Benefit-Cost Analysis for Green and Grey Infrastructure - Evaluating Cost-Effective Strategies for Flood Loss Reduction, Erosion Mitigation, and Water Quality Improvements From Projects to City-Wide Strategies

The following presentation was made to the Southern Ontario Municipal Stormwater Discussion Group on September 27, 2018. It describes benefit-cost analysis to show the return on investment (ROI) of infrastructure improvements to reduce flood damages (insured and total), and to achieve other benefits including erosion mitigation and water quality improvements. Earlier benefit cost analyses for projects ranging from the Winnipeg floodway to the Stratford, Ontario storm system master plan are shown. The benefit-cost ratio of an Ontario flood control study is shown including a comparison of grey and green infrastructure cost effectiveness - analysis shows the grey infrastructure solution can meet the current Disaster Mitigation Adaptation Fund (DMAF) benefit/cost threshold of 2:1 required to be eligible for federal funding. In addition, city-wide analysis of grey infrastructure storm and sanitary system upgrades and green infrastructure / low impact development infrastructure strategies is summarized.

Results show that the grey infrastructure solution can meet the DMAF benefit/cost threshold of 2:1 but that the benefit/cost of green infrastructure is substantially below it considering flood reduction benefits. When other benefits are considered, and targeted implementation of green infrastructure is considered (e.g., representing 25% of the urban area with limited overland drainage design standards) and considering additional benefits including a substantial 'willingness to pay' estimate for water quality improvements, costs continue to exceed benefits. The insurance industry and some affiliated research groups have suggested that natural infrastructure or green infrastructure should be considered to improve climate resilience and reduce flood damages - this analysis would suggest that approach is misguided and could misdirect scare resources to ineffective strategies. In addition, the impacts of green infrastructure that infiltrate runoff into the ground, stressing wastewater systems and increasing basement flooding have not been considered in the analysis - if considered, these wastewater system flood impacts, as well as pumping and treatments costs, and potentially the cost of increased CSO's and their water quality impairment, would have to be estimated to reduce the net benefits achieved by managing overland flooding.

Grey and green infrastructure benefits and costs. Selected scenarios are shown.

If "Climate resilience must be part of every government’s agenda" per Intact Financial and Sun Life, What is Insurance Industry's Role?

A Globe and Mail opinion piece from the insurance industry does some finger pointing on resilience, but should look in the mirror and consider its role in a public-private partnership. It is right to have resilience on every government's agenda - it is - how can the insurance industry participate and support the move toward greater resilience? Besides in writing opinions pieces from its echo chamber and foisting impractical research on governments who are trying to get things done? My response below.


These insurance executives need some new advisers, preferably from the engineering community and should realize that just as fire insurance has evolved so will flood insurance. And certainly flood resiliency is important and communities must protect critical infrastructure, but the opinion piece is just that – grossly uninformed sound bite opinions gathered and amplified in an echo chamber of the insurance industry’s own making. Someone open the door and let in some fresh air for clearer thoughts.

What is with the focus on climate change causing extreme weather events and impacts? All good engineers know that flood damages are caused by historical design standards in old communities (old small sewer pipes never designed to handle big storms at all) and historical high-risk land use decisions (long term leases to Toronto Island cottagers and commuter rail lines through the bottom of the Don River Flood Plain) ... hazards that have been in place for over a century. And despite what the insurance industry has been saying (storms are more frequent or severe due to climate change), official Engineering Climate Datasets say the opposite in regions like southern Ontario. And it has been uncovered that widely-touted “Insurance Facts” like weather events that happened every 40 years is no happening every 6 year, has been shown to be absolutely ‘made up’ by those in the engineering community who rely on data to design resilient communities. So hazards are long standing, damages may be increasing because the insurance industry is growing just like urban areas are, increasing exposure to the same ‘old normals’ we have always contended with. The ‘new normal’ is an insurance industry-affiliated research organization categorically misinforming the public on the causes and often appropriate solutions to mitigate flooding.

Saying “our industry can help find solutions” – oh, really? The engineering community begs to differ. Why does the insurance industry’s research organization promote wetlands as a solution to urban flooding, cherry-picking atypical watersheds covered with wetlands as their case studies, and prescribe them as an absolutely impractical solution to tackling today’s infrastructure capacity constraints across the country? Why does it promote green infrastructure like permeable pavement on driveways to soak rain into the ground while at the same time promoting sealing of cracks in driveways to keep it out of the ground. Conflicted? Confused? Absolutely. Never mind that permeable pavement has such as high maintenance cost it has a payback period of over 300 years if you install it. This is not helpful.

The insurance industry is so panicked, maybe because it is being misinformed on all the facts, and as KPMG recently noted did not do a good job of pricing flood insurance at all, that it is throwing everything at the wall to see what will stick as solutions. Bioswales? Aka soggy green ditches on every front yard to store runoff from the road – those are being promoted by the insurance industry from Senate committee presentations to TVO spotlights. While the insurance industry recommends these green infrastructure measures, economic analysis shows they pay back pennies in flood reduction benefits for every dollar invested in capital construction and ongoing maintenance. Are those the keen financial investments the insurance industry wants communities to make, and taxpayers to pay for? Please – if these are your practical solutions for cities STOP right now. You’re sinking them and diverting attention from solutions that can make a difference.

The insurance industry needs an “A-Team” to give them fact on what is causing flooding and the priorities for addressing it, and where they can help. Get out of the echo chamber. Insurance industry research agencies can do more harm than good in mis-identifying problems and prescribing the least technically and cost effective solutions.

Here’s an idea – all cities have to complete asset management plans to get federal gas tax funding. Why don’t insurance companies share a bit of their data, and help cities set infrastructure upgrade priorities instead of just saying there is a crisis in the papers? That’s right, be active in this ‘public-private partnership’ you write about– tell the folks down the hall to find a few big cities that you insure and go reach out and share some risk data with them so they can refine their infrastructure investment priorities. This ain’t rocket surgery. Work with city engineers on the resiliency aspect of their asset plans – insurance folks, you have risk mapping at the postal code level of details that many small municipalities do not have – some small cities don’t even have digital mapping of assets at all. Support them. Create a 'capacity-building' program to do this through FCM ... I'd volunteer for that.

In 28 years of practice I have never seen an insurance industry representative at a community meeting about local flooding and I’ve been to many. To make a public-private partnership that has to change. How? Create an insurance industry group that actively participates in the current problem solving process – maybe you will have to herd cats among all the companies to make this happen, but it will be worth the effort. In Ontario we call this the Class Environmental Assessment process, and this is how we have been defining and solving flooding and other infrastructure problems for over 30 years. Insurance industry, come on out to our meetings. Join our project stakeholder committees. Don’t just wait for big flood with in Peterborough or Windsor to show up and declare a crisis. Make it part of someone’s day job. Participate. Share. Help.

You may not know this but this type of data sharing partnership has already happened in some advanced cities where the insurance industry and cities are working together on solutions. We need to support this ground-level public-private collaboration as opposed to having insurance-industry research organizations proposing out-of-touch, impractical and costly ‘solutions’ upon cities – that will only keep us in a holding pattern and prevent us from moving ahead on effective resiliency strategies. Please stop with promoting the bioswales, permeable pavement and wetlands! As pretty as this stuff looks in the artist renderings, it would be a green blob menace to government finances and we just don't have wetland-flood-remediation opportunities in most urban areas.

So please get some fresh air. Get in touch with the local engineers and participate in finding practical solutions in all communities, at the ground level. Opinion pieces do not really solve anything, nor cherry-picked, headline-grabbing research – governments can achieve extreme weather resilience with your active participation.

Robert J. Muir, P.Eng.

Urban Flood Risk Evaluation to Guide Best Practices and Projects - Tiered Vulnerability Assessment and Risk Mapping for Storm, Wastewater and River Systems from Flood Plain to Floor Drain

A tiered vulnerability assessment framework for existing flood risk in urban systems was developed to support best practices development in Canada as published in this previous post. Risk mapping is critical to defining areas of interest for implementation of no-regrets policies and practically deployed programs that can reduce risk in a cost-effective and technically effective manner. Examples of such include stormwater management peak flow control policies, or construction by-laws, and low-cost programs to reduce stresses on infrastructure systems (e.g., sanitary or combined sewer downspout disconnection) or to isolate flood-prone properties from sewer back-up risk (e.g., plumbing protection through backwater valve installation or foundation drain disconnection/sump pump installation).

A review of vulnerability assessment methods was prepared for the Ontario Urban Flooding Collaborative to share risk mapping approaches being considered as part of an Ontario-wide strategy being developed to reduce existing urban flood risks. The September 13, 2018 webinar presentation is below:

The presentation illustrates examples of tiered vulnerability assessment in areas with existing urban flooding interests and demonstrates how progressively more advanced risk characterization methods (e.g., monitoring, modelling) are considered commensurate with the level of risk. Simple methods including mapping of reported flooding to identify areas of interest for no-regrets initiatives (policies and programs) to more advanced hydrodynamic modelling methods to support economic analysis and design/implementation of viable projects are shown.

An early example of layering of multiple risk factors related to construction practices (e.g., type of drain connections to the municipal sewer network), overland flow (pluvial) flooding risks, and storm sewer surcharge back-up potential is shown, i.e., the presentation author's Stratford City-wide Storm System Master Drainage Plan. The range of simple to advanced risk characterization methods that were combined in the overall system screening and prioritization are illustrated on the following slide:

urban flood risk mapping city of stratford vulnerability assessment
Urban Flood Risk Mapping - City of Stratford City-wide Storm System Master Plan. Dillon Consulting Limited. 

Several recent examples of multiple risk factor screening are shown in a recent blog post - the following map illustrates how era of construction (design standards inferred from dwelling age), topographic risk factors like catchment slope, overland flow path design (i.e., pluvial flooding risk) and reported historical flooding are related in a north Toronto neighbourhood:
Toronto urban flood risk mapping
Era of Dwelling Construction, Overland Flow and Catchment Slope, and Flood Report History - Risk Factors Affecting Reported Basement Flooding During Extreme Rainfall Events, City of Toronto Flood Reports.
While there are numerous examples where the risk factors explain the observed flooding, there are equally as many examples of risk factors not explaining the observed flooding. So mapped risk factors can explain overall trends, however there is considerable scatter in the data meaning a high degree on uncertainty when it comes to defining actions required to address priority flood risk reduction measures. As a result, local, detained risk assessments as part of comprehensive studies are required to support and infrastructure investments after areas of interest are screened though high level vulnerability assessment.

A holistic process of tiered flood risk vulnerability assessment to identify no-regrets, low-cost policies and programs (i.e., best practices) and then, commensurate with risks, more advanced assessments to define capital projects is shown in the following slide from the presentation above.

Urban flood risk evaluation framework, tiered vulnerability assessment, risk mapping
Process for Defining Policies, Programs and Projects for Urban Flood Risk Reduction Including Tiered Vulnerability Assessment (Risk Mapping).

Some 'best practices' can be identified with high level, simple to intermediate risk screening as shown below:

best practices for urban flood risk reduction, no-regret, low cost policies and programs
Defining No-regret, Low-Cost, Practically-Deployed Policies and Programs  for Urban Flood Risk Reduction With Simple and Intermediate Vulnerability Assessment (Risk Mapping).
When considering sanitary / wastewater collection systems, this holistic process for assessing risk and defining policies, programs and projects is illustrated below, including example risk factor thresholds that may be used to guide progression to more advanced tiers of assessment, and ultimately design and economic screening.

sanitary sewer risk assessment and urban flood risk reduction
Sanitary / Wastewater System Risk  Assessment Process to Implement Policies, Programs and Capital Projects - Simple to Advanced Risk Mapping and System Analysis to Prioritize Actions with an Urban Flood Risk Reduction Strategy 

Similarly, storm systems can be assessed in a holistic manner with progressively more and more advanced / detailed risk characterization.

storm sewer risk assessment and urban flood risk reduction
Sanitary / Wastewater System Risk  Assessment Process to Implement Policies, Programs and Capital Projects - Simple to Advanced Risk Mapping and System Analysis to Prioritize Actions with an Urban Flood Risk Reduction Strategy
For illustrative purposes, the City of Toronto financial screening threshold for basement flood mitigation projects is shown ($32k per benefiting property) to evaluate advanced evaluation projects. A benefit/cost of 2 is also shown, which is based on the eligible Disaster Mitigation Adaptation Fund project Return on Investment (ROI) threshold. Alternative thresholds for benefit/cost ratios (from under unity to 1.3) to support pubic investment in flood mitigation infrastructure are discussed by Watt in Hydrology of Floods in Canada.

A holistic approach to urban flood risk mitigation will focus on high risk areas and deliver risk reduction in a timely and cost-effective manner. Across Canada and Ontario, many communities were designed and constructed under design standards with limited flood resiliency compared to today's modern standards. The proportion of existing residential communities that have resiliency limitations can be estimated according to Statistics Canada data on dwelling construction date ("Dwelling Condition (4), Tenure (4), Period of Construction (12) and Structural Type of Dwelling (10) for Private Households of Canada, Provinces and Territories, Census Divisions and Census Subdivisions, 2016 Census"). Data tables for various geographies (i.e., Canada and Ontario), and for various ground dwelling types (i.e., single detached house, semi-detached house, row house, and other single attached house) have been used to estimate the proportion of residential development within various eras of construction to classify resiliency and risk mitigation needs. The graphs below illustrate the number and proportion of these types of dwelling construction in different construction eras based on Statistics Canada's 2016 Census data.

design standards and flood resiliency Ontario and Canada
Ground Dwelling Era of Construction - Ontario and Canada - Design Standards and Flood Resiliency. Dwelling Count and Cumulative Fraction of Dwellings Using Statistics Canada 2016 Census Data.

Ontario flood resiliency and adaptation priorities based on era of construction and design standards
Ground Dwelling Era of Construction - Ontario - Design Standards and Flood Resiliency. Dwelling Count and Cumulative Fraction of Dwellings Using Statistics Canada 2016 Census Data.

Pre-1990 construction accounts for about 65% of residential ground dwellings in Canada and in Ontario (i.e., excluding apartments). Generally, design standards after 1990 offer high resiliency (very low risk) such that risk mitigation through remediation is not a priority. Lower risks are expected in post-1980's construction where wastewater systems are fully separated (i.e., about 14% of Ontario construction), and moderate to high risks are expected in communities constructed before 1980. Tiered vulnerability assessments will typically begin with the 65% of pre-1990 areas, and progressively refine risks associated with systems within those areas.  To illustrate this, sanitary system upgrades to address flood risks in the City of Markham, determined after advanced risk assessments, may account for less that 2% of the total sanitary sewer length - modelling revealed that only 1.8% of sanitary maintenance holes exhibited surcharge during a 100-year event that would be considered a basement flooding risk.

Flooded Basement Cost in Canada - How Dwelling Size and Regional Differences Affect Cost Benefit Analysis and Return on Investment in Flood Risk Remediation Strategies

Flooded basements dominate natural hazard damages in many parts of Canada. The cost to lower flood risk varies considerably from several hundred to a few thousand dollars for simple lot-level best management practices, up to many tens of thousands of dollars per home for significant and complex infrastructure upgrades. Cost Benefit Analysis (CBA) can help guide what scale of risk reduction investment is appropriate given the implementation cost and the long term benefits of deferred damages over the lifecycle of the investment.  While CBA is uncommon in traditional municipal infrastructure planning, it has been applied in the past to identify municipal flood infrastructure priorities and is now a requirement for large scale projects under the new Disaster Mitigation Adaptation Fund. This blog post explores basement flood cost estimates that may be used to in CBA to help develop flood mitigation strategies and prioritize cost-effective best practices, programs and projects that deliver timely risk reduction.

The Insurance Bureau of Canada (IBC) has identified the cost of a flooded basement in Canada to be $40,000 - this has been described as an "average cost". This is an important number in CBA since it drives the value of deferred damages associated with flood risk reduction best practices.  A recent report "Blueprints for Action, Minimizing Homeowner Flood Risk in the GTHA"
(July 2017) prepared by Civic Action, IBC, and the  Intact Centre on Climate Adaptation indicates that value reflects damages for two particular flood events, the including the Toronto area July 8, 2013 storm and the 2013 flooding in Alberta, which included extensive river flooding - the report indicates "Combined with flooding in Alberta that same year, affected homeowners faced
a $40,000 repair bill on average." and " The average cost of repairing basements damaged by flooding in Alberta and Toronto in 2013 was more than $40,000 for each affected homeowner." citing a CBC news article in which the Intact Centre on Climate Adaptation indicates "The average cost of restoring water-logged basements in Alberta and Toronto in 2013 was more than $40,000 for each homeowner".

Correspondence with IBC suggests the value of $40,000 reflects Greater Toronto Area (GTA) costs for 2013 flooding, and not Alberta basements as noted in the Blueprints for Action report, and that values have been 'rounded up'. IBC also noted Intact Centre on Climate Adaptation relies upon US data in Forbes: "More specifically, according to the National Flood Insurance Program in the United States, a 15-centimeter flood in a 2,000-square-foot home is likely to cause about $40,000 in damage”  (Flood Insurance: Protection Against Storm Surge. 2012).

Let's review this basement damage cost and also consider how it can be used in cost benefit analysis that must take into account the frequency of these damages, that is, the return period or probability of such damages occurring.

First, what geography does the $40,000 damage value apply to? That is unclear and seems to grow over time:
i) The July 2017 Blueprint for Action report suggests it applies to the Toronto area and Alberta,
ii) A May 2017 Global News article suggests it applies to all major cities according the the Intact Centre on Climate Adaptation: "A flooded basement costs an average of $42,000 in major cities",
iii) A 2018 infographic by the Intact Centre on Climate Adaptation suggests that it applies more broadly to basements across Canada, not just major cities,

iv) Intact Centre on Climate Adaptation's presentation to the Standing Senate Committee on Energy, the Environment and Natural Resources Issue No. 38 - Evidence - February 8, 2018 (see transcript) notes ""This is very problematic, because the average cost of a flooded basement in the country right now, in urban and rural areas, is about $43,000.", expanding the damage estimate of "about" $43,000 to include rural areas too.
v) Shortly after the Intact Centre on Climate Adaptation appears to firm up the estimate on TVO's The Agenda, Assessing Ontario's Flood Risks, March 23, 2018 (see transcript) and notes "It's highly problematic. The average cost of a flooded basement in Canada right now is $43,000."

So the estimate has changed from characterizing Toronto and Alberta flooding in 2013 extreme events to applying to "major cities", then more widely to "urban and rural areas", and has changed from an estimate to a more definitive "average cost" across Canada.

How do the flooded basement damage values compare to other sources?

KPMG's report "Water Damage Risk and Canadian Property Insurance Pricing" (2014) for the Canadian Institute of Actuaries summarizes water damage trends from Aviva Canada:

"In a media release dated April 10, 2013, Aviva stated: Approximately 40 per cent of all home insurance claims are the result of water damage . . . and the average cost of water damage claims rose 117%, from $7,192 in 2002 to over $15,500 in 2012, a year in which the company
paid out over $111 million in property water damage claims. (source)

An older media release in early 2011 also highlights the concern: “Aviva Canada’s data found that
in 2000, the average cost of a water damage claim was $5,423. In 2010, it was over $14,000 – an
increase of nearly 160 percent”. (source)"

Aviva Canada has also commented that average flooded basement costs has increased in a Canadian Underwriter article stating "The average cost per residential water damage claim has increased significantly – going from $11,709 in 2004 to $16,070 in 2014, a 37% increase.". These values may reflect damages in years that did not have widespread record-breaking flood events like 2013, characteristic of more average or typical low-flooding years.

So average water damage claims at Aviva Canada have been given as:

$5,423 in 2000
$7,192 in 2002
$11,709 in 2004
over $14,000 in 2010
over $15,500 in 2012, and
$16,070 in 2014.

Why are 2013 claims cited by IBC so high? The KPMG report offers some insight:

"... good practice for property pricing requires that actuaries have the ability to link claims data with detailed exposure data. Thus, actuaries require accurate cause of-loss coding for all property claims. This coding is particularly important following the occurrence of major events such as the Alberta and Toronto floods of 2013. Many losses arising from the Alberta floods, in particular, were covered by insurers as a goodwill measure and to enhance the long-term relationship with customers and not because the peril of water damage was covered in the insureds’ policies."

So 2013 damage values include perils that were not covered, e.g., overland flooding. Why else are 2013 claims cited by IBC so high? Because of the severity of the 2013 storm event in the GTA. The chart below shows 3-hour rainfall totals in comparison to return period totals in west Toronto and Mississauga. The Gauge 1, 2 and 3 3-hour rainfall depths of 138 mm, 121 mm and 96 mm were identified by Toronto Water in a ICLR Friday Forum workshop February 19, 2016 entitled 'Reducing flood risk in Toronto' - see slide 19 in the presentation.
July 8, 2013 extreme rainfall frequency analysis for basement flood damage estimation and flood risk mitigation strategy development
July 8, 2013 Extreme Rainfall Frequency Analysis and Comparison to Design Rainfall Intensities
An excerpt from the Toronto Water presentation is shown to the right showing the location of the 3 gauges that recorded over 100-year storm amounts.

IDF data for the Toronto City (Bloor Street) gauge are used in the frequency analysis chart above. The version 2.3 datasets with 61 years of record have been extended to include 2017 data obtained from Environment and Climate Change Canada, extending the record to 70 years. Frequency analysis on the extended dataset using a the Gumbel distribution was completed by the City of Markham who uses this gauge in its stormwater management and engineering design guidelines and standards. Frequency analysis was also extended for the Pearson Airport gauge. A summary table showing the updated IDF values, including the 3-hour duration rainfall depths in the chart is shown below.

For large portions of the GTA where flooding was concentrated, the observed rainfall amount exceeded the 100-year design rainfall and at two rain gauges even exceeded the 2000-year (not 200, 2000!) rainfall statistic. So 2013 was certainly not an average year for basement flooding.

Another reason that the IBC flood damage value may be so high is that it has been vetted by comparing to much larger homes in the US. While the damage for a 2000 square foot home in the US may equate to $40,000, the most flood-prone homes in Canada are much smaller. Most flooding in Canadian cities occurs in older subdivisions build before modern drainage and wastewater servicing design practices, in general, before 1980 - a previous post shows this quantitatively. A Globe and Mail article quotes M Hanson Advisers, a U.S. research firm that caters to institutional investors, indicating "In 1975, the average size of a house in Canada was 1,050 square feet." - this is about half of the comparative house size used to vet the $40,000 damage estimate. The average claim-count-weighted US flood damage is much higher than the Canadian average event claims, perhaps reflecting the severe nature of hurricane event damage as explored in a previous post that evaluated FEMA flood damage payouts - our analysis generated inflation-adjusted claim-count-weighted average payout of $60,600, considering 116 events between 1978 and 2017.

So IBC has identified a 'rounded-up' basement flood damage cost associated with a very extreme rainfall event in the GTA and that considers extensive riverine flooding in Alberta where payouts appear to have been for uninsured perils as a means of goodwill and client retention. The Aviva Canada reported average claims suggest a lower water damage amount in average years without the unique 2013 considerations in the GTA and Alberta.

What flooded basement damage amount should be considered in deriving deferred damage benefits and in return on investment (ROI) calculations for flood remediation projects? Yes, that was how this post started. Such calculations can take two approaches, one a top-down aggregation approach to guide long-term flood remediation program spending, and another bottom-up property-by-property approach at that recognizes variability in individual property risk.

Stay tuned for our economic model of flood damages and remediation strategies!

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

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.