A dry well or soak well is an underground structure that disposes of unwanted water, most commonly stormwater runoff, by dissipating it into the ground, where it merges with the local groundwater. A dry well may be either a structural chamber and/or an excavated pit filled with gravel.
A dry well is a passive structure. Water flows through it under the influence of gravity. A dry well receives water from one or more entry pipes or channels at its top and discharges the same water through a number of small exit openings distributed over a larger surface area in the side(s) and bottom of the dry well. When a dry well is above the water table, most of its internal volume will contain air. Such a dry well can accept an initial inrush of water very quickly, until the air is displaced. After that, the dry well can only accept water as fast as it can dissipate water. Some dry wells deliberately incorporate a large storage capacity, so that they can accept a large amount of water very quickly and then dissipate it gradually over time, a method that is compatible with the intermittent nature of rainfall. A dry well maintains the connection between its inflow and outflow openings by resisting collapse and resisting clogging.
Simple dry wells consist of a pit filled with gravel, riprap, rubble, or other debris. Such pits resist collapse, but do not have much storage capacity because their interior volume is mostly filled by stone. A more advanced dry well defines a large interior storage volume by a reinforced concrete cylinder with perforated sides and bottom. These dry wells are usually buried completely, so that they do not take up any land area. The dry wells for a parking lot’s storm drains are usually buried below the same parking lot.