Catch basin under surcharge condition 3

[PAB - 1]

Hi everyone,

We're trying to help a client that had serious flooding of their building during a significant rain event last August in our area. There is a catch basin (CB) situated on the client's property. An 8 inch CMP exits from the CB and goes to a street drain manhole (DMH) about 150 ft away. The grate on the CB can be seen in the attached photo.

The DMH has a 24 inch line going into and out of it.

During the rain event, the CB was significantly surcharged. The maintenance guy told us there was a pond, approx 2 ft deep, over the CB during the event. The 8 inch line was subsequently camera-ed and found to be clear. The City Engineer also informed us that the line in the street was not overloaded during the event. Due to how the pipe crowns are situated in the DMH, there is a possibility that the 8 inch CMP going into the DMH was either partially or completely submerged during the event.

We're presently modelling the system.

My question is this: does anyone have any information on determining the flow rate of a surcharged CB grate? I have scoured the internet but have not come across anything. The CB grate is acting as an outlet for the pond and I'd like to model it to help us determine if it, or the 8 inch CMP, or both, caused the ponding and flooding.

Thank you in advance for any help on this!

 

[TigerGuy]

I'm a few years out from stormwater topics, but I thought the manufacturers have information on orifice flow through their various grates.

 

[psmart]

You should be able to use orifice flow for each opening in the grate, and then add the flow for all the openings. Your openings seem to have rounded ends, but you should be able to approximate with a rectangular opening.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

submerged grates tend to clog. reduce the area using a clogging factor. for areas with a lot of vegetation or trash, up to 50% clogged

 

[TheEnginerd]

You can model this situation in HydroCAD and similar programs. I would treat the grate as a horizontal orifice which you can set the opening percentage. You know the discharge pipe and you can figure out how much flow is getting to the low point. Separately, i know local foundries in our area (NJ) have flow rates charts through their grates and different head elevations. One such company is Campbell Foundry. I used to use those charts all the time to determine if the grate was large enough to handle large drainage areas or if i needed a double inlet, etc.

http://www.campbellfoundry.com/products/