Green Infrastructure Capital and Operation and Maintenance Costs - City of Philadelphia Clean Waters Pilot Program Final Report

A previous post summarized budget costs for Philadelphia's extensive green infrastructure program, showing budget costs of $568,00 per hectare, comparable to recent Ontario LID project tenders with an average cost of $575,000 per hectare.

The Philadelphia Water Department's has also reported extensively on green infrastructure costs and performance in their report Green City, Clean Waters Pilot Program Final Report. Highlights are presented below.

Green Infrastructure Capital Costs (Construction)

"The median construction cost per unit of impervious drainage area was $353,719/ac" - that equates to $872,000 per impervious hectare (2015 dollars).

"Median construction cost per unit of storage volume (Greened Acre) is $248,365/ac-in" - that equates to $2416 per cubic metre.

Overall costs appear to be increasing over time as shown in the following chart - to convert cost per acre to per hectare, multiply by 2.47 :

Green Infrastructure Construction Cost by Feature Type

Capital costs vary according to the type of green infrastructure (called GSI in Philadelphia). The following chart shows the variability in cost per managed impervious area for various types, suggesting some economies of scale for larger managed impervious areas.

The following chart shows the range of cost, median and average cost per managed impervious acre. A high variability in costs is shown from project to project.

Construction Cost by Loading Ratio / Efficiency

The cost efficiency of a green infrastructure project can vary according to its loading ratio, i.e., the relative size of the contributing runoff area to the project area itself. The following chart shows how project costs decrease for larger loading ratios - costs at ratios of 15 or greater are 25% less than costs for ratios of 10 and under. Also it appears that costs level-off for ratios of 15 and greater (i.e., the average cost for a loading ratio of 15 or greater is the same as for a loading ratio of 10 to 15).

Green Infrastructure Operation and Maintenance Cost

Operation and maintenance costs have been reported as well and show a wide variability. The following chart shows cost per impervious drainage area by broad type of green infrastructure, whether a subsurface or surface feature. The data indicates that surface features - those that are vegetated - cost on average more than subsurface features to maintain.

The average cost per impervious acre of $8000 equates to about $20,000 per impervious hectare. The following chart shows the variability in operation and maintenance costs according to each specific green infrastructure type. The chart shows for example higher costs for surface bumpouts and rain gardens than subsurface trenches and basins. For example, on average a bumpout costs almost twice as much as a subsurface basin.

The operation and maintenance cost appears to be approximately $20,000/$872,000 = 2.3% of capital cost. Lifecycle replacement / reconstruction of green infrastructure features, based on their deterioration over time,  would generally add to this cost and could be considered to be 1-4% of capital cost depending on the service life of the feature (i.e., features that last 25 years add 4% depreciation, and those that last 100 years add 1%).

Using these unit costs, overall lifecycle costs for Ontario-wide implementation are explored below, assuming an initial 50-year build-out period and a range of green infrastructure measures with service life durations of 25 to 100 years.

Given 852,000 urban hectares in Ontario, and assuming these are 50% impervious, the cost of green infrastructure retrofits in this province would be $370 billion dollars in capital construction cost (using $872,000 per impervious hectare) - that compares to the current Ontario stormwater infrastructure deficit of $6.8 billion. The Ontario-wide annual operation and maintenance cost for 426,000 impervious hectares would be $8.5 billion assuming $20,000 per impervious hectare - that O&M cost is over 1% of Ontario's GDP. Based on these costs, green infrastructure policies that prescribe wide-spread implementation require careful review for affordability. To recap:

Capital cost = $366 billion (using slightly lower unit cost of $860,000 per Row 12 below)
Annual O&M cost = $8.5 billion
Annual depreciation = $7.2 billion
Annual lifecycle cost (O&M + depreciation (reserve/rebuild)) = $15.8 billion

The following table summarizes the unit costs and illustrates the Ontario-wide costs that should be a cause for concern.

Ontario Green Infrastructure LID Capital, Operation and Maintenance and Lifecycle Depreciation / Reconstruction Costs - Units Costs per Philadelphia Green City, Clean Waters Pilot Program Final Report  

The follow chart illustrates the time series of costs including initial capital construction, operation and maintenance ramp-up followed by sustained operation and maintenance, reserve contributions for lifecycle asset reconstruction / rebuild according to service life (assumed 1/3 25-year, 1/3 50-year and 1/3 100-year durations), and rebuild costs (starting in year 26). It is assumed that 50 and 100-year service life assets are rebuilt over 50 a 50 year period, similar to the initial construction period.

Ontario Green Infrastructure LID Capital, Operation and Maintenance and Lifecycle Depreciation / Reconstruction Costs - 50-year initial buildout and ongoing replacement of assets beginning in year 26, funded by annual reserve.

After the initial build, the average annual operation and maintenance and depreciation costs (that are reflected in the reserve and rebuild costs) is $15.8 billion.

Some academics, including those who promote green infrastructure for amenity or other stormwater management values, have proposed green infrastructure for the purpose of flood control as well. In order to achieve flood mitigation benefits, however, widespread implementation in the sewersheds or tributaries that have flood risks is required - in that case, the costs would appear to be prohibitive to achieve quantifiable flood reduction benefits. For illustrative purposes, a York Region 100 hectare catchment has recently undergone sewer capacity upgrades at a capital cost of approximately $20M and with nominal changes in net operation and maintenance cost (larger sewers replace older ones) and a 100 year service life - implementation was over 3 years. In comparison, the green infrastructure capital costs would be in the order of $872,000 * 50% impervious * 100 hectares = $44M with additional operation and maintenance costs and lower service life durations of 25-100 years, and long term implementation (over decades) with challenges on implementation on private properties, challenges with implementation in newer tributary catchment areas with low flood risk and high existing asset value (i.e., no co-benefits of watermain replacement, etc.). Basically, the conventional flood mitigation (grey infrastructure) approach is less expensive, has a shorter implementation time and more reliably addresses the flood risk issue (i.e., green infrastructure infiltration can aggravate wastewater inflow and infiltration stresses, can adversely affect foundations, and can be unreliable in high groundwater tables areas or during saturated conditions when green infrastructure storage in ineffective).

Some further case studies and detailed assessment are required to explore where and how some green infrastructure features can contribute to Ontario urban flood risk goals in a technically effective, timely and cost-effective manner. Similarly, analysis is needed to evaluate the strategic role of green infrastructure for achieving other stormwater management goals beyond flood risk mitigation.

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How do Philadelphia GSI / green infrastructure costs compare to those of other jurisdictions? One can compare unit costs of $872,000 per hectare for Philadelphia's 1,100 projects with those in Onondaga County, New York. Costs for various types of green infrastructure measures are summarized in a recent article: http://stormwater.wef.org/2015/12/real-cost-green-infrastructure/http://stormwater.wef.org/2015/12/real-cost-green-infrastructure/.

The following chart illustrates lower unit costs with larger projects projects, similar to the Philadelphia reporting.

Green Infrastructure Unit Cost by LID (GSI) Type - Onondaga County, New York

These construction costs may be expressed as costs per area for projects. Considering projects managing 1 to 1.5 acres of impervious area the average cost per acre and hectare are summarized in the table below.

Green Infrastructure unit cost for projects managing up to 1.5 acres of impervious area  - Onondaga County, New York

 Excluding green roof projects, the average construction cost per impervious acres managed is over $368,000, or $783,000 per hectare. This cost is close to the Philadelphia cost of $872,000 per impervious hectare. Assuming 80% impervious surfaces in a catchment, the unit construction costs for project excluding green roofs in Onondaga County, New York is about $627,000 per hectare. This value is in the range of Ontario pilot projects with costs average costs of $575,000 per hectare.

The article citing Onondaga County green infrastructure costs notes that lower costs can be achieved by bundling implementation with other roadway works. In those cases costs were $320,000 per impervious hectare, or approximately $288,000 per total hectare, assuming 90% impervious coverage in those street projects.

Operation and maintenance costs for green infrastructure are summarized by CH2M as well. One observation that is similar to Philadelphia cost reporting is that vegetated systems are more costly to maintain than non-vegetated systems. The following chart summarizes costs per impervious area for various green infrastructure (LID, GSI) measures.

Green infrastructure operation and maintenance costs by type per impervious area managed.

Excluding green roof measures, a annual maintenance costs range from about $500 per impervious acre (low range for infiltration trench) to $3300 per impervious acre for tree infiltration trenches. A typical cost would be about $1500 per acre per year ($3700 per impervious hectare per year) which is 1500/368,000 = 0.4% of capital cost. This is significantly below the Philadelphia unit cost of $8000 per impervious hectare. It is also significantly below reported O&M/capital costs ratios reported in the American Society of Civil Engineers' report Cost of maintaining green infrastructure. In that report O&M costs for infiltration trenches and bioretention ranged from 5-20% and 5-7% respectively per one source (USEPA 1999 summarized by Weiss et al. 2007), and 8% for bioretention per another (Normalized UNHSC Installation and Maintenance Cost Data).

Urbanization, Runoff, Overland Flow and Flooding - How Sprawl of Ontario Cities Drives Flood Risk and Insurance Losses in Urban Areas

Readers of this blog have seen these basic process described several times: (1) rain transforms into runoff when it hits the ground, (2) runoff accumulates and flows in rivers or municipal drainage infrastructure, (3) the capacity of the flow systems determines whether flow "backs up", "surcharges", "spills", or generally flows uncontrollably to where we don't want it to go, causing flooding.

Using Environment Canada's data and research, we have shown that rainfall intensities have not increased in southern Ontario here. In fact there are more statistically significant rain intensity decreases than increased south of 44 degrees. So the rainfall influence on runoff is not increasing. But runoff has been increasing after decades of urbanization under the today's stable or decreasing rainfall intensities.

The following maps show urban expansion in Mississauga, Oakville and Burlington Ontario from 1966 to about 2000 (data varies from 1999 to 2002). The overland flow system path based on Ontario conditioned digital elevation model is superimposed on the land use map so that the impact of urbanization and runoff into the drainage system can be considered.

 The effect of urbanization in Mississauga on runoff would be most acute in the smaller watersheds (e.g., not the Credit), where the upstream urban area has increased significantly since 1966.

Likewise in Oakville - Bronte Creek, a large watershed more slightly influenced by the city's sprawl, has not been affected to the same degree as the smaller Fourteen Mile Creek to the west, where a high relative change in land use over that smaller watershed has occurred throughout the city.

Same in Burlington - many small creek watersheds originating off the escarpment have dramatically increased urbanization over three decades. Burlington is characterized by creeks that have been realigned, straightened and encroached upon. These can be expected to be more sensitive to increased runoff rates due to expanded urbanization.

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Parts of Hamilton have been urbanized up to the watershed divide (black line) by the late 1990's / early 2000's. How does this affect runoff into the old 'core' built to pre-1960's standards?

Hamilton, wider perspective. Some wetlands remaining upstream of Dundas? :

Richmond Hill (Lake Wilcox near upper middle of map). Some urbanization around the lake flows to the Humber where flow impacts would be muted, while other areas to the south flow flow to headwater tributaries of the Don and Rouge:

Land Use Change Drives Urban Flood Risk .. Yet Hydrologists Become "Useless Appendage" To More Virile Sciences

"The unsatisfactory state of hydrology is, in the final analysis, the result of the dichotomy between the theoretical recognition of hydrology as a science in its own right and the practical impossibility of studying it as a primary discipline but only as an appendage of hydraulic engineering, geography, geology, etc." Vit Klemeš in Dilettantism in hydrology: Transition or destiny?, Water Resources Research, Vol. 22, 1986.

That explains it! Nobody studies hydrology in its own right, and so this limp appendage of other scientific pursuits has been ignored. Pity - because hydrologic science and simple quantifiable land use changes can explain increased runoff and increased urban flood damages in Ontario municipalities. Why is it ignored? Because hydrology is just the run up to other things like dynamic hydraulic simulations and then infrastructure construction.

Take a look at these Ontario urban area land use maps, and see how the limits of urban areas have increased over thirty years or so from 1966 to the late 1990's. It can be dramatic. For some catchments, the increase in urban land coverage can be an order of magnitude.

Sure, some of these areas developed between 1966 and 2000 have stormwater management controls up to a point. But it is commonly accepted that the on-site local stormwater controls do not completely address cumulative volumetric runoff impacts at larger scales. And the controls are finite - not controlling the rarest storms that drainage infrastructure could see in its design life. So expanded urban areas can drive downstream flood risk. Yes, every catchment is different and maybe no property is at risk downstream of some urbanized area (i.e., dedicated drainage easements and wide regulated flood plains have maintained a wide major flow path) - but in may cities the overland flow path has not been mapped managed or preserved, so increased runoff and flood risk can result.

Tantric Hydrology
Brought to you by the Flood Guru.
OK, could a movie about hydrology be any worse than The Love Guru? I guess.....