There are some statistically-based methods called the Analytical
Probabilistic Stormwater Model that may be of some application. There
is a recent (1998) text by B.J.Adams (U of T) and Papa Urban Stormwater
Management Planning with Analytical Probabilistic Models, John Wiley &
Sons, 2000, that provide solutions to return periods, proportions of
flows detained, etc for CSOs that may be of value.

Barry Palynchuk
Barry_Palynchuk@CPR.CA