4 minute read
Gibraltar Point
Toronto, Ontario, Canada
Placing an innovative nearshore reef and sand dune for shoreline protection. Off the coast of the City of Toronto at the southwesterly tip of the Toronto Islands on Lake Ontario, Gibraltar Point has historically experienced decades of shoreline erosion and storm damage. For decades, the roughly 500-meter-long reach of shoreline from Gibraltar Point to the north has severely retreated by approximately four meters per year. At the request of Toronto and Region Conservation Authority (TRCA), Baird & Associates initiated a study to develop a nature-based design to mitigate erosion and maintain a dynamic beach shore through a long-term, sustainable sand-management approach, using minimal structural intervention and maintaining economic feasibility. In this multiphase project, TRCA initiated development, design, and construction of a nearshore reef concept that would mimic natural coastal features and include improvements in the aquatic habitat. Further improvements and mitigation efforts included construction of a sand dune to provide additional erosion control and develop habitat areas. In 2021, following completion of the reef, 50,000 tonnes of imported sand were barged in and placed to construct a two-hectare sand dune plus additional dune enhancements, including planting 4,200 trees and shrubs, 500 herbaceous species, 7,300 beach marram grass plants (Ammophila), and other native plant species. Logs, woody debris, and two songbird boxes were used as natural infrastructure to enhance the habitat value of the dune ecosystem. Construction of the dune was completed in spring 2023.
Producing Efficiencies
Traditionally, natural reefs have been mimicked by designing low-crested structures, or breakwaters, to protect against storm waves without being a visual obstruction. However, low-crested breakwaters do not stop all wave transmission but are efficient when most required, like during storm events. In this project, the reef was defined as a reef breakwater with a wide crest and gentle outer slopes built with rocks in the nearshore area. When there is not enough dissipation of wave energy through wave breaking (such as under higher water levels), the reef’s design allowed for additional dissipation via resistance against wave orbital flow through porous media.
Using Natural Processes
Naturally occurring nearshore reefs can mitigate wave transmission and sediment transport. The project’s reef mirrors the natural coastal features inherent in nearshore reefs. The submerged nearshore reef uses the natural wave breaking over a submerged structure to dissipate the wave energy before it reaches the shoreline, reducing erosion. Coupled with installation of the sand dune, the nearshore reef has the potential to not only improve coastal resilience to erosion but also enhance local ecology, provide recreational opportunities (e.g., snorkeling), and support desirable aquatic and terrestrial habitats.
Broadening Benefits
The City of Toronto is a bustling tourist destination. Only a short ferry ride from Downtown Toronto, the Toronto Islands are a popular location valued for their beaches and other recreational amenities, receiving thousands of visitors each year. The project has created a widened beach for visitor recreational use. Additionally, both terrestrial and aquatic habitats have been created and restored with construction of the submerged nearshore reef and sand dune, providing home to a variety of species.
Promoting Collaboration
Several design alternatives and approaches were considered before settling on the nearshore reef and sand-management concept, which provided the most economical and natural erosion control measures. The project was a collaborative effort that included many meetings and discussions between the design engineers, Baird & Associates, and the TRCA, on behalf of the City of Toronto. These meetings were crucial for aligning goals and objectives and creating a successful project for all stakeholders.
