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Culvert Capacity

Culvert Capacity

Culvert Capacity

(OP)
Hello All,

I probably have a real simple question. I am running the sampler version of Hydracad and I have been given the responsibility of figuring out this program so my boss can justify purchasing it. I have modeled a pond, infiltration trench, with 3 different storage area, a gravel layer with voids, a soil layer with voids, and an ponding depth at the surface. I have also modeled a culvert and a riser to allow for overflow. I assume I have set all this up correctly as I am not getting any error messages or warning.

All that to say, how do I determine the capacity of my outlet culvert(device), i.e., how full is the pipe? I have set up the outlet devices as follows. I have a culvert at the bottom of my pond which is the primary outflow device and a horizontal grate/orifice 3.5 feet above that and is being route to the culvert, device 1. Am I missing a riser?

Any help would be greatly appreciated.

Thank you.

RE: Culvert Capacity

By routing the horizontal orifice to the culvert you are creating a drop inlet at the top of the riser, just as you wanted. You can examine the behavior of the overall structure on the stage-discharge graph, which can also be used to determine the outlet capacity at any specific pond elevation.

For further details on riser setup please see www.hydrocad.net/pond1.htm

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Culvert Capacity

If you're trying to determine if the culvert size is adequate, have a look at the "device analysis" at the bottom of the pond summary. This will identify the control point for the peak flow. In your case, it will tell you if the culvert or the top of the riser is the effective flow control. Also, a careful examination of the stage-discharge curve will often show a point of inflection where it transitions from weir flow (over the top of the riser) to culvert control.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Culvert Capacity

(OP)
Yes I suppose I just want to know if the primary culvert at the bottom of the pond is adequate. I'm just trying to see how full the pipe is. Is it less than half full? I attached the stage discharge graph and pond summary. I'm sure the results are clear as day but I'm still trying to learn the program. All the help is greatly appreciated.

RE: Culvert Capacity

At the bottom of the pond summary it says the culvert "passes 0.49 cfs of 2.47 cfs potential flow", so you could reduce the culvert size without have any effect on the pond routing.

This is consistent with the stage-discharge curve, which does not appear to show a point of inflection for the culvert control. The transition point is essentially off the scale.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Culvert Capacity

(OP)
Thanks psmart. I appreciate all the help. I'm following what you're saying. I guess I was looking for a black and white area saying "you have selected a 10" culvert at invert 0.00 and the water surface is at elevation 0.37'". This will be more than sufficient. Thank you again.

RE: Culvert Capacity

Remember that culvert flow is much more complex than just flow depth inside the pipe. For example, you could have several feet of head at the inlet, but because of inlet losses, the barrel might be just half full. So being half full really doesn't tell you much. You really need to look at the capacity of the pipe at a given head to determine if the size is suitable for a given application.

Peter Smart
HydroCAD Software
www.hydrocad.net

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