Operation and maintenance (O&M) of swales

Warning: This page is in edit and testing by the wiki authors. It is not a content page for the Manual. Information on this page may not be accurate and should not be used as guidance in managing stormwater.

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).

Construction Phase O&M Considerations

Proper construction methods and sequencing play a significant role in reducing O&M problems. Some key items during the construction phase include:

• Before construction begins:
  • Install erosion control prior to excavation of swale.
  • Hold a pre-construction meeting to review the construction plans and the sequencing of construction
  • Designate a stormwater supervisor to make sure someone is responsible for erosion and sediment control.
• During construction:
  • Construct any pre-treatment devices first to trap sediments.
  • Ensure heavy equipment does not enter the footprint of the practice to avoid compaction of the growth medium
  • Ensure the correct plant species are being installed by having a detailed list of species and quantities. These should be spelled out in the plan set both in the Details and Specifications and on actual drawings. A copy should be kept on one’s person at all times during construction.
  • Inspect the practice during construction to ensure that the swale is built in accordance with the approved design and standards and specifications. Use a detailed inspection checklists that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer.
  • Conduct soil infiltration tests to ensure proper soil compaction.
• After construction:
  • Verify that the swale was built in accordance with the approved design and standards and specifications.
  • Verify that the practice properly conveys, filters, and/or infiltrates runoff. Ensure that no standing water remains within the swale 48 hours after precipitation.
  • Verify that pretreatment functions properly.
  • Use a detailed inspection checklists that include sign-offs by qualified individuals at the completion of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided here.
  • Review and discuss the plant warranty/establishment period with the plant provider to understand the conditions under which failing plants will be replaced. Divert stormwater away from BMP during establishment phase as necessary for the selected vegetation.
  • Determine if stormwater should be kept offline from the practice until the seedlings are established.
  • The design/construction team should provide the O&M team with the following information to be included in the O&M plan:
    • The plant warranty.
    • The “as-built” plans of the practice.
    • A list of conditions that might cause failure of the practice if not properly maintained.

Post-Construction Phase O&M

Effective short and long-term performance of grass swales requires a dedicated and routine maintenance plan with clear guidelines, expectations, and schedules. Proper maintenance will not only increase the effectiveness of the swale at treating stormwater runoff, but will create a viable green space supporting ecological function. A maintenance plan clarifying maintenance responsibilities should be required for all practices.

Some important post-construction considerations are provided below along with recommended maintenance standards.

• A site-specific O&M plan should be prepared by the designer prior to completing construction of the grass swale. This plan should provide any operating procedures related to the practices. The plan should also provide clear maintenance expectations, activities, and schedules, including minimum maintenance frequencies expected. Include photos if possible. Be clear about who is responsible for the maintenance and the type of expertise that will be needed for distinct O&M activities. The O&M plan should include an anticipated budget for O&M activities. The O&M plan should also include an example O&M inspection checklist and an example maintenance report. Example O&M plans and inspection checklists are provided further down below.
• A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority.
• Inspection and maintenance activities are distinct and can be done as separate activities or together. Inspection will typically assess the practice for any O&M issues, whereas maintenance will address the O&M issues identified by the inspection. A dedicated inspection effort on a large number of BMPs can help prioritize maintenance activities.
• Maintenance activities should be careful not to cause compaction.
• Ponding water that does not infiltrate within 48 hours of a storm is an indicator the swale is not functioning properly. If ponded water is observed outside of a routine O&M schedule it should be addressed immediately.
• BMP areas generally should not be used as dedicated snow storage areas. Snow storage within swales should be avoided when possible, such as in parking lots. Snow storage can cause compaction and increase sedimentation within the swale over time, and so snow storage on swales designed for infiltration should be specifically avoided. Click here for additional snow and salt considerations.

The table below provides an overview and schedule of general maintenance activities.