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.
We had our kick off meeting in the field yesterday with KWL consulting engineers who have expertise in water resource engineering. The team included a geomorphologist, a coastal engineer, a professional surveyor and a water resource engineer. The size, complexity and type of this assessment is unique. The work at Cowichan Lake will be a challenge but we have the expertise who can tackle it. Consider the variability of a lake of this size with changing beach slope angles, different geology, varying vegetation, changing winds, protected bays, exposed points, creeks and streams of all sizes as well as the work humans.
As we seek to determine the current natural boundary we need to consider a few factors: 1. The presence of water: how often water reaches a certain point on the shoreline influences what types of vegetation can grow at different elevations. 2. The action of water: the energy of waves along the shoreline depends on the exposure of the shoreline to prevailing winds or boat wakes. This wave energy determines how shoreline are eroded and vegetated over time. 3. The character of the current shoreline (slope, substrate type, vegetation) not only influences the location of the current natural boundary but also plays an important role in how the natural boundary could change as a result of the proposed upgrades and operation of the Cowichan Lake Weir.
Over the next several weeks measurements and photographs will be taken to capture existing conditions to ensure we have a solid benchmark from which to start our analyses. Our BC Professional surveyor will be using his years of experience to record the current natural boundary and key access points along the shoreline. Others will record beach substrate such as gravel, sand, mud, rock and vegetation. This new information along with existing photographs, and high resolution LiDar (light detection and ranging using laser light) data and other databases we will create a clear picture of what exists today. This information, along with a new weir design, climactic data, hydrological modelling will help us determine future impacts of raising the weir.
August is one of those months where most seem to take the time to vacation and enjoy friends and family. I also took some time to enjoy my kids and grandkids right here on Cowichan Lake. It is such a beautiful environment to relax and have some fun.
Now it’s back to work! I am really please to say that the Project has continued to progress on plan. A few highlights to share with you are that the survey results are ready to be published, that the preliminary conceptual engineering designs are starting to take shape and that part two of this project, the Cowichan Lake Shoreline Assessment work, has been awarded and work begins immediately.
Field work will begin mid September. This work will utilize a Professional Surveyor of BC to make visual observations, photograph and take some GPS measurements of current conditions of the existing shoreline. They will access the beach with a boat and work from the existing water level to the typical High-Water Mark. Those of you who live in the area will see a postcard in the mail describing this work and those who own lakefront property will receive a letter in the mail.
This shoreline assessment work will help us understand the potential impact(s) of raising the weir to the properties around the perimeter of the lake. Most people who live in the Cowichan Valley understand the affect of climate change and the necessity to raise the weir to ensure a healthy supply of water for our HERITAGE Cowichan River and all of it’s inhabitants (the fish)! Those who actually live on the lake they are trying to balance the between climate change, raising the weir and impact to their property. We hope to address this. Stay tuned.
Thank you to all who participated in the survey. I have looked at all the results and read all the feedback. There is quite a range of opinions and thoughts, however I found most to be positive and supportive. One thing I appreciate is trying to find the balance between ‘wants’ and costs. The other perspective is that this structure will be in place for the 50++ years so we do want it to have some appeal to the region for generations to come.
I’ll get back to you with some overall results and themes once I review your input with our key stakeholders (CVRD, Cowichan Tribes, Cowichan Watershed Board and Paper Excellence).
We just awarded the second key piece of work – Cowichan Lake Shoreline Assessment Study. This work will study today’s conditions and model future conditions. We hope to differentiate the impact the new weir would have on the shoreline from the impact of ongoing climate changes. Water levels and shorelines are not static systems – just a few thousand years ago this area was covered in ice! Modelling and assessing are not an exact science but with the latest evidence and best forecasting we will measure what the future will likely show us.
The drilling program and the Bathymetric Survey were both completed last on June 30.
Four core holes were drilled, two on the Weir island and one on each side of the river. We had anticipated reaching bedrock at 25-30 meters, but we found it at 37m on the island, and 47m an 50m at each abutment area. Testing and sampling were performed throughout the boreholes so now we have very, very good geological and geotechnical data to perform design analyses that meets all the provincial and federal requirements including seismology.
Glaciers on Vancouver Island about 15,000 to 20,000 years ago helped form our Cowichan Valley. As the glaciers receded they cut and formed and deposited materials throughout the valley. It is these glacial deposits that we are finding above the bedrock. Understanding the characteristics of these deposits are key to understanding design options.
A bathymetric survey exactly measures the lake and river beds. We performed a detailed survey on either side of the existing weir on June 30th. This is also necessary to inform the design.
The Public Survey has been coming in steadily with over 160 received so far. It’s too early to share the information, but it is providing valuable insight into what you think – Thank you for responding.