Major/Minor System Modelling

I am looking for different ideas for developing a rating curve (for XP-SWMM) to limit inflows to the sewer system based on the number of catchbasins in a subcatchment. In the past, we have assumed that flows up to the peak flow for a particular design storm will enter the sewer system, but we recognize of course that the standards for inlet spacing do not necessarily ensure that the inlet capacity will match the sewer capacity. Any thoughts? We will be applying whatever method we choose to a large number of subcatchments.

Thanks,
Andrea Kauppinen
akauppinen@MRC.CA

Hello Andrea:

Based on my previous literature review for similar purposes, it appears that a rather long and painfull procedure is required to accurately estimate interception capacity of stormwater inlets - one needs to consider longitudinal and side slopes, type of grate, direction of flow, etc.

There are some examples in the Ontario MTO Drainage Manual, as well as in "Municipal Storm Water Management" by Debo and Reese, Lewis Publishers, 1995.

The big question remains whether this calculation is practical at all - some municipalities in Ontario (e.g. Burlington) have standard grate types with given interception capacity.

Minor system flows are typically designed to accomodate the 5-year storm without surcharging. I think that there is a general consensus (where I'm unaware of its source) that sewer capacity under surcharged conditions should equal interception capacity of storm inlets approximately for the 10-year storm. As a result, everything above that rate is conveyed overland.

Serge
rista@OUTER-NET.COM