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Area of pond

Area of pond

Area of pond

I have heard different debates regarding this issue and so I wanted to get everyone's thoughts. When you all size detention ponds for developments to limit peak post developed rate to peak existing condition flow rate, do you all account for the area of the pond in the inflow hydrographs. For example, if you have a 20 acre development with a 3 acre pond, would the total acreage of the inflow hydrographs be 20 acres or 17 acres. If you do account for the surface area of the pond, do you develop a weighted CN value for the overall inflow hydrograph? If so, do you use a CN value of 100 to represent the pond? If you use a 17 acre inflow hydrograph and a separate 3 acre hydrograph to represent the pond, than wouldn't the time of concentration for the pond be so low that you are pushing the limits of the SCS method? You would end up with a hydrograph with a CN of 100 and a tc of 2 minutes or so. This would generate a lot of volume. I've heard some say that unless the size of your pond is large (25 acres or more) than you don't need to account for the area of the pond because as soon as a drop of water hits the pond a drop will exist the pond. How do you all handle the acreage of the pond?

RE: Area of pond

The rainfall on the pond's surface is a legitimate pond inflow, just like the runoff from the surrounding ground, so it should be included in the model.  (It is no more likely to "exit the pond" immediately than any other inflow.)

Generating a separate "runoff" hydrograph and adding it to the other "inflows" is the best approach.  Use a CN or 98+ and the smallest Tc that your model will allow.  (Some models will allow Tc=0)  This causes most (or all) or the rainfall volume to "runoff" into the pond with little or no time delay.

Combining the pond surface area with another subcat is not recommended because (1) average CN values do not produce the same runoff as adding their separate hydrographs, and (2) the Tc values are probably different.


Peter Smart
HydroCAD Software

RE: Area of pond

Used a CN = 99 and lag time = 1 min in HEC HMS.
Indeed the program don't allow CN = 100 and Lag time = 0.  

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