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

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

Overland Flood Risk - From Flood Plains to Foundation Drains

From foundation drain to floodplain there is a continuum of drainage features that affect flood risks and damages in urban areas - these are micro lot level factors and macro neighbourhood factors.

River flood risks are defined by floodplain maps (called
engineered floodplains) because of the detailed hydrologic
analysis used to define flood flows and river and bridge
surveys used to define river and valley hydraulics to define
precise flood levels. Hazards in these zones are closely
regulated, new development is not permitted, and
 redevelopment is subject to special policies if allowed.

Yes, some property reference points could identify a few flood risk factors to address, but overland flooding risk is not apparent at the property scale - that is if we agree overland flooding is not a result of poor lot grading or obvious rain entry points.  Unlike other risks like fire hazards, the structure or property does not define most of the risk when if comes to flooding. It will be interesting to see what methods Aviva has adopted for defining risk levels and overland flood endorsement premiums.

Aviva is excluding high risk properties (about 5% of properties) considering flood plain maps that identify river (and sometimes lake) flooding hazards. Micro-scale factors like the property's foundation drain and service lateral condition can affect almost any property, but issues with those do not cause overland / surface water flooding.

Overland flow areas in an urban setting.  Areas beyond blue
regulated floodplains may be several hundred hectares in size.

I'd suggest the most sensitive overland flooding occurs upstream of large floodplain-mapped valleys and up onto urbanized 'table land' where the sewer drainage system and overland drainage system are not designed to accommodate runoff from major rainfall events.  Usually this would be in catchments draining a 125 hectare or smaller area in Ontario, lying beyond the regulated flood zones above where Aviva insurance would not be available. In these areas, neither the capacity of the storm sewer system, nor the adequacy of the overland drainage system (ideally on roadways and drainage easements) is apparent at the individual property scale.

The macro-scale, overland system is only apparent from runoff accumulation and concentration from 10's of hectares to even a couple hundred hectares of table land runoff that accumulates downstream. That is, a broad neighbourhood-scale macro factor representing many hundreds upstream of properties. The map of overland flow drainage areas (red text in yellow highlight) shows the continuum of drainage areas extending beyond regulated floodplains that flow into to smaller surface features that run through the urban lot fabric.

Engineered floodplain limits are determined through
extensive engineering analysis of the valley system.  More
local overland flow limits may be determined through a
local flood remediation study that analyzes the 'major system'
of road drainage as if it is a river valley. That can be
 expensive. Alternatively the risk of being close to the
overland flow path can be estimated by defining buffer
distances from small and large runoff area flow paths.

The key overland flood risk factor is whether the property is on the overland flow path where the drainage accumulates. Maybe a property is at a sag in a road where runoff could spill toward the property (i.e., it's vulnerable to overland flooding) but if there is no large upstream area contributing overland runoff toward the property, its not exposed to meaningful risk. But with a large overland drainage area from the neighbourhood flowing into the sag there is a risk to the property.  And in that case, disconnected downspouts won't help reduce risk - downspouts drain a couple hundred square meters of rooftop, while large overland catchments can be a couple hundred hectares (meaning 10,000 times greater than the local roof area)

GIS Tools can identify major sags, or depressions,
 (called sinks) in the landscape that can indicate or amplify
overland flood risk.

Neighbourhood scale risks factors for overland flooding can be assessed with some minor effort (relative to detailed pipe-by-pipe sewer and street-by-street hydraulic simulation models that is) by just considering topography from readily available elevations models and by applying core GIS hydrology tools.

The image to the left on 'major sags' is an example showing overland flooding risks including dwellings within poorly-drained sags.  In this case it includes dwellings upstream of a railway embankment that can impede and 'back up' flows during large events.

Ranked property risk considering proximity to overland flow
path, drainage area, and major sags in topography
 (i.e., drainage-challenged during extreme runoff events).

The risk to dwellings and property within close proximity to overland flow paths can be ranked. For example a building within 3 m of a 10 hectare area runoff flow path, or 15 m from 100 hectare area flow path, would be at relatively low risk. Alternatively a dwellings within  3 m of a 100 ha flow path would be at a high risk.  A building within both a flow path and in a sag would be at the highest risk. Remember, these are estimates and are qualitative.  Nonetheless, experience shows that areas identified through these heuristic methods have be subject to overland flooding during extreme rainfall events.

The issue with overland flood insurance (urban, table-land type flooding) is that risks are concentrated with a small portion of downstream properties for whom risk-based premiums could be unaffordable, or for whom coverage would not be available.  The majority of upstream properties would not likely ever experience overland flooding nor add coverage for it. So overland flood damages are not like wind damage (path of wind can be anywhere whereas path of water is always the lowest elevations).

At a property scale, a property with poor lot grading and a reverse slope driveway may be at high risk regardless of proximity to overland flow paths or major sags in topography. Such a property would benefit from having an overland flood endorsement in its water protection insurance. Ideally homeowners in this situation can take action to mitigate risks by improving grading, installing barriers to flow from the reverse drive, keeping grates clear of debris and perhaps installing a backflow valve on the driveway drain.  Beyond these exceptions, anyone with foundation drains that could clog or a service lateral that could become root infested is at risk of flooding - but that would not be overland flooding, but rather back-up from the floor drain or seepage from the foundation wall and cracks (a different endorsement all together).

Check out new analysis of table land flood risks and insight into correlations with basement flooding / sewer back-up incidents : overland flow flood risk factors