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Trapezoidal grassed channel over a culvert

Trapezoidal grassed channel over a culvert

Trapezoidal grassed channel over a culvert

(OP)
I've got 500 feet of shallow box culvert with which I intend to convey low storms, and want to provide extra capacity above it in the form of a trapezoidal overflow channel. I would like to enter the culvert and the overflow channel both into the Outlets dialog of a no-storage pond node in HydroCAD, but there is no option for the channel.

I thought I might make a dummy weir of arbitrarily large width with capacity in excess of the channel capacity, and route the discharge from that weir to the channel as a secondary discharge, then rely on the elevation in the channel to dictate the head at the node. It doesn't seem to be working.

Are the routines for trapezoidal channels in reach nodes in HydroCAD robust enough to do what I'm doing? Or would I need to do something else? Can I be confident in my results if I approximate the overflow as a broad crested weir?

My application may be pushing the boundaries of the software, and I'd like some guidance on exactly how far I can confidently push them.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Trapezoidal grassed channel over a culvert

The most direct approach would be to use a trapezoidal weir to represent the outlet channel. Of course, this assumes the flow is governed by the weir equation, and not by flow conditions within the channel.

On the other hand, if channel flow conditions prevail, you're approach sounds reasonable: Use a weir outlet to (to model the channel entrance) and route the flow to a trapezoidal reach, which will generate a tailwater for the weir. HOWEVER, you should use the actual entrance geometry, rather than an "arbitrarily large weir". Since this approach involves a tailwater dependency you will need to use the DSI routing procedure.

Another option is to use a special outlet device to specify the stage-discharge relationship for the channel. (You can use a reach node to calculate the rating table, and then enter the values into the special outlet.)

Peter Smart
HydroCAD Software
www.hydrocad.net

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