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PROSPECT OF WHITBY

A cutting edge laser technique is being used to monitor cliff erosion in North Yorkshire.Damon Sch³nmann scans the ground.

The cliffs along stretches of the North Yorkshire coast are renowned for two things; good fossil collecting and instability. It is the cliffs' propensity to tumble down that makes for such rich palaeontological pickings.

But in Whitby, regular rockfalls from the 75m high face not only pose a major hazard but present infrastructure maintenance firm Crown Castle UK with a problem.

Community relations manager Julie Bircher explains: 'Crown Castle took on the BBC's network of transmission sites and there is a 45m stayed lattice mast that provides TV and radio for Whitby.' With the mast only a few metres from the precipice, 'We wanted the best possible evidence of the rate of erosion, ' Bircher says.

The cause for concern is real: in January this year a slide caused about 200t of material to cascade down the almost vertical face. The cliff is made up of almost horizontal beds of sandstone, shale and limestone, capped by sandstone with 3m of glacial till on top.

Accurately gauging the rate and pattern of rockface loss has historically been difficult. While scars on the face sometimes indicate where rock has fallen, it is almost impossible to measure volume, as debris scatters among the boulders.

Enter the University of Durham's department of geography, with what it claims is a new system for assessing and quantifying erosion. Monitoring began in September 2003 and is due to run until at least September 2006.

The system uses a terrestrial laser to scan the cliff, and data is used to build a 3D model of the face. Images generated at monthly intervals are compared to detect block detachments and failures. Researchers say this means they can accurately assess rates of coastal retreat.

Senior post-doctoral research associate Nick Rosser says the project is intended to improve understanding of coastal collapse processes.

'There is a massive lack of long term monitoring at high resolution of the pro cess of cliff retreat and erosion of cliff faces, ' he says.

Until now, mapping of cliff faces has typically been done by walkover or aerial surveys. The problem is that it is hard to accurately identify which part of a cliff has changed.

Rosser says: 'A cliff can look shot to pieces and you can't quantitatively say which part has come off or how big it is unless it's blatantly obvious.' Where aerial photos are used, problems occur with steep, vertical or overhanging cliffs.

The cliff being monitored in front of the mast at Whitby is only 250m wide which is too small for a costeffective aerial survey anyway.

The university is using a Measurement Devices LaserAce 600, a 'time of fl ight' laser that reads a distance by sending out radiation and timing how long it takes for the reflection to come back.

This happens 5,000 times a second, with data averaged down to 250 points a second as the laser sweeps across the cliff face.

This data is converted into a 3D image showing the cliff surface in clear detail. The team says a version of the system is already used in quarrying to take surface snapshots, but that it has made advances to show 'frankly stunning' cliff face changes over a period of time.

'What we are doing differently is the data processing, ' says university reader David Petley.

At Whitby, Rosser and Petley are using interferometry to discover the extent of changes in the rockface.

Images of the cliff are captured from exactly the same viewpoint at monthly intervals. Where the cliff remains the same from one month to the next, the laser pulse-response measurement remains the same. But where rock has been lost, the pulseresponse measurement is different, as it has slightly further to travel.

When one image is overlain on the other these differences in response time show up as areas of interference. These can be translated into contours, assigned colours, and, by joining all of the points up digitally, given volume.

Rosser says the lack of an industry standard for calibrating laser equipment means accuracy is generally measured in fairly large figures. But the system at Whitby can show detail of 100mm at 600m.

At this site, where the scanner is about 75m from the target, the accuracy is 50mm, allowing it to measure losses small enough to fit into a teacup. Consequently the team can compare well defi ed surfaces to show quantifiable change.

Results of the scheme are interesting as they fl y in the face of historic measurements. Petley says the actual retreat appears to be slower than that measured previously.

'This goes for all the sections of cliff we are working on. It maybe that the cliffs retreat slowly for say nine years then step back in the tenth year, ' he says. 'But I think it's down to the fact that the other techniques are inaccurate. I think our measurements are right and these other techniques are wrong.' A superfi cial failure in the glacial till could also threaten the mast's safety, so researchers carried out direct shear tests to assess the risk and installed six piezometers to measure water levels.

But groundwater has not reached trigger levels. Why is unclear, particularly as the till has failed a few hundred metres along the coast.

Speculation surrounds the top level of bedrock that slopes slightly down and away from cliff face, perhaps encouraging drainage. The water level may also be lowered by flow through fi ssures.

Petley is sure the transmitter's days are numbered, although he sees 'no evidence of anything dramatic happening in the short term'.

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