13.2 BMP T1020 - Wet Biofiltration Swale
A wet biofiltration swale is a variation of a basic biofiltration swale for use where the longitudinal slope is slight, water tables are high, or continuous low base flow is likely to result in saturated soil conditions. Where saturation exceeds about 2 weeks, typical grasses will die. Thus, vegetation specifically adapted to saturated soil conditions is needed. Different vegetation in turn requires modification of several of the design parameters for the basic biofiltration swale.
Wet biofiltration swales can be used to help ensure compliance with Minimum Requirement 6 - Stormwater Treatment.
The performance of wet biofiltration swales is variable between storm events and is therefore not a consistent treatment BMP.
Wet biofiltration swales are applied where a basic biofiltration swale is desired but not allowed or advisable because one or more of the following conditions exist:
The swale is located on glacial till soils and is downstream of a detention pond providing flow control.
Saturated soil conditions are likely because of seeps or base flows on the site.
Longitudinal slopes shall be less than 2 percent.
Wet biofiltration swales may be located upstream or downstream of detention facilities.
Use the same design and sizing criteria as for basic biofiltration swales except as noted below and shown in .
Wet biofiltration swales shall only be designed as offline facilities.
Adjust for extended wet season flow.
If the swale will be downstream of a detention pond or vault providing flow control, multiply the treatment area (bottom width times length) of the swale by 2, and readjust the swale length, as necessary. Maintain a 5:1 length to width ratio.
The increase in treatment area is needed to ensure pollutant removal can be achieved during extended flow conditions.
The bottom width may be increased to 25 feet maximum, but a length-to-width ratio of 5:1 must be provided. No longitudinal dividing berm is needed.
The minimum swale length is 100 feet.
If longitudinal slopes are greater than 2 percent, the wet swale must be stepped so that the slope within the stepped sections averages 2 percent. Steps may be made of retaining walls, log check dams, or short riprap sections.
A high-flow bypass (i.e., an offline design) is required for flows greater than the Qbiofil. The bypass may be an open channel parallel to the wet biofiltration swale.
Design water depth shall be 4 inches for all wetland vegetation selections.
Flow Velocity, Energy Dissipation, and Flow Spreading
Access is only required to the inflow and outflow of the swale. Access along the swale is not required.
Wheel strips may not be used for access.
A list of acceptable plants is shown in A1000 - Vegetation.
A wetland seed mix may be applied by hydroseeding, but if coverage is poor, planting of rootstock or nursery stock is required. Poor coverage is considered to be more than 30 percent bare area through the upper ⅔ of the swale after four weeks.