Bioretention: Life Cycle Costs

Parking lot bioretention with surface ponding well in foreground, Kortright Centre, Vaughan, ON. Read about the performance of this practice in the technical brief (Source: STEP, 2016).

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 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 LCCT Tool 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 assumptions are based on tool default values and STEP recommendations:

• 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.

Design 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.
  • Mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction with equipment needed already on-site.
  • 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 contents is assumed to be 16% higher than the cost for new (greenfield) development 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 construction cost breakdowns 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. No infiltration,

As can be seen the greatest cost for this practice (regardless of configuration type) will be Material & Installation, which includes costly components such as the impermeable membrane/liner appropriate planting and vegetation selection, underdrain, filter media and the stone storage layer or Internal water storage zone (IWSZ).

Life Cycle Costs

Below are both the capital and life cycle costs of the three bioretention configuration practices over a 25 and 50-year time horizon based on a detailed assessment of local input costs, maintenance requirements, rehabilitation costs and design scenarios relevant to Canadian climates. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values.

Looking at the pie charts below for each configuration we can see that they are all relatively close to being the same cost with a variation of a few hundred dollars between each over both a 25 and 50-year time horizon. The percentage of the Full Infitlration configuration is greater than the other two due to the lesser cost of the overall installation for material and Installation, (no underdrains, no gravel storage layer or impermeable membrane, etc.).

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.

Total Cost & Design Summary

As previously discussed the three Bioretention configurations total cost summary vary greatly dependent on whether you want you feature to possess full infiltration, no infiltration, or partial infiltration. In short, the most expensive of these options is the Bioretention: Partial infiltration option ($113,800.83 vs. $109,113.76 - no infiltration and $80,392.33 - full infiltration). The same can be said for construction + associated retrofit costs with each configuration design ($132,008.97 vs. $126,571.96 - no infiltration and $93,255.10 - full infiltration) This configuration allows for the greatest storage volume (187.5 m³) compared to its counterparts at 145³ for no infiltration and 107.5³ for full infiltration; all while having the exact same surface area footprint of 250m². As reminder, it is important to understand your site's surrounding native soil infiltration rate to ensure you are selecting the appropriate design.

A final note regarding the accuracy of the LCCT. A follow up sensitivity analysis study was conducted by CVC & STEP back in 2019 to test the tool's accuracy. The analysis took designs from 6 completed projects (4 bioretention, 1 permeable pavement, and 1 infiltration trench), and ran them through the tool comparing construction costing results from the LCCT to actual construction costs for the projects. The accuracy target set for the tool was plus-or-minus 30% of actual construction costs.

The analysis found that the tool was on average (±14%) to 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

Non-Infiltrating/filtration only

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

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 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 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.

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.

Credit Valley Conservation

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.

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.