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Wet Pond Storage Volume

Wet Pond Storage Volume

Wet Pond Storage Volume

I am modeling a wet pond for stormwater management and am having difficulty in getting the report to show the proper storage volume. Here is the scenario.

I have the pond invert at 96.00,a height of 6', a starting elevation of 100.00, a weir at 100.00, and an orifice at 100.50. I have automatic base flow checked to maintain the water level.

When I run a report on the pond node, I do not get any errors, but the storage volume includes the entire volume, not just the volume available. If I change the invert to 100.00, the volume is correct, but my peak flow rate increases because the "dead water" isn't there to slow down the flow.

What am I missing?


RE: Wet Pond Storage Volume

Since there will be no outflow at your starting elevation of 100' the base flow should be zero. In any case, I would turn off the base flow. All you need is the starting elevation.

The reported volume in the pond will include ALL the water from the bottom up, including the portion below the starting elevation. This is necessary for certain calculations, such as plug-flow detention time, which are influenced by the volume in the initial pool.

If you don't want the "dead storage" included in the reported values then you can start your pond storage definition at the weir. The routing calculations, including the outflow peak and volume, will be the same either way.

If you have further questions please post your HydroCAD project file.

Peter Smart
HydroCAD Software

RE: Wet Pond Storage Volume

Thanks for the info. I'm not sure what I was doing before, but I was sure that I was getting different outflow numbers if I included the "dead storage" volume. It seems to be giving me accurate numbers now.



RE: Wet Pond Storage Volume

Pa allows us to subtract the stage volume from the total outflow volume. I don't suppose that there is a way to show that in HydroCAD.

RE: Wet Pond Storage Volume

Not sure what you mean by "stage volume". But if you start with the pond empty, then the volume at the spillway will be be retained and is effectively subtracted from the outflow volume.

If you set a starting elevation, that pre-filled volume is included in the peak storage volume, but it is NOT included in the outflow volume. (unless the starting WSE is above the weir, and then it will normally draw-down to the weir and WILL contribute to the outflow volume.

If you have further questions I think we need to look at a specific example.

Peter Smart
HydroCAD Software

RE: Wet Pond Storage Volume

Thanks for your response and sorry that I haven't been very clear. I have attached the file that I am working on.

The PA Stormwater Manual allows for a volume reduction in outflow amount through a wet pond equal to storage volume between the predicted high water level elevation and the lowest outlet elevation to account for infiltration and envapotranspiration. In the example that I have attached, according to the pond node report, the water level will rise 1.11 ft during a 100 yr storm. Of course that amount would be different for each storm event. Is there a way to remove that amount from the total outflow in HydroCAD. I can easily calculate that amount separately, but it would be nice to just show that with the node report.



RE: Wet Pond Storage Volume

Sure, there are tricks you could use to subtract a given volume. But frankly, I don't see how the rise in WSE above the weir would correlate to a reduction in total outflow volume. Any reduction due to infiltration or evapotranspiration should be directly modeled, and this is easily done using 1 or 2 exfiltration outlets. This will give you an accurate simulation of the physical system behavior, which is what HydroCAD is designed to do. Arbitrarily subtracting a given volume is a regulatory provision that seems at odds with the actual system behavior.

Edit: If you want to allow for exfiltration only through the portion of the pond above the weir, then you can set a minimum elevation on the exfiltration screen. This will cause exfiltration to be applied only to the area above that elevation.

Peter Smart
HydroCAD Software

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