Stormwater design is one of the most critical components of urban engineering. Unfortunately in Canada there is no national standard of design principals used by municipalities or approval jurisdictions.
I come from Australia where there is a national standard called Australian Rainfall and Runoff that is published by Engineers Australia. Here in Canada, it seems that the majority of authorities require the rational method as the design method, which is strange, because it is regarded as one of the least accurate methods of design and one of the least sophisticated. So in these days of Civil3D and millimeter accurate design, we are using little more than a guesstimate for rainfall and runoff.
The Rational Method is very popular because it is easy to use and can be set up in a simple spreadsheet program; but the actual application of the method is very complex, something that baffles many designers, both technologists and engineers. There are designers who treat the method in the same way for every job, mainly because they don’t understand hydrology. They just don’t understand how an equation with only 3 terms in it can be used differently for different sites. A steep site behaves very differently compared to a flat site, and the frequency of the storm can impact how we treat the rational method coefficient of runoff or C-Value. In my experience, the best text for use in North America is “Municipal Stormwater Management” by Debo and Reese – and in my assessment of how others use the Rational Method, I defer to this text for guidance.
For reference, here is the most recent Castlegar IDF curve, updated in 2010.
In the past three jobs I’ve held, up until my current role, I have written up guidelines on how to incorporate engineering best practice into the design software being used at these firms. This has ensured consistency in practice and methodology across staff and has ensured that a risk based approach has been followed in drainage design.
In my experience, the rational method should only be used to calculate catchment, approximate in-pipe and open channel flows. It must not, in any circumstances, be used to size detention ponds, I’ll get to that in a future post. Ultimately any stormwater design involving pipes should be analyzing the hydraulic grade line for the design storm, with adequate losses for manholes and catchbasins.