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Monitoring Moving Mud

Bits of coal and wallpaper paste are helping to provide a better understanding of the dangerous type of liquefied landslip called a debris flow surge. Lincoln PhD student Peter Coleman, working under Dr Tim Davies, is using pieces of coal to represent the stationary coarse rocks and paste for the flowing mud material involved in such surges.

Coleman has hypothesised that the nature of a debris flow surge can be explained as a form of moving granular dam, with the rocky debris at the front of a surge acting like a dam wall to contain the mud reservoir behind.

In a conventional dam, there is a liquid-to-air interface known as the "phreatic front". Coleman has suggested that a moving granular dam produced by debris flow has a similar interface he has termed a "wetting front". Experiments conducted in a flume (a tiltable rectangular box with transparent side walls) has shown that the wetting front plays an essential role in controlling the surge's motion down a channel.

Three main forces act on the moving dam -- frictional, gravitational and hydrostatic -- the net force of which affects the rate at which the dam moves. Quantitative analyses of photos of the modelled surges have provided support for Coleman's granular theory.

Further support for the moving dam hypothesis has come from field research. An examination of three debris flow sites at Mt Thomas in North Canterbury revealed a silt layer 15-20 centimetres from the rocky surface, most likely representing the hardened remains of the suspected wetting front. Mapping the profiles of the layers and comparing them with lab results showed a significant match.

Coleman says that the concept could be very useful in interpreting the form and behaviour of one of nature's more formidable natural hazards. Such surges can occur as a consequence of heavy rain, flooding or snowfall. In New Zealand, one of the country's worst disasters, the 1953 Tangiwai train tragedy, was a direct consequence of a lahar of volcanic debris flow destroying the Tangiwai railway bridge.

There is strong international interest in such research, particularly in the area of preventing or mitigating the potentially disastrous effects of liquefied landslides.