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no outlet velocity calculated

no outlet velocity calculated

(OP)
So I am working on a detention pond checking 2-yr 24-hr, 5-yr 24-hr, 10-yr 24-hr and 100-yr 24-hr storm events. I am required to list exit velocities for each storm event. Unless things have changed when you are below 5 fps vegetation is adequate erosion control. I have not being doing this lately a whole lot but I imagine things have not changed.

So my 2-yr event the output mentions 5.2 cfs of 6.9 cfs passes through the outlet culvert. It is a riser structure with a low flow orifice, mid level orifices, and the open manhole structure type inlet at the top of the riser. 18" culvert as the outlet.

The same output for the other storm events mentions barrel controls and gives a flow rate in cfs and velocity in fps in the same area where before it gave no velocity for the 2-yr event.

Why is there no velocity mentioned for the 2-yr event? It shouldn't matter since the only velocity over 5 fps is the 100-yr event but I would imagine they will be asking unless I calculate the velocity using Manning's equation myself.

Thanks

RE: no outlet velocity calculated

HydroCAD only reports the velocity for the controlling device. For the devices that are "passing" less than their potential flow at a given head, the velocity is not known and will not appear in the report.

For details please see www.hydrocad.net/outlets.htm (scroll to the end of the page)

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: no outlet velocity calculated

For a quick estimate of outlet velocity you could assume normal flow in the pipe and use Manning's.

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

RE: no outlet velocity calculated

I think FHWA's hydraulic toolbox has a circular channel that can do that calculation or flowmater. You could always revert back to the partial flow nomograph for pipes.

RE: no outlet velocity calculated

You can also do the calculation in HydroCAD. Just setup a pipe reach and examine the stage-discharge curve. No inflow is required - just an isolated reach.

Peter Smart
HydroCAD Software
www.hydrocad.net

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