I'm interested in the history of the Priessman slot and future plans in its development. When was it first introduced? Was it when Extran was first conceived? Where can I read more about this slot so I can develop a better understanding of how it works? I looked in the SWMM Manual and other collected SWMM Sources but the only thing I found is in an old XPSWMM manual.

Reason: I recently ran a Extran model to test the time to dewater a 50 mile long tunnel that normally operates with greater than 600 feet of static water depth above the invert. Tunnel diameter is 13.5 feet. I ran a simulation to set the static depth at the same elevation all across the tunnel and just to make sure the tunnel conduits were completely filled and the model was stable and with good continuity. I began to notice that the conduit volumes in the model's output file were 20% larger than hand calculations. I checked the developer who confirmed that it was the Priessman slot that was adding the addtional volume. I've confirmed that by hand calcs. and what history I've been able scape up on the slot. I thought the solution was easy, just make the slot as small as possible, but when I did that the model became very unstable even with a 0.5s time step. The software developer is crunching on the code now about this problem.

As I recall Mr. Dickinson, presented a paper on the proposed modification of the slot for SWMM 5. I believe, is about forming the slot with the cross section in the form of a triangle to help improve the stability. Is this correct or is it my imagination,or what?

John Minnett
JMinnett@PIRNIE.COM