What is the history of the Priessman Slot in SWMM

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

Robert Barkau's UNET 1-D unsteady flow model includes the Preissman (note spelling) slot in Chapter 7 of the users manual. UNET is a Corps of Engineers model. In addition to a brief discussion of theory and application, there are references:

Streeter & Wylie, 1967
Parmakian, 1963
Liggett, 1975
Cunge et al, 1980.

Neil Jordan
NJordan@EXPONENT.COM

Hi,

In Advances in Hydroscience, Vol 14, 1986, by Ben Yen; there is a good explanation on how the analysis of surcharged sewers evolved from the use of open channel hydraulics equations to the development of the Preissmann slot, including the selection of the slot width.

Cheers,
Mauricio Herrera
mherrera@UOGUELPH.CA

John,

The transition zone for the slot at the bottom is parabolic and not trapezoidal but that area has mimimal impact if you are talking about a 600 foot tall slot. I estimate an increase of 28.29% in volume if you use a slot width of 0.005 times the pipe width of 13.5 feet. If you lower the slot width to 0.0001 times the pipe width then the increase in volume is only 0.57%. Of course you will have to use a much small time step to make your model stable.

A ten foot tall slot only increases the volume by 0.47% with a slot width of 0.005 times the pipe width and only 0.01% with a slot width of 0.0001 times the pipe width. I do not know if the slot ( a mathematical construct to connect the top of the pipe to the ground surface so that the St. Venant equation can be used for all flow situations) was intended to be so much taller than the pipe diameter.

Regards,
Bob Dickinson
DickinsonRE@CDM.COM

Hello all,

Alexandre Preissmann (note spelling, double n please, as used by Mauricio below) was a mathematician who worked at Sogreah for most of his career. He retired in 1981, and the book "Engineering applications of computational hydraulics" (Pub by Pitman, 1982) was dedicated to his retirement.

The Preismann slot is not yet officially incorporated in any versions of SWMM4.3 or 4.4 - I understand that it will be in SWMM5, as detailed at the Toronto conference last February.

However a version of EXTRAN with many enhancements including the Preissmann slot is available in PCSWMM (actually PCSWMM2003) - perhaps this is where the confusion arose?

Bill James
wjames@UOGUELPH.CA

Bill,

Thanks for the additional information about the spelling of Preissmann. I have a tendency to spell it both ways. The original Preissmann slot question was actually a question about the slot width in XP-SWMM but John M's experience with the slot width has important implications for SWMM5.

Regards,
Bob Dickinson
DickinsonRE@CDM.COM

Hello John;

There's a brief yet v. useful discussion on Preissmann (hypothetical piezometric) Slot vs. Standard transient pipe flow approach by B.C. Yen & A.O. Akan in chapter 14 of the Larry Mays "Hydraulic Design Handbook", Mc Graw Hill,1999.

Good Luck!
Mostafa Bozorg-Zadeh(ICE)
bozorg@KOSAR.NET

[unswjeb]

Bill & Bob,
If John's original question relates to the width of the slot, he may wish to refer to the 3ICUSD Proc. from Gotenburg in Sweden where A Sjoberg presented a paper discussing the impact of the slot width on the wave propagation and on continuity issues associated with a slot.
Regards
James Ball