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A dry well, also called a soakaway, is a passive water runoff management system. The dry well is a covered, underground reservoir that usually is filled with gravel. It receives and directs excess runoff water away from a building and allows the water to percolate underground into the soil. Dry wells help control stormwater runoff from roofs, basements and other structures. The dry well basin is connected to a pipe or a combination of pipes that collect and disperse excess water away from the foundation of the structure.
A moderate rainstorm pummeling an average-size house roof can produce more than 1,000 gallons (more than 3,785 liters) of water. This runoff water pours into the soil that surrounds the foundation. If the soil has poor drainage or the foundation has cracks or openings, runoff water could permeate the basement, causing it to flood and eroding the strength of the foundation walls. The dry well passively captures the roof runoff water before the water reaches the foundation subsoil, and it directs the water away from the structure through a system of pipes and basins.
Dry wells are situated in a lower geographic area than the structure it serves, such as at the bottom of a slope, to allow gravity to direct the water without the need for mechanical equipment and maintenance. For maximum results, a dry well is generally 15 feet (4.6 meters) or more from the structure. The dry well basin is placed underground but above the water table. A dry well is rarely effective in compacted, clay soil or impermeable soils that are filled with bedrock or granite.
There are other drainage systems besides the dry well for managing excess stormwater runoff. The French drain is an uncovered dry well system, usually installed along the interior perimeter of basement walls to direct and expel moisture from a basement. A cesspool is a covered dry well system that disperses sewage underground. A recharge basin, also called an infiltration basin, is an uncovered pit or open pond that receives stormwater runoff through a network of pipes. The recharge basin typically is used in urban areas as part of a larger, stormwater runoff and flood management system.