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An eye for detail

A scanning technique being trialled in North Yorkshire could yield new insights into cliff erosion, Damon Schünmann discovers.

The cliffs along stretches of the North Yorkshire coast are renowned for two things - fossil collecting and instability. It is the cliffs' propensity to tumble down that makes for such rich palaeontological pickings. But in Whitby, the regular rockfall from the 75m high face poses a major hazard.

Cliff erosion at Whitby is especially interesting to infrastructure maintenance company Crown Castle UK.

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.' This is just a few metres from the precipice. 'We wanted the best possible evidence of the rate of erosion.' Concern over the site stems from partial collapses such as that in January, when around 200t of material cascaded down the almost vertical face. The cliff is made up of interbedded sandstone and shales with narrow limestone strata almost perfectly horizontally bedded.

The geology is capped by sandstone with 3m of overlying glacial till.

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 rockfall volume, as debris scatters among the boulders.

But the University of Durham is bringing what it claims is entirely new technology to Whitby, in a bid to assess and quantify erosion in almost pinpoint detail. The monitoring project began in September 2003 and will run until September 2006.

The system uses data gathered through successive terrestrial laser scanning to build a three-dimensional model of a cliff face. Images generated at monthly intervals are compared to detect block detachments and failures.

'There is a massive lack of long-term high-resolution monitoring of cliff retreat and cliff face erosion, ' says senior post-doctoral research associate Nick Rosser.

Mapping of cliff faces has typically been done using walkover or aerial surveys. A problem with these methods is it is hard to say which part of a cliff has changed or by exactly how much. 'A cliff can look shot to pieces and you can't quantitatively say which part has come off or how much it is unless it's blatantly obvious, ' Rosser says. Where aerial photos are used, problems occur with steep, vertical, or overhanging cliffs.

Additionally, the surface being monitored in front of the mast at Whitby is only 250m wide - too small for a cost effective aerial survey.

Durham University is using a Measurement Devices' LaserAce 600 - a robotic total station and time of flight laser. It works in a way that is analogous to sonar, by zapping out radiation and timing the bounce-back of the signal to give a measurement of distance. This happens 5,000 times a second, but averages down to 250 points a second as the laser sweeps across the cliff face.

A ree-dimensional oordinate is given to each point, building up an image showing the surface in clear detail.

The team says that 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 different is the data processing', notes University reader David Petley.

At Whitby, Rosser and Petley are using a technique known as 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 time of flight laser pulseresponse measurement remains the same. But where rock has been lost, the pulse-response 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 three-dimensional volume.

Rosser explains that the absence 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 to 100mm at a 600m range, and precision is directly proportional to distance.

At this site, where the scanner is about 75m from the target, the accuracy is 50mm - it can measure losses small enough to fit in a teacup.

Consequently the team can compare well defined surfaces to show quantifiable change.

Results of the scheme are interesting as they fly in the face of historic measurements, Petley says. 'The actual retreat we are measuring is not as fast as other data sets and this goes for all the sections of cliff we are working on. It may be that the cliffs retreat slowly for, say, nine years then step back in the tenth year. 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.'

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