something to consider....what is the highest groundwater elevation in relation to your underground storage facility? If the ground water elevation is above the bottom portion of your storage facility, a percentage, if not the entire storage facility, will become ineffective for the duration of the increase in grounwater elevation. If depth is an issue due to groundwater, i would agree with cvg, under ground storage may not be the best alternative, however depending on the storgae volume required, ground water elevation, etc there are products available that provide storage volumes at shallow depths. Obviously the footprint of your facility would increase but it's an option to consider. If your interested in the shallow storage facilities types let me know, I will forward on.
If you want to infiltrate, but need a shallow system that can be build above the water table, you can use:
* Perforated pipe (steel or plastic)
* Plastic chambers
* Open-bottom concrete products
If you don't want to infiltrate, you can use a sealed system that extends below the water table:
* Non-perforated pipe (steel or plastic)
* Plastic chambers wrapped in a waterproof membrane
* Concrete products (tanks, arches on a sealed slab)
Each has it's trade-offs. The HydroCAD Chamber Wizard makes it easy to model and compare different options.
it won't float, but it also won't be able to store any runoff if the groundwater rises that high. In addition, you may have some issues with contamination of ground water when you introduce runoff from pavement or turf areas into the underground storage and then allow it to perc directly to the high groundwater.
If the ground water rises above the flowline of the perforated pipe, then the draining of the pipe out the orifice will lower the ground water elevation.
in which case, your retention system becomes a dewatering well. I don't think you will be able to get a permit for that. and i don't think we can determine that the perforated pipe would be sufficient to permanently draw down the water table. this would depend on the capacity of the perforations, the elevation of the orifice and the capacity of the soil to drain the groundwater into your pipe. either way, it still does not function well for storm water storage if groundwater is seeping in and filling part of the pipe that is expected to store stormwater. the perforated pipe option simply is not well advised if the groundwater depth is that shallow.
injection wells (dry wells) - shallow or deep can be used to dispose of water in a retention basin. But generally do not have sufficient capacity to reduce stormwater peak flows. A basin or storage facility is still required for that. Additionally, sediment needs to be settled out before going into the drywell.
I agree with CVG's comments. We see a lot of folks that expect infiltration systems to help with peak flow management, but unless you have exceptionally high infiltration rates it has a minimal effect on peak flows. In most cases you'll still be using detention as the primary means of peak reduction, and this requires sufficient storage. Of course, infiltration can be a viable option for emptying the pond over a longer time period. And these are often combined in an underground detention/infiltration system using some type of chambers or pipe.
