Operation and maintenance (O&M) of swales
Overview of Typical O&M Issues
A grass or dry swale is a general term for filtration or infiltration conveyance practice, but there are a wide variety of designs depending on the site requirements. The key functions of swales are to convey water with reduced peak velocities and to perform water quality treatment with filtration and infiltration. Swales have the benefit of providing ecological function in areas dominated by development and impervious surface. These practices are ideal for urban settings, which creates an opportunity for humans to impact their function through vandalism, littering, and vehicular traffic.
These practices, like other green stormwater infrastructure, require regular maintenance to ensure proper and long-lasting ecosystem and stormwater function. If properly maintained, swales can operate effectively for 20-50 years, depending on the design type. Vegetated dry swales may perform properly for up to 50 years (Malvern Borough, 2013). The most frequently cited O&M concerns for swales include: *
- Standing water in the swale after a 48 hour timeline to infiltrate runoff.
- Loss of volume storage due to accumulated sediment.
- Clogged soil media.
- Clogged inlet and outlet structures, or underdrains.
- Growth and proliferation of weeds or other non-native species that threaten the long-term effectiveness of the swale.
The sections below describe best practices to prevent or minimize these common problems.
Design Phase O&M Considerations
Swales, like other infiltration practices in urban areas, may be subject to higher public visibility, greater trash loads, pedestrian traffic, vandalism, and vehicular loads. Designers should design these practices in ways that prevent or minimize O&M issues. Examples include:
- Designing easy-access points for maintenance personnel and equipment.
- Limiting the contributing drainage area and sizing the practice in accordance to its contributing drainage area.
- Adding pre-treatment and trash racks to reduce sediment and debris loads.
- Providing a vegetation design plan, emphasizing the following considerations:
- Utilizing native plantings that enhance pollinator and wildlife habitat, improve infiltration and evapotranspiration, reduce urban heat island effect, provide optimized carbon sequestration, and provide climate adaptation. Native plantings typically require less maintenance and replacement than non-native plantings because they are adapted to the local climate (see Plants for Stormwater Design).
- Site-specific plantings that take into account sun exposure, shade, proximity to traffic corners (visibility issues), interior vs exterior plantings, salt-tolerant plants, etc. The selection of plantings suitable to their immediate surroundings will minimize long-term care and replacement frequency.
- Providing educational signage for swales in residential areas to increase public awareness.
Designers should consult and include any local requirements regarding green infrastructure. O&M considerations often depend on whether the practice is located on public land, private land, or in the public right of way. For example, plantings in the public right of way that conflict with any traffic safety considerations could require increased O&M, such as pruning or complete removal. Swales are particularly subject to traffic considerations and regulations, given their frequent placement in public right-of-way.
The designer should also provide a site-specific O&M plan that includes the following:
- Construction inspection schedule and checklists
- Post-construction routine maintenance schedule and checklists
- Operating instructions for the practice (if applicable)
For more information on design criteria for dry swale (grass swales).