The city will postpone work on the middle Colliery Dam until 2023 while data is gathered to redefine extreme flood danger for the Chase River. (News Bulletin file photo)

Further work postponed on middle Colliery Dam

New study suggests extreme flood flows might be less likely than previously estimated

The city will postpone further work on the middle Colliery Dam until at least 2023 while more data is gathered on the Chase River.

In a press release issued Tuesday morning, the city said the completion of the lower Colliery Dam auxiliary spillway partially address outstanding requirements from the Provincial Dam Safety Section order, which also required the city to address safety risks at the middle dam.

The city reviewed flood and seismic risks for the middle dam. The new study suggests that extreme flood flows through the middle dam might be lower than previously estimated.

“This new analysis takes advantage of river level data that wasn’t previously available,” said Poul Rosen, city senior manager of engineering, in the press release. “The additional data gained by continuing to monitor river levels will provide more certainty of the results and ensure that any future decisions on the middle dam can be made with confidence.”

The study looked at several years of river level data gathered through the early warning system installed at the dams in 2014 to make predictions about extreme flood events and also compared runoff rate data from other watersheds in the region to provide the basis for the decision to defer further risk assessment until 2023.

“Since this new model only uses a few years worth of data, it is not possible to make definitive predictions about extreme events,” Rosen said. “However, the lower flow results suggest that years of further data collection is warranted before any significant decisions are made or any potential future work at the middle dam planned.”

As a result of the study, the city and province agreed the best strategy regarding the middle dam will be to continue gathering river flow data for a hydrology model update in 2022.

For more information on the Colliery Dams visit www.nanaimo.ca/goto/dams.

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