Exfiltration Question ?

[TerraSouth]

I have a retention pond that I plan to design with a Sand Seepage Chimney. Due to the site being flat, I have to design a retention pond rather than a detention pond.

The clay in the pond bottom blows out at 15' to pure sand.
I plan to cut a pit (chimney) throught the clay and back fill it with course sand down to the existing sand layer of sand. The area of the chimney is yet to be determined. But from my calculations of pure coarse sand I expect a Saturated Hydraulic conductivity of approximatley 25 gpd/sq.ft. or a perc rate of 3 min/in.

I plan to design the area of the Chimney to drain the pond within 72 hours of the 25 year storm event.

My question is how do I model a specific Area in the bottom of my pond for this exfiltration? or is this possible.

Thanks

 

[psmart]

If most out your storage is in the upper clay-lined volume, the wetted-area of sand will be fairly constant, so the final exfiltration will be constant. In this case, you can just specify a constant CFS for the exfiltration outlet.

However, if the exfiltration is subject to change due to varying wetted area, then you could use the option for exfiltration VELOCITY. To prevent exfiltration from occuring through the upper (clay) part of the pond, use a separate storage definition for the upper and lower volumes (both in the same pond), and uncheck the option to "Allow Exfiltration" in the upper volume. When you're done, check the stage-storage graph to be sure it displays the intended behavior.

For general information on exfiltration options please see

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