Acceptable Stormwater Treatment Practices
A stormwater manual needs to identify a set of stormwater treatment practices (STPs) that meet water quality goals. The Manual Builder defines a set of five major STP groups, and then identifies specific practices within these groups. The manual should typically be accompanied by a technical support document to justify selection or elimination of a particular practice. The Manual Builder focuses on practices that have demonstrated ability to remove 80% of Total Suspended Solids (TSS).
The following practices are included in the manual builder. The Performance Criteria section describes these practices in greater detail.
Group 1: Ponds
Stormwater ponds are practices that have a combination of a permanent pool, extended detention or shallow marsh equivalent to the entire WQv. Design variants include:
Note that the Dry Extended Detention Pond is not included in this group. Available data suggest poor pollutant removal for this practice.
* Note that very little monitoring data are available to assess the pollutant removal performance of the micropool extended detention pond. This practice is a modification of the Dry Extended Detention pond. The major difference is that the micropool extended detention pond incorporates a small micropool at the outlet, and a wet forebay. The practice is ideal for cold water stream systems, where a practice with a large permanent pool may cause stream warming. It is assumed that this practice performs similarly to wet ponds.
Group 2: Wetlands
Stormwater wetlands are practices that create shallow marsh areas to treat urban stormwater and often incorporate small permanent pools and/or extended detention storage to achieve the full WQv. Design variants include:
The Gravel-Based Wetland, a design variant of the stormwater wetland has not been included in the Manual Builder. While this design has not been tested extensively, available monitoring data suggest good pollutant removal. For more information on this practice, consult Article 96 or Article 97 from the Practice of Watershed Protection.
Group 3: Infiltration Systems
Stormwater infiltration practices capture and temporarily store the WQv before allowing it to infiltrate into the soil. Design variants include:
Porous Pavement systems are not included in the Manual Builder, because field application has shown a very high failure rate. Porous Pavement may have some application in very low traffic areas, however. Article 103 from the Practice of Watershed Protection provides some information about porous pavement.
Group 4: Filtering Systems
Stormwater filtering system capture and temporarily store the WQv and pass it through a filter bed of sand, organic matter, soil or other media. Filtered runoff may be collected and returned to the conveyance system, or allowed to partially exfiltrate into the soil. Design variants include:
* Little data is available to assess the performance of bioretention systems. It is assumed that they perform similarly to other filtering systems, or to the dry swale, which has a similar design.
Group 5: Open Channel Systems
Open channel systems are vegetated open channels that are explicitly designed to capture and treat the full WQv within dry or wet cells formed by checkdams or other means. Design variants include:
Grass channels are included in the list of practices for the Manual Builder, although monitoring data suggest slightly less than 80% sediment removal. They are best applied to residential developments. Alternatively, this practice can be included in a manual as a Stormwater Credit.
STP Groups Not Included in the Manual Builder
Some groups of STPs that are not typically included as stand-alone water quality practices include the following: