The results for one of my models with a weir appears to be very strange. What I am seeing is the weir works fine under normal conditions, but at the point in time when the weir goes to a surcharge condition the flow drops dramatically. The flow then remains low, much lower than it should hydraulically, and then when the weir is no longer surcharged the flow will jump up again.

Currently I am using XP-SWMM and I have not had an opportunity to put together a model in EPA SWMM to compare results. But I was curious to see if anyone knows of any problems with the SWMM engine or has experienced similar problems or if someone has some suggestions on what I should try.

At first glance it also looks as though surcharged weirs may be modeled differently between the different SWMM engines. My understanding of how surcharged weirs are modeled, according to the documentation, is that:

EPA SWMM calculates a weir surcharge coefficient using the equation:

Qw = Csur * Lw * (Ytop -Yc) * SQRT(2 * g * h' )

It calculates the Csur at the time step before the weir goes to surcharge conditions. It then uses this equation (which is basically an orifice equation) to calculate flow.

XP-SWMM does the same thing as the EPA version but then the documentation says it goes on to convert the weir to an equivalent conduit by equating the conduit and weir discharge equations:

(m/n) * A * R^(2/3) * SQRT(S) = Cw * W * H^(3/2)

where: S = H / L

n = R^(2/3) / (Cw * SQRT(L)), where R is taken when the head is half way between Ycrest and Ytop.

L = based on the Courant number.

So if I understand it correctly, XP-SWMM really just converts a weir to a rectangular conduit for surcharge conditions.

Any suggestions/comments/ideas are welcome and appreciated.
Thanks
Daniel P. Christian, P.E.
Dan.Christian@McNamee.com