The Municipality of Chatham-Kent has released the geotechnical assessment, created by Golder Associates Ltd., regarding Erie Shore Drive.
Chatham-Kent declared a State of Emergency for the area late last week, held an emergency meeting on Friday about the issue and Council voted on Monday to close a portion of Erie Shore Drive until further notice.
“A recent visual assessment of the embankment indicated a recent slope failure, the development of new longitudinal tension cracks and the widening of existing cracks indicating that approximately the north half of the embankment is moving northward toward the adjacent municipal drain,” the Golder report states.
“Based on the visual assessment, the results of new slope stability analyses, predicted lake levels and the historical storm frequency in the late winter/early spring months, it is considered that there is approximately a 5 to 40 per cent chance for overtopping conditions to develop that would render the dyke unstable with progressive failures potentially leading to a significant breach.”
Although the probability of a major breach is less than 50 per cent, the consequences of such a breach would be severe given the low ground surface elevations to the north and the low elevation of Erieau Road, the Golder report indicates.
“Erieau Road provides the only access to the Village of Erieau and flooding due to a major breach of the dyke would effectively isolate the community,” the Golder report states.
“In addition, the overtopping events and any resulting failure of the embankment would preclude direct emergency vehicle access to the subject section of Erie Shore Drive and the residences to the south.”
The consultant’s report says in the near term, work is required to raise the embankment grade to prevent uncontrolled overtopping.
“Work is also required to provide more robust erosion control measures to reduce erosion/scour at the locations of discharge channels,” the Golder report indicates.
“Concrete blocks/earth fill used to raise the grade should be placed as close to south edge of roadway as possible (so as not to destabilize the north slope) without imposing loads on existing underground utilities that would cause compression of the underlying organic soils and resulting damage to the utilities.”
Golder officials added: “Given the lake level forecasts, the historical late winter/early spring storm frequency, and the severity of the consequences of a breach, there is some urgency in completing these efforts as soon as reasonably possible. As noted previously, the embankment serves as both a transportation corridor and a flood control structure. Given the current spatial and geometric constraints (width of right of way, proximity of Lakeshore Drain, slope inclinations), the grade raise required to prevent overtopping and the flatter slopes required for the discharge spillways preclude the continued use of the dyke structure as a transportation corridor for the short- to medium-term.”
In the longer term, Golder officials say work is required to also protect the embankment from saturation during flood events, to reduce the flow velocities in the discharge spillways, and to reinforce and/or flatten the north slope to enhance stability and further reduce spillway discharge velocities.
“Flattening the slope would require the relocation of the municipal drain further to the north,” Golder officials said in their report.
“For both the short-term and long-term mitigative measures, the timing and duration of construction activities would be highly weather dependent. If uncompleted works are exposed to severe weather/flooding events, the stability of the dyke would be further compromised potentially leading to severe erosion and failure. Accordingly, construction activities should be carried out when weather permits and the work should be carried out in relatively short sections that can be completed during the anticipated windows of suitable weather.”
Golder officials added: :Alternatively, the flood control function could be performed by a new seawall and armouring system installed along the Lake Erie shoreline. However, such an alternative would be technically complex and could prove to be economically infeasible.”
To read the full report from Golder, click here.
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