Cofferdam – Control the Flow
By: Cathy Vaughan
Reprinted with permission from Chebucto News
Installation of a cofferdam is an essential phase of Build Nova Scotia’s construction project to replace the deteriorating, 255-year-old dam on Williams Lake.
A cofferdam is a temporary dam built on the lakebed next to the old dam to create a dry, stable, safe work area for crews and heavy construction equipment to operate. It looks like a sculpted rock pile of clean, densely compacted rocks, gravel, clay, and sand and may include a geotextile, waterproof impermeable liner. The density and weight of the rocks provide a stable foundation for the watertight barrier around the construction area.
on the lakebed next to the old dam.
Once the cofferdam is installed, the lakebed can be cleared of rocks and natural debris and crews and equipment can begin to dismantle and remove the old dam. Then work will begin to excavate the site down to bedrock to prepare for construction of the new concrete dam.
As part of this construction phase, lake water levels will be temporarily lowered to not only accommodate the installation of the temporary cofferdam but to ensure that it remains stable throughout the project. Water levels will be continuously monitored and adjusted as evaporation and rain events naturally occur.
The cofferdam has specifically engineered sloped compacted rock walls which face the lake waters and fend off waves, lake currents, and weather-caused erosion. The huge water-diversion pipes next to the cofferdam will allow excess water to drain into the existing Lawson’s Creek. Once the new dam is built, this temporary cofferdam and pipes will be removed.
There are several important safety features of cofferdams covered by Canadian Dam Association guidelines, federal environmental and in-water regulations, provincial engineering codes, and national dam occupational health and safety guidelines.
Cofferdams must be watertight to keep the construction site as dry, stable, and safe as possible; able to withstand existing lake water pressure, waves, and major meteorological events and use materials that are clean and easy to dismantle and recycle. Build Nova Scotia’s team continues to work closely with Dexter Construction Company the on-site contractors to ensure the required safety and environmental standards are achieved.
Completion of the new dam is projected for this fall, with restoration and replanting of the area scheduled for the spring of 2027. As with any construction build of this complexity, conditions do change throughout the phases of this project. Adjustments may be required and will adhere to environmental standards and safety guidelines.
For updates on this important project, check Build Nova Scotia’s website and follow Williams Lake Conservation Company social media (listed on the website’s front page).