The coastal cliffs along the Mediterranean are eroding at an average rate of 10 centimeters a year, a much slower rate than previously thought, according to a new study.
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Previously, the rate of collapse of the cliffs was estimated to be at least 20-30 centimeters a year. But a study by the Geological Survey of Israel, a unit within the National Infrastructures Ministry, challenges that view and concludes that most of the cliff erosion is due to natural processes, as opposed to man-made causes, such as the construction of breakwaters.
The study’s findings, which are to help prevent collapse of coastal cliffs, were presented last week in a seminar held by the Geological Survey at the Israel Museum in Jerusalem. The study was led by Dr. Amit Mushkin and Dr. Oded Katz, together with five other institute scientists.
Israel’s coastal cliffs rise 50 meters high, stretching along close to 50 kilometers, 13 of them in cities and communities. In recent years, the collapse of cliffs has sharpened awareness of the risks this poses to property and lives.
The study, focusing on the cliff section between Herzliya and Givat Olga, consisted of gathering archaeological and geological findings pertaining to the processes along the cliff-lined beaches. The scientists dated the rocks that fell to the shore during previous collapses and considered the location of abrasion platforms indicating the cliff’s original location. They also calculated the natural rate of sand removal from the cliff base to the sea following the rocks’ collapse over the years. As long as this sand is not removed, the cliffs are naturally protected and not significantly eroded.
The scientists believe the cliffs’ average retreat rate is no higher than a few centimeters a year, and has remained stable over the last few hundred and even thousands of years. In recent years the rate has been lower than 10 centimeters a year. Most of the erosion occurs during the collapse of large cliff sections, which can cause the cliff face to retreat several meters on a single occasion. But such events take place only once every few hundred years.
In recent years the pace of retreat has not accelerated, despite the fact that other studies found that building structures such as breakwaters have blocked the natural sand stream. This narrowed the sandy beach strip that protects the cliffs from erosion as a result of contact with the waves.
Geological Survey scientists are working on a method to estimate the risks posed by the cliffs’ collapse as a result of natural and artificial processes.
So far the effectiveness of walls built at the cliffs’ base is not clear. Some of the walls cause the coast further damage because they effect the sand movement at the cliffs’ base.
The survey advises the Mediterranean Coastal Cliffs Preservation company, set up by the government in 2013 to preserve the cliff-lined coasts. Company CEO Yaacov Bachar said the company is concentrating on protecting coast sections where buildings are in danger of collapse. The company is advancing a plan to protect and strengthen the coast cliffs in Ashkelon, Netanya, Herzliya and Beit Yanai, he said.