Tailwater in Hydraflow StormSewers

[Anonymous]

I have a 15" pipe going into a 36" pipe (subsurface detention) at 0.5% minimum slope. I've entered in the inverts, rim elevations, calculated rate going into the inlet. When I run the system (Known Qs only), I put in the starting downstream HGL as the maximum water surface elevation in the 100-year storm based on Hydraflow Hydrographs.

Downstream inv. = 500.08

Water surface elev. = 503.00

Storm Sewers is defaulting the HGL to critical Depth (500.50), which doesn't allow me to see if there is a surcharge in the inlet.

I recognize that Tc wise, the pipe should be empty by the time the basin reaches it's maximum WSE, but why is the program just now limiting me to critical depth, when we've used pressure flow in the past?

[pperales2]

Check the design codes

[Anonymous]

Here are the design codes, but regardless changing the minimum depth shouldn't affect anything since the critical depth calculated is less than the tailwater I want to use.

[pperales2]

Can you post the .stm?

[Anonymous]

It's not letting me attach the file. 

[pperales2 · Accepted solution]

Well if I understand what you are doing which is use the 36" pipe as a detention pond and the 100yr WSE is 3' basically the 36" pipe is full then set the starting HGL at the crown.

[pperales2]

[Anonymous]

Thanks!

[fcernst]

Place it in a zipfile..

You should be able to model your high tailwater condition.. the software just won't allow you to specify an elevation BELOW critical depth...

[Anonymous]

See attached. Thanks.

[fcernst]

There's only one 12" line in there..?

[Anonymous]

Yes -- this is just a one-off pipe and is only connected to a basin not to another network. This is not for an actual conveyance calculation (yet), but for now is being used to make sure that the basin design is adequate. My fault though in that it should be 15". But that doesn't really matter, as I just tried again with that edit and it still doesn't let me assign a higher TW condition than the crown.

Thanks again.

[fcernst · Accepted solution]

Ok.. yes I can get the 503.00 to work.. make sure you are pressing ENTER after typing the tailwater elevation...

[Anonymous]

Thanks, this has never happened to me before, but I appreciate the effort to help figure out such an obvious solution.