Systems Planning

Dry wells

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​​Dry wells are stormwater management structures that can play an important role in reducing total runoff volume. Dry wells are underground structures that store water in the void space between crushed stone or gravel and allow the water to slowly percolate downward into the subsoil. Dry wells are installed by digging a deep hole and setting up a perforated tank in it. Gravel is usually placed​ around the outside of the tank and a lid put on top. This structure often 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 on the sides and bottom into the soil that surrounds it. Dry well install This Old House.jpg
Because of their limited size, dry wells structures are best used to infiltrate the first inch/half inch of runoff from frequent small storms; they are not effective for infiltrating the runoff from large storms. Dry wells must be equipped with an overflow or bypass device to divert runoff in excess of their capacity without causing erosion and/or property damage. ​

Applications and design principles 

The excavated hole, 3 - 12 feet deep, must be lined with filter fabric and backfilled with washed, crushed stone 1.5 - 3 inches in diameter. 

The dry well must be lined with a material that provides void space. The simplest method is to use perforated drain tile. Pre-manufactured concrete rings with holes in them are also available. The rings are typically three feet high and six feet in diameter. The dry well may be buried completely, so that it does do not take up any land area 

Where dry wells accept roof runoff through a system of gutters and downspouts, screens must be installed at the top of downspouts. 

For runoff from paved surfaces, runoff must pass through a grass swale or filter strip to pre-treat stormwater before it is discharged to the dry well. 

​Requirements for credit

​​​​Installing a dry well​ can result in a savings of $6.35 per quarter.​​ Only one credit can be taken for utilizing​ a dry well or cistern. 

Roof Drainage

At least 50 percent of your property’s roof area should drain to the dry well or the dry well must capture runoff from impervious area on your property that is equal to 50 percent ​of your roof area. 


66 cubic feet OR 500 gallons of capacity 


A completely full dry well must completely infiltrate into the ground within 24 hours.

To test ground before installing​: 

  1. Dig an 18 inch hole and fill it completely with water. 
  2. Let it drain completely
  3. Fill again. If the second time it takes less than 24 hours to drain, the soils are adequate for the dry well.