Keeping Tidal Forces at Bay

In 2020, Ducks Unlimited Canada (DUC) and the Canadian Wildlife Service (CWS) restored a wetland located at the Wallace Bay National Wildlife Area (NWA) that was experiencing a breached and eroding dyke from sea level rise. Wallace Bay in northern Nova Scotia is perched on the shores of the Northumberland Strait and the NWA consists of more than 1,433 acres (580 hectares) of marshes, fields, and forests. In the 1970s, DUC and CWS created managed wetlands that became home to species like the ring-necked duck, blue-winged teal, and American black duck. Impounded freshwater wetlands were developed over a large section of lands that were once drained and dyked for agriculture. However, with recent sea level rise, tides were topping the dyke, speeding up its erosion, and making it harder to maintain. The mix of saltwater and freshwater in the ecosystem wasn’t as welcoming to plants and wildlife that originally called the area home. In 2016, management goals and objectives were established through the Wallace Bay National Wildlife Area Management Plan. To address the impacts of climate change, the plan’s management approach included identifying and removing barriers to promote inland migration of salt marshes in response to rising sea levels, and identifying and mitigating areas of high erosion on impoundment dikes. The restoration project in 2020 involved removing a dyke and water control systems to reintroduce the tidal influence to a wetland impoundment, allowing reconnection of the tidal regime. The free flow of tidal water provided conditions for salt marsh restoration to occur. The project resulted in approximately 23 hectares of saltmarsh habitat restored. DUC has partnered with the Confederacy of Mainland Mi’kmaq (CMM) to monitor both the Wallace Bay site and a control salt marsh, located just a kilometre away.

Understanding and Assessing Impacts

Over the last 50 years, wetlands were created over a large section of land that were once drained and dyked for agriculture at Wallace Bay NWA. This marine wetland is made up of tidal channels and salt marsh; biodiversity has flourished at Wallace Bay, with DUC and CWS working together to monitor habitat conditions, adjust water levels and maintain the infrastructure. While the Wallace Bay NWA is generally protected from the most severe storms due to its location at the headwaters of Wallace Harbour, over time, with sea levels rising, one of the segments, Wallace Bay #3, became more challenging to oversee. Tides were topping the dike, speeding up erosion, and making it harder to maintain. A management plan for Wallace Bay NWA was completed by the Canadian Wildlife Service in 2016. The management plan identified a number of challenges and threats including predicted sea level rise resulting in erosion of the expansive salt marshes within Wallace Bay. The management plan notes that, as the uplands rise quickly in elevation around the existing wetlands, the loss of salt marsh along the coast is unlikely to be matched by gains at the salt marsh-upland interface. With increasing frequency and strength of storms, this may cause erosion to increase with eventual significant loss of wetland habitat. At Wallace Bay 3, marsh transgression was not possible because of dyke infrastructure that excluded tidal processes.

Identifying Actions

Over the last 50 years, wetlands were created over a large section of land that were once drained and dyked for agriculture at Wallace Bay NWA. This marine wetland is made up of tidal channels and salt marsh; biodiversity has flourished at Wallace Bay, with DUC and CWS working together to monitor habitat conditions, adjust water levels and maintain the infrastructure. While the Wallace Bay NWA is generally protected from the most severe storms due to its location at the headwaters of Wallace Harbour, over time, with sea levels rising, one of the segments, Wallace Bay #3, became more challenging to oversee. Tides were topping the dike, speeding up erosion, and making it harder to maintain. A management plan for Wallace Bay NWA was completed by the Canadian Wildlife Service in 2016. The management plan identified a number of challenges and threats including predicted sea level rise resulting in erosion of the expansive salt marshes within Wallace Bay. The management plan notes that, as the uplands rise quickly in elevation around the existing wetlands, the loss of salt marsh along the coast is unlikely to be matched by gains at the salt marsh-upland interface. With increasing frequency and strength of storms, this may cause erosion to increase with eventual significant loss of wetland habitat. At Wallace Bay 3, marsh transgression was not possible because of dyke infrastructure that excluded tidal processes.

Implementation

Management approaches were established to ensure wetland habitats will continue to provide habitat for migratory birds including waterfowl, waterbirds, and marsh birds under a changing climate. These approaches include monitoring habitat changes and coastal erosion within the NWA through annual aerial photography, identifying and removing barriers to inland migration of salt marshes in response to rising sea levels, and identifying and mitigating areas of high erosion on impoundment dikes.

In this case, continued dyke maintenance at Wallace Bay #3 was no longer feasible due to continued scour and erosion from tidal hydrodynamics. Furthermore, restoration of the marsh platform and tidal regime was required to make room for tidal marsh migration inland. Flood risk scenarios, LiDAR DEM, topographic maps, current and historic aerial imagery, and solicitation of local knowledge, were used together to advise the restoration plan. Adjacent landowners were involved and supportive of the project goals. Following consultation, DUC and CWS agreed to reintroduce tidal influence on the impoundment by removing the hard infrastructure. The free flow of tidal water would provide conditions for salt marsh restoration to occur and allow free fish passage by virtue of channel reconnections. Three attributes were critical to restoration success: reconnection of tidal channels, adequate sediment supply, and suitable marsh elevation. Presence of a halophytic vegetation seedbank was also considered before proceeding.

Outcomes and Monitoring Progress

In September 2020, CWS and DUC began work to restore the site to the original salt marsh through managed dyke breaches. The densely vegetated access road was mulched to allow equipment to transit. Impoundment water levels were slowly reduced to reveal the marsh platform. Following dewatering, excavation equipment leveled the dyke material, depositing the excess soil back into the impoundment borrow pit. Later, two associated water control structures were also removed, and the main and secondary channel were reconnected to the tide. By reintroducing the tidal influence on this impoundment, conditions were established for salt marsh restoration to occur. Reconnection of tidal channels was also important to re-establish dynamic equilibrium and increase soil solidification rates on the marsh.

As the tide comes into the salt marsh, soil particles flocculate out of the water and deposit on the surface of the marsh, building up the marsh over time. With the tide flowing, salt water is pouring back into the marsh, revealing mud flats and letting the seeds of salt grasses buried deep in the soil pop back to life. In patches of marsh that are shallower, freshwater plants and cattails might hold on for years before giving way to reedy cordgrasses. As the salt marsh develops, the vegetation will grow and change along with it. There’s the first colonizer cordgrass species that takes root in the initial plunge of new water. As the marsh rises and elevates with more sediments, the successional plants come in. And all the while, these changes are crucial for combatting climate change.

Next Steps

Managers are hopeful that no other intervention action will be necessary at the site. Ecological monitoring efforts aim to reveal how the restoration is progressing and will identify any adaptive management required. Managers hope to see a return of several fish species, including striped bass, killifish, trout, and gaspereau. In addition, CMM is hoping to see a resurgence of sweetgrass, a common salt marsh plant, and one that’s particularly important to the Mi’kmaq and is used ceremonially and for smudging.

Resources

Link to Full Case Study

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