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DATE: November 13, 2020
TIME: 4:00 PM MST
SUBJECT: Hydrological Assessment of Exshaw Groundwater Event
The Council for the Municipal District of Bighorn (“the M.D.”) has received the anticipated hydrological assessment of the Exshaw groundwater event that occurred in June of this year. Mr. Roger Towsley, Senior Hydrogeologist from McElhanney, presented the report to Council.
The McElhanney assessment states that the primary cause of flooding appears to have been from melting of an abnormally high snowpack, and that flood conditions may have been worse if precipitation had been at or above normal amounts between late May and early July. “The importance of the effects that the engineered debris flood mitigation structures on Exshaw Creek have on local groundwater levels in the North Community of Exshaw are unclear, but effects appeared to be minor as compared to the impact from the melting snowpack. The completion of the structures in 2019 and flooding the following spring was likely coincidence rather than cause. Further study is warranted to better understand how or if the structures cause increased infiltration and exacerbate rising groundwater levels from environmental causes.”
Mr. Towsley also reiterated many times during his presentation that hydrogeology beneath the hamlet is very complex and further studies could be undertaken to gain additional knowledge of groundwater flows. He suggested in his presentation to Council, to continue groundwater monitoring and expand the groundwater monitoring network to assess the effects of Bow River water levels on local groundwater levels and the effects of impoundment of water in the sedimentation pond on local groundwater levels.
The McElhanney report corroborates the findings of a previous AEMA study that noted the flooding in east Exshaw “appears to be linked to rainfall and snow melt events and seems to be worse with melting of larger snowpacks and heavier rainfall events”. According to the AEMA study: “Large scale channelization on Exshaw Creek was recently completed to mitigate surface flooding. Many affected residents feel that these works contributed to this year’s flooding. My contacts and my observations seem to indicate these works most likely did not contribute to this year’s groundwater intrusion.”
In the first week of June 2020, the hamlet of Exshaw experienced a high groundwater event that affected the streets of Pigeon Mountain Drive and Mount McGillivary. Groundwater flooded residential properties and basements, including the east lift station and the Exshaw lagoon. Council had approved the groundwater investigation on June 9, 2020 to determine what caused the high groundwater levels and if the Exshaw Creek flood mitigation project was the cause of the groundwater flooding. “Council and Administration needed to understand what caused the flooding and if the Exshaw Creek project was the source of the high groundwater levels affecting east Exshaw” said Reeve Dene Cooper.
The MD is currently reviewing the effectiveness of a temporary drainage system employed in June to help residents move groundwater away from residences on Mt. McGillivray Drive and Pigeon Mtn. Drive. “What we’ve learned is that the June, 2020 groundwater levels were not a statistical anomaly. We need to be as prepared as possible for the next high groundwater event, whether it’s caused by an exceptional snowpack or high precipitation levels or the Bow River”, said CAO Robert Ellis.
At the regular meeting of Council on Tuesday, the Council heard from Director of Operations, Bill Luka who said “designs plans have been prepared to re-grade three lanes in east Exshaw to improve drainage flows created by storm water and surfacing groundwater and convey the water south towards Veteran’s Way. Residents in east Exshaw will be contacted to provide feedback on the proposal. Council approved Mr. Luka’s recommendation for the lane regrading and feedback from residents.
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FOR MORE INFORMATION, CONTACT:
Dene Cooper, Reeve
Municipal District of Bighorn
P.O. Box 310 / 2 Heart Mountain Drive
Exshaw, Alta., T0L 2C0
Robert Ellis, Chief Administrative Officer
Phone: 1-403-673-3611 (ext. 229)