This research could contribute to improving the design of boulder-based breakwaters placed offshore to absorb wave energy and mitigate erosion across the world.
Every year, around 24 percent of the world’s sandy shoreline erodes away, causing significant financial loss from damage to property and infrastructure. With around 40 percent of the global population living within 100 km of the coast, erosion presents a real and immediate challenge. Coastal erosion is a costly affair. In the US, for example, coastal erosion causes an estimated $500 million USD in property loss and damage every year.
Cutting-edge environmental research in Israel is taking advantage of new technologies and the unique environment of the Dead Sea to better understand the processes behind coastal erosion and sediment transport.
“Understanding coastal erosion and the transport and sorting of coarse sediments along shores is highly important for assessing coastal stability,” says Haggai Eyal, a Ph.D. candidate at the Hebrew University of Jerusalem. He is supervised by Professor Nadav Lensky from the Geological Survey of Israel and Professor Yehouda Enzel from the Hebrew University of Jerusalem.
Ideal Conditions for Capturing Sediment Movements During Individual Winter Storms
Eyal’s Ph.D. focuses on the fluvial (relating to rivers) and coastal movements of sediments in the Dead Sea, a hypersaline lake located between Israel, Jordan, and the West Bank. The Dead Sea is primarily fed by the Jordan River and does not drain or discharge to the ocean, making it a terminal lake. Sediments from the Jordan River and other channels make their way to the Dead Sea through a variety of sediment transport processes.
“Fluvial channels supply unsorted sediment to the lake during winter storms,” says Eyal.
During these storms, waves transport sediments from the channel mouth along the shore, creating berms—beach ridges that extend along the shore.
During the dryer and hotter summer months, the lake level drops about 1 m.
According to Eyal, because of the dramatic seasonal drop in water level, these berms are unique to each winter season, creating an environment where sediments from each storm are preserved separately. This is unlike other areas where the water level is relatively stable from one season to the next.
Because of the Dead Sea’s dramatic seasonal drop in water level, the berms—beach ridges that extend along the shore – are unique to each winter season, and sediments from each storm can be identified separately. (Photo credit: Josh Gold)
Understanding How Coastal Erosion Relates to Atmospheric and Oceanographic Processes
To understand the role different atmospheric and oceanographic processes play in coastal erosion and apply that knowledge to other parts of the world, real-world oceanographic measurements are necessary.
“This is where the science of coastal erosion is challenging, specifically where waves break,” says Lensky. “You need the wind measured properly, which is easy enough, but then you need to measure currents and waves, which is very hard to achieve.”
Nortek’s Signature ADCPs provide crucial quantitative measurements of the currents throughout the water column and directional waves simultaneously.
“The ability to measure directional waves with the Signature1000 enabled us to correlate the wind direction and the wave directions,” Eyal explains.
Wave direction is an essential characteristic of sediment transport along the coast.
“The interaction between waves and coasts and the angle between them dictates the amount of longshore sediment transport,” he says.
By using the Signature1000’s three-dimensional current measuring and wave profiling capabilities, the researchers were able to measure nearly the full water column.
To account for the area closest to the seabed missed by the Signature because of its up-looking orientation, the researchers filled the gap by measuring currents using an Aquadopp 300 m current meter.
Researchers in Israel take advantage of ADCPs and other advanced technologies deployed in the unique environment of the Dead Sea to better understand the processes behind coastal erosion and sediment transport. (Photo credit: Josh Gold)
Understanding Coastal Erosion and Gravel Transport
The measurements from the study revealed some key insights about the processes that dictate transport in the Dead Sea.
“We see that the transport is dictated mainly by the waves, not the current,” says Eyal. “This combination of using measurement of wind as atmospheric drivers, directional waves and current profiles and turbidity profiles using the Nortek Signature1000, and motion measurements from smart boulders is unique. It allowed us to quantify the transition of momentum from wind to waves to coastal erosion and gravel transport along the coast,” he adds.
Prof. Lensky and his student Shai Abir prepare the Signature1000 ADCP installed in a gimbaled tripod for deployment on the shore of the Dead Sea. (Photo credit: Nadav Lensky)
Applying Coastal Erosion Research With ADCPs to the Rest of the World
The study was published in Geophysical Research Letters in collaboration with researchers from the University of California in the USA and Technische Universität Braunschweig in Germany. This paper offers insights into sediment transport processes beyond the Dead Sea. Using the observations from the study, the researchers demonstrated how sediment sorting is directly related to the wave climate. They developed a new model for quantifying the wave height needed to move a given mass of gravel and formulated how the distribution of wave heights during a storm sorts gravel along the shore.
This information is helpful in designing boulder-based breakwaters, structures built from boulders placed offshore intended to absorb wave energy.
“You want these boulders to stay in place. In our research, we quantify the size of the boulders that move at a certain wave intensity distribution,” says Lensky. “If we properly understand the physical relation between the waves and the underlying processes that move the boulders, you can generalize these processes to other regions.”
Lensky notes the importance of repeating their findings in other areas. “To verify that these insights are valid for other coasts, we need to have similar measurements from coasts with a different wave climate,” he adds.
Boulders are often used to create breakwaters that absorb wave energy. Knowing what size boulders are needed to stay in place helps ensure the longevity of breakwaters. (Photo credit: Monika Schröder – Pixabay license)
