This is a file that was developed and used in a court case involving drainage. In this file I go into some depth in using the Rational Method to compute the peak flow from a catchment. I felt that this catchment was made for the Rational Method.
Re: Rational Method peak flow Wilkerson v OPSB.xmcd
That is a very nicely crafted worksheet. The math, the diagrams, the presentation, and your explanations are clear and logical, which means the attorneys probably almost understood it.
I'm keeping a copy for my own archives. I don't do a lot of storm drainage work, but what work I do almost always uses the Rational Method, or what I like to call the Irrational Method. I also haven't had to testify in court, so it's good to see an example of how it's done.
In the late 1980s, for a project I was doing downslope of Interstate 5 in central California, I reviewed a hydrology report done by the California Department of Transportation in the mid-1960s for the freeway design. I was surprised to find that their engineers had applied the Rational Method (including the Kirpich Equation) to watersheds up to 5 square miles in my project area and over 10 square miles outside my project area. These watersheds were mostly rolling hills covered by very thin grasses (typical for the eastern side of the Coast Ranges). I would have expected what was then the world's premier highway agency to be more sophisitcated about their hydrology. How naive I was.