Dry wells are shallow chambers installed in excavations and filled or surrounded with stone to intercept and temporarily store stormwater runoff until it infiltrates into underlying and surrounding soils. Installed underground, dry wells do not take away landscaping space on a property while providing significant reductions in stormwater runoff.Explore how this innovative structure reduces runoff and provides valuable insights into sustainable stormwater management practices. Learn about its environmental impact and contribution to urban ecosystems.
See the in Action
At the demonstration site, the dry well receives passive drawdown and overflow from the cistern (Stop 8). Water enters the dry well through a surface drain connection, fills the stone reservoir and well chamber, and then slowly infiltrates into the surrounding and underlying soils. A pop-up emitter delivers any overflow to the lawn area near the rain garden during exceptionally heavy storm events.
Practice Type: Subsurface infiltration and temporary storage
Configuration: 50-gallon dry well system (such as NDS Flo-Well) in stone-filled excavation
Stone: 1 ft. base and 1 ft. perimeter of crushed, washed 3/4 in. No. 57 stone
Soil Requirement: Infiltration rate of 0.50 inches per hour or greater
Drainage Target: Completely drain within 24 hours of end of rainfall
Water Source: Receives passive drawdown and overflow from the cistern (Stop 8)
Maintenance
Task
Frequency
Details
Sediment pre-trap
At least annually and after large storms
Check sediment pre-trap and clean out accumulated material
Drain entry
At least annually and after large storms
Check for leaf and yard debris accumulating near drain entry; clear as needed
Gutters and downspouts
Regularly
Clean gutters and downspouts to keep debris out of the dry well system
Access lid
At least annually
Verify access lid is not buried or missing and can be easily opened
Interior inspection
At least annually
Check sides of dry well and drain pipes for integrity; check that sediment is not more than 1/3 full; clean out sediment and debris
Drainange test
After significant rain events
Monitor that there is no standing water in dry well after 72 hours. If standing water persists, the system is clogged and needs cleaning or stone/soil replacement at the bottom of the excavation
Erosion check
At least annually
Check for soil erosion around drain inlet, forebay area, and outflow pop-up emitters; repair with new stone or turf vegetation
Sizing
To design a dry well system, first consider the goal of your application. In most residential landscapes, the dry well is used to prevent flooding or serious soil erosion downhill of a rooftop or other impervious surface. A 10-foot by 10-foot rooftop that receives 1 inch of rainfall will deliver just over 60 gallons of water. If you need to collect more stormwater, either from a larger roof or heavier storm, you will need to scale up with a larger well or a series of small wells connected. Consider including some other stormwater control measure for overflow.
Construction Details
• Excavation must be at least 2 feet wider in diameter than the dry well and at least 1 foot deeper
• 1-foot-thick base of crushed and washed 3/4-inch stone (No. 57 stone)
• 1-foot-thick perimeter of the same stone around the outside of the dry well
• Erosion control filter fabric lining the excavation to prevent soil from filling the stone reservoir
• Native soils at the bottom tilled to 3–4 inches depth before placing stone
• Top of the dry well or observation port located as close to the surface as possible for accessibility
• Surrounding the cap: a layer of well-drained soil and turf grass or herbaceous plants
Overflow Management
Dry wells should be designed so that overflow stormwater drains away from buildings to prevent foundation damage or basement flooding. An overflow pop-up drainage emitter should be installed and designed to deliver excess stormwater from heavier rainfall events to another space on the landscape with acceptable infiltration rates (0.5 inches per hour or greater).
Pre-Treatment
If used to receive rooftop runoff, dry wells should be preceded by a leaf screen installed in the gutter or downspout to prevent clogging. If used to receive lawn or driveway runoff, dry wells should be preceded by a pea-gravel-filled catchment forebay and a vegetated filter strip to capture silt and sand before it clogs the interior surfaces. To help prevent soil compaction, heavy vehicular and equipment traffic and foot traffic should be kept out of the dry well area before, during, and immediately after construction.
Dry wells are an effective solution for homeowners dealing with localized flooding, downspout erosion, or saturated lawn areas. The installation process may require heavy digging machinery, such as a small excavator, to investigate soils below the dry well location. Key considerations include:
• Test your soil infiltration rate before committing to a dry well installation
• Size the well based on the impervious surface area contributing runoff
• Consider connecting the dry well to other practices (such as a cistern or rain barrel overflow) for a system approach
• Ensure the overflow pop-up emitter directs excess water to a safe, pervious area away from structures
• Plan for periodic inspection and cleaning to maintain long-term function
The Georgia Stormwater Management Manual, Volume 2, Section 4.7 (page 199) provides detailed engineering specifications for dry well systems in stormwater compliance applications, including sizing calculations for required runoff reduction targets.