Drywell — Stormwater Infiltration and Permit Requirements

A drywell is an underground structure filled with gravel or fitted with a perforated barrel that receives and slowly disperses collected stormwater into the surrounding soil.

What It Is

A drywell is a subsurface infiltration structure designed to receive and temporarily store stormwater or clean roof runoff so it can percolate gradually into the ground rather than running off the surface. It typically consists of a large perforated barrel, chamber, or gravel-filled pit located at a low point in a drainage system, receiving inflow from gutters, downspouts, yard drains, or French drains.

The drywell works by accepting water into the pit and allowing it to seep outward and downward through the perforated wall or gravel fill into the surrounding native soil. For this to function properly, the soil must have sufficient permeability — sandy or loamy soils work well, while clay soils drain too slowly for a drywell to be effective. A percolation test is used to determine whether a site is suitable before installation.

Types

Precast concrete or polyethylene barrel drywells are the most common residential type. They are typically 3 to 4 feet in diameter and 3 to 6 feet deep, with perforated sides and a solid lid at the top. Modular drywell chambers made of lightweight plastic can be assembled in various configurations to increase capacity. Gravel pit drywells are simply large excavations filled with washed stone and lined with filter fabric — lower-cost but less durable and harder to service. Some jurisdictions restrict or prohibit drywells and require connection to storm sewer systems instead.

Where It Is Used

Drywells are used to manage roof runoff from residential homes when no practical surface discharge path exists, to control ponding in yards where the soil can accept infiltration, and as the termination point for French drain systems, curtain drains, and area drain pipe runs. They are particularly common in rural and suburban settings without municipal storm sewer access. They must be located away from the foundation, wells, septic systems, and property lines per local regulations.

How to Identify One

A drywell is usually not visible at the surface except for its access lid or inspection port set flush with the grade. In some installations the lid is hidden under mulch or lawn. The presence of a downspout that disappears underground or a pipe outlet that ends in a buried structure with no visible daylight outlet often indicates a drywell at the terminus. Excavating or camera inspection of the pipe can confirm its presence.

Replacement

Drywells become ineffective when the surrounding soil becomes saturated with fine particles that clog the infiltration zone — a process called clogging failure. Early signs include slow drainage, surface ponding near the drywell location, and water backup in the connected drain system. Rehabilitation may involve excavating and repacking the gravel with fresh filter fabric, replacing a failed precast barrel, or relocating the drywell to an area with fresher soil. Adding a larger drywell or converting to a surface discharge outlet may be necessary if the soil condition cannot support infiltration.