Robin Davidson-Arnott | Bernard Bauer – Predicting the Impacts of Rising Sea Levels on Sandy Coasts

Sandy beach systems are complex because they evolve through the interplay of ocean waves, currents, and tides, and wind action on land. Many beaches are backed by sand dunes that formed when sand was transported from the beach inland by strong winds over many years. But dunes can be quickly eroded by waves during storms, returning some of their sand to the sea in a matter of hours.

However, beaches and dunes will rebuild, as sand in the nearshore is brought back to the beach during normal conditions between storms. Dry sand on the beach surface is readily blown inland by wind, rebuilding the dunes and completing the cycle. The impact of sea-level rise on this cyclic process is difficult to predict, but the answer to this is key to understanding how sandy beach systems will evolve in response to rising sea level.

Since the 1960s, coastal managers and engineers have relied on the Bruun model, which predicts the relationship between sea-level rise and shoreline recession. A critical outcome of the model is that sand is continually moved offshore as the shoreline retreats, giving rise to a loss of sand from the beach. In 2005, Robin Davidson-Arnott of the University of Guelph proposed an alternative model – the RDA model – that envisions sand moving progressively onshore from deep water toward the beach, and then to the dunes. In the RDA model, sandy beach systems will move landward and increase in elevation, keeping pace with sea-level rise.

With his colleague Bernard Bauer of the University of British Columbia, Davidson-Arnott published a paper in 2021 updating the RDA model, taking into account that the evolution of sandy beach systems in response to rising seas is driven by processes controlling the nearshore-beach-dune cycle.

They reviewed studies of sea-level rise in the recent and geological past showing that sand transfers between the nearshore, beach and dunes were rapid compared to the slow rate of sea-level rise. Thus, most beach-dune systems can easily keep pace with sea-level rise and maintain their integrity, as long as there are no obstacles preventing shoreline migration.

Davidson-Arnott and Bauer also investigated which conditions might complicate the applicability of the RDA model. Specifically, the model assumes that sea level rises by a few millimetres each year, as is currently the case. If the annual rate is ten-fold greater, as can be found in areas of coastal subsidence, there is insufficient time for sand transfers to keep pace with the rapid inland migration of the shoreline.

They also note that human infrastructure, such as buildings, roads and seawalls, can constrain the natural migration of the beach-dune system, likely leading to coastal drowning and the loss of sand to other sections of the coast.

A key control on shoreline migration in the RDA model is the slope of the beach following a major storm. Very steep beaches will experience minimal migration whereas very shallow beaches will likely be over-run by the sea eventually. However, for average beach slopes, and even with expected increases in atmospheric carbon, the researchers predict that sandy shorelines will move inland by less than 20 metres by the end of this century. This can be easily accommodated with some planning by coastal managers.