The Bras d'Or Lakes, Nova Scotia: seafloor topography, backscatter strength, coastline classification, and sensitivity of coasts to sea-level rise

Research was carried out in the Bras d'Or Lakes, Nova Scotia under the aegis of Project X-29 in NRCan's Geoscience for Ocean Management Program, with additional funding from the Climate Change Impacts and Adaptation Program,. The goal was to provide the scientific information that would help resolve management problems arising from the expected impact of accelerated sea-level rise on the coasts of the Bras dOr Lakes. The specific objectives of the work were: Define the recent, present and future trends of water-level increase in the Bras dOr Lakes. Knowing the recent (last 5000 yr) trend would allow us to understand how rising water levels triggered changes to coastal environments in the lakes. Knowing the modern trend would help us to understand coastal changes over the past 100 years. Some idea of future water levels would be a prerequisite to assessing how the modern coasts will change. Map the modern coastal environments. Assess future impacts on the range of coastal environments, particularly those environments that we suspected to be most sensitive and hence vulnerable. Transfer of information on coastal vulnerability to sea-level rise to stakeholders in GIS formats suitable for their systems. The results of the work are summarized as follows: 1) The lakes were fresh until ca. 6,350 calendar years ago, when rising sea level crossed the -25 m sill and connected them with the ocean. The rate of sea-level rise at the start of inundation was 79 cm/century, and has declined throughout the past 6000 years. Coastal landforms such as spits, barrier beaches, and cuspate forelands were submerged when exposed to the high rate of relative sea-level rise. Submerged shores are visible on multibeam sonar imagery, mainly in the southern lakes, where sediment supplies were abundant. Submerged river networks occur in St. Patricks Channel and Denys Basin. 2) The trend of modern sea-level rise in the region is 36.7 cm/century. Assuming the median increase predicted by the International Panel on Climate Change (2001) (48 cm/century from 1990-2001) and assuming it is distributed equally around the globe, then sea level in the Bras dOr Lakes will increase by 75 cm over the period 1990-2100 AD. The rate of increase will be 60 cm/century by 2030 AD, 99 cm/century by 2080 AD, and 115 cm/century by 2100 AD. 3) Total shoreline length is 1272 km, or 14.4 % of the Nova Scotian coastline (8811 km). Shorelines are grouped into eleven classes: three types of rock shore, seven types of nonrock shore (unconsolidated), and artificial shores. Coastal barriers make up 12 % of the shoreline. 39 % of these barriers are building and established, 44 % are in the breakdown and collapse phase, 13 % are in transition, and 4 % are artificially constrained. 4) We group the shoreline types into three sensitivity classes, depending on the likelihood that changes will be triggered by sea-level rise. 18.8 % have high sensitivity, 73.9 % have moderate sensitivity, and 7.3 % have low sensitivity. The most sensitive shoreline types are unconsolidated cliffs, coastal barriers, and artificial shores. 5) Coastal barriers will continue to change in their natural cycles of growth and decay over the coming century, but at higher rates. There will be an increasing tendency for complete submergence of coastal barriers by 2030 AD, and a strong likelihood of submergence by 2045 AD. We predict accelerated unconsolidated cliff erosion and increasing effort and expense to maintain coastal defenses, particularly those on barrier beaches that would otherwise migrate or submerge. 6) The recommended response to these future changes is to allow them to take place with as little interference as possible, that is, to allow the coast to respond in a natural way as it did in the past. Having a natural coast will ultimately benefit the region more than having a coastline constrained by coastal protection structures.

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