The height of waves must be taken into account in rising sea levels on the coast.

The height of waves must be taken into account in rising sea levels on the coast.

© IRD/ H. Bataille

Waves disrupt sea levels

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Updated 20.02.2019

Research by ocean and coastline experts shows the hitherto unrecognised influence of waves on the sea level on the coast. Their contribution to overwash is at least equivalent to that of sea level rise linked to melting ice and ocean warming.

Coastal erosion, submerged shorelines, migration of island populations: sea level rise has major consequences. The role of waves in this phenomenon has long been underestimated... The research conducted by a team of French scientists based in Toulouse could be a game changer. “We highlight their substantial contribution to sea level variations”, explains coastline physicist Rafael Almar. He is the co-author of a recent publication on the subject1.

Rising sea levels result from variations on different spatial and time scales. Some of these variations, global and slow, contribute to changes in the overall sea level. They notably include water transfers from land masses to the oceans - melting glaciers and ice caps, variations in inland water reserves - the thermal expansion of the oceans and the mass, salt and heat redistribution caused by oceanic circulation patterns. Other local and lower-magnitude variations are involved on a coastal scale, such as tides, storm surges relating to atmospheric pressure and waves.

Storm surge and superposition

The water level on the shore is therefore the superposition of the total ocean level and variations recorded locally, explains the researcher. Previous studies on the subject did not take these coastal processes into account. The water level on the coast was estimated as a function of the average sea surface height”. Tide gauges, devices designed to record the sea surface height, are installed a little offshore or on sites protected from the destructive action of waves, thereby measuring a total value, not the effective shoreline value. This is why the contribution of waves has remained unknown, except in the specific interpretation of extreme events, storms, cyclones or tsunamis.

Waves are caused by wind on the sea surface, states Rafael Almar. Their movement is often formed hundreds or thousands of kilometres offshore, before travelling to the coast”. All great bodies of water are subjected to winds, as a result of which their coastlines are hit by waves of varying sizes.

1m offshore, up to 20cm on the coast

To assess the waves’ overwash effect, and therefore their contribution to changes in sea level on the coast, we used an empirical model shared by the oceanographic community. This model estimates this storm surge at 1/10th to 1/5th of the height of offshore waves, depending on the characteristics of deep-water waves and the shoreline topography”, says the expert. For example, 1m waves of offshore result in a 10-20cm rise in sea level on the coast. When applied on a global scale over the 1993 to 2015 period, the calculation reveals the importance of the waves’ contribution to interannual and decadal variations in sea level on the coast. “We show that their contribution may be of the same order of magnitude as, if not greater than that linked to thermal expansion and mass loss from glaciers and ice caps”, claims the scientist. This research advocates for the integration of this overlooked component into sea level studies. This is essential in the current context of climate change, where changes in sea wind conditions could modulate the impact of waves on the coastlines.

 


Note :

1. Melet A., Meyssignac B., Almar R., Le Cozannet G., Under-estimated wave contribution to coastal sea-level rise, Nature Climate Change, 24.01.18


Contact rafael.almar@ird.fr