VIRGINIA DCR STORMWATER
DESIGN SPECIFICATION No. 11 

WET SWALE

VERSION 1.6
September 30, 2009

 

SECTION 1:  DESCRIPTION

Wet swales can provide runoff filtering and treatment within the conveyance system and are a cross between a wetland and a swale.  These linear wetland cells often intercept shallow groundwater to maintain a wetland plant community.  The saturated soil and wetland vegetation provide an ideal environment for gravitational settling, biological uptake, and microbial activity.  On-line or off-line cells are formed within the channel to create saturated soil or shallow standing water conditions (typically less than 6 inches deep).  While wet swales do not provide runoff reduction, they do provide moderate pollutant removal depending on their design (see Table 1).  Wet swales are particularly well suited for the flat terrain and high water table of the coastal plain.


Table 1:  Summary of Stormwater Functions Provided by Wet Swales

Stormwater Function

Level 1 Design

Level 2 Design

Annual Runoff Reduction

0%

0%

Total Phosphorus Removal 1

20%

40%

Total Nitrogen Removal 1

25%

35%

Channel Protection

Limited. Reduced Time of Concentration; and partial CPv can be provided above the TV within allowable maximum ponding depth

Flood Mitigation

Limited. Reduced Time of Concentration

1 Change in event mean concentration (EMC) through the practice.  Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate.
Sources: CWP and CSN (2008), CWP, 2007

 

SECTION 2:  LEVEL 1 AND 2 DESIGN TABLE

The major design goal for Wet Swales is to maximize nutrient removal.  To this end, designers may choose the baseline design (Level 1) or an enhanced Level 2 design that maximizes nutrient removal.  

Table 2:  Wet Swale Design Criteria

Level 1 Design (RR:0; TP:20; TN:25)

Level 2 Design (RR:0; TP:40; TN:35)

Tv = [(1.0”)(Rv)(A)/12] – volume reduced by upstream RR BMP

Tv = [(1.25”)(Rv)(A)/12] – volume reduced by upstream RR BMP

Swale slopes less than 2%1

Swale slopes less than 1%1

On-line design

Off-line swale cells

No planting

Wetland planting within swale cells

Turf cover in buffer

Trees within swale cells

1 Wet Swales are generally recommended only for flat coastal plain conditions with a high water table. A linear wetland is always preferred to a wet swale. However, check dams or other design features that lower the effective longitudinal grade of the swale can by applied on steeper sites to comply with these criterion.


SECTION 3:  TYPICAL DETAILS

Figure 1 provides a standard plan and profile detail for an on-line wet swale with an off-line wetland cell.  Figure 2 shows a typical plan, profile, and section for a wet swale.

Figure 1. Wet Swale Details

Figure 2.  Typical Wet Swale Schematics

SECTION 4: PHYSICAL FEASIBILITY AND DESIGN APPLICATIONS

Wet swales can be implemented on development sites where development density, topography, and soils are consistent with the following criteria:

  • Contributing Drainage Area:  The maximum contributing drainage area to a wet swale should be 5 acres, but preferably less.
  • Space Required:  Wet swales usually consume about 5-15% of their contributing drainage area.
  • Site Topography:  Site topography constrains wet swales; some gradient is needed to provide water quality treatment, but not so much that treatment is impeded.  Wet swales generally work best on sites with relatively flat slopes (e.g., less than 2% gradient).
  • Depth to Water Table:  It is permissible for wet swales to intersect the water table.
  • Soils:  Wet swales work best on more impermeable Hydrologic Soil Group (HSG) C or D soils.
  • Hydraulic Capacity:  When Wet Swales are used as an on-line practice (Level 1) they must be designedwith enough capacity to convey runoff from the 10-year design storm event, and be non-erosive during both the two-year and ten-year design storm events. This means that the surface dimensions are largely determined by the need to pass these larger storm events.

When wet swales are used as an off-line practice (Level 2), a bypass or diversion structure must be designed to divert the large storm: flow rate and/or volume in excess of the water quality Treatment Volume (Tv) to an adequate channel or conveyance system. The wet swale is then designed to provide the required volume and meet the velocity and residence time criteria for the Tv.

  • Hotspot Land Uses:  Wet Swales are not recommended to treat stormwater hotspots, due to the potential interaction with the water table and the risk that hydrocarbons, trace metals, and other pollutants could migrate into groundwater.  For a list of designated stormwater hotspots, consult the Design Specification No. 8 (Infiltration).
  • Highway Runoff: The linear nature of wet swales makes them well suited to treat highway or low- and medium-density residential road runoff, if there is adequate right-of-way width and distance between driveways.

SECTION 5:  DESIGN CRITERIA

5.1. Sizing of Wet Swales

Wet swales should be designed to capture and treat the treatment volume (Tv) remaining from the upstream runoff reduction practices Runoff treatment credit can be taken for any temporary or permanent storage created within each wet swale cell. This includes the permanent wet storage below the normal pool level and up to 12 inches of temporary storage created by check dams or other design features.  Designers must also demonstrate that the on-line wet swales have sufficient capacity to safely convey the 10-year design storm event.  The hydraulic design methods outlined in Design Specification No. 3 (Grass Channels) should be consulted.

(NOTE:  As DCR moves forward, the above requirement will be driven by the Va. SWM Regulations (4 VAC 50-60-66 A 1 and B 1).  MS-19 will be superseded.)

5.2. Swale Pretreatment and Geometry

The wet swale should follow the general design guidance contained in Design Specification No. 3 (Grass Channels).

5.3. Other Design Issues for Wet Swales

  1. The average normal pool depth (dry weather) throughout the swale should be 6 inches or less.
  2. The maximum temporary ponding depth in any wet swale single cell should not exceed 18 inches at the most downstream point (at the check dam or driveway culvert). 
  3. Check dams should be spaced as needed to maintain the effective longitudinal slope identified for the Level 1 or Level 2 as appropriate. A typical plan and profile for the check dams is provided in Figure 3 below.
  4. Individual wet swale segments formed by check dams or driveways should generally be at least 25-40 feet in length.
  5. Wet swale side slopes should be no steeper than 4:1 (H:V) to enable wetland plant growth.  Flatter slopes are encouraged where adequate space is available to aid in pretreatment of sheet flows entering the channel.  Under no circumstances are side slopes to exceed a gradient of 3:1 (H:V).

 

5.4. Planting Wet Swales

Designers should choose grass and wetland plant species that can withstand both wet and dry periods as well as relatively high velocity flows within the channel.  For a list of wetland plant species suitable for use in wet swales, refer to the wetland panting guidance and plant lists provided in the main part of this Design Specification No. 13 (Constructed Wetlands).  If roadway salt will be applied to their contributing drainage area, swales should be planted with salt-tolerant and non-woody plant species.

5.5. Material Specifications

Please consult Design Specification No. 3 (Grass Channels) for suitable materials for check dams and other swale features.

SECTION 6: REGIONAL AND SPECIAL CASE DESIGN ADAPTATIONS

6.1. Karst Terrain

Wet swales are not generally feasible in karst terrain, since the water table rarely reaches the land surface.

6.2. Coastal Plain

Wet swales work well in areas of high water table, and consist of a series of on-line or off-line storage cells.  Designers should design cells such that underlying soils are typically saturated, but do not cause standing water in between storm events.  It may also be advisable to incorporate sand or compost into surface soils to promote a better growing environment.  Wet swales should be planted with wet-footed species, such as sedges or wet meadows.  Wet swales are not recommended in residential areas due to concerns about mosquito breeding.

SECTION 7:  WET SWALE CONSTRUCTION SEQUENCE AND INSPECTION

Please consult the construction sequence and inspection checklists as outlined in Design Specification No. 3 (Grass Channels) and Design Specification No. 13 (Constructed Wetlands).

SECTION 8: WET SWALE MAINTENANCE

Wet swales have maintenance needs similar to dry swales, although woody wetland vegetation may need to be removed periodically.  Please consult the maintenance inspection checklists and ongoing maintenance tasks as outlined in Design Specification No. 10 (Dry Swales), Design Specification No. 3 (Grass Channels) and Design Specification No. 13 (Constructed Wetlands).

SECTION 9:  COMMUNITY AND ENVIRONMENTAL CONCERNS

The main concerns of adjacent residents are perceptions that wet swales will create nuisance conditions or will be hard to maintain.  Common concerns include the continued ability to mow grass, landscaping preferences, weeds, standing water, and the risk of mosquitoes breeding.  For these reasons, wet swales are not recommended in residential settings, because the shallow, standing water in the swale is often viewed as a potential nuisance by homeowners.


SECTION 10:  DESIGN REFERENCES

Claytor, R. and T. Schueler.  1996.  Design of Stormwater Filtering Systems.  Center for Watershed Protection. Ellicott City, MD.

CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD.

Maryland Department of Environment (MDE). 2000. Maryland Stormwater Design Manual. Baltimore, MD  http://www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.asp

Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD

Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD www.chesapeakestormwater.net

Virginia Department of Conservation and Recreation (VA DCR). 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA