Combined Detention and Wetpool Facilities

5.4.29 Combined Detention and Wetpool Facilities

5.4.29.1 BMP Description

Combined detention and runoff treatment wetpool facilities (BMP T10.40 in the Ecology Manual) have the appearance of a detention BMP but contain a permanent pool of water as well. Site considerations, setbacks, and other typical siting and design considerations for combined BMPs are the same as specified for each individual BMP, unless noted below. The following combined BMPs are addressed in this section:

There are two sizes of the combined wetpond, a basic and a large, but only a basic size for the combined wetvault and combined stormwater wetland. The BMP sizes (basic and large) are related to the treatment performance goals (see Vol I–4.2.6 Minimum Requirement #6: Runoff Treatment).

5.4.29.2 Performance Mechanism

The intent of a combined detention and wetpool BMP is to provide runoff treatment in addition to flow control. The three types of combined facilities provide runoff treatment as follows:

A combined detention pond/wetpond provides pollutant removal via settling and biological uptake.
A combined detention/wetvault provides pollutant removal via settling.
A combined detention/stormwater wetland provides pollutant removal via settling, biological uptake, filtration, and soil adsorption.

5.4.29.3 Application and Limitations

Combined detention and runoff treatment BMPs can be efficient for sites that also have detention requirements, but for which infiltration is infeasible (Vol II–5.3.2 Determine Infiltration Feasibility). The runoff treatment BMP may often be placed beneath the detention BMP without increasing the BMP surface area. However, the fluctuating water surface of the live storage will create unique challenges for plant growth and for aesthetics alike.

The basis for pollutant removal in combined facilities is the same as in the standalone runoff treatment BMPs. However, in the combined BMP, the detention function creates fluctuating water levels and added turbulence. For simplicity, the positive effect of the extra live storage volume and the negative effect of increased turbulence are assumed to balance and are thus ignored when sizing the wetpool volume. For the combined detention/stormwater wetland, criteria that limit the extent of water level fluctuation are specified to better ensure survival of the wetland plants.

Unlike the wetpool volume, the live storage component of the combined detention and wetpool BMP shall be provided above the seasonal high water table.

Combined detention and wetpool BMPs can be applied to partially or fully satisfy runoff treatment and flow control requirements as shown in Table II-5.32.

Table II-5.32. Combined Detention and Wetpool BMP Applicability.
BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
BMP	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus	MR #7: Flow Control
Combined Detention and Wetpond			X	X^a		X^b	X
Combined Detention and Wetvault			X	X^a		X^b	X
Combined Detention and Stormwater Wetland			X	X^a		X^b	X
Notes: Can be used to meet enhanced runoff treatment requirement as part of a two-BMP treatment train; see Volume III, Chapter 1 in the Ecology Manual. Can be used to meet phosphorus runoff treatment requirement as part of a two-BMP treatment train; see Volume III, Chapter 1 in the Ecology Manual.

5.4.29.4 Site Considerations

See Volume V, Chapter 8, BMP T10.40 in the Ecology Manual for site considerations. Also see the setback requirements for detention ponds (Vol II–5.4.26 Detention Ponds), wetponds (Vol II–5.4.22 Wetponds), wetvaults (Vol II–5.4.23 Wetvaults), and/or stormwater treatment wetlands (Vol II–5.4.24 Stormwater Treatment Wetlands), depending on the combined detention and wetpool BMP type.

5.4.29.5 Design Information

See Volume V, Chapter 8, BMP T10.40 in the Ecology Manual for detailed design guidance and criteria on combined detention and wetpool BMPs.

5.4.29.6 Minimum Construction Requirements

Construction requirements are the same as for wetponds (Vol II–5.4.22.6 Minimum Construction Requirements), wetvaults (Vol II–5.4.23.6 Minimum Construction Requirements), or stormwater treatment wetlands (Vol II–5.4.24.6 Minimum Construction Requirements), depending on the combined detention and wetpool BMP type.

5.4.29.7 Operations and Maintenance

Refer to operations and maintenance for detention and wetpool BMPs (see Vol II–5.4.22 Wetponds and Vol II–5.4.26 Detention Ponds) for required maintenance practices for combined detention and wetpool BMPs.