Nearshore Sediment Transport in a Changing Climate: North Shore of Prince Edward Island, Canada

Manson, Gavin
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University of Guelph

Two impacts of changing climate that are affecting much of Canada's coast are decreasing sea ice and changing storminess. The historical trends and projections in sea ice are well known, but there is less consensus on historical and projected storminess. The general purpose of this research was to investigate nearshore sediment transport in a changing climate including various scenarios of wave storminess and sea ice concentrations off the central north shore of Prince Edward Island (PEI). Specifically, nearshore sediment transport was simulated using the Delft3D hydrodynamic modelling software to: help explain the persistence of some coastal sedimentary landforms; develop a technique to simulate wave- and current-driven nearshore sediment transport in the presence of nearshore sea ice; and simulate nearshore sediment transport under a range of wave and ice conditions to assess the relative impact of each on predicted nearshore sediment transport. The model employed two nested grids covering the central north shore of PEI. Waves currents, water levels, and sediment transport were modelled in open water conditions. The results were found to accurately simulate measurements collected during autumn field studies. In open water simulations in autumn, bedload comprised approximately 60% of total transport and was directed onshore. Suspended load comprised the remaining 40% and was directed offshore and alongshore to the east. Results from sediment transport modelling help explain the persistence of an offshore sand-starved zone, sand in the nearshore, and sediment supply to other transgressive geomorphological features such as large banks, flood tidal deltas and onshore dunes. To include sea ice, sediment transport in open water simulations was modified using a model of the exponential attenuation of the energy of waves entering nearshore sea ice. When compared to data collected during a field experiment measuring waves in sea ice, the model successfully simulated nearshore wave energy attenuated by sea ice. It was found that when sea ice was present, a semi-empirical model of the exponential decay of wave energy in sea ice can be applied at the coast. In 49 simulations of storminess and sea ice, sediment transport shows much less sensitivity to changing storminess than to changing sea ice. Scenarios of sediment transport in variable wave storminess are not significantly different from each other while scenarios of sediment transport in variable sea ice differ significantly. In the simulations varying sea ice, sediment transport increased to approximately 30% concentration, nearshore sediment transport doubled. As open-water storminess increased from present day through possible future scenarios, nearshore sediment transport increased a further 50%. When sea ice concentration decreases below approximately 30%, storminess will then become the important factor in rates of nearshore sediment transport On ice-affected coasts, understanding the implications of decreasing sea ice is of greater immediate concern for managing adaptation to coastal climate change than is understanding the potential implications of uncertain changing storminess.

sea ice, storminess, wave attenuation, coastal change