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:

Urban Flood Risk Mapping - Tiered Vulnerability Assessment in Risk Mitigation Frameworks for Existing Communities from Robert Muir

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 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:

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.

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:

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 / 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.

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.

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.

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.