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Science Headlines - Landslips and Erosion

Science Headlines - Landslips and Erosion

An information service for media in New Zealand

17 August 2006

As water-logged soils fall onto roads and unstable slopes endanger buildings, scientists comment on landslides and erosion in New Zealand

1. Grant Dellow, an engineering geologist at GNS Science, works on an inventory of past landslides and a catalogue of current landslides.

"I don't think there are any more landslides this year than in previous years. The reason this year appears different is because they're spaced out in time - the majority of landslides usually occur during one or two extreme events every year, either rain storms or earthquakes. At the moment, the ground is so saturated from prolonged rainfall over a couple of months that it doesn't take much more rain to cause yet another landslide."

"This year they also seem to be occurring more in urban areas - last year there was a lot of damage in the Orongorongo Ranges, for example, but of course it didn't affect humans so much."

2. Professor Michael Crozier - a geomorphologist at the School of Geography, Environment and Earth Sciences, Victoria University of Wellington.

Professor Crozier will be speaking on this topic at the 7th Natural Hazards Management Conference 2006, From Science to Practice, in Christchurch, from 23-24 August.

"When a property is damaged by a landslide or some other hazard it is not uncommon to hear a statement like 'what a stupid place to build - why did the council let them build there?' But accurate prediction of unstable sites is the business of probabilities rather than precise forecast. However, careful reading of the ground has shown that there are certain hotspots in the landscape that will be periodically subject to catastrophic events and which should be treated with great care. These should be a priority in any hazard and risk assessment.

"When it comes to identifying the cause of landslides there are many myths:
-It must be climate change.
-It is because the trees were taken off the slope
-It happened because it was a steep slope
-It has failed once before at this location therefore it will happen again.
-Now that the slope has failed it will be safe.

"All of these are wrong and all of these are right - it depends on the situation. There is no single factor explanation for landslide occurrence. Landslide hazard assessments can't be applied universally - they need to be tailor-made for each locality."

3. Dr Mauri McSaveney, a geomorphologist at GNS Science

"The numbers of landslides we're seeing this winter have one basic trigger - rain. When there's a lot of water in the soil you get more landslides. But it's not quite as simple as that - water lubricates the grains of soil, and that makes them move, but some water also stops grains pulling apart so easily. It's like making a sandcastle: really dry sand flows like a liquid, sand with some water sticks together well, and sand with too much water slides apart.

"All landslides are grainflows - solid particles that move relative to each other like dry salt, sugar, and wet concrete. There's some interesting recent research that shows that the stresses in moving landslides are not what we think they are, and the stresses are what determine how landslides travel."

4. Dr Aleksey Sidorchuk - a leading scientist at the Geographical Faculty, Moscow State University, Russia, spent 2001-2005 at Landcare Research on an erosion carbon project, as well as working on a Marsden-funded project researching the basic mechanics of erosion

"New Zealand loses soil through erosion at about ten times the rate of other countries. On the West Coast that's because of natural factors: a mountainous landscape, a short distance to the ocean and huge precipitation. But in the North Island it's also because of land use. On the slip prone East Coast, erosion is 10 times higher than it was before humans arrived."

"But most erosion isn't in the form of slips, which often only move soil a short distance. The main agent to remove soil from slopes to the rivers is the invisible detachment of individual soil particles from small and large patches of bare ground, including those of landslips. We've been studying the actual mechanics of how this works and with our computer model, we should be able to predict soil loss in different environments and for various types of land use. We'll also be able to take sedimentary strata and accurately work out past erosion rates and erosion landscape evolution."

5. Dr Nick Preston - a lecturer in physical geography at the School of Geography Environment and Earth Sciences, Victoria University of Wellington, is also available for comment on the long term effects of sediment loads and erosion.

Science Headlines is a service managed by the Royal Society of New Zealand and funded by the Ministry of Research, Science and Technology.


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