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UNSEEN CHANGES THE CLIFFS of the coast of the North York Moors National Park are predominantly formed of hard rocks from the Jurassic and Cretaceous periods. They appear stable, with the exception of abrasion caused by incoming tides and waves. But closer inspection reveals that the cliffs are not only eroding at the toe, but across the entire cliff face, although this is easy to miss when observing them by eye alone. Changes in coastal cliffs are difficult to spot because they are sometimes so slow or subtle that they go undetected. Using aerial photography for surveying cliffs is limited in practice, particularly on steep rock faces that are obscured when viewed from above. In order to understand these unseen changes taking place in the coastal cliffs that can lead to collapse, new techniques are needed that allow researchers to monitor coastal cliffs more accurately and over long time periods. Fortunately, there is an advanced technology available that helps geoscientists to monitor the cliffs in ways never before possible. This technique is known as Terrestrial Laser Scanning (TLS). 3D point cloud of cliffs at North York Moors National Park.
Terrestrial laser scanner.
A TERRESTRIAL LASER SCANNER is equivalent to a 3D camera but instead of capturing a coloured pixel for every position upon the cliff, as with a digital camera, it uses a laser to measure the shape of the cliff to build a 3D point cloud of the surface. Comparing this data month to month helps researchers to monitor the underlying changes in rock cliffs that are otherwise invisible to the eye. ‘It’s accurate’, says Rosser, ‘it’s fast and convenient because you don’t have to go near the cliff, and we can capture extensive sections of the coast within one survey. The system captures data at very high-resolution, measuring the cliff surface every 3–5 cm across the face’. Rosser’s research team travel to the same part of the coastline in the North York Moors National Park each month to capture the shape of the cliffs. The laser scanning system is fast enough to allow researchers to scan 3 km of the coastline in only one visit. They have been doing this from the foreshore during low tide for 11 years in total, and now have the longest monitoring dataset of coastal cliff erosion. This makes their research site the most intensively monitored coastal cliffs in the world.
Left: Laser scan of cliff face along the coastline of North Yorkshire.
The data collected using laser scanning of the cliff was in some ways surprising in comparison to previous research on hard rock coastal cliffs. Erosion at the cliff toe is normally used to explain deterioration of the cliff via rock fall, but based on research by Rosser and his team, there is clearly more happening at the cliff face than previously thought. While the cliff face may appear solid and stable, it is ‘constantly evolving’, says Rosser, ‘you can’t necessarily relate erosion or subsequent retreat to any simple or single environmental condition using conventional approaches’. Rosser adds that some aspects of cliff behaviour are predictable. For example, sequences of rock fall are observed via a progression from small to medium to large rock falls. At present, coastal erosion models that only consider how wave action drives erosion may be ‘a bit misleading’, he says. Instead, Rosser and his team suggest that monitoring data, which is inclusive of the rock mechanics that determine collapse, and that operates over the timescales of change observed at the coast, is essential to building an accurate model of how rock falls occur along coastal cliffs.