Surcharged Weirs

> It's kind of ironic. I was reading your message, Dan, and thinking to myself that I would suggest replacing the weir with an equivalent rectangular conduit. Then your message states that's exactly what XP-SWMM does. I've used this technique many times in EPA's SWMM without any difficulties. However I have yet to experience a surcharged weir in XP-SWMM. Perhaps you should attempt manually replacing the weir with an equivalent rectangular conduit rather than letting XP-SWMM do it. I've never had a pipe's flow drop to zero due to surcharging.

> On the other hand, it is a fairly simple process for someone proficient with a spreadsheet to convert the XP- SWMM .dat file into an EPA-SWMM input file. I would be interested in comparing the results of a surcharged weir using the two computational engines.

> ssticklen@hntb.com


The recent discussion on surcharged weirs has suggested that the output from XP-SWMM and EPA SWMM (for the same input data) must be compared . Although an XP file can be manually converted to an EPA file, the process is laborious for large projects.

I have previously asked XP Software to enable creation of an input file for EPA SWMM. XP-SWMM's input interface is unparalleled in the industry and this capability alone will be worth buying the software. "Create EPA SWMM Input File" has been a menu option in XP-SWMM for a while but it does not work. A message indicates that it will be implemented in the future. I check every new release with great anticipation and curiosity without any luck. Although XP-SWMM can be run using a built-in EPA SWMM engine, the results are rarely the same as one would get if running the EPA SWMM outside XP-SWMM.

As always, I highly recommend to let your vendor know what you want in the software. I'm sure most vendors will honor the wishes of their user group.

Uzair Shamsi
ushamsi@chester-engineers.com

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

Dear Uzair,

I have always liked reading your comments on the swmm users group and think that this latest comment is a good one. A luck would have it I recently recreated our epa converter for version 4.4 of SWMM. The attached file xptoepa.exe will create a file called epaconv.exe which will convert a xp-swmm32 input file to an epa swmm verion 4.4 input file. It is a DOS program that you run in the following way:

epaconv xpswm.dat
the name for the xp-swmm32 created input file

the program epaconv will make a new file called xpswm.epa automatically.

This version of epaconv will convert all of the water quality data in runoff to epa swmm format, all of the transport links to epa swmm format, and exports the entrance and exit losses to the c1 line of transport and extran. It also adds the comment lines forr the individual data lines. It, however, does not convert the stats and rainfall data files to epa swmm format

Best Regards,
Robert Dickinson

Uzair,

You wrote:

> However, there is still one solution. The interface can provide an option to run the model using EPA's or Dr. Huber's EXE file. Users with special interests in SWMM enhancements can continue to use the EXE file developed by the interface vendor.

Uzair has contributed many interesting ideas over the years to epa swmm and to us at XP SOFTWARE. We try to have an open architecture or as open as we can at XP SOFTWARE concerning our code and epa swmm code. It would be possible to do what you suggested in the solve=dos mode of xp-swmm32. Trying to keep epa swmm as a black box with xp-swmm32 as a preprocessor and post processor would be possible with a few modifications. These changes would include putting the xptoepa converter in epa swmm and adding our binary graphics files for post-proccessor graphics to epa swmm. Another possiblity would be two external programs to do the conversion and graphics etc. If any one else has any other appraches to this I would like to hear from them. I could see some potential benefits to this approach.

Best Regards,
Bob Dickinson