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City sets sights on new utility for dealing with climate change induced storm water events

Timothy Schafer Local Journalism Initiative Reporter
By Timothy Schafer Local Journalism Initiative Reporter
February 10th, 2022

The effects of climate change are moving the city toward the creation of its own storm water utility to deal with the increase in significant rain events.

In his Oct. 29 budget meeting presentation to city council the director of Public Works, Colin Innes, presented an overview of the city’s storm water system and the challenges it faced from climate change and infill building throughout the city.

From that discussion city council directed staff to investigate the option of creating a storm water utility — in order to deal with the anticipated costs of improving the system — and to bring that finding forward to the 2023 budget discussions.

The challenges — related to a changing climate — have to do with the storm water system not being overwhelmed when the significant rain events occur, said Innes.

“Modeling of the storm water system predicts that as precipitation events are impacted by climate change, some segments of the storm water system will face capacity challenges,” he said.   

But work has already begun on the storm water system and how it can continue to accommodate climate change-induced rainfall events, said Innes. In 2018 SNC-Lavalin performed modelling work on the city’s storm water system.

Design rainfall events were created for 10-year (10 per cent probability of occurring) and 100-year events (one per cent chance), with over 100 years of historical climate data available to create the hyetographs — a graphical representation of the distribution of rainfall intensity over time — for the city.

“We also know that with climate change there are some changes coming to precipitation patterns, as well as the intensity, duration and frequency events that are expected to be occurring as a result of climate change,” he said at the time.

“I think that we are probably seeing some of those events and effects already starting to creep into things.”

In order to account for the effect of climate change, the city’s design intensity-duration-frequency (IDF) curves were modified using a tool developed by the University of Western Ontario. The tool used nine different climate models and three different emission scenarios to predict changes up to the year 2100, with simulations to see what causes the greatest impact to the urban storm sewer system.

“It allows us to see how out system is going to change over time,” Innes said. “Not surprisingly, because we are in a built up urban environment, really it’s those high intensity, short duration storms that the situation that is impacting” the city the greatest.

“You are getting this sudden surge of flow and that’s what is creating all of the trouble,” he added.

In order to mitigate predicted challenges, Innes explained, there are two primary means:

• Increase the capacity in areas where capacity challenges have been predicted (ie: provide more pipe capacity); and

• Attenuate the amount of run off/the rate of run that off enters the storm system.  

“A combination of these two approaches will be required,” he said. “The areas where attenuation can be utilized requires additional design.”

The avenue to address the problems won’t be without a cost. In all, the SNC report identified $19.6 million of conventional structural mitigation works.

As a result, the city is in the process of applying for a grant to aid in funding this work over a three-year period. 

The city’s storm water system includes 866 catch basins, 867 storm water pipes, 23 outfalls to Kootenay Lake and 32 outfalls to creeks. The city’s storm water system functions as 21 separate catchment areas covering the 645 hectares of urban development.

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