case study

Halting erosion and monitoring habitat restoration along Victoria’s Ross Bay seawall

When the City of Victoria moved to halt seawall erosion and re-nourish the beach along the historic Ross Bay waterfront, Archipelago joined the project to help minimize any impact from construction, and assess the restoration of habitat above and below the tide line over the long term


 

Located along the south coast of Victoria, British Columbia, the Ross Bay seawall was built in 1911 to protect the historic Ross Bay Cemetery from erosion caused by waves pounding against the shoreline. To halt the encroachment and protect the cemetery, a recurved concrete seawall designed to redirect the wave energy was constructed. Shore–side graves were relocated, and Dallas road, which had originally diverted traffic north around the cemetery, was extended the length of the shoreline between the seawall and the cemetery.

seawall-erosionThough protecting the cemetery from further erosion, the seawall introduced problems of its own. The action of the waves relentlessly scoured the beach at the base of the seawall, and storms brought waves and logs crashing across the road, requiring temporary road closures and removal of debris afterwards.

Over the decades, city crews tended to the ongoing erosion of the seawall toe, and continually repaired failures of the aging concrete along its length, but over 80 years, one to two metres of beach was eroded away from the toe of the seawall. By 1990, the intertidal beach had disappeared completely.

In 1993, a larger concrete seawall was installed along a portion of the shoreline, replacing the curved backwall with a deeper, stepped design intended to accommodate foot traffic along the esplanade. While the angular structure proved popular with runners, walkers, and beach goers, nearby residents noted increased noise and vibration.

In an effort to identify a practical, yet environmentally sensitive solution, city planners contacted Vancouver–based Sandwell Engineering, and Archipelago Marine Research of Victoria. After examining the problem and experimenting with detailed physical modeling, Sandwell’s coastal engineers recommended a three–part plan: raise the approach to the wall with gravel and beach fill; establish a series of perpendicular groynes jutting into the bay to reduce erosion of beach surfaces; and implement a program of long–term monitoring to determine whether erosion had decreased, while assessing the restoration of habitat above and below the tide line.

Sea wall erosion rehab

Archipelago habitat biologists evaluated potential impacts to eelgrass habitat, crab population, and juvenile fish, and adopted a series of mitigating strategies to minimize any negative impacts from construction, while ensuring the preservation and well–being of the nearshore community. In addition, Archipelago worked with Sandwell and the City of Victoria to design a long–term monitoring plan that would assess the efficacy of the gravel fill design.

Beach nourishment RBPhase 1 began in 1995, during which 19,000 tonnes of gravel was placed in front of the wall to raise the existing beach to the top of the lower step on the reconstructed seawall. In 1998, Phase 2 added 60,000 tonnes of gravel beach fill to the mid and eastern part of the bay, three large rock groynes (installed perpendicular to shore) to help keep the gravel in place, and a habitat berm situated parallel to shore.

Now more than two decades on, long–term monitoring has confirmed the success of this process. While the re–nourished beach has met the design objective of protecting the seawall, erosion has virtually ceased; in fact, gravel was found to be moving onshore at a slightly higher rate than anticipated—eliminating the need to replenish gravel at a later date.

Rocky areas have since regenerated, and as a result, offshore rock sills have proven to be excellent habitat for bladed kelp and bull kelp. Home to gulls, otters, blue herons and more, the Ross Bay seafront is a popular destination year round for residents and visitors alike.

For more information on this or similar projects, contact Archipelago.

Beach nourishment - 10 years after

 

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