how do you calculate drainage pipe sizes for proposed pipes on a site plan?

Answer 1

This is the bread and butter for civil engineers. You have to determine the basin for each inlet and then determine what flows each basin produces. There are different methods to do this and different criteria to determine what event (100yr, 25 yr) to use. You also need to determine a time of concentration for each basin.

Once this is done, you determine the flow for each reach. Then you use Mannings equation for open channel flow and what slope you have to size each pipe. Depending on the site, I would use a minimum of 10" and 12" for most runs.

Answer 2

You need to first determine what size storm you are planning on servicing. The pipe size will vary vastly depending on this. Once you calculate the flow from this storm that the pipe will have to carry (use q=ciA, a standard eq, to calculate the flow) Then, use a civil engineering reference manual to help match a pipe size to this flow rate. Your choice will also depend on the pipe material and slope.