Low Impact Development Capital Costs - City of Philadelphia 2018 Annual Report Compiles Green Infrastructure Costs and Service Area

Green Infrastructure (GI), Low Impact Development (LID) Stormwater Best Management Practice (BMP) measures are widely promoted to achieve water management goals including baseflow management (i.e, through groundwater infiltration / recharge) and erosion stress reduction (i.e., through runoff volume reduction for small storms).

The capital cost of GI / LID / BMPs is of considerable interest to those considering implementation on a system-wide basis. In general, the capital cost can be related to the size of the measure, often dictated by the size drainage area it serves.

The City of Philadelphia has been implementing GI / LID / BMP retrofits for many years and has compiled the cost of measures. These have been recently reported in this document (see pdf page 151). This is an excerpt:


Data in the table has been compiled to support cost benefit analysis for flood resiliency assessments. The chart below shows how costs increase as a function of drainage area size.


The slope of the cost vs. drainage area trend line is $295,000 per acre, or $728,000 per hectare considering 85 projects, some with multiple sub-projects.

The 2018 report includes projects completed between 2006 and 2018. When capital costs are adjusted for changes in construction costs, costs can be expressed in 2018 dollars. An adjustment in Philadelphia project costs was based on data from RSMeans (link). The following chart shows green infrastructure capital costs in 2018 dollars vs. drainage area.

 The average cost per acre is $320,000 while the cost per hectare is $792,000. There is a very strong correlation of cost with drainage area serviced by the green infrastructure feature. Weighted by the overall program cost of $56.8M and total drainage area of 164 acres (66 hectares), the program costs per acre and hectare $347,000 and $857,000 respectively. In a previous post the budgeting considered by Philadelphia was $350,000 per impervious acre, or $865,000 per impervious hectare. These costs suggest a lower cost per impervious area than those budget amounts.

The cost of providing storage is indicated in the chart below. Again the overall project or bundled project cost is highly correlated to the storage volume provided by the green infrastructure measures. The average cost is $52.7 per cubic foot or $1850 per cubic metre of storage. Note that the weighted overall program cost is different - the $56.8M cost for 907,000 cubic feet, or 25,700 cubic metres, of storage works out to unit costs of $62.6/cu.ft, or $2200/cu.m
It is worthwhile noting the average storage depth across the drainage areas serviced by the green infrastructure features. The average depth is 25,700 cu.m / 66 hectares = 25,700 / 660,000 = 39 mm. This reflects the range in design of 1 to 2 inches (25-50 mm) intended for CSO control. It is above the depth that is typically used for achieving watershed or catchment water management goals, e.g., water balance management for erosion mitigation, groundwater recharge, or non-point source water quality management. The draft Ministry of Environment, Conservation and Parks Low Impact Development guidance manual proposed control volumes up to 32 mm in Ontario, or 18% below the average depth in the Philadelphia projects.

Unit costs can be used to support high level green infrastructure program and project budgeting. There is scatter within the charts for a given drainage area or storage volume, meaning the cost of an individual project should allow for a high contingency above the average unit costs, until design details are available.

Unit costs can be assessed for various LID types - cost per drainage area for permeable pavement, rain garden, infiltration trench and tree trench projects are shown in the chart below with infiltration trenches having the lowest cost per acre and permeable pavement having the highest. The unit costs of rain gardens and tree trenches are above those of infiltration trenches.



Cost per storage volume for permeable pavement, rain garden, infiltration trench and tree trench projects are shown in the chart below. Again, infiltration trenches having the lowest cost per cubic foot of storage and permeable pavement having the highest cost per storage unit. The unit costs of rain gardens and tree trenches are above those of infiltration trenches.



The inset text boxes on the charts illustrate unit costs (i.e., the slopes of the trend lines):

Unit Cost per Drainage Area Summary:
Infiltration Trench: $260k/acre ($642k/hectare)
Pervious Pavement: $594k/acre ($1.47M/hectare)
Rain Garden: $408k/acre ($1.01M/hectare)
Tree Trench: $318k/acre ($790k/hectare)

Unit Cost per Storage Volume Summary:
Infiltration Trench: $49.0/cf ($1730/cu.m)
Pervious Pavement: $97.2/cf ($3430/cu.m)
Rain Garden: $60.1/cf ($2120/cu.m)
Tree Trench: $58.9/cf ($2080/cu.m)

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Green Infrastructure Strategies?

In Ontario, there are approximately 852,000 urban hectares that have no green infrastructure controls  in the year 2000 (per SOLRIS GIS land use data). Implementing green infrastructure across all these areas, similar to Philadelphia, would cost 853,000 x $857,000 = $730 billion - this is 100 times greater than the entire Ontario stormwater infrastructure deficit, and a similar magnitude to Ontario's annual GDP ($854B in 2018). Philadelphia is targeting a specific need for CSO control to justify its green infrastructure investment. Given this magnitude of cost, to make green infrastructure affordable in Ontario certainly requires a strategic assessment on where it is implemented. The expected annual flood damage loss in Ontario is $292M, meaning that if flood control green infrastructure reduces the cost by half each year for 50 years (assumed service life), the total savings is $7.3B - why would one invest 100 times that loss ($730B) in green infrastructure?

Ontario's Draft LID (Low Impact Development) Manual sets a generic goal of implementing LID / green infrastructure in new development and retrofit development (including linear infrastructure). Cost concerns with a generic approach have been identified by the Ontario Society of Professional Engineers in comments on Bill 139 assuming unit cost of $400,000 per hectare based on a small set of Ontario tender costs. Those costs were for small volume installations, whereas the Philadelphia program costs are for high volume installations.

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