GeoLog

Palaeo-shorelines

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.

Imaggeo on Mondays: Arid lands and ancient lakes

Palaeoclimatologist Annett Junginger takes us to one of the hottest and driest places on Earth in this week’s Imaggeo on Mondays…

“Suguta Showers” by Annett Junginger, distributed by the EGU under a Creative Commons licence.

“Suguta Showers” by Annett Junginger, distributed by the EGU under a Creative Commons licence.

The picture was taken in 2010 during the third in six expeditions to the remote Suguta Valley in the northern Kenya Rift. This unbelievably beautiful place is located just south of Lake Turkana and is one of the hottest and driest places in equatorial Africa. Temperatures often reach 50°C in the shade, but there is no shade. Evaporation is so high that occasional rainfall over the valley escarpments only fills the rivers temporarily, and water often evaporates from the valley slopes faster than it can reach the deepest part of the valley to form a lake. Only during the rainy seasons there is enough water to allow the development of the small Lake Logipi (5 x 10 km across and approximately 3 m deep), which is a key destination for millions of Flamingos.

Due to the remoteness and hostile conditions of the valley, it is largely uninhabited, allowing the perfect preservation of all kinds of geological information. To make the most of this preservation, the Suguta Valley Project has been set up to investigate the valley’s late Cenozoic structural development and lacustrine deposits. The aim is to better understand environmental changes over the course of the region’s volcano-tectonic evolution and the climate fluctuations of the last five million years.

From left to right first row: Lake Logipi during the rainy season; a palaeo-shoreline 300 m above Lake Logipi approximately 10,000 years old (credit: M. Trauth); Lacustrine sediments of the same age. From left to right second row: Flamingos (pink dots) along the shore of Lake Logipi; Mid-Pleistocene lacustrine sediments; Erosion of Holocene lacustrine sediments. From left to right third row: Lake Eight, a maar lake in the southern part of the valley; Sand dunes in the foreground and palaeo-shorelines on the Namarunu volcano in the back; palaeo-shorelines on a small crater in the centre of the valley approximately 5,000 years old. (Credit: Annett Junginger)

From left to right first row: Lake Logipi during the rainy season; a palaeo-shoreline 300 m above Lake Logipi approximately 10,000 years old (credit: M. Trauth); Lacustrine sediments of the same age. From left to right second row: Flamingos (pink dots) along the shore of Lake Logipi; Mid-Pleistocene lacustrine sediments; Erosion of Holocene lacustrine sediments. From left to right third row: Lake Eight, a maar lake in the southern part of the valley; Sand dunes in the foreground and palaeo-shorelines on the Namarunu volcano in the back; palaeo-shorelines on a small crater in the centre of the valley approximately 5,000 years old. (Credit: Annett Junginger)

It may sounds strange that lacustrine deposits can be found here, when the valley experiences such hot and dry conditions. But this is case! Palaeo-shorelines and up to 40 m thick lacustrine sediments are the remains of the huge palaeo-Lake Suguta, 300 m deep and 2,200 km2 large, that existed between 15,000 and 5,000 years ago – from a geological perspective, that’s not long ago at all! We also found lacustrine deposits up to 1 million years old that explain multiple humid periods in the past. The investigation of these palaeo-climate records may help to provide new insights into (1) the spatial and temporal synchronicity of the Early Holocene humid period in East Africa, (2) the role of the tropics in glacial-interglacial transitions during the Pleistocene, and (3) the links between climate changes and human evolution.

By Annett Junginger, Institute for Earth and Environmental Sciences, University of Potsdam

Reference:

Junginger, A., Trauth, M.H., Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15–5 ka BP), Glob. Planet. Change (2013)

Imaggeo is the EGU’s open access geosciences image repository. A new and improved Imaggeo site will be launching soon, so you will be able to peruse an even better database of visually stunning geoscience images. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

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