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 conducted life cycle costs estimates for each of bioretention's 3 design configurations which can be viewed below. To design your own life cycle cost estimates that can be adapted to fit your project budget and unique development needs access the updated LCCT Tool here.

Design Guidance

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.

Tool defaults based on STEP recommendations:

• Maximum drainage area to surface area ratio of 20:1
• Default depth of 0.75 meters.
• Default mulch depth of 75 millimeters.
• An underdrain (minimum 200 mm perforated pipe) is only needed when native soil infiltration is less than 15 mm/hr or infiltration is precluded.

Design Notes

• Designs include pretreatment through stone diaphragms at curb inlets. Pretreatment through settling forebay and vegetated filter strip not included.
• The tool calculates costs for new designs and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal. Mobilization and demobilization costs not included. The tool adds 10% contingency and additional overhead.
• Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
• Unit costs are based on 2018 pricing; the tool automatically adds inflation. See the Assumptions sheet in the tool for further details.
• The cost of retrofitting is ~16% higher than the cost of new construction.
  • Retrofit costs are included in the 'Costs Summary' section and can be added to the Total Construction Cost for increased accuracy.
    

Note: Bioretention (all 3 design scenarios): Includes pretreatment sump at inlet and assumes that rehabilitation of filter media bed is performed at 25 years.

Construction Costs

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

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

Construction Costs Per Unit Drainage Area (CAD$/m²) - No Infiltration Design, 25 mm Treatment

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

Above you can find a cost breakdown of a 250m² in three different configurations:

1. non-infiltrating/filtration only Bioretention,
2. Bioretention: Partial infiltration
3. Bioretention: Full infiltration

As can be seen the greatest cost for this practice (regardless of configuration type) will be Material & Installation, which include 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 costs of maintenance and rehabilitation (Life cycle costs) are set at "Present Value" of these activities in 2022.

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: Please click on each image to enlarge to view associated life cycle cost results.

50-Year life cycle cost break down

Bioretention: Full infiltration

Bioretention: Non-infiltrating

Bioretention: Partial infiltration

Note: Please click on each image to enlarge to view associated life cycle cost results.

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 this 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 a note it is important to understand your site's surrounding native soil infiltration rate to ensure you are selecting the appropriate design.

A final note in 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.

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

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 pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

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.

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 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.

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.