10.4 BMP 730 - Drywells
Drywells are subsurface concrete structures that convey stormwater into the underlying soil.
10.4.2 Design Criteria Specific to Drywells
See Figure 4 - 12: Typical Infiltration Drywell - Type 1 and Figure 4 - 13: Typical Infiltration Drywell - Type 2 which show common drywell configurations.
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
The base of the drywell shall be a minimum of 5 feet above the seasonal high groundwater table or other impermeable layer.
Drywells area typically 48" in diameter and 10-15 feet deep.
Depending on local soil conditions, geotextile may need to be placed on top of the drain rock and on sidewalls to prevent migration of fines into the drain rock. The non-woven geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Low Survivability, Class C.
Multiple drywells shall not be placed closer than 30 feet on center or twice the drywell depth, whichever is greater.
Figure 4 - 12: Typical Infiltration Drywell - Type 1
Figure 4 - 13: Typical Infiltration Drywell - Type 2