Three of four dams belonging to the Sunshine Coast Regional District (SCRD) are going to require upgrades and repairs at a potential cost of $2 million, the SCRD board of directors has been told.
Deficiencies were identified following a comprehensive review of the dams conducted by the Vancouver engineering firm WSP Canada Inc. Representatives from the company outlined the report’s findings in a delegation to the Dec. 17 online meeting of the board’s infrastructure committee.
Two dams are located in Tetrahedron Provincial Park on the lower Sechelt Peninsula, at Chapman Lake and Edwards Lake, about 1,000 metres above sea level. The two others, one made of concrete, are on McNeil Lake, south of Madeira Park and at an elevation of about 170 metres. Chapman and Edwards Lake provide the bulk of the water supply for the southern part of the lower Sunshine Coast, while McNeil Lake is a water source for the northern end. The so-called saddle dam on McNeill Lake is called an embankment dam and does not appear to require work.
The dams were constructed sequentially over the past 55 years, most recently on Edwards Lake, in 1991. Chapman Lake’s is the biggest of the four dams, measuring 35 metres long and 3.7 metres high
While the dams need upgrades, the report did not cite any deficiencies that require urgent action. The concrete on all the structures was “in generally good condition,” the report said. Key improvements would help deal with potentially damaging ice and wood debris. There is also a need for security fencing and remote surveillance systems.
In addition to assessments of the dams’ condition, WSP developed maintenance and surveillance manuals for district staff, prepared emergency plans, and outlined the consequences of the failure of any of the structures.
Failures of the dams, while not considered likely, could in some cases be life-threatening and cause serious physical damage downstream. One safety concern is ice on Chapman Lake, which can form between December and May, WSP engineer Vincent Cormier told the meeting.
“Since the dam is high up in altitude, there’s a pretty thick ice cover that can develop on the lake and that pushes against the dam,” said Cormier. “In Canada, there’s some minimum requirements to withstand those kind of forces and Chapman dam doesn’t meet them, unfortunately.”
Cormier told the meeting that a major failure of the Chapman Lake or Edwards Lake dams could unleash a sudden, high volume of water downstream toward Big Maple mobile home park on lower Chapman Creek and on the highway bridge that spans it. “It’s the only link between Roberts Creek and Gibsons to Sechelt. So, if the bridge fails, there is no connection between those,” Cormier noted.
He said the threat could be reduced by stabilizing and reinforcing the structure with rebar “that will be drilled in the concrete dam all the way to the bedrock underneath.”
While the McNeil Lake concrete dam also needed upgrades, ice formation is not an issue due to the lake’s lower elevation.
Total upgrade costs were estimated at $764,250 for the Chapman Lake dam and $633,250 for the dam on Edwards Lake. About one-quarter of the total is for helicopter services required to access the deeply remote lakes. The cost of upgrading the McNeil Lake dam, which can be accessed by road, is estimated at $580,150.
Design and permitting would take place in 2021 and actual construction would begin in 2022, SCRD staff said in its report. The report and recommendations will go to the SCRD board for consideration.
