What Is a Drywell for Stormwater Drainage?
A drywell is an underground chamber — typically a perforated concrete or plastic structure surrounded by gravel — that receives stormwater and lets it infiltrate into the surrounding soil. It is the standard solution where surface discharge is not possible and the soils percolate well. Civil engineers size drywells from infiltration testing; in clay soils or high-groundwater areas, they do not work and the design must go elsewhere.
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How drywells work
Roof leaders or area drains pipe into the drywell chamber; water fills the gravel void space and percolates outward into the native soil. Pretreatment (a catch basin or filter) keeps sediment and debris from clogging the gravel — a clogged drywell is just an expensive buried box. Overflow provisions route extreme storms somewhere safe when the soil cannot keep up.
Sizing and testing
Sizing starts with a percolation or infiltration test at the proposed depth — the measured rate (inches per hour) determines the chamber volume needed for the design storm. Jurisdictions specify the design storm (often the 10-year or 100-year event) and require setbacks from foundations, property lines, and septic systems. Infiltration rates below ~0.5 in/hr generally rule drywells out.
Maintenance and failure modes
Drywells fail from sediment clogging (no pretreatment), roots invading the chamber, and groundwater rising into the system. Maintenance is simple but non-optional: clean pretreatment devices annually, inspect after major storms, and watch for surface ponding above the drywell — the telltale sign it has stopped infiltrating. Replacement means re-excavation, so build the pretreatment right the first time.
The bottom line
Drywells infiltrate stormwater where soils percolate — sized from infiltration tests, protected by pretreatment, set back from foundations. They fail in clay; maintain the pretreatment annually.
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