Operation and maintenance of dry swale (grass swale)

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Green Infrastructure: Dry swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

The most frequently cited maintenance concern for dry swales is surface soil/media and underdrain clogging caused by organic matter, fine silts, hydrocarbons, and algal matter. Common operational problems include:

• standing water;
• clogged soil/media surface;
• clogged inlet, outlet or underdrains; and
• invasive plants out-compete native vegetation.

Design phase maintenance

Implicit in the design guidance is the fact that many design elements of filtering and infiltration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include:

• limiting drainage area;
• providing easy site access (REQUIRED);
• providing pretreatment (REQUIRED); and
• utilizing native plantings (see Plants for Stormwater Design).

For more information on design information for dry swales, link here.

Construction phase maintenance

Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). In particular, with construction of filter and infiltration practices the most important action for preventing operation and maintenance difficulties is to ensure that the contributing drainage area has been fully stabilized prior to bringing the practice on line (this is a REQUIRED practice).

Warning: It is required that the contributing drainage area has been fully stabilized prior to bringing the practice on line

Inspections during construction are needed to ensure that the filter/infiltration practice is built in accordance with the approved design standards and specifications. Detailed inspection checklists should be used 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. An example construction phase inspection checklist is provided below.

Dry swale construction inspection checklist.
Link to this table
To access an Excel version of form (for field use), click here.

Project:
Location:
Site Status:
Date:
Time:
Inspector:
Construction Sequence	Satisfactory / Unsatisfactory	Comments
1. Pre-Construction
Pre-construction meeting
Runoff diverted (Note type of bypass)
Facility area cleared
Soil tested for permeability
Soil tested for phosphorus content (include test method)
Verify site was not overdug
Project benchmark near site
Facility location staked out
Temporary erosion and sediment protection properly installed
2. Excavation
Lateral slopes completely level
Soils not compacted during excavation
Longitudinal slopes within design range
Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence
Verify stockpile is not causing compaction and that it is not eroding
Was underlying soil ripped or loosened
Size, location, and inverts per plans
Side slopes stable
Groundwater / bedrock verified
3. Structural Components
Stone diaphragm installed per plans
Outlets installed pre plans
Check dams installed per plans
Underdrain installed to grade
Pretreatment devices installed per plans
Soil bed composition and texture conforms to specifications
Inlets installed per plans
Underdrain installed per plans
4. Vegetation
For native dry swales, plants and materials ordered 6 months prior to construction
For native dry swales, construction planned to allow for adequate planting and establishment of plant community
Complies with planting specs
Topsoil complies with specs in composition and placement
Soil properly stabilized for permanent erosion control
5. Final Inspection
Dimensions per plans
Pretreatment operational
Check dams operational
Inlet/outlet/underdrain operational
Soil/media/filter bed permeability verified
Effective stand of vegetation stabilized
Construction generated sediments removed
Contributing watershed stabilized before flow is diverted to the practice
Comments:
Actions to be taken:

Post-construction operation and maintenance

Proper maintenance is critical to the successful operation of a filtration/infiltration practice. Without regular maintenance, filtration/infiltration system soil/media can become clogged, losing its ability to conduct and infiltrate water at the designed rate. This can lead to stagnant water, mosquito breeding habitat, and reduction or elimination of pollutant removal capacity.

Warning: A maintenance plan clarifying maintenance responsibility is REQUIRED. Effective long-term operation of filtration/infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value.

Inspection and maintenance planning

A maintenance plan clarifying maintenance responsibilities is REQUIRED. Effective long-term operation of filtration/infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value. Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards.

• A site-specific O&M plan that includes the following considerations should be prepared by the designer prior to putting the stormwater practice into operation:
  • Inspection and routine maintenance checklist (see below)
  • Operating instructions for any outlet components
  • Vegetation maintenance schedule

Dry swale operation and maintenance checklist.
Link to this table
To access an Excel version of form (for field use), click here.

Project:
Location:
Site Status:
Date:
Time:
Inspector:
Maintenance Item	Satisfactory / Unsatisfactory	Comments
1. Debris Cleanout (Monthly)
Contributing areas clean of litter and vegetative debris
Filtration or infiltration facility clean
Inlets and outlets clear
2. Vegetation (Monthly)
Vegetation maintenance complies with O&M plan
Vegetation meets performance standards (including control of specified invasive species)
Plant composition according to O&M plan
Minimum mowing depth not exceeded
No evidence of erosion
3. Dewatering (monthly)
Dewaters between storms within 48 hours
4. Sediment Deposition (Annual)
Area clean of sediment
Contributing drainage area stabilized and free of erosion
Winter accumulation of sand removed each spring
5. Outlet/Overflow Spillway (Annual, After Major Storms)
Good condition, no need for repair
No evidence of erosion
No evidence of any blockages
No evidence of structural deterioration
6. Other (Monthly)
Encroachment on easement area (if applicable)
Complaints from residents (if applicable)
Any public hazards (specify)
Comments:
Actions to be taken: