The report relied on input including this blog post prepared for the Intact Centre authors' consideration: Reducing Flood Risk from Flood Plain to Floor Drain Developing a Canadian Standard for Design Standard Adaptation in Existing Communities https://www.cityfloodmap.com/2018/02/reducing-flood-risk-from-flood-plain-to.html
(e.g., as noted in the March 2018 draft of the Intact Centre report)
The Intact Centre Weathering the Storms report has some gaps regarding the key question of the role of green infrastructure, natural infrastructure and engineered low impact development practices, in terms of mitigating existing urban flood risks. In other reports, Intact Centre has advocated strongly for green infrastructure as a core flood risk mitigation measure - as in the 2018 report Combating Canada’s Rising Flood Costs: Natural infrastructure is an underutilized option, co-written with IBC and IISD which stated (page 4):
“Nature conservation and climate resilience go hand in hand,” said Craig Stewart, Vice-President, Federal Affairs, IBC. “This report emphasizes that coastal and inland flood risk can be reduced by conserving and restoring natural infrastructure, such as wetlands and coastal marshes, and that the return on investment of natural infrastructure can at times exceed that of built infrastructure, such as dams and dikes. Nature can be our best friend in lowering the risk of exposed communities.”
That Combating Canada's Rising Flood Cost report also promoted "green infrastructure" measures like open creeks, natural ponds and underground storage tanks to mitigate existing flood risks saying (see page 4):
"Fortunately, as documented in this report, flood risk can be limited through conservation and restoration of natural infrastructure features, such as ponds, wetlands and vegetated areas. This report demonstrates how to quantify the benefits and costs of these natural features as a strong complement or a viable alternative to grey infrastructure option for flood mitigation."
"natural infrastructure merits consideration alongside grey infrastructure solutions as a means of limiting flood risk across all levels of government and all jurisdictions"
Unfortunately, the Combating Canada's Rising Flood Cost report had many gaps in terms of the rigour in which flood benefits were assessed in case studies, such that analysis was characterized as "number stretching" and "concept massaging" in a review in the Financial Post (see Terence Corcoran article here). My review of those case studies agrees with the Financial Post assessment, since engineering and economic analysis was lacking - for example, one-time capital cost differences were reported as recurring annual ecosystem services for one case study (Oakville natural channel), local flood damages were replaced with higher, arbitrary 'meta-analysis' values for another (Manitoba wetland). In addition, the report's key case study (Brampton Metrolinx parking lot) misclassified a conventional grey infrastructure storage tank as green infrastructure, and relied on arbitrary river flood damage values a tributary with no actual river flood damage risks. A review of these and other case studies was in my 2018 CWWA annual conference presentation - see presentation here.
And in a recent report, TOO SMALL TO FAIL: Protecting Canadian Communities from Floods, Intact Centre notes "Flood Risk Mitigation Projects" include rain gardens, bioretention, and permeable pavement and that these are practical and can be applied nationally calling these (page 13):
"practical adaptation solutions that could be replicated in communities across Canada to limit flood risk at a local level"
That earlier report concludes that (page 38):
"If future projects are deployed on scale, the additive benefits could materially reduce the costs of flooding at the provincial and federal level."
But, despite the strong promotion in earlier reports, the new Weathering the Storm report only weakly advocates for natural infrastructure preservation and low impact development practice (green infrastructure) consideration alongside grey infrastructure to address overland flooding, and the cost-effectiveness is left up in the air. The new report notes (see page 34):
"Maintain natural infrastructure (e.g., wetlands and watercourse corridors) and consider low impact development practices to complement grey infrastructure solutions for stormwater management"
The report notes the Capital Cost is "Med" (i.e., medium) and the Ease of Implementation is "Moderate". This is misleading as it conflates two completely different strategies. Maintaining wetlands and natural corridors in existing communities has no capital cost, not medium capital cost, and is certainly an effective damage mitigation planning policy. Historically, it has been proven that Ontario's floodplain management policies have lowered flood damages relative to other jurisdictions that do not have proactive management and preservation of valley corridors (see example comparison with Michigan). But low impact development practices have a very high capital cost and a very high operation and maintenance cost too. To put a range of risk management measure capital costs into perspective we can look at Markham's '"green" land use policies, lot-level best practices and programs, and its sewer capacity upgrade programs (grey infrastructure), and compare those unit costs to low impact development (green infrastructure) unit costs (updated 2019-01-26):
Preserving Wetlands and Watercourse Corridors: $ 0 / hectare (lands are dedicated to City)
Sanitary Downspout Disconnection & Backwater Valve Subsidy Program: $ 1,300 / hectare
Sanitary Sewer Capacity Upgrades: $ 11,000 / hectare
Storm Sewer Capacity Upgrades: $ 120,000 / hectare
Low Impact Development / Green Infrastructure Retrofits: $726,000 / hectare
So the Intact Centre report mixes a no cost land use policy with a very high cost physical intervention, and averages a medium cost. This can only create confusion as to what types of green infrastructure are truly cost-effective. While earlier reports cite specific practices as flood control measures, e.g., with the Too Small to Fail report identifies i) Mississauga's water quality bioswales and permeable pavement, ii) Dionis-Désilets retention basin vegetation planting, and iii) Halifax St Mary's Boat Club shoreline stabilization, the Intact Centre Weathering the Storm report does not even define what low impact development practices it has considered, and only refers to green infrastructure once in a case study (see page 29), however that was in the context of non-flood considerations of "drinking water supply, water quality, soil erosion, water balance, and overall public well-being". Hopefully there can be clarifications in follow-up work.
As the new Intact Centre report applies to "Existing Communities" it is unclear if "Maintain natural infrastructure" reduces risk at all or just maintains existing risk. The title to Section 4.4 Selected Physical Interventions to Reduce Flood Risk, suggests reduction was intended.
Comments shared by those in the acknowledgement list for the report included the need to downplay the role of low impact development measures, and the need to consider cost effectiveness of measures and not just cost. Those comments do not seem to be reflected in the report but the limitation is acknowledged that (page 33) "Further, capital cost rankings are not normalized with
consideration of performance effectiveness." - this is in contrast with recent Intact Centre reports that make strong conclusions on cost-effectiveness, and relative effectiveness compared to conventinal practices - By including low impact development measures as a medium cost flood mitigation measure, the Intact Centre report appears to support earlier Intact Centre reports (like Too Small to Fail) that had essentially "reclassified" water quality improvement projects, such as Mississauga's Lakeview permeable pavement and bioretention units, as a flood mitigation projects. The earlier Intact Centre/IBC/IISD report Combating Canada's Rising Flood Costs, also "reclassified" a conventional grey infrastructure storage tank on a Metrolinx parking lot as a "green infrastructure" project. Clearly there is a need for more clarity on what low impact development practices or green infrastructure measures are, and what their individual costs are. Without this, it is challenging to determine what practices or measures are truly cost-effective and to what degree they should be considered in overall risk reduction strategies.
To help look at cost-effectiveness in this report, I had provided comments to the authors from August to October, 2018, however these were not accepted for consideration. This included earlier analysis on grey and green cost effectiveness that has been refined and is that is summarized below. This analysis could fill that gap left in the Intact Centre report, showing how considering low impact development to complement grey infrastructure would affect total cost and benefit/cost ratios for flood mitigation. In addition the effectiveness at achieving other watershed benefits such as water quality improvements and erosion mitigation can also be measured, considering the broader triple bottom line benefits of green infrastructure. This analysis summarized below is based on retrofitting Markham's pre-1980 service areas that are being evaluated as part of its city-wide Flood Control Program and its city-wide Wastewater Servicing Master Plan. The study area is equivalent to an area of approximately 2360 hectares, or 25% of the city's urban area. Further results of this analysis will be presented in the Spring 2019 Water Environment of Ontario and conference and the 2019 TRIECA conference.
The table below illustrates a range of flood mitigation strategies within 25% of the pre-1980 urban area:
Strategy A - 100% grey infrastructure,
Strategy B - 100% green infrastructure, and
Strategy C - 90% grey + 10% green infrastructure,
The table includes the benefit/cost ratios associated with each strategy. Ratios are shown for flood damage mitigation, water quality improvement and erosion repair mitigation. Contact me if you would like to see the detailed calculations behind these values, or wait for distribution of the WEAO conference paper.
"Increase the size of deficient storm and sanitary sewers to allow for additional conveyance capacity"
has a "High" capital cost and that the ease of implementation is "Complex". In fact the capital cost and lifecycle cost of grey infrastructure are significantly lower than that of green infrastructure / low impact development practices. Further, the cost-effectiveness to provide flood damage reduction, shown in the benefit/cost ratio, is high which implies a low cost for the performance achieved.
The following chart shows how incorporating a range of green infrastructure (between Scenario A and Scenario B) affects total costs and benefit/cost ratios. In brief, given the high cost of green infrastructure, implementation of any amount in a strategy only lowers the overall cost-effectiveness (lower benefit/cost ratio). While some water quality improvement and erosion mitigation benefits accrue as more green infrastructure is implemented, the triple bottom line benefits do not offset the additional costs. This holds true even with generous water quality benefits that consider an intangible 'willingness to pay' for such improvements.
|Correction Note: Scenario B (25% City-wide Green Infrastructure) corresponds to third row in table above (i.e., Scenario C)|
It is a long-standing principle that flood mitigation measures be cost effective, and that benefits of deferred damages exceed project costs. Watt promotes this in Hydrology of Floods in Canada and Eckstein has done so since his founding work at Harvard in the 1950's. Maximizing benefits requires that incremental measures in a strategy proceed only where incremental benefits exceed incremental costs. Given that most urban flood reduction strategies are founded on proven, low cost engineering methods (i.e., grey infrastructure capacity upgrades), any further consideration of green infrastructure would have to demonstrate further incremental benefits exceeding cost - this consideration does not appear to be justifiable given the unit cost for implementation, especially within a strategy that has already reduced damages with core measures, making further incremental benefits harder to realize. The analysis above shows that even adding 10% green infrastructure to a strategy (10% of pre-1980's urban areas, which is only 2.5% of total urban areas) drops cost-effectiveness for flood control by over two thirds. Given the limited resources available for infrastructure upgrades to address flood risks, and most likely some funding limits, it would appear to unwise to limit total flood mitigation benefits that could be achieved by considering green infrastructure in the strategy. Keeping spending constant, 25% of a city could be retrofitted with Strategy A - 100% Grey Infrastructure, while only 8% of that city could be retrofitted with Strategy C - 90% Grey Infrastructure + 10% Green Infrastructure with similar funding. Higher costs with Strategy B - 100% Green Infrastructure would allow only 1% of urban areas to be retrofitted -note this reflects operation and maintenance costs as well as capital costs.
The Intact Centre Weathering the Storm report provides some very limited estimates of green infrastructure cost-effectiveness, and implies a favourable benefit/cost performance. But the report fails to identify the flood benefits relative to other benefits, and it ignores local flood damage data in published research, substituting instead IBC/Intact Centre/IISD's 'meta-analysis' presented in the Combating Rising Flood Cost report. The Weathering the Storm report states (page 42):
"Performance monitoring results: In 2017, University of Saskatchewan researchers assessed the net economic benefits of Pelly’s Lake project at $3,700,148 CAD, assuming a 20-year life cycle and 3% discount rate. This assessment reflected the value of flood attenuation, nutrient load reduction (phosphorus and nitrogen) and carbon dioxide offset benefits."