EXSHAW – A third-party review of the design for flood mitigation for Jura Creek near Exshaw has raised a number of concerns – including legal questions for some elected officials.
BGC Engineering conducted the independent analysis of the debris-flood mitigation design by Golder Associates for the MD of Bighorn. The municipality posted the review on its website, along with a response from Golder, which led to a discussion at the Bighorn May 11 council meeting.
Councillor Paul Ryan questioned whether or not the municipality should seek a legal review, as the report found several significant issues with the design as proposed.
"I would like to have legal advice of what the jeopardy is to the MD of Bighorn after they commissioned a report from an internationally accredited engineering firm and then do not implement the recommendations in the third-party review," Ryan said.
"And I want the report to come in front of council, so it becomes a report that council receives, not Golder receives."
Coun. Lisa Rosvold agreed with her colleague's concerns and desire to seek a legal review, as well as an opportunity for council to ask questions about the two reports.
"If I put myself into the shoes of the people in east Exshaw, I do not know if I would have confidence that the mitigation work that is currently suggested for Jura Creek would provide the protection that is needed," Rosvold said. "I would like to move forward with confidence knowing what we are doing is in the best interest for our residents and best for our municipality."
Reeve Dene Cooper disagreed with his fellow council members' take on the third-party review. He said Golder is the engineering firm of record and it has accommodated changes to the design based on the third-party review.
"The report was provided to the engineers of record and they provided their evaluation of that document and accommodated changes that were affordable and were considered effective," Cooper said. "I do not think council is at jeopardy at all – we are not engineers."
Council passed a motion 3-2 to direct administration to seek an opinion from its legal counsel.
"To be specific on potential consequences of the MD of Bighorn of not acting on the recommendations of the engineering firm it commissioned to do the third-party review," Ryan said.
Jura Creek debris flood mitigation became a priority for the MD of Bighorn after the 2013 flood. Both Exshaw and Jura creeks swelled with runoff mixed with debris after a significant rainfall event.
The project has three phases, the first of which was completed in 2016. The second phase set for this year is the construction of flood mitigation in the creekbed upslope of the 1A Highway. When it comes to the space between the highway and the Bow River, the report stated a channel, like what is in place upstream, will not be used. However, there are more culverts under the railway line proposed, although the work is under the purview of the province, not the municipality.
BGC's scope of work set out to look if the design incorporates best practices for steep creek erosion control; maximizes cost-benefit ratio; is durable and low maintenance; and minimizes the risk of groundwater inadvertently being redirected to east Exshaw.
The objective of the mitigation work is to reduce the frequency of flooding for homes along the eastern edge of the hamlet; however, the third-party review determined the design would increase the likelihood of flooding due to several factors.
The design's components include an upstream channel (20-metres wide and three-metres deep); sediment pond (three metres to seven metres deep) upstream of the highway; a weir and transition to the culvert; and culverts under the highway.
Issues identified by BGC included erosion and sediment redistribution on the alluvial fan, which was substantial during the 2013 flood. These processes can also occur during annual floods and the report noted the design proposed will "likely substantially increase the erosion and sediment redistribution volume on Jura Creek fan compared to the 2013 debris flood."
This results in, according to BGC, maintenance costs, reduced upstream channel capacity during a debris flood and rapid filling of the sediment pond during a debris flood. An implication that raised concerns is that it would further decrease the discharge capacity of the mitigation. If the sediment pond fills rapidly, it will increase the chance of the highway culverts being blocked.
"If the channel capacity is less than the flood discharge, water will overtop the channel and bypass the downstream works," stated the third-party review.
BGC concluded these factors could increase the frequency of water overtopping the mitigation and flooding east Exshaw. Woody debris getting stuck in the culverts could contribute to the same issue, with BGC noting the design assumes the culverts are fully open and provides no solution to protect them from being blocked.
The design is proposed to handle debris floods that exceed a 100-year return period, but events with an up to 500-year return period would be managed by breaching Highway 1A.
For the typical flows seen in the creek each year, the BGC review raises questions about how the design would affect groundwater levels.
"A significant portion of the water arriving at the pond will infiltrate into the ground given the relatively high permeability of alluvial fans," stated the report. "For example, [the MD of Bighorn] reported that the entire spring runoff flow in 2020 at Exshaw Creek sediment pond infiltrated into the ground and did not overtop the weir.
"Similar performance is expected at Jura Creek. This infiltration will increase the total volume of groundwater, but BGC has not assessed if this infiltration could affect groundwater levels at east Exshaw."
BGC concluded in its opinion that the proposed design is unlikely to protect east Exshaw in the way it was intended from debris floods. The third-party review also concluded that it does not minimize risk of groundwater flooding either.
"The unlined sediment pond increases the volume of water that infiltrates into the ground," stated the report, which also made 12 recommendations for design changes. "A hyrdrogeological assessment would be needed to estimate impacts to groundwater levels in east Exshaw.
"Until a detailed hydrogeological assessment demonstrates otherwise, it is conceivable that the proposed design could increase groundwater levels in east Exshaw during annual runoff and debris floods."
In its response to the review, Golder highlighted the design approach taken, which included finding a low-cost flood mitigation solution and that woody debris as an issue is "of less significance than at Exshaw Creek."
Golder also stated that the design capacity of the channel would not be compromised if the creek eroded and changed shape.
"Golder's design anticipates re-configuration and evolution of the constructed Jura Creek channel," stated the response.
Golder stated the upgraded box culverts under the highway also significantly reduced the risk of a blockage. However, it noted there would also be a need to upgrade the flow capacity at the railway embankment as well.
"Golder contends that all four key elements raised by BGC were addressed in the conceptual design report and considered in the evaluation of the various design options, including the selection of the preferred option," stated Golder. "The proposed design will protect east Exshaw from debris flooding in Jura creek for events significantly more extreme than the 100-year return period flood."
However, the report noted the risk of groundwater effects to east Exshaw were not assessed as part of the design, as it was deemed "to be unnecessary and the purpose of the project as defined by the MD of Bighorn was to reduce the risk of debris flooding, which is a greater hazard compared to groundwater impacts."
The MD held a public information session on the mitigation work being done on Jura Creek this year on March 30. The third-party review, and Golder's response, was brought up by residents, as they had not been provided a copy in advance. They were both posted on the MD's website on April 13.