Bypass Storm Sewer Flowing Through Outlet Structure

[cstorrar]

I have a situation where I am connecting an detention basin outlet structure to an existing storm sewer. The existing storm sewer is capturing offsite area that is bypassing the detention basin. I am worried about the existing storm sewer surcharging and backing up into my basin through the outlet structure in large storms. Currently I have the basin modeled with orifice outlets that are routed to a CB that is the existing storm sewer. The offsite runoff watershed also enters the existing storm sewer CB. Is this correct? Running with the Dyn-Stor-Inds will make sure tailwater is accounted for, but what about overflowing out of my outlet structure into my basin? Should I use a Sim-Route model with a reverse flow from the basin to the CB? I'm worried Sim-Route will not work with the CB. Thanks for any help you can provide

 

[gbam]

Have you considered a flap gate or duckbill back flow preventor?

 

[cstorrar]

I have considered those, but was hoping to avoid one if possible because it would require an additional structure. Currently the outlet structure is proposed to drop right on top of the existing sewer. To have a flap gate would require the outlet structure to be a separate structure and then tie into the existing sewer at another structure.

 

[psmart]

Are you getting a warning message about potential reverse flow? HydroCAD checks for tailwater and reverse head at each time step, and will alert you if this should occur. If there's no warning message, you probably don't have a reverse flow situation.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cstorrar]

Yes, I am getting a warning about possible reverse flow. The basin is exceeded by almost 5.5' according to the model.

 

[psmart]

You might be able to model the reverse flow using SimRoute, but this ONLY works between two ponds with some storage. It won't work from a zero-storage CB back into the pond. For details see

http://www.hydrocad.net/simroute.htm

But before you attempt to model this, make sure the reverse flow will actually be allowed by your stormwater regulations. If the timing of the sewer fills your pond early in the storm, you won't have any volume available for peak management of your on-site flow.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cstorrar]

Thanks for all of your help. I looked at the Sim-Route option and it gives me reasonable results (no oscillations, etc.) but I understand it is not to be used for catch basins. I was surprised because it does allow for flow back into pond from the basin (although it is a rather small amount). The reason I want to model the back flow into the pond is to account for it in my peak detention calculations. The storm sewer only backflows in large event storms so that is where I need to check it.

 

[beej67]

Used to model this sort of thing all the time with XP-SWMM.

I'm less than a year in to learning HydroCAD, but as I understand it, what you want to do is use Sim-Route, make sure you don't have any oscillations, and make your catch basin a very small storage node so that HydroCAD processes the reverse flow properly. Is that correct, Peter?



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[psmart]

As long as the model is stable you should be OK. You'll generally get a warning if there are any problems.

Peter Smart
HydroCAD Software

http://www.hydrocad.net