|Mr. Spock favours facts|
--SPOCK, Star Trek: The Original Series, "Space Seed"
Flooding from extreme rainfall is dangerous. But if facts related to the cause of flooding are insufficient, the solutions to flooding will be misguided ... illogical.
As noted in an earlier post, when Environment Canada recently reviewed Southern Ontario rainfall statistics to see if there have been any measurable trends in rainfall patterns and intensities they found the following:
- Significant increases, as well as decreases, were detected at some stations in a number of the extreme precipitation indicators. However, the majority of station trends were determined to be non-significant and no consistent geographical patterns for increases or decreases were observed across Canada. In most cases, the magnitude of the observed changes was also very small.
- On sub-daily or short duration rainfall intensities of less than 24 hours (this is the data / or trends municipalities would use for planning, design and management of most drainage and stormwater systems that respond to flashy storms): The majority of the trends were determined to be non-significant with no simple patterns or uniform rates of change evident in the short duration rainfall. Trends were determined to vary with duration and regional location.
Here are some insufficient non-facts:
"The August 19th downpour was just one of 8 extreme rainfall events in the past 20 years that resulted in basement flooding and other damages. The August 19th storm exceeded the rainfall expected in a 1-in-100 year storm. Most of the other storms were estimated previously as having a 1-in-25 year to 1-in-50 year chance of occurring. Taken together, these events indicate that intense rainfalls are already on the rise in the Toronto area and that design standards for stormwater management need to be revised."
|Mr Data favours data|
Once you have eliminated the impossible, whatever remains, however improbable, must be the truth.
--SPOCK, Star Trek (2009)
|Dusty old data|
|Decades old analog chart for tipping bucket rain gauge|
It is possible that infill and intensification in Toronto has increased runoff rates and volumes. Anecdotally, I need only to look around my house to see the residential intensification in the last 45 years: on my block the rear laneway has been urbanized (used to be gravel, now it is concrete with storm sewer), the two smallest homes across the street are now monster sized, the third smallest is now the vestibule for the massive new home in the old back yard, the semi I'm attached to has an extension on the back as do several others, smaller homes on large lots that had some greenspace have been replaced with multiple homes (includes the neighbours to the north of me, and the neighbours north of them, and the neighbours north of them). The latter two homes now have 90% paved front yards for parking and many other homes have added or expanded front yard parking (more impermeable surfaces = more runoff). Factually, as part of flood remediation hydrologic model refinements, I have quantified increases in rooftop area of several percentages over 10 years in a GTA municipality using GIS data. Additional driveway increases, not measured, would add more pavement. Over multiple decades infill and intensification increases runoff.
|Example intensification, just the 8 addresses north of me since the late 1970's ... add it up.|
It is possible that infill an intensification in Toronto has obstructed major overland flow paths, aggravating flood damages during extreme rainfall events. Factually, I have observed drainage easements encumbered by sunroom additions and landscaping such that drainage of low points in residential roads is compromised, and have observed overland flow paths encumbered by monster home rebuilds. This occurs because overland flow paths (unless there is an old easement) are not regularly mapped or managed by municipalities, yet are prevalent in areas built pre-1980's when overland drainage was not engineered/designed to stay on right-of-ways and away from homes.
It is possible that removal of sanitary overflows for environmental protection has increase basement flooding risks by removing the "relief valves" from the system during extreme events. See Ministry of the Environment's Procedure F-5-5. Factually, the basement flood remediation Class Environmental Assessment (EA) Study for my neighbourhood (Toronto Remediation Area 32) identified the adverse impact of sewage overflow measures on basement flooding. These measures built in the early 1990's were effective at achieving water quality improvement for small storms (see Council report - Eastern Beaches Water Quality - Effectiveness of the Kenilworth Avenue and Maclean Avenue Detention Tanks (Ward 26)). But the Kenilworth tank and related flap gates were cited in the Class EA as the cause of flooding (excerpt from the Class EA study):
The solution to flooding includes reestablishing sewerage overflows as highlighted in the causes and remedial measures summary for Area 32 flood cluster area 1:
The Toronto Area 32, flood cluster 1 shows environmental improvements implemented in the 1990's to reduce sewage overflows aggravated basement flooding during the extreme August 19, 2005 storm. This is often referred to as the "law of conservation of poop" - poop either it spills to the river / or lake or backs up in the system and potentially into basements - it is an incompressible liquid and it has to go somewhere. Area 32 is one specific example in my neighbourhood, but you can find many others.
Another example is in Hamilton, Ontario, in relation to their Ward One Combined Sewer Overflow Master Plan Class Environmental Assessment. The 2003 report to the Committee of the Whole recommends implementation of overflow reduction measures to prevent pollution in Cootes Paradise Hamilton Harbour - it is also to address MOE Procedure F-5-5 control requirements. The report notes the risk of interfering with the law of conservation of poop as follows in the analysis of Class EA alternatives on page 4:
"• Combined Sewer Overflow Regulator Adjustments
Adjustments to combined sewer overflow regulators to send more flow to the treatment plant can reduce combined sewer overflows, but must be carefully investigated to prevent basement flooding and overloading of the wastewater treatment plant."
The recommended solution for the Sterling outfall :
"For the Sterling outfall the recommended solution is a regulator adjustment, which would involve the raising of the existing weir within the combined sewer system to increase the amount of wet weather flow going to the wastewater treatment plant and reduce CSOs. This is a Schedule A project and may proceed to implementation."
Translation: Schedule A = no further study required as impacts are assumed to be small per the Municipal Class EA Process. The law of conservation of poop would tell us that reducing the overflow of sewage at the Sterling outfall by raising the height of the overflow weir will increase the volume of poop in the sewer system, will raise the level of poop in the system and can put basements at higher risk of flooding. It is not clear if those flooding impacts have been quantified - they may be low or they may be high, just like the Toronto Area 32 impacts. Google F-5-5 and see what other municipalities have tightened up their sewer systems to achieve pollution reduction but may also have unintended impacts.
The challenge in large municipalities is that there can be competing interests and programs. And it is possible that right hands are not fully informed of what left hands are doing or of impacts of actions or inaction. A good example of that is in Ottawa where Petrie Island beach closure in August 2006 were likely caused by a major unreported sewage spill at the Keefer regulator site adjacent to the John Street-Sussex Drive intersection. The 2008 audit report is available online. Like in Toronto Area 32 or Hamilton Ward 1, the management of sewage spills will affect the system - too much regulation in Toronto and perhaps Hamilton can adversely affect basement flooding, while too little regulation as in Ottawa will cause sewage spills/pollution. Municipalities are caught between a rock and poop.
To sum it up:
- There is factual, scientific analysis of rainfall extremes that shows no increasing trends. There are also anecdotes (MR Spock's dangerous insufficient facts) about a lot of big storms that are turned into misguided conclusions - this is really like "Jenny McCarthy vaccine science".
- Without more intense rainfall, there are also many factual explanations for increased flooding in urban areas based on i) tangible, even measurable, runoff factors (infill and intensification), and ii) the regulation/modification of the underlying sewer systems to meet other often competing objectives like pollution prevention (F-5-5).
- Insufficient facts could cause us to connect flooding solely with climate change, and cause us to more fully invest in green energy and carbon pricing to reduce greenhouse gases as a mitigation measure - or as Mr. Spock says 'invite danger' by trying to solve the wrong problem, and misdirecting focus and resources.
- Scientific facts should cause us to promote greater runoff source controls for infill development, and to preserve overland flow paths in old neighbourhoods with easements, grading or infrastructure improvements.
- Scientific facts should cause us to carefully trade off pollution control objectives and natural environment impacts with social and human impacts due to flooding, and to restore overflows where pollution controls have caused flooding (see Area 32 structural recommendations above).