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.
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
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
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.

Basement Underpinning and Sewer Back-up Risks - How Lowering Basements Increases Flood Damage Potential in Canadian Cities Undergoing Intensification

Underpinned Basement - Typically Basemnent Floors Are
Lowered By 2 to 2 and a half feet (60 to 75 cm).
Many explanations for flood damage losses in Canadian cities have been identified - these include urbanization that extended new development areas and intensification within existing development areas, sometimes with limited runoff control. But another type of development can have a clear impact on basement flooding, or sewer back-up risk, and that is the lowering of basements. As property  homeowners strive to gain the most out of their living spaces in older urban areas where basement heights were often limited, underpinning of foundations can support the lowering of the floor slab to increase headroom and maximize the use of a finished basement.

But what does lowering the floor slab do to flood risks? Essentially it reduces the safety factor against sewer back-up but putting the floor and finished contents closer to the elevation of the municipal sewer in the street. In older areas, municipal sewer can be prone to surcharge during extreme rain events, such that sewage and extraneous water rise up well above normal flow levels. So lowering a basement reduces the 'freeboard', or buffer, between a home's valuables and the the level of wastewater in the municipal collection system. Typically a basement is lowered by 2 to 2 1/2 feet (i.e., 60 to 75 centimetres) to give a finished basement height of 8 feet (2.4 metres).

Toronto Open Data provides statistics on building permits and description of works that may include underpinning. As house prices have increased in Toronto, homeowners are clearly motivated to increase the amenity value of their basements, whether as second suites or elaborate "man-caves". Trends from 2001 to 2017 are shown below. Several hundred more permits were issues for projects including 'benching' but are not shown. Overall the number of lowered basements increased consistently from less than 200 properties in 2001 to over 1700 properties per year in 2017 - in total 14,000 properties were lowered (adding benching projects to underpinning projects shown).

Toronto Basement Underpinning Permits 2001 to 2017 - Lower Basements Can Result in Higher Flood Risks 

The following cross sections illustrate how lowering a basement reduces the 'freeboard' safety factor, or clearance, between the finished basement floor and the municipal sewer system.


Where sewer elevations are not favourable, ejector pumps for sewage may be proposed which would create a good hydraulic break between the municipal sewer and the lowered / underpinned basement.

In any case, where a basement is lowered, especially when the new space is then finished and filled with valuable contents, a backwater valve should be installed to reduce back-up risk. This can only be considered where downspouts and weeping tiles drains (foundation drains) are separated from the sanitary lines, to avoid flooding the home upstream of the backwater valve with local drainage.

Decrease in Southern Ontario Design Rainfall IDF Curves Matches Trends in Observed Storms - Decrease in Both Frequent and Rare Short Duration Intensities - Overall Decrease in Small Storms, Large Storms Mixed

Are Ontario Rainfall Trends a Nothing-Burger?
Read This Post and Find Out !
Previous posts reviewed trends in observed maximum series of observed rainfall, showing more decreasing trends in Southern Ontario than increasing trends (see post). That observed trend analysis is part of  Environment and Climate Change Canada's Engineering Climate Datasets, Version 2.3. Design rainfall intensities are derived from these observations to create intensity-duration-frequency (IDF) curves, by fitting a probability distribution to the observations. A sample of the change in design intensities over time was presented at the National Research Council's February 27, 2018 Workshop on adaptation to climate change impact on Urban / rural storm flooding  (see slides 9 and 10):

The sample IDF review showed no change in 2-year to 10-year return period intensities over durations of 5-minutes to 2 hours. The slide content was also featured in a previous post which includes links to the earlier 1990 datasets used in the comparison (for those who have thrown out those old 5 1/4 inch floppy disks with the 1990 data).

This post shows the change in IDF values for these Southern Ontario climate stations for all durations and all return periods. The chart below summarizes the change in IDF values for the 21 stations, each with 30 years of record or more. It shows the range in IDF change for each return period, across all durations. The changes for each station have been weighted by the duration of the climate station record, so that a station with a record of 60 years is given double the weight of a station with 30 years of record.

Ontario IDF Trends for Extreme Rainfall Climate Change Effects
Southern Ontario IDF Trends - Decreasing Frequent Storm Intensity, Mixed Infrequent Storm Intensity, Overall Decrease in Average Rainfall Intensity Values for Engineering Design. 5-Minute to 24-Hour Durations.
Looking into the details, the next chart shows the change in rainfall intensity for each duration within each return period as well.

Ontario IDF Trends for Extreme Rainfall Climate Change Effects Details
Southern Ontario IDF Trends - Decreasing Short Duration Storm Intensity (5 minutes - dark red bars), Decreasing Moderate Duration Storm Intensity (1-2 hours - green bars), Negligible Change in Long Duration Storm Intensity (12-24 hours - dark blue and purple bars).
The take-aways from the IDF update comparison :

i) small frequent storms (2-year, 5-year, 10-year return periods) used to design storm sewers, for example, are consistently smaller now than in the 1990 dataset,

ii) large infrequent storms (25-year, 50-year, 100-year return periods) used to design major drainage systems and infrastructure networks are mixed with some increases and some decreases since 1990 but no appreciable change that would affect design (any changes are less than 1%, which is negligible in engineering design),

ii) there is an overall average decrease in IDF values of 0.2 % across all return periods and durations.

Percentage IDF change values shown in the detailed chart are summarized in the following table for 5-minute, 10-minute, 15-minute, 30-minute, 1-hour, 2-hour, 6-hour, 12-hour and 24-hour durations, and for 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year return periods.

Ontario IDF Update Trends in Rainfall Intensity and Frequency
Southern Ontario Rainfall IDF Trends From 1990 to Current Version 2.3 Engineering Climate Datasets (Average Values for 21 Long-Term Climate Stations Below 44 Degrees Latitude - Individual Station Percentage Changes Factored by Length of Climate Station Record).
Percentage IDF change values for 'unweighted' station changes (i.e., short records are given the same weight as long records) are summarized in the following table - same overall pattern as the record-length-weighted table above.

Ontario IDF Update Trends in Rainfall Intensity and Frequency Unweighted
Southern Ontario Rainfall IDF Trends From 1990 to Current Version 2.3 Engineering Climate Datasets (Average Values for 21 Long-Term Climate Stations Below 44 Degrees Latitude - Individual Station Percentage Changes Factored by Length of Climate Station Record).
So what are we to make of this? The media, the insurance industry, and those who are exercising their 'availability' bias instead of looking at storm statistics, have regularly reported that storms are bigger, or more frequent, or both, but the local Ontario data shows the opposite (Northern Ontario will be a different story as AMS trends were up in the north, unlike the south). The Ontario government is website is even out of step wit hthe data.

The new Progressive Conservative government in Ontario has just renamed the Ministry of Environment and Climate Change the Ministry of the Environment, Conservation and Parks, taking out 'climate change', but the content under it has not been updated.

Ontario Ministry of the Environment, Conservation and Parks replaces former Ministry of the Environment and Climate Change. New name but content still reflects climate change effects on storms that is inconsistent with data.
If we look at rainfall trends in Southern Ontario it would seem appropriate to now de-emphasize the change in 'climate' or, regarding storms, the change in weather statistics. The current "MOECP" website reflects the earlier MOECC, and indicates that climate change has caused extreme weather issues in the province.

Ministry of the Environment and Climate Change website links extreme weather with climate change.
Specifically, the website indicated (as of July 2, 2018):
"It damages your property and raises insurance premiums:
  • the severe ice storm in December 2013 resulted in $200 million of property damage in OntarioToronto lost an estimated 20% of its tree canopy during the storm
  • Intact Financial, one of Canada's largest property insurers, is raising premiums by as much as 15-20% to deal with the added costs of weather-related property damage
  • Thunder Bay declared a state of emergency in May 2012 after being hit by a series of thunderstorms, flooding basements of homes and businesses due to overwhelmed sewer and storm water system"
While we cannot comment on ice storms, the official datasets for rain storms show no change, and therefore raised insurance premiums must be due to other factors instead of climate change. Blog readers will point to our review of  urbanization, intensification, etc. as a key cause.

KPMG has also commented in "Water Damage Risk and Canadian Property Insurance Pricing" (2014) for the Canadian Institute of Actuaries that prior to 2013, flood insurance pricing was inadequate, so the 15-20% increase by Intact Financial is just catching up to the market pricing for that service. It also reflects the higher value of contents and finishing of basements that are flooded / damaged during extreme weather.

City of Waterloo Flood Risk Factors - Historical Design of Sanitary Sewer and Overland Flow Paths Help Define Neighbourhood-scale Flooding Risk During Extreme Rainfall

This post summarizes risk factors affecting urban flooding and explores the example of flood risk in the City of Waterloo, Ontario.

Two key factors explain basement flooding risks in many urban areas:

1) sanitary sewer design practices, and
2) overland flow design practices.


Virtually all urban properties have gravity-drained sanitary sewer connections to the municipal sanitary sewer systems, and this collects wastewater from homes as well as infiltrated groundwater from foundation drains in most pre-1980 areas and occasionally direct rain and melt water inflows thorough illicit collections to the home plumbing and drainage systems and ultimately the municipal sanitary sewer system.  Because of this connection, any surcharging of municipal sanitary sewer systems during extreme weather can back-up into low-lying floor drains, flooding basements.

So the capacity of the municipal sanitary sewer system will partially define basement flooding risk. Design standards in Canada have evolved over time as described in a previous post. While each municipality is a little different, we can consider 1975 as a year in which systems became fully-separated, with no more foundation drain connections that serve to overwhelm the system with infiltration and, more importantly, provide a pathway for illicit inflow connections, like from rooftops or other property drains (in York Region we once even found an outside kitchen sink connected to foundation drains - it was near the garden and used to rinse vegetables!).

Overland drainage began being considered in urban drainage design in the late 1970's - the former Town of Markham's design standards recognized overland 'major' system design requirements in 1978, under the guidance of University of Ottawa's Dr. Paul Wisner. Many other municipalities in Canada adopted dual minor-sewer/major-overland drainage design standards throughout the 1980's. Historical development grading and old subdivisions that did not integrate overland flow are prone to flood stresses due to i) water entry into building openings via windows, doorways, recessed walkouts/stairs, and reverse-sloped driveways, ii) storm sewer surcharge that backs up into foundation drains and through basement walls and under flood slabs, and iii) sanitary inflows into maintenance hole lids (e.g., at roadway locations with deep ponding over the lids pick-holes and edge). The insurance industry refers to overland flooding pluvial flooding, an unheard of term in Canadian engineering design (this is to distinguish between urban overland flooding and 'fluvial' riverine flooding that occurs in valleys).

Show me !

The City of Waterloo has an extensive Open Data portal that includes information on sanitary sewer installation date. This GIS data has been used to characterize neighbourhood flood risk according to era of construction and engineering design practices.

Overland flow risks can be mapped in many ways with increasing complexity on aspects of:

i) Input Data - e.g., elevation model detail and conditioning as input to the hydrologic and hydraulic analyses can be based on coarse provincial datasets (raster cell sizes suitable for macro-scale neighbourhood assessments), local datasets such as detailed 3D breaklines used for other image rectification (raster cell size of a metre or two for master drainage planning), to LiDAR datasets (to generate sub-metre cell size for fine-scale lot-by-lot, or gutter-by-gutter analyses),

ii) Defining Risk Zones / Hazard Area - e.g., this can involve the simple delineation of flow accumulation paths and definition of sinks (ponding areas), to setting of buffers around flow paths based on drainage area size (a surrogates for hydrology and hydraulics but good for screening), or more advanced flow spread calculations (i.e., applying hydrologic and hydraulic principles) to identify risk zones.

Data to the above can include province of Ontario processed topographic data (through Land Information Ontario (LIO)), including a conditioned elevation model and flow direction raster grid that has been used to map overland flow paths and spread across much of the province.


Simple Flow Path and Ponding (Sink) Delineation: My City-wide Storm System Master Plan for the City of Stratford in 2003 was one of the first applications of major drainage system / overland / pluvial flood risks using ESRI's Spatial Analyst and the emerging hydrology tools (that would later become the familiar ArcHydro tools), and first introduced by the University of Texas as an extension to ArcView 3. The following map illustrates the assessment of overland flow path drainage issues and ponding issues. No base data was available for the analysis and the elevation model was derived from half-metre AutoCAD contours to generate a 2-metre DEM raster for analysis. The integrated GIS-modelling approach was subsequently presented at the 2004 AWRA conference in Nashville, Tennessee.

Major Overland Pluvial Flood Risk
Stratford City-wide Storm System Master Plan - Major Overland Flow / Pluvial Flood Risks Based on GIS-based Flow Paths Delineation and Ponding Areas using ArcView GIS Spatial Analyst Extension.
Buffered Flow Paths and Ponding (Sink) Delineation: A similar approach was taken in Markham, Ontario in 2013 to conduct a screening-level identification of properties in close proximity to flow paths or within potential ponding areas. This was shown in a previous post. The images below illustrate some of the outcomes that were subsequently aggregated over catchments to identify areas for detailed study. In this example 3D breaklines from a recent orthophoto rectification were used to generate the DEM raster within the city - this was integrated with a more-coarse elevation model outside of the city boundaries to ensure a complete watershed delineation. The final DEM was refined after extensive manual editing of the 3D breaklines and reprocessing of overland flow paths and ponding areas/sinks.

Overland flow / pluvial flooding risk defined by buffers on overland flow path as a function of drainage area. 

Overland flow / pluvial flooding risk defined by buffers on overland flow path and ponding with building pluvial flooding risk risk estimated by proximity to flow buffer or to ponding area..
Hydrologic-Hydraulic-Based Overland Flow Paths: Analysis of City of Toronto overland flood risks was completed in 2015 using a pre-conditioned provincial DEM - as it is conditioned it cannot be used to generate ponding limits. Simplified rational method hydrology was applied considering individual cell-by-cell time-to-peak and individual 100-year design rainfall intensities, along with a standard runoff coefficient. Overland hydraulics to define flow spread were applied on a derived vector-based overland flow network that considered 100-year flow along each overland reach and flow spread defined by longitudinal slope and uniform flow conditions for a typical roadway cross section. The presentations below illustrates the overland flood hazard / flow spread that was then used to explain the location and density of reported basement flooding during recent extreme rainfall events.

Refined Hydrologic-Hydraulic-Based Overland Flow Paths: The Toronto-based overland risk mapping approach was refined using SOLRIS land use classification to derive cell-by-cell weighted rational method runoff coefficients, for a more precise hydrology. This was required as both rural and urban areas across south-west and central Ontario were assessed. The analysis was completed in 2016 as summarized in a previous post. The result is an overland drainage network with over 800,000 flow segments (reaches) with an individual 100-year design flow rate and flow spread. A snapshot of the analysis is shown below.
Ontario Overland Flow / Major Drainage / Pluvial Flood Risk Assessment

This last overland flood risk analysis approach is used to help assess City of Waterloo flood risks. The map below shows flow paths in the western part of the city and and highlights buildings (in red) that intersect the overland flow path - in this analysis flow paths with 3 hectares of contributing drainage area (i.e., 30,000 square metres or more) are shown. The presence of modern stormwater management and drainage design, as suggested by the municipal stormwater ponds in the western-most areas, would mitigate the possible impact of these overland flow paths by capturing and controlling the release of major flow during extreme events. In addition, modern minor systems in these modern, post-1980 subdivisions may be designed to capture and convey runoff generated by extreme rainfall.

City of Waterloo - Example Overland Flow Risk (Urban Major Drainage / Pluvial Flood Risk) - Buildings along Flow Path Highlighted (Surface Flooding and Sanitary Inflow Risk)
Multiples of the 100-year flow spread are shown for catchments of 3 to 1000 hectares. For larger areas, only the flow centreline is shown and those assessing valley-feature overland flood risk should refer to regulated floodplain limits that are determined through more advanced hydrologic and hydraulic analyses.

The next map shows installation date of sanitary sewers with pre-1975 sewers shown red (highest risk for infiltration and inflow stresses during extreme weather), 1975-1989 sewers shown in orange, and post-1990 sewers shown in green.
City of Waterloo - Sanitary Sewer Installation Date  (Inflow and Infiltration Risk) - Pre-1975 sewers (red), 1975-1989 sewers (orange), 1990 and newer sewers (green).
The map suggests that sanitary sewer replacement has occurred in the older core ares to the east (new green sewers surrounded by older red sewers).

This next map illustrates the intersection of overland flow path attributes onto sanitary sewer features that they intersect. Specifically the drainage area is assigned to each sewer segment it crosses and the sum of the intersected overland flow is aggregated to each segment and then weighted by the age of the sewer - post 1990 sewers have the area reduced by a factor of 5 considering modern drainage design and low infiltration and inflow stresses in modern fully-separated systems, while 1975 to 1990 sewers have the area sum divided by a factor of 2 considering lower fully-separated systems stresses. This is an approximate screening method, of course, but consistent with industry understanding of risk factors based on more detailed studies. The width of the red highlighting surrounding sanitary sewer segments illustrated thee age-factored sum of intersected flow area.

City of Waterloo - Overland Flow Impact on Sanitary Sewer Systems - Intersection of Major Drainage Flow Path Areas To Sanitary Sewer Segments, Factored by Age of Construction.

Red highlighted areas are or interest for further study. It is clear that in some core areas with predicted flood risks, sanitary sewer replacement has already occurred (i.e., newer green sewers in eastern areas), meaning that some flood risks may have already been mitigated.

The last map adds average age of dwelling construction in census areas. Clearly, the is a strong correlation to the sanitary sewer age risk factor and overland drainage design risk factor and the average age of construction. It is interesting to note that the broad, census-area neighbourhood risk does not account for local sanitary sewer upgrades, nor does it help identify individual properties that are at risk of significant overland flooding, as those buildings are isolated to the major overland flow path hazard area.

City of Waterloo - Urban Flood Risk Factors and Average Age of Dwelling Construction