Will it or Won’t it?

Will the new weir raise the winter high water levels or won’t it?

One of the more common questions I have heard is regarding lakefront owners concern for flood levels. Many of the older structures were built before the current development rules (to build above the 200 year flood level) came into place and as such some of those buildings see flood conditions on occasion. They don’t want to see an increase in frequency or higher flood elevations if possible.

This brings up the question on whether or not the new weir will impact flooding conditions. Well, this is exactly one of the concerns we are trying to determine within our current projects (Weir Design and Shoreline Assessment). The shape, location and operability of the weir along with modelling historical, current and future hydrologic conditions will help us determine the impact the new weir would have during every part of the year. The answer, at this point, is we do not know yet. The weir is in the middle of it’s design process and has to account for a number of design inputs with the priority being a structure that optimizes environmental (fish) flows and fish passage from river to lake and vise versa.

The current weir is submerged throughout the winter months and the gates are laying flat on the river bottom. In the winter high water condition you can’t even tell a weir exists as it is submerged and the water easily passes overtop. That is because the weir itself is flooded due to the restriction at the Greendale Trestle where the river canyon narrows and the river bed rises. After the weir is taken off of control in early November the river flow, and therefore the elevation of the lake, is controlled by other restrictions such as the narrow valley at the Greendale Trestle. The new weir will also be submerged during the wetter winter months – as such it should become the narrow valley that continues to control flooding conditions. Our analyses will help us determine whether or not that is true.

4 thoughts on “Will it or Won’t it?

  1. Considering the only months that the river flow becomes critical for fish and supplying the pulp mill, has anyone considered the notion of approaching the mill on the idea of having the mill owners develop a desalination plant? I understand the mill owners have very deep pockets and wouldn’t it be great if they were to give back to the larger community by providing themselves and the community the water security each party needs?

    1. I understand (unofficially) that Crofton has looked at desalination but the costs are very prohibitive. The other point to consider is that the mill’s withdrawal system is near Duncan. The stretch of river from the Town of Lake Cowichan to Duncan would continue to experience low flow and drought conditions which would continue to affect environmental conditions and fish habitat if a new weir was not constructed.

  2. It seems odd that the design would come before a shoreline assessment. I realize that elements of the design would affect the potential impact on the upstream shoreline but the basic principle of raising the weir level should be enough to model lake levels – shouldn’t it?
    My concern, like many, is the impact on maximum lake levels during the winter.
    The fact that this project has been long in the works, and that the design comes first, makes me concerned that the shoreline assessment is unlikely to cause the project to be shelved or substantially altered.
    Is there any potential result of the shoreline assessment that would cause the project to be shelved? Are there design elements that can be changed that significantly change the upstream impact? (ie greatly increasing flow at high water levels?)

    1. The original plan was to have both projects (weir design and shoreline assessment) run in parallel. The weir design projected was able to be scoped and awarded fairly quickly and the timeframe to do the design came in shorter than planned. The shoreline assessment project took longer to scope and award and is taking the anticipated timeframe to complete. Also, the shoreline assessment modelling needs to take into account the weir design and it’s operating characteristics so the sequencing of the two projects is valid. Note that the elevation of the new weir is based upon the 2018 Water Use Plan recommendations.
      Both of these projects will be used to inform the future water license holder and government approval bodies of the cost, timing and the implications of a new weir and they can decide if there need to be further adjustments or other considerations.
      note: At high winter water levels (above 163.07m) the water would flow over the weir.

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