CityFloodMap.com overland flood risk analysis across the City of Toronto has been used in ESRI's ArcScene software to visualize at risk neigbourhoods and vulnerability zones where the correlation of urban overland / pluvial flooding and surcharged sanitary sewers resulting in basement flooding has been established. Flood risks exist beyond large regulated valley systems with a continuum of risk existing from river flood plain to individual building floor drain.
Additional visualization below includes identification of building structures within the estimated overland flood risk zone.
Showing posts with label flood plain. Show all posts
Showing posts with label flood plain. Show all posts
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.
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 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!
PBO Flood Damage Report Inaccurate Rainfall Trends for Prairie Storms - No Significant Increases vs. "Anecdotal Statistics"
The Office of the Parliamentary Budget Officer released a report Estimate of the Average Annual Cost for Disaster Financial Assistance Arrangements due to Weather Events. It is great to focus attention on flooding but the explanation of causes has gaps, particularly overstating long duration rainfall trends. But it seems the scientists have been muzzled from sharing real data and so some truth is lacking.
Its also is great that the insurance industry, who had input the the PBO report, is no longer saying rainfall intensities are increasing across the country due to climate change and causing flooding. But the report notes on page 26 under Effects of Climate Change:
"One last factor, which is likely affecting the intensity of floods in the Prairie Provinces, is climate change. The warming in the arctic has been associated with persistent weather systems in the mid-latitudes as well as extreme weather events. Consistent with this, multiple-day rain events have significantly increased in the Prairie Provinces and in the Rockies. The recent record setting multiple-day rainfalls in south-eastern Saskatchewan in 2010 and 2014 are likely examples."
What does Environment and Climate Change Canada data show about the intensity of multi-day rainfall? They don't publish multi-day statistics in the Engineering Climate Dataset but they do track trends up to the 24 hour periods - we can look at those as an indicator of longer duration trends:
(updated to distinguish Alberta and BC trends above:) In Manitoba there are no significant trends, only mild trends up and down. In Saskatchewan, there are some statistically significant downward trends (Davin 5 and Diefenbaker International Airport stations - see charts at right) and mostly non significant up and down trends. In southern Saskatchewan, there are more decreasing than increasing trends. In Alberta there is one statistically significant upward trend in 24 hour rainfall volumes - this is for Edmonton City Centre Airport, but is for records up to 1993 (i.e., not a recent trend). In BC, statistically significant increases include Yoho Nat Park Boulder Cr, but that is a short term record of only 14 years, ending in 1988 (i.e., short and not recent), and also include Blue River Airport and Creston WPCC (both more recent records). So the PBO statement "multiple-day rain events have significantly increased in the Prairie Provinces ..." appears inaccurate but could apply to isolated areas. This is a link to an interactive map: 24 hour rainfall trends.
Note that short duration climate records can have an upward bias in trends that is not related to underlying changes (like due to climate) because the probability distribution of rainfall extremes is skewed (minimum bound of zero and long 'right tail' extreme events). To look past this statistical sampling bias, we can review longer record climate stations. The following table shows trends for Manitoba, Saskatchewan, Alberta and British Columbia stations with more than 25 years of record, observations since 2000, and locations east of longitude 120 degrees west (about east of Kamloops) to avoid the coast (note table is updated).
Some Saskatchewan climate stations have decreasing rain intensity trends over most durations, including 24 hours - these stations include Buffalo Narrows Airport in the north and Weyburn in the south.
The map below includes the south eastern Saskatchewan area that the PBO report notes has had "recent record setting multiple-day rainfalls". The 24 hour rainfall trends do not show increases in observed rainfall trends, however this may not account for the noted 2014 event. It is unlikely that the PBO report statement "multiple-day rain events have significantly increased in the Prairie Provinces" is accurate. Long duration increases are more prevalent in eastern British Columbia and Manitoba, while Alberta and Saskatchewan have mixed trends including downward and upward intensities.
And derived return period rain intensity values, i.e., representing extreme storm probabilities, do not necessarily increase following a record event as shown in the following progression of IDF values over the past decades. These 24 hour design intensity values are derived from the observed trends and include values with probabilities from 2-year up to 100-year (1% chance per year):
***
Its also is great that the insurance industry, who had input the the PBO report, is no longer saying rainfall intensities are increasing across the country due to climate change and causing flooding. But the report notes on page 26 under Effects of Climate Change:
"One last factor, which is likely affecting the intensity of floods in the Prairie Provinces, is climate change. The warming in the arctic has been associated with persistent weather systems in the mid-latitudes as well as extreme weather events. Consistent with this, multiple-day rain events have significantly increased in the Prairie Provinces and in the Rockies. The recent record setting multiple-day rainfalls in south-eastern Saskatchewan in 2010 and 2014 are likely examples."
What does Environment and Climate Change Canada data show about the intensity of multi-day rainfall? They don't publish multi-day statistics in the Engineering Climate Dataset but they do track trends up to the 24 hour periods - we can look at those as an indicator of longer duration trends:
 |
| Engineering Climate Dataset Version 2.3 - 24 Hour Rainfall Trends and Statistical Significance. |
 |
| Saskatoon - significant decrease in 24 hour rain. |
 |
| Davin 5 - significant decrease in 24 hour rain. |
Note that short duration climate records can have an upward bias in trends that is not related to underlying changes (like due to climate) because the probability distribution of rainfall extremes is skewed (minimum bound of zero and long 'right tail' extreme events). To look past this statistical sampling bias, we can review longer record climate stations. The following table shows trends for Manitoba, Saskatchewan, Alberta and British Columbia stations with more than 25 years of record, observations since 2000, and locations east of longitude 120 degrees west (about east of Kamloops) to avoid the coast (note table is updated).
The table of long term stations with recent data shows the following:
 |
| Buffalo Narrows Airport climate station - consistent decrease in annual maximum rainfall over all durations. |
- British Columbia trends show more increases than decreases for the 24 hour period (9 vs. 2). Blue River Airport is the only station with a statistically significant increase in 24 hour rainfall trend out of all 39 long term recent stations in the area of interest.
- Alberta trends are more evenly up and down over 24 hours (5 vs. 3), but shorter durations of 2, 6, and 12 hours have decidedly more decreasing observed intensities.
- Saskatchewan has more decreasing intensities than increases over 24 hours but a near even split (6 vs. 5) - like Alberta, shorter durations have more decreases.
- Manitoba has more increases than decreases (6 vs. 3) over 24 hour periods.
 |
| Weyburn climate station - consistent decrease in annual maximum rainfall over all durations. |
The map below includes the south eastern Saskatchewan area that the PBO report notes has had "recent record setting multiple-day rainfalls". The 24 hour rainfall trends do not show increases in observed rainfall trends, however this may not account for the noted 2014 event. It is unlikely that the PBO report statement "multiple-day rain events have significantly increased in the Prairie Provinces" is accurate. Long duration increases are more prevalent in eastern British Columbia and Manitoba, while Alberta and Saskatchewan have mixed trends including downward and upward intensities.
 |
| No statistically significant increases 24 hour rainfall observations in south-eastern Saskatchewan. Source: Environment and Climate Change Canada's version 2.3 Engineering Climate Datasets (ftp://ftp.tor.ec.gc.ca/Pub/Engineering_Climate_Dataset/IDF/, file: idf_v2-3_2014_12_21_trends.txt, in IDF_Additional_Additionnel_v2.30.zip). |
The problem with the PBO report is that it seems to make the common mistake in rain intensity reporting - i.e., that the plural of anecdote = data. Of course it does not equate to data, and so two events should not be cited as the rationale for identifying significant trends. Citing two storms to make a trend conclusion demonstrates the "availability bias" in reactionary thinking that we have explored previously. Evidence-based policies for flood risk mitigation will require Kahneman's System 2 thinking with detailed, methodical analysis and not "anecdotal statistics".
Single events, even record-setting ones do not necessarily change rainfall trends. Recently the July 8, 2013 storm set record rainfall amounts in Mississauga, Etobicoke and west Toronto areas. Despite this record, the statistical trend is downward for the Pearson Airport climate station, as shown in the following graph:
 |
| Recent record events do not necessarily define trends. |
 |
| Intensity-duration-frequency trends for Pearson Airport climate station shows decreasing return period intensities over many intervals despite record recent event (July 8, 2013). Data are Up to 2013 per Engineering Climate Dataset v2.3, to 2007 per v2.2, to 2003 per v1, to 1990 per R. Muir hardcopy records. |
Apparently for many one data point is enough for a theory.
For the PBO report, two data points can define a significant trend across an entire region, skipping over mounds of data in between.
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.
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.
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.
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)
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.
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
 |
| 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. |
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. |
 |
| 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 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
Subscribe to:
Posts (Atom)