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Geosciences Column: Do coastlines have memories?

do coastlines have memories

Did you know that the shape of coastlines is determined by the angle at which waves crash against the shoreline. It has long been thought that fluctuations in the wave incidence angle are rapidly felt by coastlines, which change the shapes of their shores quickly in response to shifting wave patterns.

Or do they?

Researchers at the British Geological Survey, Duke University (USA) and Woods Hole Oceanographic Institution in Massachusetts, have performed experiments which show that spits and capes hold ‘a memory’ of their former shapes and past wave climates, influencing their present geomorphology. The findings have recently been published in the EGU’s open access journal Earth Surface Dynamics.

Gradients in sediment distribution within wave-driven currents and shoreface depth play an important role in shaping coastlines. But the angle between an offshore wave crest and the shoreline is chief among the parameters which shape coasts worldwide.

Low-angle waves – those with approach the coast at an angle of 45° or less – have a smoothing effect on the coastline and keep its shape relatively steady. On the other hand, high-angle waves – those with slam against the shore at an angle of 45° or more – introduce instability and perturbations which shape the coast.

The figure shows the experimental set-up used in the study. It also nicely illustrates how coastlines are shaped by the angle of the incoming wave. The arrows indicatenet flux direction under waves incoming from the left; arrow lengths qualitatively indicate the flux. Sand is not transported through cells which are in shadow for a particular wave. From C. W. Thomas et al., 2016.

Alterations to the patterns of shorelines are caused by enhanced erosion and/or deposition, driven by changes in wave climate. Ultimately, coastline geomorphology evolves depending on the relative degree of high and low-angle waves in the wave climate, as well as the degree of irregularity in the wave angle distribution.

Climate change will alter the wave climate, particularly during storm events, so we can expect shorelines to shift globally. Predicting how coastlines will adapt to changing climatic conditions is hard, but more so if coastlines retain a memory of their past shapes when responding to changing wave regimes.

Flying spits (finger-like landforms which project out towards sea from relatively straight shoreline) and cuspate capes (a triangular shaped accumulation of sand and shingle which grows out towards sea) are particularly susceptible to climate change. They form when high angle waves approach the shore at a slant. Animal communities living within fragile marine and estuarine ecosystems largely depend on the protection they offer. They are also of socio-economic importance as many shelter coastal infrastructures. Understanding how they will be affected by a changing climate is vital to develop well-informed coastal management policies.

To understand how changing wave climates affect the evolution of flying spits and cuspate capes (from now on referred to as spits and capes), the team of researchers devised experiments which ran on a computer simulation.

They generated an initially straight shoreline and set the wave conditions for the next 250 years (which is the length of time it takes in nature) to allow the formation of spits and capes.

To test whether pre-existing coastal morphologies played a role in shaping coastlines under changing wave climates, over a period of 100 years (which is loosely the rate at which climate change is thought to be occurring under anthropogenic influences), the scientists gradually changed the angle at which waves approached the coast.  After the 100 year period the simulation was left to run a further 650 years under the new wave conditions.

The investigation revealed that when subjected to gradual changes in the angle at which waves approach the shoreline, capes take about 100 years to start displaying a new morphology. The tips of the capes are eroded away and so they slowly start to shrink.

Spits adjust to change much more slowly. Even after 750 years the experimental coastlines retain significant undulations, suggesting that sandy spits retain a long-term memory of their former shape.

Snapshots of simulated coastline morphologies evolved under changing wave climate. U is the fraction of waves which are approaching the shoreline at 45 degress or higher. Coastlines evolved for 250 years under initial conditions. (aii, bii)> The U values of the changed wave climate show the coastline morphologies evolved 200 and 500 years after the wave climate is changed at 250 years, and the morphologies evolved over 1000 years under static wave climates with the same U. From C. W. Thomas et al., 2016. See paper for full image caption. Click to enlarge.

The implications of the results are far reaching.

Be it implicitly or explicitly, many studies of coastal geomorphology assume that present coastal shape is exclusively a result of present wave climate. The new study shows that even with steady wave climate conditions at present, coastline shapes could still be responding to a past change in wave climate.

Reconstructions of ancient coastal geographies and paleo-wave climates might also be approached differently from now on. The researchers found that as spits adjust to changing wave climates they can leave behind a complex array of lagoons linked by beach bridges. Though there are a number of process which can lead to the formation of these coastal features, researchers must also consider alterations of coastlines as a response to changing wave climate from now on.

The findings of the study can also be applied to the management of sandy coastlines.

Currently, forecasts of future shoreline erosion and sediment deposition are made based on observations of how coasts have changed in recent decades. The new study highlights these short observation timescales may not be enough to fully appreciate how our beaches and coasts might be reshaped in the future.

This is especially true when it comes to climate change mitigation. Decisions on how to best protect the world’s shores based on their environmental and socio-economic importance will greatly benefit from long-term monitoring of coastal geomorphology.

But more work is needed too. The experiments performed by the team only consider two types of coastline morphology  (spits and capes) and only two types of wave climate. While the experiments provide a time-scale over which spits and capes might be expected to change, other factors not considered in the study (wave height, shoreface depth, etc…) will alter the predicted timescales. The time-scales given by the study should be used only as a guideline and highlight the need for more research in this area.

 

By Laura Roberts Artal, EGU Communications Officer

 

References

Thomas, C. W., Murray, A. B., Ashton, A. D., Hurst, M. D., Barkwith, A. K. A. P., and Ellis, M. A.: Complex coastlines responding to climate change: do shoreline shapes reflect present forcing or “remember” the distant past?, Earth Surf. Dynam., 4, 871-884, doi:10.5194/esurf-4-871-2016, 2016.

Geosciences column: Shelter island – building a barrier to protect the coast

The latest Geosciences Column features recent research into tsunami hazards and explains how island building out to sea can help protect buildings on the shore…

Barrier reefs are well known for holding off the wrath of the ocean and sheltering the serene lagoons that stretch between them and the mainland. Barrier islands possess the same protective power, taking the impact of waves that have built up across the ocean and dissipating their energy before they break on the continent. Now, a team of Spanish and Columbian scientists have shown how this barrier island effect can be harnessed to protect communities from the worst of ocean waves – the tsunami.

Tsunamis are generated when vertical faults beneath the seabed slip, causing a large earthquake (over magnitude 5 on the Richter scale) and displacing a huge volume of water. They pose a greater hazard than earthquakes alone, and in seismically active coastal areas they are a significant concern. One such area is the seismic belt that shadows the coastline between Ecuador and Columbia, where the Nazca Plate subducts beneath the South American.  There have been six major quakes along the belt in the last century, the most recent of which was a magnitude 7.7 quake that resulted in a devastating tsunami and the destruction of an entire island within the Mira River Delta in 1979.

Spot the difference. Top: the island of El Guano prior to the 1979 tsunami; bottom: the same area following the tsunami. (Credit: Otero et al. 2014)

Spot the difference. Top: the island of El Guano prior to the 1979 tsunami; bottom: the same area following the tsunami. (Credit: Otero et al. 2014)

In the Columbian department of Nariño alone, the 1979 tsunami resulted in the loss of over 450 lives and 3080 homes. But the devastation would have been greater if it weren’t for El Guano, a sandy barrier island that was once present just off the country’s Pacific coast. By modelling tsunami as it happened, and how it would unfold if it occurred again today, Luis Otero and his colleagues from the University of Norte, Columbia, and the Environmental Hydraulics Institute IH Cantabria, Spain, showed just how good a barrier the island was – cutting the energy transferred to the island city of Tumaco by up to 60%.

It’s not the first time natural defences have been shown to protect the coast. Indeed, studies of the 2006 Boxing Day tsunami in Indonesia have shown that reefs, mangroves, beaches and dunes all provide the coast some protection by absorbing the tsunami’s initial impact and slowing the speed of the advancing wave.

How flooding would differ if El Guano island was present: (a) shows the current situation and (b) shows what would happen if the island was recreated. The white regions represent the areas that are not flooded and the black line shows the shoreline. (Credit: Otero et al. 2014)

What a difference an island makes: (a) shows the current situation and (b) shows what would happen if the island was present. The white regions represent the areas that are not flooded and the black line shows the shoreline. (adapted from Otero et al. 2014)

Tsunami hazard in this region is both high and likely, and the team show that rebuilding the island would be a worthwhile engineering effort if the government hopes to afford the area the same protection it had in ’79 in the future. Elongating the island would increase its protective potential even further, as would reshaping the it to form three similarly shaped barriers to cut the energy transferred to the Columbian coastline beyond.

Otero’s tsunami model showed such engineering would offer tremendous protection to Tumaco and the other inhabitants of the Mira River Delta in the event of a tsunami – particularly one that occurred at high tide. But because Tumaco is such a sizable coastal city, some unprotected areas would remain.

Currently, the government’s focus is on establishing a swift and effective early warning an evacuation strategy, but a barrier island could provide a big boost to the safety of the local population and the security of local infrastructure.

By Sara Mynott, EGU Communications Officer

Reference:

Otero, L. J., Restrepo, J. C., and Gonzalez, M.: Tsunami hazard assessment in the southern Colombian Pacific basin and a proposal to regenerate a previous barrier island as protection, Nat. Hazards Earth Syst. Sci., 14, 1155-1168, 2014.

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