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determining detention time w/exfilter system

determining detention time w/exfilter system

determining detention time w/exfilter system

Hello all,

I am working on a storm facility located in sandy soils, so I plan to use infiltration practices.  One of the design criteria is the detention of a 1-year storm event over a 12 hour period.

When modeling my pond without infiltration, I can size a control orifice that provides the 12 hours I need.  Once I add the exfiltration component, though, the detention time drops to around 3 hours (based on the Center of Mass det time in the report).

It seems that the COM calc doesn't take exfiltration into account.  Either that or the calc defines the structure right at the pond bottom.

The regulators are concerned with the impact of the storm on the receiving water, and the 1-year storm standard is made to prevent scouring of the stream channel.  However, the infiltrated water will be as non-erosive as you can get, as it just goes down to the water table and helps out base flow.

How should I structure my model, or explain it, to prove my compliance with the standard?  It's ridiculous to say that a pond without infiltration is more compliant with the standard than a pond with infiltration.


RE: determining detention time w/exfilter system

The calculated detention time considers the total discharge from the pond, including any infiltration.  In your case, you probably need to do a separate assessment of detention time, considering only the orifice discharge.

If there is zero orifice flow for the 1-year event, it means everything is being retained and infiltrated, and you've attained an effectively infinite detention time.

If there is some orifice flow, you can calculate the COM detention time by subtracting the time of peak inflow from the time of peak orifice flow.

Peter Smart
HydroCAD Software

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