The Culture and Community Centre in Ste. Agathe hosted round two of a three-part consultation series regarding the raising of the community’s ring dike.
On March 7, residents were invited to afternoon and evening sessions in order to provide much-needed feedback on several proposals that would effectively increase their flood protection to one-in-200-year levels.
Agents from Manitoba Transportation and Infrastructure (MTI), KGS Group, and Scatliff, Miller and Murray were on site to guide Tuesday’s meetings. They represent experts in the field of landscape engineering and urban development.
Most of these same professionals were also on hand at the introductory meeting held in December 2022.
The third and final phase of community consultation will take place sometime this spring where residents will be able to view final, detailed designs that reflect much of the feedback that is gleaned throughout the process.
The hope is that construction will begin shortly afterward in order to complete the project by March 2025. Achieving this deadline will allow the province to maximize federal funding, which is currently available through the Investing in Canada Infrastructure Program grant.
Raising the Dike and Improving Drainage
Since the initial December meeting, engineers and design teams have been busy conducting a variety of analyses on the area’s topography, including soil composition and drainage concerns in and around Ste. Agathe.
An environmental review was also conducted, including a look at the preservation of the aquatic habitat and prevention of shoreline degradation.
Finally, resident feedback from the initial meeting was considered prior to the creation of preliminary dike designs. This early feedback included concerns over the loss of river views and yard space, as well as the potential for exacerbating existing drainage problems.
Residents also looked for reassurance that public space along the riverbank would be enhanced and access to the riverbank, especially around the boat launch, would be maintained.
Jared Baldwin is originally from St. Adolphe and is the MTI project manager for the Ste. Agathe initiative.
“The province will successfully apply for federal funding,” says Baldwin, “but in order to get as much money as possible, we have to do our utmost due diligence.”
According to Baldwin, based on the analysis, the dike will be raised in almost every area where it currently exists. In spots where there is no dike currently, the team strongly recommends that something permanent be built.
In order to determine what is needed to achieve a one-in-200-year flood protection level, each area of the dike has been assessed based on a number of factors, including its ability to withstand maximum overland water levels plus an excess of two feet for environmental impacts such as wind that causes the water to crash against the dike.
As a result of their findings, the team is recommending that the dike be raised as little as a third of a meter in some areas to as much as 1.5 meters in others.
Bruno Arpin, project manager for KGS, says that the dike along the northeast corner of the community provides the least amount of concern. The existing dike there is plenty wide and will require minimal raising.
By slightly relocating that dike in two locations, the team says that internal drainage can be significantly improved.
The balance of the dike surrounding the community is much narrower, he says, and will require more work to shore it up.
In terms of gaps in the dike where highways pass through, Arpin says the goal is to make these areas easy to close off with the least amount of work.
“This program isn’t about raising Highway 75 or PR 305,” Arpin says. “It’s about raising lower-lying land [surrounding them] and reducing the effort required when there is a need to provide that flood protection.”
In some areas, Arpin says, appropriately sized and gated culverts can be installed. He described gated culverts as passive systems with flaps on the exterior side that operate through gravity. They open automatically to release water from inside the dike when pressure is detected without letting external floodwater back in.
Doubling up on the safety measures could mean adding a manual slide gate on the inside of the dike culvert in case of an outside gate failure.
The open stretch of land along the riverbank has the greatest potential for environmental impact, according to Baldwin. A dike was never installed here in the past due to concerns about negatively impacting the riverbank’s stability.
For the small community, this means that every time river levels rise, a temporary dike needs to be erected and then deconstructed later.
Using modern technologies, the engineers and design teams have come up with a variety of permanent flood protection options.
“Riverbank stabilization, under any circumstances, is necessary,” Baldwin says. “There’s no way around it except to do nothing or to do a half [measure], but honestly I’m not keen on a half solution.”
Without stabilization, he adds, a permanent dike would have to be erected farther from the riverbank, in the area where buildings and the community’s main street currently sit.
“Once this is all said and done, we want to be able to demonstrate that we’re leaving the environmental conditions better off than when we started,” Baldwin says.
Riverbank Dike Options
Arpin introduced three different options, appealing to the residents for feedback on each one, or recommendations on a variation of them.
“What we are presenting today may be a hybrid or a combination thereof,” said Arpin. “Once we understand what people like or don’t like, we can start to advance to a preferred design solution.”
All three options include a permanent dike in one form or another, as well as built-in erosion protection along the outside of the dike nearest the riverbank.
Option one includes an earthen dike similar to that of the existing dikes around the community. To the east of this new dike, two parallel-running inground shear keys would be installed to provide protection against riverbank erosion.
A shear key involves trenching deep into the clay along the riverbank and filling it with rock.
“Essentially we’re taking weaker materials and replacing them with stronger materials,” Arpin said.
At the same time, he indicated that shear key construction can be an expensive endeavour given the tonnage of rock that would be required.
“The extent of riverbank improvement required for option one is approximately twice as much as what’s required for options two and three,” he added.
The second option for residents’ consideration involves a mechanically stabilized earth (MSE) wall which creates a much smaller footprint than an earthen dike.
MSE retaining walls are composite structures consisting of alternating layers of compacted backfill and soil reinforcement elements fixed to a wall facing. The MSE wall lends itself well to the inclusion of decorative terraces and natural-looking steps for access to the riverside.
The installation of two shear keys is required here, but in this case they can be narrower, reducing the amount of rock fill.
The final option would replace the earthen dike and MSE wall with a bin wall that is driven deep into the clay beneath the soil. Examples of bin wall dikes can be found in St. Jean Baptiste and Emerson.
These walls have the appearance of corrugated metal but, for improved aesthetics, it can be clad with a variety of weatherproof finishes.
Like option two, the bin wall dike has a much smaller footprint. The obvious advantage here is that only one narrow shear pin along the river is needed.
“At this stage, it’s more about presenting the concepts and options and receiving the feedback,” Arpin says. “The detailing [after this] is where we get into the cost comparisons… We really didn’t want price to be the driver for this discussion.”
Attendees at the public meetings were encouraged to provide verbal feedback and fill out response forms available at the community hall.
Alternatively, residents can also send feedback by email to jared.baldwin@gov.mb.ca.
