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:
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:
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.
