Can a retention/detention pond and a parking-lot be constructed in the same site foot-print?

Question

Retention/Detention Ponds are often added to a site-design to mitigate storm-water runoff. Enhanced Porosity Concrete (EPC), aka Pervious Concrete, can be used as a parking-lot pavement that allows storm-water to flow through it. If a detention-pond was covered with a filter-fabric, and clean-gravel was used to fill the pond on top of the filter-fabric, then EPC could be used to cover the gravel such that a parking-lot would be created on top of the pond, and using the same foot-print as the pond. The gravel would support the pavement loads and the water-storage capacity of the gravel-filled pond would be a function of the amount of void-space in the gravel.

Because parking-lots and detention-ponds both take up a large part of land area of a building-site it would seem that combining the two in the same foot-print would save money and land area for the building-site developer. The pavement would also keep debris out of the pond and create a safer environment.

Answer 1

Yes, Underground Detention is a thing. That Wikipedia article even mentions placing them under parking lots.

The exact method that you propose is a little different than preconstructed vaults, but it is the same in the end. Pervious concrete on top of gravel with voids serves the same purpose but with much lower capacity. It is not really speeding up the infiltration of water into the ground.

Answer 2

Underground storage can be used, but it is much more expensive. There are a variety of options such as concrete tanks, R-tanks, or piped storage. Whatever you go with, you need to ensure that you are able to collect stormwater from your site and convey it to your tanks. Using pervious pavement is usually difficult since it's hard to drain a large area to your pervious surface. Using inlets is more common. Lastly, when draining underground storage, deeper is more expensive so you should keep it shallow. This makes it tougher to route storms with appropriate release rates, not the same as designing a basin. So due to cost, normal basins are frequently used unless you have significant site restraints.