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Modeling open-grate manhole as riser - Autodesk SSA

Modeling open-grate manhole as riser - Autodesk SSA

Modeling open-grate manhole as riser - Autodesk SSA

I'm trying to model the outlet control for a detention pond and wanted to use an open-grate manhole as a riser with an orifice sized for 2yr 24hr and a barrel sized for the design storm event. I'm a bit confused on how to model the water entering the riser. Would this be a weir with a length = circumference of the manhole? Is there another structure in SSA for this application? I've attached a schematic of what I want to achieve. It seems pretty basic, I'm just new to SSA.

My current model is connecting the storage node to a junction with an orifice sized for 2yr and a link that represents the barrel sized for the design event. The junction then links to an outfall with the same barrel configuration. Not sure if this is the best method and I'm curious how to best show this. For some reason, my barrel link does not receive full flow during the design storm and my pond overtops. Something like the max capacity of the barrel is ~50cfs, but it's only seeing ~18cfs during the storm. The barrel is set 1' below the top of the pond elevation.

My rough flows are Q2yr = 20.61cfs, Qult = 54cfs

RE: Modeling open-grate manhole as riser - Autodesk SSA

54 cfs through a grated manhole cover seems a bit ambitious, especially if you make a conservative assumption that it is 50% clogged. 18 cfs sounds about right unless this is a very large manhole

RE: Modeling open-grate manhole as riser - Autodesk SSA

What diameter grate for the 18cfs assumption?

RE: Modeling open-grate manhole as riser - Autodesk SSA

I used a rectangular grate 4.4' x 2.4' using the weir equation and got 13 inches depth. I didnt check orifice flow, at this flow depth it might transition to orifice conditions

RE: Modeling open-grate manhole as riser - Autodesk SSA

I was planning on using a 44" grated manhole cover. How would I model that in SSA? Do I just assume it's a weir with length = to the circumference of the opening?

RE: Modeling open-grate manhole as riser - Autodesk SSA

no, the grating blocks part of the length and that needs to be subtracted from the circumference. clogging should also be considered to block part of the length. your effective, unblocked, unclogged weir length will be much less than the circumference

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