lAKE MAJOR dAM REPLACEMENT

• Meco

» Scope of Project

Meco was engaged by Halifax Water to replace the Lake Major Dam to achieve long term security of the Dartmouth Water Supply.  The existing timber crib dam sustained damage during an early winter flood in 2014/15 that altered the operating options for the dam.  The replacement dam was designed in 2015/16 and constructed in 2018/19.  Meco worked with Halifax Water through the initial stabilization, operational adjustments for fish passage, options analysis for a replacement design, detailed engineering and construction support and administration.  Other partners on the project include Envirosphere Consultants (Fisheries) and Lindsay Construction (Builder).

» Execution

The dam was constructed with reinforced concrete for the operational areas, and a concrete core-wall embankment fill for the abutments and an adjacent saddle dam.  Meco was the principal engineer for concept development, feasibility, hydrology and hydraulics, structural and geotechnical design, detailed engineering and contract administration. 

There were several challenges to construction because the reservoir is the water supply for the City of Dartmouth and had to remain operational during construction.  The temporary works during construction included a diversion system that could discharge a 20 year flood event without impacting construction. 

Similarly, fisheries migration had to be maintained throughout construction.  Egress was achieved through use of an innovative use of fish pump technologies adapted from the aquaculture industry.  The fish pump system was in place from 2015 to 2019 with effective results.

» Project Highlights

The replacement design was governed by hydraulic controls upstream and downstream of the dam.  The Lake Major Road bridge was replaced in 2011 but the opening caused backwater effects at the spillway that compromised hydraulic efficiency.  As well, the foundation level for the pump house to deliver water to the residents of Dartmouth was located near the flood operating level.  To manage the upstream and downstream controls, Meco designed a high efficiency Labyrinth type spillway that provides more downstream discharge for higher frequency events but will increase reservoir surcharge when backwater impacts occur from the bridge.  This is the first use of the Labyrinth type spillway in Atlantic Canada.  The design was able to accommodate competing demands while meeting safety criteria of the Canadian Dam Association.

The replacement dam also includes a low level under-sluice and variable head pool and weir type fish-way to connect the marine ecosystems upstream and downstream, and an energy dissipation basin to manage erosion.  The spillway channel embankment is stabilized with willow, an innovative use of vegetation that is beneficial to fish populations and displaces use of coarse rock.