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Exshaw groundwater study highlights high water table, need to avoid building into ground

A long-awaited study nearly two years in the making on the impact of groundwater flooding in Exshaw was presented to MD of Bighorn council, with a future open house to be held for the public.

MD OF BIGHORN – A long-awaited study nearly two years in the making warns against digging into the ground due to the risk of groundwater flooding in the hamlet of Exshaw.

And while there’s no immediate recommendations that will act as a silver bullet in the Matrix Solutions report presented to the MD of Bighorn council, it offered potential policy changes as options that could aid future development to avoid the annual June basement flooding faced by the majority of Exshaw residents.

“Future developments should really avoid pushing too far into the ground,” said Nathan Green, a hydrogeologist and project manager. “We know that water table is naturally very high there, so if there’s an avoidance if at all possible of subsurface developments, then it shouldn’t impact the groundwater system at all.”

As part of the lengthy study, Matrix Solutions analyzed to see if the sediment pond – as well as the influence of storms, snowpacks, creeks and the Bow River – that was built as part of the flood mitigation on Exshaw Creek played a role in groundwater flooding.

Among the data used to create the model was climate information from 2010-22, topography, geology, land cover and boundaries. It looked at three scenarios of what it was like prior to the construction of the sediment pond, mitigation through dewatering and mitigation through flow barrier.

“We can’t tell what those conditions would’ve been before the sediment pond was constructed and put in place, so for that we need a way to back calculate or recreate that scenario,” said Green. “We went with the numerical modelling for that reason.”

And while anecdotal evidence from residents suggested flooding has been worse since construction of the sediment pond – with Bighorn Reeve Lisa Rosvold saying she had heard from many residents “it’s become a bigger event in recent years” – the study couldn’t find information that the pond exacerbated the flooding.

“We cannot make out any significant difference between the groundwater system with and without the sedimentation pond,” said Chris Gabriel, the senior environmental consultant and the lead in preparing the modelling.

Modelling, as well as field data, showed the surface water in the sediment pond had a minimal influence on groundwater levels in the aquifer and that high groundwater was likely the result of naturally rising seasonal water table within the entire alluvial fan aquifer.

The study found the area has low groundwater from late fall to early May, but that it slowly increases in early spring.

Water eventually flows from the sediment pond, goes into the aquifer and flows south into the Bow River.

“It’s interesting to think of this pond as a reflection of the water table in a way. If the event happens really fast, water will be transmitted through the creek and be in the pond and then because the pond is holding that water, it will just leak away back into the aquifer below,” said Jonathan Kerr, a senior hydrogeologist for Morrison Hershfield and an advisor for the MD of Bighorn on the project.

Bighorn council approved requests for proposal in June 2021 for an engineering firm to be selected to study the causes of high groundwater flooding in Exshaw.

At the time, municipal staff noted there were more than a dozen reports between 2009-21 that would contain information on the review.

The municipality also had piezometers – which are used to measure the pressure of groundwater – that record additional data in areas such as Exshaw and Jura creeks.

Matrix Solutions was selected by council in September, 2021 and Morrison Hershfield did the third-party consulting for a total of about $200,000.

The intent was for the study to provide more information on the causes of high groundwater flooding, the influence of storms, snowpacks, creeks and the Bow River in the area, particularly for east Exshaw residents.

It was anticipated the report would be completed in 2022 to include the annual spring freshet, with monitoring happening at nine well locations and four surface water locations.

Matrix would review data such as water level, analyze precipitation and snowpack data and finish modelling to further understand surface and groundwater as one system. Additionally, the study would look at groundwater at Exshaw Creek prior to the flood mitigation work to see the impact on groundwater in Exshaw.

The final report was delayed and expected to be released in March, but pushed back further until the May council meeting.

The municipality had previously hired McElhanney Engineering, which submitted a technical memo that summarized recommendations from a November 2020 physical hydrogeological assessment report.

However, McElhanney left the project and a third-party review by BGC Engineering raised concerns on the debris-flood mitigation design that had been completed by Golder Associates for Jura Creek. It also had council raise issues early in 2021.

Council was unable to ask questions on the third-party review, which added to concerns.

In addition to hearing the Matrix Solutions presentation, Bighorn council also directed municipal staff to rescind the east Exshaw high groundwater emergency response plan and move forward with constructing the west lane of the east Exshaw stormwater improvements. Council directed staff to defer the re-grading of the north and east lanes due to increased costs.

The approved budget was $210,000, but for all three lanes. However, the two quotes received were for $314,000 and $400,000.

The municipality did have a capital budget of $750,000 for additional stormwater and groundwater improvements.

The Matrix Solutions modelling showed flood mitigation by dewatering would require a municipal-scale pumping system, but it, as well as mitigation through sub-surface flow barrier was a “significant financial commitment” and “financially unfeasible."

Green noted the intent is to keep some wells that were part of the study action to continue long-term monitoring and collection of information.

The modelling, however, will allow a better idea of how groundwater could impact future development and infrastructure.

“That’s where the value of the numerical model comes into play. We can test different scenarios and tweak what that level could be if we had to put a number to it for a development policy,” said Green.

Kerr noted historical data of when water hits a certain elevation level, groundwater will begin appearing in basements with the target level being 1,287 metres above sea level.

“It’s a real number. It’s not a model number. We have that elevation. It’s really up to policymakers to how much of a cushion they want to put in on that," Kerr said.

"If we see, for example, fluctuations come up and we’re not comfortable with water coming to the close of the bottom of the foundation or footing, then we could drop that number even further to require buildings to be placed higher, but that number seems to be a good number to more or less maintain a dry condition year-round for the bottom of the basement.”

Though council didn’t set a date, an open house for the public to get information and ask questions on the study will take place.

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