Infiltration Trench: Life Cycle Costs

Infiltration trench located at the Honda Campus, Markham, ON. Read about the performance of this practice in the technical brief (Source: STEP, 2015).

Overview

Given that Bioretention is a widely used name and a general "catch-all" term for most vegetated LID practices that temporarily store stormwater runoff in depressed planting beds there are a number of configurations of the feature that can be used in various contexts and environments. Depending on native soil infiltration rate and physical constraints, the facility may be designed without an underdrain for full infiltration, with an underdrain for partial infiltration, or with an impermeable liner and underdrain for no infiltration/filtration only (i.e., a stormwater planters or biofilter) design. STEP has prepared life cycle costs estimates for each design configuration, based on a 2,000 m² asphalt 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

Bioretention is an ideal technology for fitting functional vegetation into urban landscapes and treating runoff collected from nearby impervious surfaces. Components include: a 'filter bed' with filter media, storage layer of reservoir aggregate, planting and a finishing surface layer of mulch and/or stone. Additional components include an underdrain to remove excess water and soil additives to enhance pollutant removal.

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)
• Maximum impervious drainage area to permeable surface area (I:P area) ratio of 20:1.
• Length to width ratio of bioretention area is 10:1.
• Default filter media depth of 1.0 metres to accommodate trees in planting plan.
• Default mulch depth of 75 millimetres.
• An underdrain (200 mm diameter perforated pipe) is included in Partial Infiltration and No Infiltration design configurations only.

Notes

• Designs include pretreatment through stone diaphragms at curb inlets and two inlet structure sumps (Rain Guardian bunkers).
• Operation and maintenance cost estimates assume rehabilitation of the filter media bed 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) - No Infiltration Design, 25 mm Retention

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 breakdowns of construction costs by expense type for 250 m² surface area facilities (I:P area ratio of 8:1) for each design configuration:

1. Bioretention: Full infiltration
2. Bioretention: Partial infiltration
3. Bioretention: No infiltration

As can be seen, regardless of design configuration, Material & Installation expenses represent the largest portion of total construction costs (75 to 79%).

Life Cycle Costs

Below are capital and life cycle cost estimates for the three bioretention 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 between 25 and 30% of total life cycle costs over the 25-year evaluation period, and increase to between 38 and 46% of total life cycle costs over the 50-year period, due to filter media bed rehabilitation costs assumed to be required after 25 years of operation.

25-Year life cycle cost break down

Bioretention: Full infiltration

Bioretention: Non-infiltrating

Bioretention: Partial infiltration

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

50-Year life cycle cost break down

Bioretention: Full infiltration

Bioretention: Non-infiltrating

Bioretention: 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 three Bioretention configurations vary substantially with the Partial Infiltration design being highest ($113,800.83), followed closely by No Infiltration design ($109,113.76), and Full Infiltration design being the lowest ($80,392.33).

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

STEP staff member conducting performance analysis of a bioretention feature at Kortright Centre in Vaughan, ON. (Source: STEP, 2016)

Partial Infiltration

No Infiltration (Filtration Only)

Sustainable Technologies Evaluation Program

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.

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.

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 perforated pipe used to assist the draining of soils.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

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 excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

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

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

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.

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.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry 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.

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

Penetration of water through the ground surface.

The technique of removing pollutants from runoff as it infiltrates through the soil.

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.