Enhanced Swales: Life Cycle Costs

Enhanced swale with rocky check dams and a metal overflow grate in Northgate Mall parking lot, Seattle. Photo credit: MLSmith.

Overview

Enhanced swales (also referred to as Enhanced Grass Swales - EGS) are vegetated open channels designed to convey, treat and attenuate stormwater runoff. Simple grass channels or ditches have long been used for stormwater conveyance, particularly for road drainage. Enhanced swales incorporate design features such as modified geometry and check dams that improve the contaminant removal and runoff reduction functions of simple grass channel and roadside ditch designs. Where development density, topography and depth to water table permit, swales are a preferable alternative to curb and gutter and storm drains as a stormwater conveyance system. STEP has prepared life cycle costs estimates for each design configuration, based on a 2,000 m² road drainage area, runoff control target of 25 mm depth and 72 hour drainage period, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated LID Life Cycle Costing Tool (LCCT) here.

Design Assumptions

Enhanced grass swales (EGS) are an ideal technology for sloped sites and cheaply retrofitting and improving the performance of existing grass swales. Components include: graded channel, resilient turf grass or other planting, check dams to facilitate short term ponding. Additional components include, amended soil or filter media to increase infiltration to soils below, and turf reinforcement to prevent scour. Swales can provide pretreatment for other BMPs (bioretention, soakaways, exfiltration trenches) or be designed in series with other practices as part of a treatment train.

Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:

• Native soil infiltration rates for Full, Partial and No Infiltration Design scenarios were assumed to be 20 mm/h, 10 mm/h and 2 mm/h, respectively, and a safety factor of 2.5 was applied to calculate the design infiltration rate.
• Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, removal of sediment from pretreatment structures annually, and removal of weeds twice a year (where applicable). Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ surface infiltration rate testing (where applicable)
• Designed with a trapezoidal or triangular cross section.
• Length between culverts should be 5 meters or greater.
• Designed with a bottom width between 0.6-2.4 m (allows shallow flows and adequate water treatment).
• Longitudinal slope should be between 0.5 and 4%. Check dams should be incorporated on slopes greater than 3%.
• Side slopes should be flat to aid in providing pre-treatment for lateral incoming flows and maximize swale filtering surface.
• Steeper side slopes may be susceptible to erosion from incoming lateral flows. A maximum slope of 2.5:1 is recommended (Tool default).
• Maximum flow depth should be 50% of grass height for regularly mown swales, to maximum of 75 mm.
• For infrequently mown swales, STEP recommends maximum flows of 33% of vegetation height.
• Designed for a maximum velocity of 0.3 m/s.
• Convey locally required design storm (~10 mm year storm) at non-erosive velocities.

Notes

• The Tool assumes an in-line system.
• The Tool assumed curb-and-gutter around the circumference of the facility with curb-cut inlets every 6 to 0.5 meter squared and stone splash pads.
• The greater the slope, the more check dams placed by the Tool. The unit cost for check dams is for the amount of material required, not number of dams.
• If configuration include driveway (2 laned), a culvert pipe connects the enhanced grass swale (EGS) and an extra 0.1m of gravel is added beneath culvert.
• Swales which cross multiple driveways or smaller driveways can be accounted for by multiplying or dividing these costs in the "Additional Costs" cells.
• The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.
  • Land value and equipment mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction.
  • Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
  • The tool adds 10% contingency and additional overhead as default.
• All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
• Unit costs are based on 2018 RSMeans standard union pricing.
• Additional costs associated with retrofit or redevelopment contexts is assumed to be 16% of the cost estimate for new (greenfield) construction contexts.
  • Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.
    

Construction Costs

Construction Costs Per Unit Drainage Area (CAD$/m²) - Full Infiltration Design, 25 mm Treatment

Construction Costs Per Unit Drainage Area (CAD$/m²) - Partial Infiltration Design, 25 mm Retention

Note: Please click on each image to enlarge to view associated construction cost estimates.

Above you can find a cost breakdown of a 1000m² in two different configurations:

1. Infiltration Trench: Full Infiltration
2. Infiltration Trench: Partial Infiltration

As can be seen, regardless of design configuration, Material & Installation expenses represent the largest portion of total construction costs (75 to 76%).

Life Cycle Costs

Below are capital and life cycle cost estimates for the two infiltration trench configurations over 25- and 50-year time periods. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values. Operation and maintenance costs are predicted to represent 38% of total life cycle costs over the 25-year evaluation period, and increase to 51% of total life cycle costs over the 50-year period, due to increased levels of litter removal, replacement of filter cloth and disposal of collected sediment (every 4 - 8 years), cleaning out the catchbasin and the Oil and Grit Separator/Hydrodynamic Separator annually, as well as flushing the internal pipes every 10 years.

25-Year life cycle cost break down

Infiltration Trench: Full Infiltration

Infiltration Trench: Partial Infiltration

Note: Click on each image to enlarge to view associated life cycle cost estimate.

50-Year life cycle cost break down

Infiltration Trench: Full Infiltration

Infiltration Trench: Partial Infiltration

Note: Click on each image to enlarge to view associated life cycle cost estimate.

Cost Summary Tables

Total life cycle cost estimates for the two infiltration trench configurations vary substantially with the Infiltration Trench: Partial Infiltration design being highest ($85,288.88), followed closely by the Infiltration Trench: Full Infiltration design being ($69,805.90).

It is notable that a sensitivity analysis was conducted in 2019 to compare construction cost estimates generated by the tool to actual costs of implemented projects. The analysis found that tool estimates were typically within ±14% of actual construction costs

Full Infiltration

Good use of geotextile, being limited to preventing fines from entering the trench from above. Read more about the performance of this feature in comparison to bioretention] BMPs in Vaughan, ON. (Source: STEP, 2016)

Partial Infiltration

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Movement of water from one location to another.

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

Sustainable Technologies Evaluation Program

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

The natural ground material characteristic of or existing by virtue of geographic origin.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Soil particles with a diameter less than 0.050 mm.

Credit Valley Conservation

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.