Mountain Station reservoir comes under scrutiny in safety review

The city is dammed if they do and dammed if they don’t.

And it is a good thing to be dammed, in the case of the city’s Mountain Station reservoir, especially since the reservoir is now classified as a “dam” by provincial authorities.

The regulation of water reservoir dams in British Columbia is a provincial responsibility, and under the Water Sustainability Act and the associated regulation, the Dam Safety Regulation, the reservoir was the topic of conversation with provincial regulators four years ago.

After it was determined that Mountain Station was a dam, it meant a number of submissions were required from the city, including the condition of the concrete and the soil material underneath it.

“It’s a very high consequence dam,” said Colin Innes, city director of Engineering, Capital Works and Special Projects.

A consequence classification is an assessment of the risks associated with and the consequences of failure for the structure — especially considering its location above the city. Therefore, it was imperative that the city really take a look at how this structure is performing, Innes pointed out.

Some preliminary work has been done on the dam to assess the condition of the concrete, and what was used as a foundation and base.

“Even though it really seems, based on the age of the structure and how well it has really held up, it looks like things were constructed very well, but there is not a lot of record of all of that,” said Innes.

“And that’s part of the problem, not knowing what level of quality assurance and quality control was used to put this structure here.”

The data collected from investigations in early October by SNC Lavalin — based on the drilling of three bore holes on the crest of dam — showed right away that there wasn’t any shear plane.

“So we’ve got to do some intrusive work here to make sure we know what this thing is sited on and how the materials underneath it are performing,” Innes said. “The last thing you would want is to have a lot of moisture moving underneath it, creating a sheer plane and then the potential for a landslide that could be triggered by a seismic event.”

The results will be put into a computer model that will be compiled into a report, ultimately landing on the desk of the provincial dam regulator by Dec. 31.

Owning the regulation

The Dam Safety Regulation specifically addresses the responsibilities of the dam owner for the safe operation of a dam and prescribes documentation requirements for the dam.

It also prescribes surveillance activities, dam safety reviews, and operational testing of flow control equipment.

A dam owner is responsible for carrying out dam safety reviews as outlined by the Dam Safety Regulation. Dam owners are required to comply with this legislation by having a qualified professional engineer carry out a dam safety review.

The qualified professional engineer is expected to prepare a dam safety review report which will be submitted to the regulatory authority for acceptance.

Source: Colin Innes, director of Engineering, Capital Works and Special Projects

Dam safety inspection

The dam safety inspection (DSI) is a visual inspection that provides comments and recommendations regarding dam safety — which was provided to the safety regulator in early 2022 by the city.

Next on the list is for the city to provide a dam safety review (DSR), a legislated review of the safety assessment of water reservoir dams carried out by a qualified professional engineer.

The DSR involves a systematic review and evaluation of all aspects of the design, construction, maintenance, operation, processes and systems affecting a dam’s safety, including the dam safety management system.

“In general terms, ageing and wear and tear present constant challenges to dam owners, and new threats to the safety of the dam can sometimes emerge,” said Innes in his report.

A dam safety review assesses if there is any significant deterioration in the level of safety — which can be estimated in terms of an increased risk position — has occurred since the last dam safety review.

The DSR will ultimately determine if the overall level of risk is being maintained within limits considered to be tolerable, said Innes.

“British Columbia’s natural environment, climate, and associated natural hazards require that the dam safety review pays particular attention to possible meteorological, geological, environmental, and seismological events,” he said. “These are generally considered in terms of floods and landslides, although such simple categorization masks the complexity of these hazards, which can act in combination.”

Source: City of Nelson Oct. 25 agenda

Timothy Schafer, Local Journalism Initiative Reporter, The Nelson Daily