.g. to determine storm sewer requirements for a proposed change in land use. The Rational Method is based on the equation

$$q_p = C\cdot i\cdot A$$ (1)

where $q_p$ $\rm\frac{ft^3}{sec}$ is the peak discharge from a drainage area of $A$ acres given rainfall intensity $i$ $\rm\frac{in}{hr}$ and runoff coefficient $C$, based on watershed slope, land-use, and hydrologic soil type. Tabulated values of $C$ essentially give a typical unit hydrograph for various watershed and soil types. As in the class examples of SCS Curve Number ($CN$), a composite runoff coefficient should be used for heterogeneous watersheds. For a heterogeneous watershed that can be subdivided into $J$ homogeneous subareas $A_j$ with associated curve number $C_j$, (1) becomes:

$$q_p = i\cdot \sum^J_{j=1}C_j\cdot A_j$$ (2)