Infiltration Chamber: 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

Infiltration chambers include a range of proprietary manufactured, modular structures installed underground (embedded in clean, crushed angular stone) to create large void spaces that temporarily store and infiltrate runoff into the underlying native soil. Typically installed under parking or landscaped areas, they can be used in various configurations. They are well suited to sites where available land area is limited, or where it is desirable for the facility to have a minimal surface footprint. They can be designed with enough load bearing capacity to support the weight of structures above them, meaning that they can be installed below parking lots, sports fields, etc. STEP has prepared life cycle costs estimates for each design configuration, 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

Infiltration chambers are an ideal technology for installing below any type of surface or landscape and for receiving and infiltrating large volumes of water. Components include: washed 25 or 50 mm diameter crushed angular stone to create a suitable storage reservoir, proprietary chambers, vaults, crates or perforated pipes that provide large water storage volume per unit area, geotextile, perforated pipe or underdrains and access structures.

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)
• Designed with an impervious drainage area to treatment facility area ratio of between 5:1 and 20:1.
  • A maximum ratio of 10:1 is recommended from facilities receiving road or parking lot runoff.
• Facilities receiving road or parking lot runoff should not be located within the two year time-of-travel of wellhead protection areas.
• Facilities cannot be located on natural slopes greater than 15%.
• The bottom of the facility should be vertically separated by one metre from the seasonally high water table or top of bedrock elevation.
• Facilities should be setback a minimum of four metres from building foundations.
• Pretreatment device options include leaf screens for roof runoff, and vegetated filter strips, grass swales or oil and grit separators for road runoff.
• The tool automatically includes an OGS for facilities receiving road runoff.
• The inlet and overflow outlet to the facility should be installed below the maximum frost penetration depth to prevent freezing.
• The overflow outlet can be the pipe inlet that backs up when capacity's reached (discharging to pervious area), or it can be a pipe connected to a storm sewer.
• Outlet pipes must have capacity equal to or greater than the inlet.
• Capped and vertical non-perforated pipes connected to the inlet and outlet pipes are recommended for inspecting and flushing as part of routine maintenance.
• Manholes and inspection ports should be installed in infiltration chambers to provide access for monitoring and maintenance activities (Tool defaults).
• Compaction, erosion and sediment control are main concerns during construction. Facilities are vulnerable to failure during the construction phase. Construction sediment can clog the excavation if construction instructions incorrectly followed.

Notes

• Designs include pretreatment through hydrodynamic separator (Oil and Grit Separator).
• 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) - 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 a cost breakdown of a 1000m² in two different configurations:

1. Infiltration Trench: Full Infiltration
2. Infiltration Trench: Partial Infiltration

As can be seen, regardless of design configuration, Material & Installation expenses represent the largest portion of total construction costs (75 to 76%).

Life Cycle Costs

Below are capital and life cycle cost estimates for the two infiltration trench 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 38% of total life cycle costs over the 25-year evaluation period, and increase to 51% of total life cycle costs over the 50-year period, due to increased levels of litter removal, replacement of filter cloth and disposal of collected sediment (every 4 - 8 years), cleaning out the catchbasin and the Oil and Grit Separator/Hydrodynamic Separator annually, as well as flushing the internal pipes every 10 years.

25-Year life cycle cost break down

Infiltration Trench: Full Infiltration

Infiltration Trench: Partial Infiltration

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

50-Year life cycle cost break down

Infiltration Trench: Full Infiltration

Infiltration Trench: 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 two infiltration trench configurations vary substantially with the Infiltration Trench: Partial Infiltration design being highest ($85,288.88), followed closely by the Infiltration Trench: Full Infiltration design being ($69,805.90).

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

Good use of geotextile, being limited to preventing fines from entering the trench from above. Read more about the performance of this feature in comparison to bioretention] BMPs in Vaughan, ON. (Source: STEP, 2016)

Partial Infiltration

Sustainable Technologies Evaluation Program

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 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.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

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.

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

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.

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.

Systems designed to remove trash, debris and some amount of sediment, oil and grease from stormwater runoff based on the principles of sedimentation for the grit and phase separation for the oil.

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.

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.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

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

Penetration of water through the ground surface.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Soil particles with a diameter less than 0.050 mm.

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

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.