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 an enhanced swale located on highly permeable native soil (Full Infiltration design), based on a 2,000 m² road drainage area, runoff control target of 25 mm depth and 72 hour drainage period, 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 rate for Full Infiltration Design scenario was assumed to be 20 mm/h, and a safety factor of 2.5 was applied to calculate the design infiltration rate.
• Ditch inlet catch basin structure located at furthest downgradient end, with connection to storm sewer pipe, included as the outlet.
• Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, removal of sediment from curb inlets and outlet catch basin annually, and removal of weeds twice a year. 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.
• Impervious drainage area to pervious swale footprint area of 10:1 (tool default value).
• Designed with a trapezoidal cross section with a top width of 4.75 m, bottom width of 0.75 m, and 2.5:1 side slopes (tool default values).
• Swale depth of 0.5 m, bottom length of 42.11 m and surface area at top of 200 m2.
• Longitudinal slope of 3%.
• Includes concrete curb check dams (0.3 m in height each) for every 10 m of swale length (4 in total).
• Includes a 8.4 m wide driveway and 300 mm diameter culvert crossing.
• Designed for a maximum flow velocity of 0.3 m/s.

Notes

• Design includes pretreatment through stone diaphragms at curb inlets.
• Operation and maintenance cost estimates assume rehabilitation of the swale surface is required after 25 years of operation.
• 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

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

Above you can find a breakdown of construction costs by expense type for a 200 m² surface area facility (I:P area ratio of 10:1):

1. Enhanced Swales: Full Infiltration

Material & Installation expenses represent the largest portion of total construction costs (55%).

Life Cycle Costs

Below are capital and life cycle cost estimates for the Enhanced swales Full Infiltration configuration 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 39% of total life cycle costs over the 25-year evaluation period, and increase to 54% of total life cycle costs over the 50-year period, due to swale surface rehabilitation costs assumed to be required after 25 years of operation.

25-Year life cycle cost break down

Enhanced Swales: Full Infiltration

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

50-Year life cycle cost break down

Enhanced Swales: Full Infiltration

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

Cost Summary Tables

Total life cycle cost estimate over the 50 year evaluation period for the Enhanced swale Full Infiltration design configuration is ($65,021.40), making it one of the cheapest LID BMP options.

Full Infiltration

Enhanced Grass swale in CVC headquarters parking lot, Mississauga ON.

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

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

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

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.

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

Ground depression acting as a flow control and water treatment structure, that is normally dry.

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.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

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 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.

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.

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

Penetration of water through the ground surface.

Low Impact Development. 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.

Credit Valley Conservation