As the sea level slowly rises, we may wonder: what will happen to our beaches? Will they change, maybe even disappear, or will they simply survive the inevitable rise in the sea level? These questions are relevant to the vast majority of Aotearoa New Zealand’s population: they directly affect cultural sites and communities, and have large societal and economic implications.
The first of the many problems we face when addressing these questions is that there is not one single and conclusive answer. NZ has an incredible variety of coastlines and beach types: from the sandy white shores in the Coromandel to the black sand of the West Coast, from the gravel coastlines of the South Island to cliffs or rocky shores. There are also many differences in the geological settings. Some of our coastlines are rising well faster than the mean sea level. To complete and complicate the mix, the “drivers” of erosion (waves and currents) change around the islands, from the continuous roaring of the West Coast waves to tropical cyclones irregularly hitting the East Coast. And we can only try to guess what the future waves will be!
Every beach has its own history and will have its own future but there are commonalities. And, since the sea level is rising (and won’t stop rising for the foreseeable future), we can try to anticipate beach response.
Scientists have long debated the effect of sea level rise and the only consensus is that our ability to predict is poor. The prediction horizon of today’s models rarely exceeds a few years. We organised an international “blind” modelling competition to predict the shoreline evolution of Tairua beach in the Coromandel. Modellers were given three years of wave data and had to submit predictions of the corresponding beach changes. All participating scientists met to discuss the quality of their predictions (it was a “blind” competition and before coming to New Zealand none of the participants knew how their model had performed). We realised that while many of the models managed to provide comparable results and somehow reproduced the observed changes, extending model predictions to the year 2100 resulted in extremely variable results. Again, this is not entirely surprisingly, given the daunting task of predicting this far into the future.
In 2020, a new study from a European research group hit the news and claimed it was possible to model and predict that up to 50% of the world’s beaches would go “extinct” by 2100. The study got huge attention from the media (probably more than the authors anticipated) especially in the Southern Hemisphere since many beaches in Australia were predicted to disappear. The results also prompted a response by scientists worldwide who highlighted the shortcomings of the assumptions of the study and clarified some points that required more attention, particularly in the context of long-term beach change and sea level rise.
The first point to make is that local geological setting can play such an important role that all processes related to waves, currents and sediment transport can be completely overwhelmed. The most obvious case is given by landscapes that are rising faster than the sea level as a result of geophysical processes, for example an earthquake. But the geological conditioning can be more subtle, and recent studies show that sediment supply and the overall shape of the landscape can also overcome marine processes.
Aside from geological effects, the concomitant effect of long-term sea level changes, tides, waves and currents will certainly affect our beaches. As much as we try to generalise future beach response, it is important to make a clear distinction between beaches backed by a structure (be it a natural cliff or an artificial seawall) and open coasts. Studies show that often the natural response of a natural beach to sea level rise is to literally “roll” shoreward. The beach will retreat and migrate onshore, and provided there is enough space and sand available, it will not go extinct and might instead maintain its width. Thousands of years ago when the sea level was higher than present, the world was full of beaches. But there are some grimmer possibilities too, particularly if the beach is backed by a vertical structure, be it a cliff or a seawall, which is the case for many beaches in urban areas. Sediment supply and other factors might slow or even offset the effect of sea level rise, but those beaches are almost inevitably going to recede. In many cases, this will heavily affect our communities and infrastructures. The process cannot be stopped without engineering or adaptation options.
Quantifying if and when a beach will be entirely lost is extremely hard. Many of today’s techniques used to predict beach evolution rely on unrealistic assumptions, and their blind application (for example, with no consideration for geological conditioning) gives predictions of limited usefulness. Together with a group of scientists around the world (USA, Spain, Australia), we are trying to develop new methodologies that specifically focus on useful predictions of beach evolution for the year 2100.
What we recently discovered is that if we want to try to predict beach changes over the long-term, we need to look beyond the traditional set of drivers (usually we use wave characteristics like wave height and period) and also consider climatic variations. Predictions of beach change appear to improve when weather patterns (for example the sea level pressure for the region surrounding the location of interest) are also considered. Our approach allows us to extract recurrent patterns from observations and relate them to the occurrence of storms, to the presence of larger waves in winter than in summer, and to multi-year patterns related to climatic oscillations.
Testing on beaches in New Zealand, Australia and France is extremely encouraging but despite the advances, we have to accept that response to sea level rise remains site-specific and extremely variable, and predictions far into the future will always be filled with uncertainties. Overall, it is evident that all beaches will undergo changes in the future. While we know that some of New Zealand’s most iconic and wild beaches will evolve and survive sea level rise, we also know that where beaches are backed by cliffs or structures, their existence is threatened.
- Cooper, J.A.G., Masselink, G., Coco, G., Short, A.D., Castelle, B., Rogers, K., Anthony, E., Green, A.N., Kelley, J.T., Pilkey, O.H. and Jackson, D.W.T., 2020. Sandy beaches can survive sea-level rise. Nature Climate Change, 10(11), pp.993-995.
- Montaño, J., Coco, G., Cagigal, L., Mendez, F., Rueda, A., Bryan, K.R. and Harley, M.D., 2020, A multi‐scale approach to shoreline prediction. Geophysical Research Letters, p.e2020GL090587.
- Montaño, J., Coco, G., Antolínez, J.A., Beuzen, T., Bryan, K.R., Cagigal, L., Castelle, B., Davidson, M.A., Goldstein, E.B., Ibaceta, R. and Idier, D., et al., 2020. Blind testing of shoreline evolution models. Scientific Reports, 10(1), pp.1-10. doi.org/10.1038/s41598-020-59018-y
- Vousdoukas, M.I., Ranasinghe, R., Mentaschi, L., Plomaritis, T.A., Athanasiou, P., Luijendijk, A. and Feyen, L., 2020. Sandy coastlines under threat of erosion. Nature climate change, 10(3), pp.260-263.