Vegetated Filter Strip: Life Cycle Costs

Filter strips between the bioretention facility and the adjacent parking lot, used to reduce sediment loading that would otherwise end up on the surface of the bioretention cell. Dennis Ave Health Center, Wheaton, MD. Photo credit: Moreau1.

Overview

Vegetated filter strips (i.e., buffer strips, grassed filter strips) are gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They slow runoff velocity and filter out suspended sediment and associated pollutants, and provide some infiltration into underlying soils. Vegetated filter strips are generally used as pretreatment to other LID practices. Filter strips also provide a convenient location for snow storage and treatment during the winter months. STEP has prepared a life cycle cost estimate for a vegetated filter strip 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

Filter strips function by slowing runoff velocities and filtering out sediment and other pollutants. Filter strips may be comprised of a variety of trees, shrubs, and vegetation to add aesthetic value as well as water quality benefits.

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

• Filter strip length of 5 metres.
• Maximum acceptable impervious drainage area to pervious footprint area ratio of 4:1.
• Swale (0.3 m depth) and ditch inlet catch basin connected to storm sewer as 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, mowing five times a year, removal of weeds twice a year, and core aeration of filter strip surface every 5 years. Verification inspections are included every 5 years to confirm adequate maintenance.

Notes

• Design includes pretreatment through stone diaphragms at curb inlets.
• Operation and maintenance cost estimates assume rehabilitation of the filter strip 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 the 500 m² surface area vegetated filter strip (I:P area ratio of 4:1)

The 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 Vegetated filter strip design 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 46% of total life cycle costs over the 25-year evaluation period, and increase to 60% of total life cycle costs over the 50-year period, due to filter strip surface rehabilitation costs assumed to be required after 25 years of operation.

25-Year life cycle cost break down

Vegetated Filter Strips: Full Infiltration

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

50-Year life cycle cost break down

Vegetated Filter Strips: Full Infiltration

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

Cost Summary Tables

Total life cycle cost estimates for a Vegetated filter strip, Non-Infiltrating configuration is ($41,448.47).

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

Exampel of a mature filter strip beside a major highway, Washington State, U.S. (Photo Source: Washington State Department of Transportation, 2018)

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.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

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.

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

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.

Sustainable Technologies Evaluation Program

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

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 gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

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.

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

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

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.

Credit Valley Conservation