Pond Storage Question

[cfj104]

I have modeled a pond with an overflow weir as the secondary outlet at the top of the pond elevation and a culvert discharge as the primary outlet, located at the base of the pond. The question I have is that when I look at the Hydrograph table, there is water flowing through the overflow weir (secondary discharge) before the pond capacity has been reached. Can you please tell me why this is occuring and what this means? I have double checked that the base of the weir is at the top of the pond storage so it is not that the storage volume should be reduced due to the outlet location.

 

[psmart]

I suspect you have an overfilled condition, which will be flagged by one or more warning messages. Please click each message for details.

In general, you need to define enough storage to cover the entire range of possible operation. In the case of a weir outlet, this means that you must define storage to some point above the weir. Otherwise the pond will effectively be "capped" at the weir crest, and will not produce the behavior you expect.

To define storage above the weir, include the controlled volume upstream of the weir, bounded by an imaginary wall above the spillway. For further details please see

http://www.hydrocad.net/overfilled.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cfj104]

While writing my response to you, I figured out the answer. I went back through and checked my model and it does not appear to have any warnings. On the hydrograph, the peak elevation in the pond is below the top elevation of my pond. I have the top of my pond set at 230.5, the weir crest at 230, and the resulting peak elevation in the pond is 230.08. My pond storage is still coming up in the hydrograph table as being less than what I inputted into the storage tab and, I just realized this is because the storage at pond elevation 230 is less (duh) than the storage at 230.5 and is in fact very close to what I am seeing in the table. Thanks for helping me talk myself through the problem.