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

Infiltration trenches typically consist of linear-oriented, rectangular- or trapezoidal-shaped trench excavations between 0.6 and 2.4 m wide that are lined with geotextile, backfilled with clear stone aggregate and located underground. They are well suited to sites where available space for infiltration is limited to strips of land between buildings or properties, or along road right-of-ways. Trench depth is determined by the infiltration rate of the underlying native soil and maximum acceptable time to fully drain the facility (i.e., drainage time). STEP has prepared life cycle costs estimates for infiltration trenches located on highly permeable native soil (Full Infiltration design), and moderately permeable native soil (Partial Infiltration design) scenarios for comparison. Cost estimates for both design scenarios are based on a 2,000 m² asphalt drainage area, runoff control target of 25 mm depth and 72 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 trenches are an ideal technology for installing below any type of landscape with requirements to infiltrate excess stormwater while safely conveying excess flows during major storm events. Components include: reservoir aggregate to store runoff water, perforated pipe to deliver and distribute flow to the trench, geotextile, inlet structures, pretreatment devices or features and monitoring wells to track drainage performance of the facility over its operating lifespan. 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, and Partial Infiltration Design scenarios were assumed to be 20 mm/h and 10 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. Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ trench water level monitoring during natural storm events.
• Trench width of 2.0 metres.
• Maximum impervious drainage area to permeable surface area (I:P area) ratio of 20:1.
• Trench depth of 580 mm and surface area of 198.73 m² (I:P area ratio of 10:1) on 20 mm/h native soil (Full Infiltration Design)
• Trench depth of 290 mm and surface area of 366.58 m² (I:P area ratio of 5.5:1) on 10 mm/h native soil (Partial Infiltration Design)

Notes

• Designs include pretreatment through two (2) hydrodynamic separators (i.e., oil and grit separators) located upstream of trench inlets, and two (2) 150 mm diameter monitoring wells per trench.
• 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

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

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

Above you can find breakdowns of construction costs by expense type for each infiltration trench design scenario:

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 (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 39% of total life cycle costs over the 25-year evaluation period, and increase to 52% of total life cycle costs over the 50-year period, due flushing of inlet, distribution and outlet 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 over the 50 year evaluation period for the two infiltration trench design configurations vary substantially with the Infiltration Trench: Partial Infiltration design being highest ($183.197.71), followed closely by the Infiltration Trench: Full Infiltration design being ($150,128.87).

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 about the performance of infiltration trenches in comparison to bioretention. (Source: STEP, 2016)

Partial Infiltration

Sustainable Technologies Evaluation Program

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.

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.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

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

The period between the maximum water level and the minimum level (dry weather or antecedent level).

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.

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.

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.

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.

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.