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

Underpinned Basement - Typically Basemnent Floors Are
Lowered By 2 to 2 and a half feet (60 to 75 cm).

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

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

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

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

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

 

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

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

Climate Change Impacts on Ontario Highway Infrastructure Design Shows High Resiliency - Wastewater Systems Too - Media Reports Ignore Engineering Design Practices and Intrinsic Resiliency / Design Safely Factors

There is a tendency to jump to conclusions when considering climate change impacts on drainage infrastructure. For example, if storms 80 years from now are predicted to be 30% bigger (more rain depth), do sewer pipes, culverts and bridges under our our highway's need to be 30% bigger to handle the additional runoff?

No. Effects of rain are attenuated through the system due to hydrology and hydraulics. Plus there are intrinsic design safety factors to account for uncertainty in design already.

Hydrology mitigates the impact on rainfall on runoff, so 30% more rain depth results in less than 30% higher peak runoff flowing into drainage systems. This is due to things like storage on the surface, in ditches, etc. before the rain can become runoff.

Hydraulics mitigates increases in peak flows as well due to non-linearities in how flow rates show up as flow depths in channels - there is not a 1-to-1 relationship between peak flow and depth. And sometimes hydraulics throttle how much flow can enter into systems, for example sewer grates at the surface can control how much flow gets into underground sewers.

The Ontario Ministry of Transportation completed a review of their drainage system vulnerability to climate change, showing quantitatively that the overwhelming majority of storm sewers, roadway gutters, culverts and bridges meet design standards under projected future climate conditions:

Addressing Climate Change through Engineering Practice from Robert Muir

Projected rain intensity increases are 10-30%. Assuming a 20% increase, with no change in today's designs:

- 96% of storm sewers already meet design standard of flowing less than 100% full
- the average flow spread in roadway gutters increases 5-7%
- 93% of culverts meet headwater depth requirements that relate to upstream flood depths
- 96% of culverts meet exit flow velocity criteria related to erosion potential
- all bridges are assessed for regulatory (historical) storms that exceed return period events, and "These storms are generally in excess of the design storm used in determining the size of the structure
opening and erosion protection measures."

So future climate change rainfall intensities do not cause today's highway drainage systems to fail - the majority of features still meet design requirements / standards / criteria. This is in direct contrast with media reports that incorrectly assume that rainfall changes translate into infrastructure changes - in fact, recently Gord Miller stated that all culverts and sewer pipes are too small:

https://tvo.org/article/current-affairs/there-will-be-floods--and-ontarios-not-ready-for-them

Specifically: “I don’t think here in Canada we understand what’s coming,” said Miller during the talk. “We have no predictability any more. One has to look from the perspective that all culverts are undersized. All sewers are undersized.”

All culverts undersized? All sewers undersized? Obviously that is incorrect based on the Ministry of Transportation's careful and comprehensive resiliency analysis.

What about wastewater systems? Are all sanitary sewers undersized too? I co-wrote a paper for the Water Environment Association of Ontario annual conference on this topic, looking at sanitary sewer system resiliency under higher potential climate change rainfall intensities. It shows that most new post-1980's subdivisions are resilient with no sewer back-up risk with potential higher future rainfall. Here is the paper with the details:

https://drive.google.com/open?id=15pc52qgbwOasSP5O1YU2GgEQLfqkjwbW

And here is the presentation:

WASTEWATER COLLECTION SYSTEM PERFORMANCE UNDER CLIMATE CHANGE – SAFETY FACTORS AND STRESS TESTS FOR FLOOD RISK MITIGATION from Robert Muir

Analysis of all sewer pipes in the City of Markham shows that very few locations are at a risk of flooding for today's or for future climate as shown in the following table from the paper:

So less than 2% of old sewers are flood prone. Just more than 1% of new sewers are flood prone. This is shown on the following map:

Blue dots show where maintenance hole surcharge to basement levels with today's climate, concentrated largely in older, partially-separated sewer service areas. With higher potential future rainfall intensities, there are more at-risk maintenance holes / locations, concentrated again in the older areas. So over 98% of sanitary sewers are not undersized with the future predicted climate.

Media reports (like the TVO article with the Gord Miller quote above generally do not rely on engineering data or comprehensive analysis to make broad statements about climate change impacts. Too bad. We deserve better or else public policy geared to climate adaptation and mitigation will be uninformed and resources to address risk will be misdirected.