Pond Discharge Question

[cfj104]

I am modeling my basin as two separate lines on the storage tab since stormwater will flow into the first cell and then overflow into the second cell. There is not a controlled discharge from the first cell to the second, just a gabion overflow weir. The second cell has an overflow spillway and a riser pipe with orifices.

My question is that when I run the model I am getting a discharge rate for the 1 and 2 year storm events eventhough the capacity of the first cell is 14,646 cf while the runoff volume is only 11,194.92 cf (1-yr event). I just want to be sure that I am interpreting the results correctly, it just seems off to me. I am well below the required rate control but I was hoping to be able to claim the volume reduction credit and do not want to state anything that is not correct in my plans.

Also, when I tried to model as two separate ponds I got a discharge rate larger then my inflow rate which is why I included the volumes as two lines.

Thanks for any help or advice.

 

[TerryScan]

Is the 14,646cf capacity you describe in the first cell all below the weir to the second cell?

How did you have your outlets configured when you get unexpected flows or higher flows than expected? (what was your outlet from cell1 to cell2 when modeled as 2 ponds?)

One usually gets higher outflow than inflow when the outlet is smaller than available storage and ficticious head builds up in the pond (you will see a peak elevation far greater than your highest storage).

 

[psmart]

If the inflow volume is less than the storage volume below the oulet invert, you shouldn't have any outflow. Everything will be retained.

I think your problem is that the "gabion overflow weir" DOES control the flow from the first cell to the second. This means the water levels will be different (especially for small inflow volumes) and the two cells should not be modeled as part of the same pond.

Although a pond can include multiple storage volumes, this is only appropriate if they always have the same water surface elevation. If the elevation is significantly different, they are NOT part of the same "level pool", and cannot be modeled as such. In short, you may need to use two ponds, one for each cell.

For details see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cfj104]

Thanks for the input. I pulled the two storage areas out as separate ponds again and everything seems to be working fine this time.

I guess my outlet must have been inputted incorrectly the first time b/c I am not getting any abnormal errors or warnings.

Thanks!