Infiltration Chamber: Life Cycle Costs

Read about the performance of infiltration chamber systems in the STEP technical brief (STEP 2015) and full report (Young et al. 2013)).

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 a life cycle cost estimate for infiltration chamber systems located on highly permeable native soil (Full Infiltration design) for comparison. Cost estimates are based on a 2,000 m² asphalt drainage area, runoff control target of 25 mm depth and 82 hour drainage period, 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 suitable for receiving and infiltrating large volumes of stormwater. Components typically include proprietary chamber system parts that provide large water storage volume per unit area, clear stone aggregate to construct base and embed chambers, geotextile, pretreatment devices, and structures to access inlets, outlets, pretreatment devices and the chambers themselves for operation and maintenance. Optional components include a flow restrictor to control the release rate of the facility, and surface drains to safely convey flows in excess of the storage capacity of the design.

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 Infiltration Design scenarios were assumed to be 20 mm/h, 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, and removal of sediment from pretreatment structures annually. Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ chamber system water level monitoring during natural storm events.
• Infiltration chamber system length of 5 metres and composed of 22 chambers in total.
• Infiltration chamber height of 0.762 metres.
• 50 mm dia. clear stone aggregate bedding depth (below and above chamber system) of 152 millimetres.
• Two (2) maintenance holes providing access to infiltration chamber system inlet and outlet structures.

Notes

• Design include pretreatment through hydrodynamic separator ( i.e., oil and grit separator) and isolated chamber row.
• 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 Retention

Note: Click on image to enlarge to view associated construction cost estimates.

Above you can find a breakdown of construction costs by expense type.

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

Life Cycle Costs

Below are capital and life cycle cost estimates for the infiltration chamber design scenario over 25- and 50-year time periods. The estimate of maintenance and rehabilitation (life cycle) cost represents net present values. Operation and maintenance costs are predicted to represent 37% of total life cycle costs over the 25-year evaluation period, and increase to 49% of total life cycle costs over the 50-year period, due to cost associated with removal of sediment from the isolated chamber row pretreatment devices every 8 years.

25-Year life cycle cost break down

Infiltration Trench: Full Infiltration

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

50-Year life cycle cost break down

Infiltration Chamber: Full Infiltration

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

Cost Summary Tables

Total life cycle cost estimate over the 50 year evaluation period for infiltration chamber system on highly permeable native soil design scenario is $123,670.89.

Full Infiltration

Example of "crate style" Infiltration chambers being installed in East Gwillimbury. Photo credit: [https://www.makeway.ca/products/stormwater-management-systems/ Make-Way Environmental Technologies Inc.

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.

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.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

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 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.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

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.