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.

Why?

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.

Examples:

Simple Flow Path and Ponding (Sink) Delineation: My City-wide Storm System Master Plan for the City of Stratford in 2004 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.

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.

Toronto Overland Flow & Basement Flooding – ArcScene 3D Visualization of Fluvial, Pluvial and Sewer Flood Risk from Robert Muir

Urban Flood Risk from Flood Plains to Floor Drains from Robert Muir

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

Construction Era, Infrastructure Standards and Extreme Rainfall Flood Resiliency and Risk

Flooding occurs largely based on clear and quantifiable municipal infrastructure design standards that have evolved over the last century, steadily reducing flood risk. Certainly factors such as urbanization and intensification affect hydrology and increase runoff and risks, even when rainfall intensities have not changed (e.g., southern Ontario for example) - but fundamental design characteristics that may or may not account for extreme weather effects are the predominant factor affecting urban flood risk.

From Flood Plains to Floor Drains, my unifying theory of urban flood risk, described how planning and design practices have evolved within the realms of riverine flooding (i.e., flood plain management), pluvial flooding (i.e., major overland drainage design), and sewer back-ups (i.e., wastewater and stormwater sewer design). These systems may also interact during extreme weather through processes not explicitly considered in the design and that may accentuate core design limitations, or lower levels or service, in the related system. For example, flood plains may back up into sewer systems (e.g., Carp River, Ottawa or Etobicoke Creek, Toronto). Or overland drainage systems may overwhelm sanitary sewer systems with inflows ... insert your local 'Lost River' example here.

So what are the construction eras and infrastructure servicing standards that characterize extreme weather flood resiliency or risk? Here is an approximate grouping that I developed to support a white paper on Core Public Infrastructure knowledge gaps and research needs for the National Research Council last year. That work did not look into riverine systems but those are included here:

Servicing Era 1 - 1960 and before

Median Flood Risk = HIGH (4 out of 5)

Level of Service Profile:

• Riverine flood risks are not uncommon, unless structural controls have been put in place (e.g., dams, berms, etc) or land use planning has relocated original at-risk dwellings.
• Pluvial / overland flood risks exist as it was not a common design practice to accommodate major system flows during extreme rainfall events.
• Sewer back-up risks exist due to high extraneous flow stresses during extreme rainfall events in combined wastewater systems and partially-separated systems (i.e., with foundation drains / weeping tiles connected to the sewer system). Risks may increase if there is reliance on mechanical and electrical systems (e.g., pumping stations in the collection system) or may decrease if hydraulic relief is available through combined sewer overflows (CSOs) or storm sewer overflows (SSOs).

Other Considerations:

• CSO or SSO hydraulic relief may have a relatively-smaller moderate flood risk (3 out of 5)
• Systems serviced by pumping systems that have finite capacity or that are affected by flood plain encroachment may have a relatively-higher highest risk (5 out of 5).

Servicing Era 2 - 1960 to 1980

Level of Service Profile:

• Riverine flood risks vary overall according to natural hazards land use planning (provincial or local policies) and vary locally based on the spatial extent of flood risk mapping (i.e., have large drainage areas (up to about 125 hectares) been mapped or estimated including through urban areas.
• Pluvial / overland flood risks exist as it was not a common design practice to accommodate major system flows during extreme rainfall events.
• Sewer back-up risks exist due to high extraneous flow stresses during extreme rainfall events in partially-separated systems (i.e., with foundation drains / weeping tiles connected to the sewer system). Risks may increase if there is reliance on mechanical and electrical systems (e.g., pumping stations in the collection system).

Median Flood Risk = HIGHEST (5 out of 5)

Other Considerations:

• Systems with good overland catchment slopes may have a relatively-lower high flood risk, despite no explicit overland drainage system (4 out of 5).
• Systems serviced by pumping systems that have finite capacity or that are affected by flood plain encroachment may have a relatively-higher risk.

Servicing Era 3 - 1981 to 1990

Level of Service Profile:

• Riverine flood risks may be significantly reduced according to natural hazards land use planning (provincial or local policies) and can vary locally based on the spatial extent of flood risk mapping (i.e., have large drainage areas (up to about 125 hectares) been mapped or estimated including through urban areas.
• Pluvial / overland flood risks may exist from jurisdiction to jurisdiction as this design practice was introduced to accommodate major system flows during extreme rainfall events, often in combination with master drainage planning at the early land use planning stage.
• Sewer back-up risks are limited due to low extraneous flow stresses during extreme rainfall events in fully-separated systems (i.e., no foundation drains / weeping tiles connected to the sewer system). Risks may increase if there is reliance on mechanical and electrical systems (e.g., pumping stations in the collection system).

Median Flood Risk = MODERATE (3 out of 5)

Other Considerations:

• Systems with good overland catchment slopes, often through explicit dual-drainage design for major system design, may have a relatively-lower low flood risk (2 out of 5).
• Systems serviced by pumping systems that have finite capacity or that are affected by flood plain encroachment may have a relatively-higher highest risk (4 out of 5).

Servicing Era 4 - 1990 to today

Median Flood Risk = LOW (2 out of 5)

Level of Service Profile:

• Riverine flood risks are typically significantly reduced according to natural hazards land use planning (provincial or local policies) . Local and downstream risks may be reduced through integrated land use and watershed/subwatershed planning.
• Pluvial / overland flood risks are limited where the dual-drainage design practice was introduced to accommodate major system flows during extreme rainfall events, often in combination with master environmental servicing (including drainage) planning at the early land use planning stage.
• Sewer back-up risks are limited due to low extraneous flow stresses during extreme rainfall events in fully-separated systems (i.e., no foundation drains / weeping tiles connected to the sewer system). Risks may increase if there is reliance on mechanical and electrical systems (e.g., pumping stations in the collection system).

Median Flood Risk = MODERATE (3 out of 5)

Other Considerations:

• Systems with no explicit dual-drainage design for major system design may have a relatively-higher high flood risk (4 out of 5).
• Systems serviced by pumping systems that have finite capacity or that are affected by flood plain encroachment may have a relatively-higher highest risk (4 out of 5).

Servicing Era 4 Plus - Added Enhanced Best Practices to Servicing Era 4

Median Flood Risk = LOWEST (1 out of 5)

OK, so what are the enhanced best practices that create Era 4+ ? Basically take the good practices in Era 4 and add measures that provide enhanced resiliency in each of the realms.

Enhancements:

• Riverine flood risk reduced through adoption of higher return period events (e.g., above 100-year level of service) or significant freeboard allowances (safety factors in design).
• Pluvial / overland risk reduced through provision of adequate freeboard on major drainage system to prevent entry to properties.
• Sewer back-up risks reduced through mandatory plumbing system isolation (backwater valves and sump pumps), or robust hydraulic design of gravity systems to consider extreme rainfall stresses above 100-year level (e.g., including future projected rainfall intensities) and to consider freeboard to basement systems during extreme events, and inlet control devices to limit storm sewer system surcharge.

A longer list of enhanced-level best practices is found in the Intact Centre on Climate Adaptation seed document on Best Practices for New communities:

http://www.intactcentreclimateadaptation.ca/wp-content/uploads/2017/09/Preventing-Disaster-Before-It-Strikes.pdf

So how do we know these Servicing Eras are relevant and really do affect flood risk? By using data to track reported flooding and correlating the flood density to the servicing era or the characteristic within the servicing era. An example of this is my assessment of overland flow characteristics, and catchment slope characteristics on reported Toronto flood density:

https://www.slideshare.net/RobertMuir3/urban-flood-risk-from-flood-plains-to-floor-drains

And reviews of flood density in Toronto based on era of construction (I used watermain installation date as a surrogate) (see slide 36):

https://www.slideshare.net/RobertMuir3/infrastructure-resiliency-and-adaptation-for-climate-change-and-todays-extremes

Or in Markham (see slides 39 and 40) in the above link (I used storm sewer installation date).

So what can we expect when we look at the flood risk profile across Canadian cities based on these Servicing Eras? First, we can expect to see a vast variation from city to city based on its growth and servicing history. Using Statistics Canada data on from the 2016 census for Ontario, we can use the date of housing construction as a surrogate for the date of municipal servicing - this approach has limitations because housing may be in place before servicing in isolated cases (i.e., servicing is newer than the housing), and servicing may be upgraded over time (i.e., capacity upgrades to original servicing). The graph below looks at housing/construction eras for census metropolitan areas (CMAs) or smaller geographic units.

We see that areas with more growth (e.g., Milton, Barrie) have less than half the proportion of high and highest risk housing stock (Eras 1 and 2) compared to other low growth areas (e.g., Sudbury, Belleville). The Milton breakdown is shown on the left. Milton has over 70% Era 4 resilient housing built and serviced after 1991 (green shaded slices of the pie), whereas Sudbury has less than 20% of housing in the group. The Sudbury breakdown is also shown on the left.

Milton shows significant growth in the 2001-2005, 2006-2010 and 2011-2016 periods once water and wastewater servicing was available to this part of the Halton Region (previously well water supply limited growth). In contrast, Sudbury shows limited growth post 2006, reflecting perhaps a slowdown in the resources/mining sector following the 2008-2009 recession.

The bar chart totals were for some entire census areas encompassing several municipalities (e.g., the Toronto CMA includes may municipalities including in York Region, Peel Region and Halton Region). Within the CMA or municipalities themselves, the infrastructure construction era will also vary (see Toronto watermain installation date in the slide noted above). For example in the Toronto CMA 47 % of housing is within Era 1 and 2, while in the City of Toronto itself 64 % of housing is within those eras, reflecting older housing stock and servicing in Toronto compared to newer communities such as Mississauga, Markham, Vaughan, etc.. Toronto also has less resilient Era 4 housing stock compared to its CMA, i.e., 26 % vs 39 %, respectively. 

The type of housing must be considered where there is a high proportion of condominium / apartment dwelling types that do not have basements with the same single family dwelling back-up risks and that typically have no riparian flood risks. The City of Toronto housing units are 39% apartment, including many new condominium units, while in the broader Toronto CMA, apartments account for 29%. As a result, the resiliency of areas with a high proportion of newer apartments may be slightly overestimated.

What's next - mapping the Servicing Era and flood risk profile at a census tract level perhaps and netting out the effect of apartments and then adding in other local risk factors that are readily available.

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And here we go with some neighbourhood variability in era of construction to show the variability in land use planning, subdivision infrastructure servicing and dwelling construction practices across several regions in Canada. The average neighbourhood age is based on Statistics Canada data at a census tract scale - sometimes that is too small a scale to assess risk and sometimes it too big, and sometimes it does not align with infrastructure system servicing boundaries. So what I'm saying is its a high level general characterization of resiliency, and one would have to drill down into specific infrastructure systems to see street by street resiliency :

Vancouver:

 Calgary:

 Golden Horseshoe / Toronto / Hamilton:

 Ottawa / Gatineau:

Montreal:

Halifax:

Other factors that characterize flood risk can also be considered, including overland flow and topographic slopes that contribute to direct surface flooding and also inflows to sanitary sewer systems (e..g, via reverse slope driveways and low opening, or windows and lower level walkouts that ultimately drain to floor drains and the sanitary/wastewater collection system).

The following images show the estimated 100-year overland flow spread for drainage areas up to 10 hectares in Toronto. Catchments with low slopes are also shown as these have been shown to influence the maximum flood density reported following extreme rainfall events.

The first map shows the Newtonbrook area of North York where the overland conveyance limitations are clearly apparent. It would appear that the topographic drainage limitations (low slopes, no overland outlet) combined with the age of construction (with partially-separated sanitary sewers) results in the high relative flood risk.

I have to say it is tempting to cherry pick the map area to prove my hypothesis is correct in terms of flood risk factors. The west end Toronto map shows the limitations in flood risk factors. For example the greatest flooding in May 2000 was not in the low slope catchments or the oldest construction area but rather in a cluster north west of Eglinton Ave West and Islington Avenue. The oldest area to the south shows one flood cluster on the overland flow path but not throughout the old area for the May 2000 event. So is construction age alone a good indicator of risk? Overall it is (analysis of all Toronto flood reports proves it is relevant), but it breaks down at the neighbourhood or sub-neighbourhood level as a predictor of risk. The best indicator of risk? Past flooding. Why? Because the complex reality of hydrologic and hydraulic systems and building construction cannot be readily simplified into list of risk factors - there is too much variability, too many exceptions and too much interaction between known and unknown factors to identify risk at a fine spatial scale.

The final map below shows another west end area where 10 hectare overland flow paths help explain some flood clusters but not others. Slope does not seen to be a driving factor where the overland drainage area is small (near Trethewey Drive), but may be a more significant factor on Jane Street where the overland flow area is more significant. What this shows is that it is combinations of factors that accentuate overall flood risk. If one factor is quite severe, it can trigger a flood cluster like east of Dufferin Street, north of Lawrence Avenue West where the age of construction is newer, slopes are good but the overland flow system behaviour alone is enough to trigger risks. Like there previous map, this one shows cluster that are off the major overland flow paths - so these do characterize risk on an aggregate basis overall, but not always at a local spatial scale.

This is perhaps the best example of overland flow risks coming to life like a 'giant tiger' during a severe storm. The picture is from Twitter at .. ummm .. the Giant Tiger store on Kipling Avenue. The store is immediately on the overland flow path - not a huge drainage area, but the building effectively blocks the flow path. The low slopes (orange polygon) around the store suggest it is in a 'bowl' (like Newtonbrook in the image above), which means the overland flow has 'nowhere to go' - good overland slopes help 'move' runoff during an extreme event. 

Looking at Mississauga, and reported flooding from July 8, 2013 one can see higher concentrations in older areas closer to Lake Ontario and in Malton (top of inset map). Newer areas that appear to have low densities of flood reports may in fact just be commercial properties with no basements and with owners or tenants who did not report flooding to the City after the storm event (e.g., areas surrounding Pearson Airport). It does appear that relatively less flooding was experienced in newer subdivisions to the west (e.g., Erin Mills).

Adding other factors such as overland flow paths to the Mississauga flooding and construction era maps shows again, like in some Toronto maps, that the combination of factors drives flood risks. Clearly higher densities of flooding within older neighbourhoods (census tracts) can be found along overland flow paths (note regulated floodplains are not shown). The tributary west of Cawthra Road between the QEW and the CNR in the Mineaola neighbourhood shows a clear line of reported flooding along the overland flow path. Other local areas in the Mineola neighbourhood do not have a high degree of reported flooding, despite the older age of building construction and servicing standards - some flooding in that neighbourhood is associated with Cooksville Creek riparian flooding risks.

Edmonton has mapped overland flooding risk areas to help educate residents on flood risks. The map below illustrates the variability in dwelling construction date across census tracts.

The following map shows areas with surcharged sewers and surface ponding risks based on the interactive map available here: http://edmontonjournal.com/news/local-news/first-defence-new-epcor-ranking-scheme-may-wake-up-neighbourhoods-to-flood-risk.

What does it show? Old areas south of the rail tracks have higher sewer surcharge risk (red pipes), corresponding to the old 'grid pattern' development. Newer areas to the north, especially north of 153 Avenue NW have very few sewer surcharge risk (i.e., fully separated sanitary sewer servicing) - those are the areas with the modern 'wiggly' road patterns. Its all coming together !

The City of Windsor experienced extensive flooding in both 2016 and 2017. The following map illustrates August 2017 flood reports.

The era of construction is shown in the map below. It would appear that oldest areas had the highest flooding reports, and newest areas (e.g., in the west) had lower flood density.

Looking back at Mississauga, we have analyzed the flood density in residential development areas by era of construction, using weighted average construction date in each census tract. The flooding locations were estimated through digitizing and therefore likely underestimate reports in the highest density areas due to overlapping symbols. Nonetheless, a strong trends appears in the data with a lower density of flooding for 1980-1989 construction compared to pre-1980 construction - this reflects the benefits of full sanitary sewer separation and more advance master drainage planning. Post-1990 construction areas show even lower flood density that 1980-1990. The ratio of flooding density in the three eras of pre-1980, 1980-1989, 1990-present was 3.1 - 1.8 - 1. Unlike the Markham densities above that are based on total dwelling counts, the Mississauga densities area are-based and therefore if modern dwelling densities are higher, the relative flooding would be even lower for more modern construction (i.e., more dwelling per area, resulting in even lower flooding per dwelling). The following map illustrate the residential areas (with the exception of Malton), colour-coded by construction era and estimated flood locations.

Looking a little closer at Mississauga and the major overland drainage system, con can see the influence on reported flooding. Older construction areas do not have uniform flood risks - while risks are higher overall, on an aggregate basis in older vs modern drainage systems, with the older areas there are distinct clusters of flooding. Often  those clusters are along a regulated valley feature or along an major overland flow path upstream of the regulated area - several examples can be seen in the map. The inset at the top right is the Malton area of Mississauga - flooding clusters are apparent along the major overland flow path beyond regulated areas - this does not necessarily mean flooding was overland, pluvial flooding, but that the major system conveyance limitations stressed the sanitary sewer system, for example, with extraneous inflows in an area that already has high infiltration flows.

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Two key flood risk factors are combined in the following map showing the City of Waterloo. Spatial analysis of overland flow path hydrologic characteristics are intersected with city sanitary sewer system age and inferred infiltration and inflow risk to identify risk areas of interest - these can be assessed through further study, whether that be investigation of critical system conditions (e.g., CCTV inspection), monitoring of flow stresses to confirm inflow potential sewer surcharge risk magnitude,  to modelling / quantification of flood risks, or further risk characterization through investigation of past flood claims and reports. Average dwelling age of construction in census areas is labelled to show the general correlation of broad neighbourhood risk factors (i.e., construction era is a surrogate for sewer and overland drainage design standard resiliency). Individual dwellings near the overland flow path are shown in red, indicating specific local risks within new and old subdivisions.

City of Waterloo - Example Urban Flood Risk Factor Review Considering 100-Year Overland Flow Risks and Sanitary Sewer Resiliency Based on Construction Era and Inflow / Infiltration Potential (Excludes Riverine Flood Risk and Properties in Regulatory Flood Plain)

Methods for assessing urban and riverine flood risks from "Flood Plains to Floor Drains" are discussed in a previous post.

City of Toronto Overland Flow Map - 100-Year Storm Major Drainage System Spread & TRCA Watershed Major Drainage Centreline

CityFloodMap.Com presented GIS-based, hydrologic and hydraulic overland flow analysis over the

Newtonbrook area overland flooding in urbanized areas and
historical flood reports (May 2000, August 2005, July 2013).

past couple years (the "2015" Toronto overland flow analysis and spatial analysis correlating overland flood limits to reported basement flooding in 2000,2005 and 2013, and then the "2016" southern Ontario-wide analysis that includes refined hydrologic parameters). This provides an important insight into urban flood risk management and the influence of surface flooding on sanitary/wastewater system inflows (e.g,. via doors, windows, walk-outs, depressed driveways) - the extraneous flows that particularly stress partially separated sanitary systems, causes sewer surcharge and basement sewage back-up.

Below is an interactive map of the overland flow analysis clipped to the City of Toronto including the estimated 100 year flow spread and a 2x 100-year flow spread estimate. The mapping also includes overland flow paths within Toronto (the centreline of the major drainage flow path) and outside of Toronto covering the entire TRCA watershed areas ((c) CityFloodMap.Com). The major drainage system flow spread in Toronto is essentially a buffer from this centreline considering each overland flow reach's hydrology (rational method) and hydraulics. A standard road cross section assumed to estimate the spread using each overland flow segment's longitudinal average slope (or a minimum to avoid zero's for segments across filled sinks that have no slope). More details on the analysis including the basement flooding correlation is described here:

Urban Flood Risk from Flood Plains to Floor Drains from Robert Muir

Social media geotagged surface flooding pictures like the one below were found to corresponds to the mapped overland flow path in some areas:

The interactive map is below (c) CityFloodMap.Com. Note, approximate TRCA regulation boundaries were estimated from camaps.ca georeferenced image features, and TRCA shoreline/slope regulation areas have been excluded to focus more on where river flood risks exist:

View larger map

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

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

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

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

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

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

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

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

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

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

Ontario Overland Flood Risk Mapping - Risk Screening Mapping to Identify Urban Flood Risk Zones Beyond Regulated Valleys

Overland flood risks often result in water damage in vulnerable urban areas. New Ontario mapping of surface drainage flow paths can identify the highest risk areas, specifically those around buildings and beyond river flood plains.

Why Is Overland Flood Risk Mapping Needed?

Analysis of historical flooding in Toronto in May 2000, August 2005 and July 2008 has revealed that basement flooding is correlated with overland flow and topographic risk factors. A building's footprint within the overland flow path is an obvious indicator of surface water damage potential - that is, water encompassing a building and entering its openings. But the proximity to the overland flow path, and its ability to negatively influence the neighbourhood wastewater system with extreme weather inflows, has also been show to be an indicator of sewer back-up risk. In this manner the overland flow spread influences flood risks on a broader spatial scale beyond the narrow overland flow path alone.

How Does Overland Flood Risk Mapping Relate to Flood Plan Maps?

Overland flood risk mapping is the natural extension of river risk mapping, up beyond the valley flood plain limits, and across 'table land' as they say in the development industry. Typically in Ontario, regulated valley areas incorporate a range of natural heritage features and hazards including flood plain, watercourse meander belt width, and unstable valley wall slopes. On table land, overland flood risk hazards run across roadways and the developed lot fabric of our cities, sometimes confined in drainage features, or sometimes not. Overland flow zones typically do not coincide with natural heritage features. like vegetated valley flood plains do.

Who Maps and Manages Overland Flood Risks?

Sometimes nobody. After all, without natural heritage features, there is less to protect under Ontario's provincial policy statement. And because the overland risks emerge on such an infrequent basis (during the most extreme rainfall events), they are not top of mind, nor are they easy to define. Progressive cities like the City of Toronto has an aggressive basement flood reduction program that assesses overland drainage systems and identifies risk management alternatives. But these overland systems are typically developed only in specific remediation areas, incorporated into InfoWorks models and characterized in Class Environment Assessment reports.

Nobody?

Well, in some isolated cases overland flood risks are mapped and managed in the same manner as regulated valley flood plains by Ontario conservation authorities. Typically these are areas of isolated watercourse enclosure where extreme rainfall runoff overwhelms the sewer or culvert conveyance system and flows over land. Almost exclusively, however, flood plain risk maps stop at the conveyance system outlet (i.e., headwall / outfall) and do not extend further up onto table land.

What About Insurance Industry Mapping?

Overland surface flooding flood risks, sometimes called pluvial flood areas, are mapped by companies such as JBA and used by insurance companies as input to insurance business decisions (where to insure, setting appropriate risk-based premiums) - but mapping is proprietary, and results are not used for regulation or risk management purposes. Rather, surface flood risk mapping is a business service.

Where Are Ontario City's Predominant Flood Risks?

In overland flood risk zone, not flood plains - in fact in Toronto 98% of flooding in the last three large storms was beyond river flood vulnerable areas. This is consistent with Conservation Ontario figures that identified the percentage of Ontario properties in flood plains to be in the low, low single digits.

Show Me !

Below are a couple images of overland flood risk zones derived for the Ontario South-West digital elevation model zone. The first is the chronically flooded Newtonbrook area in Toronto, and the second is the chronically flooded Brydges-Elgin area in Stratford (subject of a settled class action lawsuit).

The large map shows the topography used to identify overland flow path alignment / upstream slope for hydrologic peaking factor / contributing drainage area / overland network reach conveyance slope, and land use used to assess contributing area composite runoff coefficient.

The inset maps shows the overland flow path spread during a 100 year peak flow, and multiples of the flow path that can indicate risks to adjacent properties connected by wastewater systems. The overland flow network is defined for all drainage areas over 3 hectares in size up to 1000 hectares in size. Typically, flood plain mapping is available for the largest drainage areas and would overlap the overland flow path limits. The inset maps shows the overland flow path on an Open Street Map base, revealing where the overland flow path affects buildings and built-up areas.

Currently overland risk zones are refined for south and south western Ontario (excluding the Ottawa River basin). This represents over 800,000 overland flow segments in the major drainage network. We are evaluating distribution methods in order to share these results as well as input layers that can be used to support refinements by others. Stay tuned!