Onslow-North River Managed Dyke Realignment and Tidal Wetland Restoration Project

In October 2021, the Nova Scotia Departments of Public Works, Agriculture, and Environment and Climate Change, in collaboration with CB Wetlands and Environmental Specialists (CBWES), Saint Mary’s University (SMU), and Queen’s University, implemented a dyke realignment project to address the rising sea levels and coastal flooding occurring on the Salmon River.

The Onslow-North River Managed Dyke Realignment and Tidal Wetland Restoration Project (Onslow-North River project) is located along a section of dykeland in the County of Colchester, encompassing the towns of Truro and Stewiacke, and the Millbrook First Nation. The lower Salmon River floodplain, the site of this project, was converted from salt marsh to agricultural land through the use of dykes and aboiteaux, to keep flood waters out. An aboiteau is a culvert with a hanging gate that allows freshwater from precipitation to drain to the sea while closing during high tides to prevent salt waters from flooding the dykelands. However, these are no longer equipped to deal with flood risks when heavy rainfall events coincide with high tide.

Dyke realignment was determined to be the most cost-effective long-lasting approach. The realignment process involved breaching the dyke to allow the return of tidal water to the floodplains. This reintroduction of tidal waters allows sea marshes to grow along the dykes. It will gradually assist the land in transitioning back into a tidal wetland ecosystem over three years. These wider floodplains and foreshore marshes can absorb large amounts of water and reduce the force of waves. In doing so, the process results in the creation of fish and wildlife habitats that contribute to reducing flood risk, enhancing local biodiversity, and supporting climate resilience in the region.

Understanding and Assessing Impacts

Climate change has led to warmer and wetter conditions in Nova Scotia, leading to more frequent and intense rainfall events and sea level rise. This has resulted in increased coastal flooding where the Salmon River meets the North River, negatively impacting coastal communities such as Truro and Colchester.

The scope and causes of the increased flooding were analyzed in a risk assessment by CBCL Limited, an engineering firm. This began with field data collection where the ground topography and river bathymetry were surveyed. Tipping bucket rain gauges were installed in 2014 to monitor rainfall, water level gauges were installed at the Salmon River, North River, and McClure’s brook to monitor tide levels and sediment samples were collected from five locations. Data collected was used in a range of hydraulic, two-dimensional, and three-dimensional models and geographic computer tools to estimate impacts. CBCL further projected possible flood risks up till 2100. They marked the area around the river banks in three categories – Floodway, Floodway 1:20 and Floodway 1:100. 1:20 identified the probability that floodwater reached the mark once in the next 20 years and 1:100 identified the probability for 100 years. This helped demarcate areas of higher and lower risk.

A Social Vulnerability Assessment was undertaken to determine how different regional populations were at risk of flooding and other related hazards. Through an analysis that utilized the Canada Index of Multiple Deprivation for the area, it was revealed that the variables of Residential Instability and Economic Dependency in Truro had significantly higher values than those for the Onslow side of the river. This was related to factors such as a higher number of senior residents, a larger number of homes needing repair, and a more significant number of residents living at home. Factors such as these affect the population’s ability to respond to and cope with hazards such as flooding, which helps determine how best to support them in adaptation.

Identifying Actions

While traditional approaches to flood management, such as strengthening dykes and constructing runoff storage structures had been implemented in the past, they were expensive, and the municipalities believed a more comprehensive regional approach was needed. Salt marshes help reduce flooding by widening the floodplain, allowing them to accommodate more water during high river discharge events.

Excess water will be absorbed and drained through the creek network, preventing water from backing up. This is a nature-based climate adaptation strategy that uses the natural ecosystem of the salt marshes and tidal wetlands to help communities adapt to flooding, which is a more cost-effective long-term solution.

The aforementioned CBCL study also modelled over 40 flood scenarios and options to reduce risks through advanced mapping and flood simulation technologies. This involved identifying a combination of adaptation measures such as realignment and pumping, and other measures such as land use planning and regulation changes to reduce the priority areas that would experience flooding by 30%. Implicit in this was a recognition that future floods could not be prevented.

Design, consultative education, a collaboration between various stakeholders, the buy-in of impacted parties, and, aligning of complementary needs were the methods successfully used in the project planning and execution. The various stakeholders (Federal, Provincial, Municipal bodies, land owners, citizens, and infrastructure providers,) were consulted on the Joint Flood Advisory Committee. The design was adapted to create minimum disruption to infrastructure (rail and power), protect a heritage cemetery, and factored in any relocation required.

Implementation

The process began with archeological investigations from 2016 to 2018 to clear the area for dyke work and tidal flooding. It was then followed by the breaching of the dyke by excavators to restore tidal flow. The new dyke was built, and three of the four aboiteaux were removed. The old dyke was flattened, and a new aboiteau was built to cater to improve drainage to protect the CN Rail line. Dyke breaching was rescheduled from 2020 to the summer of 2021 to allow NS Power to relocate infrastructure. The tidal flow began in 2021 through the remaining aboiteau, which was finally removed in November 2021. The TransCoastal team measures sediment concentrations and deposition to assess whether the project has succeeded.

After the breaching, the area will initially look like a mudflat as sediment is deposited from the Bay of Fundy tides and existing plants die off. With time, wetland grasses and plants will begin to grow, and wildlife will return to the area, transforming it into a productive tidal wetland.

This was complemented by the municipalities of Truro and Colchester designating certain areas as unsuitable for development or only permitting low-intensity development such as trails and camping facilities to improve land development. Effective land use planning helps to support dyke realignment and protect the newly created tidal wetlands. The planning and regulation process is still underway.

The project was estimated to cost $1,655,559 (in 2018 CAD), with the purchase of the land costing almost $800,000 and the earthworks and breaching activity costing $625,000. It was funded by a number of departments, including the Department of Transportation and Infrastructure for monitoring and planning costs, the Department of Agriculture and the National Disaster Mitigation Program. The Flood Risk Infrastructure Investment Program also provided $400,000 for Truro’s comprehensive flood risk study.

Outcomes and Monitoring Progress

The outcomes can only be estimated as the project began its implementation in 2021. The salt marsh is expected to gradually be established over three years and move through three phases: transition, establishment, and equilibrium.

The project is expected to result in reduced flood risk as the wider floodplain will be able to absorb and slowly drain more water during high river discharge events. This increased flood resilience would help protect people, infrastructure, and homes. It will also lead to increased carbon sequestration, improved water quality, and the creation of fish and wildlife habitats which will increase biodiversity.

Some challenges emerged during consultations which were addressed by the project team. For example, locals were concerned that mosquito populations would increase. This was addressed by providing accurate information to the public and being as transparent as possible during consultations, including inviting mosquito experts to discuss mitigation strategies. Another was the potential impact on infrastructure, resolved by moving a utility line and providing additional protection to a rail line.

The CBWES and SMU will be conducting pre- and post-monitoring to track changes to the project site over time. This will span five years and aims to ensure proper habitat recovery. It will focus on a range of biophysical variables, including sediment accretion and carbon sequestration potential. It will be funded by the Nova Scotia Department of Public Works.

Next Steps

The project has begun devoting public resources to public education, as evidenced by the development of a website that outlines the details of the project and will continue to explore other public education approaches, especially with other organizations. Education resources promote stakeholder engagement, more informed decision-making, and increased collaboration.

Additionally, the area will be used as a living laboratory, promoting the scientific study and supporting environmental education on such managed realignment techniques and tidal wetland restorations.

Dyke realignment and tidal wetlands restoration cannot solve flooding on their own. The municipalities of Colchester and Truro will need to continue to assess their land development patterns, planning, regulations on zoning and use of various designated areas, and floodproofing requirements to ensure the dyke realignment is and remains effective at flood risk reduction. The Coastal Protection Act regulations for managing development at the shoreline will be a useful tool for municipalities in policy making. Additionally, stormwater control and management were recommended by CBCL.

Resources

Link to Full Case Study