A dry well is an underground structure to drain storm water runoff and release it to the groundwater table. It may be a buried concrete structure, or a simple excavation filled with gravel, that catches storm drainage and then allows the stormwater to seep out and infiltrate down to groundwater.
Stormwater management is used to drain storm water runoff from some location and allow it to dissipate slowly either to groundwater or to surface water. A dry well is an underground structure that takes in storm drainage and releases it to the groundwater below it, so the stormwater doesn't go to a river or stream. The structure might be a concrete or plastic tank or might simply be an excavation filled with gravel, rubble or something similar that has adequate volume to catch storm drainage and allow it to seep out.
Dry Well Types
There are several types of dry well that are in use to drain storm water runoff. For a larger dry well, a buried concrete or plastic tank can be used, as shown in the diagram at the left. The concrete or plastic tank must have holes in the sides and bottom, as shown in the picture, to allow collected storm drainage to seep out. The figure at the right shows another option, simply a hole, lined with filter fabric, and filled with 1.5" to 3" stone fill. The figure shows this type of rock-filled dry well being used to catch downspout stormwater and release it to groundwater. A simple rock-filled dry well can also be installed under a soggy spot in your yard to make it dry out faster.
Will a Dry Well Work in my Location
The proper test to check on whether a dry well will work to drain storm water runoff at your location is a percolation test, also shortened to "perc test." To begin the perc test dig a four foot deep hole in the place you hope to put in a dry well. A posthole digger or soil auger works well for this task. If the hole fills in with groundwater, then the groundwater table is too high for a dry well to be used, because it won't drain. When the hole has been dug, pour at least 5 gallons of water into the hole. Measure the time taken for each one inch drop in water level. If the water level drops at 3 minutes per inch or faster, it is a good candidate for a dry well. If the water drains very quickly, fill the hole again check the rate that it recedes. Very dry soil of any type will absorb water very quickly. For good storm drainage the second 5 gallons should drain at a good rate also. If the water drains very slowly or the hole remains full for a long time with no perceptible drop in level, then a dry well won't work in that location.
Guidelines for Dry Well Installation and Use
Here are some commonly used guidelines for dry well installation:
- The top of the dry well should be far enough below the ground surface to prevent freezing. A depth of at least 36 inches is often used.
- The bottom of the dry well should be at least 2 feet above the seasonal high groundwater level.
- For a plastic drum, 1 inch holes drilled in a random pattern around the side and bottom of the tank are needed for drainage.
- Filter fabric should be wrapped around the top and sides of a dry well tank, so that fine soil won't clog the holes.
- For a rock-filled hole, use filter fabric around the sides of the excavation and over the top of the rocks to prevent soil from clogging up the rock fill.
- At least 6 inches of drain rock should be placed under a dry well tank.
- If rock fill is used, as in a rock-filled hole dry well, 1-1/2" to 3" diameter washed rock is a good size to use.
About the Author
Dr. Harlan Bengtson is a registered professional engineer with 30 years of university teaching experience in engineering science and civil engineering. He holds a PhD in Chemical Engineering.
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