5.4.17
5.4.17.1 BMP Description
Drywells (BMP T7.50 in Volume V, Chapter 5 of the Ecology Manual) are subsurface concrete structures, typically precast, that convey stormwater runoff into the soil matrix. They can be used as standalone structures, or part of a larger drainage system (i.e., the overflow for a bioretention swale).
Drywells are UICs and shall follow the applicable requirements in Volume I, Chapter 4 in the Ecology Manual.
5.4.17.2 Performance Mechanism
Flow control occurs through temporary storage of stormwater runoff in the spatial voids of the aggregate material, and subsequent infiltration of stormwater into the underlying soils.
5.4.17.3 Application and Limitations
A drywell can be designed to partially or fully satisfy on-site stormwater management and flow control requirements (see Table II-5.20).
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
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|
Drywells |
X |
|
X | ||||
|
Notes:
|
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5.4.17.4 Site Considerations
Site considerations for the applicability of infiltration BMPs including drywells are provided in Vol I–5.3.2 Determine Infiltration Feasibility. In addition:
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Drywell bottoms shall be a minimum of 5 feet above seasonal high groundwater level or impermeable soil layers (see Volume V, Section 5.6 in the Ecology Manual).
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Drywells are typically a minimum of 48 inches in diameter and approximately 5 to 10 feet deep, or more.
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Filter fabric (geotextile) may need to be placed on top of the drain rock and on trench or drywell sides prior to backfilling to prevent migration of fines into the drain rock, depending on local soil conditions and local jurisdiction requirements.
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Drywells shall be no closer than 30 feet center to center or twice the depth, whichever is greater.
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Drywells shall not be built on slopes greater than 25% (4:1).
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Drywells may not be placed on or above a landslide hazard area or slopes greater than 15% without evaluation by a engineer licensed in Washington State with geotechnical expertise or licensed geologist and jurisdiction approval.
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Drywells shall be sited at least 100 feet up-slope and 20 feet down-slope from building foundations.
5.4.17.5 Design Information
Minimum requirements associated with drywell dimensions and layout include the following:
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The minimum depth of a drywell (aggregate and cover) varies between the two types of Drywells. Type 1 Infiltration Drywells shall be a minimum of 5 feet with a 9‑foot minimum depth as required to penetrate the pervious strata. Type 2 Infiltration Drywells shall be sized based on drainage analysis, but shall be a minimum of 13 inches deep (see BMP T7.50 in Volume V, Chapter 5 of the Ecology Manual).
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Drywells are typically a minimum of 48 inches in diameter and approximately 5 to 10 feet deep, or more.
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Spacing between drywells shall be a minimum of 30 feet or twice the depth, whichever is greater
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The drywell can be placed under a pervious or impervious surface cover to conserve space.
Drywells shall be filled with uniformly graded, washed gravel with a nominal size from 0.75‑ to 1.5‑inch diameter. The minimum void volume shall be 30%.
Non-woven geotextile fabric shall be placed around the walls, bottom and top of the drywell aggregate. A 6‑inch minimum layer of sand may be used as a filter media instead of geotextile at the bottom of the well, but geotextile is still required on the sides and top of the aggregate material.
The minimum measured subgrade infiltration rate for drywells is 5 inches per hour. If runoff from any PGHS is directed to the drywell, underlying soil shall meet the site suitability requirements for treatment outlined in Volume V, Section 5.6 in the Ecology Manual.
During construction the subgrade soil surface can become smeared and sealed by excavation equipment. The design shall require scarification or raking of the side walls and bottom of the BMP excavation to a minimum depth of 4 inches after excavation to restore infiltration rate.
Flows shall be delivered to the drywell aggregate using a pipe with a 4‑inch minimum diameter. Stormwater inflows shall be routed through a catch basin or yard drain with downturned elbow (trap).
Drywells that are designed to meet flow control requirements and receive runoff from contributing areas of 5,000 square feet or more shall be equipped with an observation port to measure the drawdown time following a storm and to monitor sedimentation to determine maintenance needs. Observation wells shall consist of a 4‑inch minimum diameter perforated or slotted pipe that extends to the bottom of the drywell (i.e., to the subgrade) and is equipped with a secure well cap.
Drywells shall have an overflow designed to convey any flow exceeding the capacity of the BMP. If overflow is connected to the public drainage system, a catch basin shall be installed prior to the connection to the public drainage system to prevent root intrusion into public drainage main lines.
To prevent damage to overlying pavement, drywells located beneath pavement shall be constructed with a trench pipe overflow connected to a small yard drain or catch basin with a grate cover. Design shall be such that, if the drywell infiltration capacity is exceeded, the trench pipe overflow would occur out of the catch basin to an approved point of discharge. The vertical elevation difference between the pavement surface and the trench pipe overflow invert shall be 1‑foot minimum.
5.4.17.6 Minimum Construction Requirements
During construction, it is critical to prevent clogging and over-compaction of the subgrade. Minimum requirements associated with drywell construction include the following:
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Aggregate Placement and Compaction – Place the stone aggregate in lifts and compact using plate compactors. A maximum loose lift thickness of 12 inches is allowed. The compaction process aids in adhering the geotextile to the excavation sides, thereby, reducing soil piping, geotextile clogging, and settlement problems.
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Potential Contamination – Prevent natural or fill soils from intermixing with the aggregate. Remove all contaminated aggregate and replace with uncontaminated aggregate.
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Overlap – Following the stone aggregate placement, fold the geotextile over the stone aggregate to form a 12‑inch minimum longitudinal overlap. When geotextile overlaps are required between rolls, overlap the upstream roll a minimum of 2 feet over the downstream roll in order to provide a shingled effect.
5.4.17.7 Operations and Maintenance
Remove debris and sediment from the drywell grate on a semi-annual basis, or as required to prevent the buildup of materials that could inhibit infiltration. General maintenance requirements for infiltration BMPs apply to drywells; see BMP T7.50 in Volume V, Chapter 5 of the Ecology Manual.