Numerous regulatory agencies in the US allow, or require, what is called here the Modified Rational Method for sizing small detention facilities. In this method a series of triangular and trapezoidal hydrographs are created beginning with the triangle with the peak flow occurring at the time of concentration.
A. Finding the Maximum Volume
The question now becomes "assuming a constant outflow, for what duration storm will the maximum storage volume be required?" In the sketch above, the shaded area represents that volume. The task of finding that duration and volume lends itself particularly well to a spreadsheet solution.
Click the icon to see this solution.
MAXSTOR.XLS (1 page)
Cell XX,YY contains the solution using Excel's =maximum function.
B. Limitations
The Rational Method was developed in the US in Rochester, New York and based on studies of a small number of urban drainage basins. The original studies used only small basins. For these reasons, most reviewing agencies limit its use to small drainage areas. For example, the Federal Emergency Management Agency (FEMA) accepts the Rational Method only for drainage areas of less than 100 acres. Local agencies typically adopt even lower limits; on the order of 25 acres or so.
