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.