Green Infrastructure - Capital Costs to Implement Low Impact Development Stormwater BMPs

The National Research Council of Canada (NRC) has released the National Guidelines on Undertaking a Comprehensive Analysis of Benefits, Costs and Uncertainties of Storm Drainage and Flood Control Infrastructure in a Changing Climate.

The full guidelines are available here to download:

The guidelines delve into many topics to support comprehensive benefit cost analysis. Previous posts explored these topics:

1) Historical extreme rainfall trends related to flood damages (see post:

2) Insurance industry loss data to assess of direct and indirect flood losses, i.e., the potential benefits of new/upgraded flood control infrastructure (see

The guidelines also present costs for a range of flood control and drainage measures in Appendix F, Life Cycle Costs of Storm Drainage Infrastructure, Annex F1 including for measures such as:

  • Regional Storage
    • Dams 
  • End of Pipe Storage
    • Dry Ponds
    • Wet Ponds
    • Engineered Wetlands
    • Infiltration Basins
  • Local Storage/Management
    • Underground Storage
    • At-Source Solutions
    • Enhanced Grass Swales
    • Infiltration Trenches
  • Conveyance and Conveyance Controls
    • Storm Sewers
    • Channel Improvements
    • Bridges
    • Culverts
    • Pump Stations 
  • Area Protection
    • Berms/Dykes
  • Building Protection
    • Backwater Valves & Downspout Disconnection
This post shares some cost data to support the planning-level estimation of green infrastructure costs, i.e., local storage/management involving at-source control measures. Some comparisons with the unit costs of other measures referenced in the NRC guidelines is provided as well.

Alternative terminology for engineered measures with at-source storage were noted to include:

Green infrastructure enhanced assets & engineered assets – Defining Municipal Natural Assets (Municipal Natural Assets Initiative, 2017)

Green stormwater infrastructure – Green City Clean Waters, The City of Philadelphia’s Program for Combined Sewer Overflow Control (Philadelphia Water Department, 2011)

Lot level and conveyance controls – Stormwater Management Planning and Design Manual (Ontario MOE, 2003)

Stormwater best management practice – Best Management Practices Guide For Stormwater, Prepared for Greater Vancouver Sewerage and Drainage District (Vol 1a and 1b) (Dayton & Knight Ltd. et al., 1999)

The follow charts and tables present capital and operating costs for many at-source stormwater control measures as presented in the NRC guidelines. Capital costs are shown as a function of storage volume provided or drainage area served - that is to support a 'top down' assessment of costs that can be used in planning studies and strategy development, in advance of any preliminary or detailed design and implementation. These costs were developed based on extensive data was analyzed from the City of Philadelphia's Green City, Clean Waters Pilot Program - that program includes comprehensive planning, implementation and assessment of green infrastructure for CSO control, using a consistent design volume target. The costs are based on actual costs of implemented projects within the program, that have undergone preliminary design, screening for affordability/adaptation, detailed design and construction.

Note that because the Philadelphia program achieves as consistent design volume, costs by area served are also related to design volumes achieved. Other projects analyzed included those within the U.S. EPA BMP Database (where costs and design volumes were available), and projects implemented in the province of Ontario and also Alberta. Those projects did not have consistent design volumes (i.e., targets) and on average implemented smaller control volumes based on available data.

The following chart from the NRC guidelines present capital costs for construction of green infrastructure in the Philadelphia program as a function of design volume provided. The costs, expressed in 2018 USD for the projects analyzed averaged $56.4 / cubic foot, or $2000 / cubic metre of storage provided, aggregating across all BMP types.

As an aside and for comparison, the cost for large-scale centralized dam/reservoir storage was identified as follows in the guidelines (based on Petheram & McMahon, 2019):

        Cost = 13138 x^-0.555

Where “Cost” is the cost of a mega-liter (ML) of storage capacity (presented in 2016 Australian dollars) and x is the storage volume in gigalitres (GL). So for a 10 ML reservoir, equivalent to 10,000 cubic metres, the cost would be $3660 or $0.36 per cubic metre, which is several orders of magnitude less than the average cost of at-source storage ($2000 per cubic metres). This is simply to illustrate the economies of scale of larger, centralized controls such as regional reservoirs in comparison with small, at-source green infrastructure controls.

To compare with smaller, local end-of-pipe storage measures such as wet ponds, the following costs were identified (based on a Greater Vancouver Sewerage and Drainage District study Best Management Practices Guide For Stormwater (Dayton & Knight Ltd. et al., 1999)):

        Cost = $28.90 × (35.31 × V)^0.70, where V = cu.m of storage volume  

The study noted that typical wet pond construction costs range from $26 – $53 per cu.m of storage volume - unit costs would be higher in 2018 dollars, e.g., $50 – $100 per cu.m of storage, scaled using Statistics Canada's building construction price index for the Toronto area. Of note is that the unit costs for end-of-pipe controls are an order of magnitude less than the smaller cost of at-source controls, again showing the economies of scale associated with larger, more-centralized works.

The next chart shows green infrastructure capital costs in the Philadelphia program as a function of total drainage area served, or controlled. Note that the Philadelphia projects provided an average of 38.9 mm of storage across drainage area served. The average cost was $346,000 per acre or $856,000 per hectare, expressed in 2018 USD. 

As another aside, in comparison with end-of-pipe storage such as wet ponds, the NRC guidelines present the following table with wet pond construction costs (2020 CDN $):

The capital cost per hectare served for wet ponds is an order of magnitude lower than the cost of at-source controls when expressed as a cost per hectare (i.e., $25,000 to $52,000 per hectare for wet ponds and over $800,000 per hectare for green infrastructure measures).

The following chart breaks down green infrastructure capital cost by low impact development (LID) BMP type, including infiltration trenches, pervious pavement, rain gardens and tree trenches. Note that some projects includes multiple BMP measures and therefore the classification can represent the core measure implemented when multiple types were combined. The costs per volume show higher costs per permeable pavement than other LID BMP types. 

The next chart shows the variation in costs as a function of drainage area served.

Below the overall costs are in the Philadelphia Clean Waters Pilot are compared to costs for projects in the USEPA International BMP Database, and projects implemented in Ontario and Alberta, Canada. The Philadelphia program costs represented the largest area controlled and the largest design volume provided.

The USEPA BMP Database cost per drainage area was a fraction of Philadelphia project costs, however, the average volume provided was significantly lower as shown above. In the case of the Ontario and Alberta projects, limited data on design volume was available but it is expected that targeted volumes would be between the Philadelphia and USEPA BMP Database volumes, based on common practice to target small storm events in many jurisdictions to manage water balance, water quality control and erosion stress reduction benefits.

Operating costs can represent a significant proportion of the present value of flood control and drainage infrastructure, and these costs vary by infrastructure type. The following chart from the guidelines illustrates how the present value of infrastructure, including both capital and operating cost, varies according to the operation and maintenance (O&M) requirements and the service life of the infrastructure.

The Best Management Practices Guide For Stormwater (1999), Prepared for Greater Vancouver Sewerage and Drainage District provides the following percentages for various at-source storage (and other end of pipe storage) measures:

• Dry Detention Basins: budget 1% of construction cost per year

• Wet Pond: budget 3% to 6% of construction cost per year

• Roof Downspout Systems: budget 3% to 6% of construction cost per year

• Infiltration Basin: budget 1% to 3% of construction cost per year

• Bioretention And Dry Swale With Underdrains: budget 5% to 7% of construction cost per year (assumed the same as grassed channels and wet swales)

• Sand Filters: budget 11% to 13% of construction cost per year

When operating costs are over 5% of capital cost, for example for bioretention measures, and where the service life is 25 years, the chart above indicates that the present value of O&M costs are over twice the value of the capital cost. This point emphasizes the importance of considering O&M costs when evaluating varied flood control alternatives.

These percentages representing annual O&M/capital cost can be contrasted to the operating costs for other measures regional storage and conveyance measures. The guidelines report lower percentages for dams/regional storage. An Australian study noted operation and maintenance costs that range from 0.08% to 0.86% of final (capital) costs, with a median value of 0.21%. As a specific example, for the Springbank Off-Steam Flood Storage Site the operation and maintenance costs were noted to be in the order of 0.3% of construction (including engineering) costs. While there is limited information available for rehabilitation costs for the Springbank Off-Steam Flood Storage Site, rehabilitation (capital) costs were determined to be in the order of 3.7% of initial construction (including engineering) costs, incurred every 10 years, adding 0.37% per year. From this, it appears that operation and maintenance and rehabilitation costs for larger scale works, such as dams/reservoirs is less that that of smaller local storage or at-source controls.

In absolute costs, annual O&M costs for various green infrastructure measures were summarized as follows for the Philadelphia program, expressed as a function of impervious surface controlled:

With a capital cost of $320,000 per acre (2018 USD) identified in the chart above, and assuming 50% impervious, the capital cost per impervious acre would be in the order of $640,000 per acre. The tabulated O&M costs are equivalent to less than 1% of those capital costs. As the O&M costs in the table above are in 2008 USD, the percentage assessed in 2018 USD would be expected to be higher. 

The table below presented in the guidelines presents capital and operating costs for various green infrastructure types in the Philadelphia program and suggests an average percentage of O&M cost/capital cost of 2.3%.

Lastly, the guidelines present cost estimates for different implementation scenarios including retrofits and redevelopment. As shown in the table below, the cost for retrofits can be significantly higher that the cost for redevelopment.