Quick Dips

Landslip Risks Predicted

Bernard Carpinter, Victoria University

The risk of landslips can be predicted by a new computer model developed by Associate Professor Mike Crozier of Victoria University's Geography Department. His program, although still in the preliminary stages, can assess the probability of landsliding on any particular day, with a capability of providing predictions over a few days into the future.

"The model bases its predictions on knowledge of which climatic and groundwater conditions have produced landslides in the past, and assessing how close we are to the critical threshold at present," Crozier says. "This allows an assessment of how much rain is needed to trigger landslides; the probability of this rain occurring can then be calculated from climatic records."

In 1996 the wet winter increased the likelihood of landslides on Wellington's steeper slopes. As well as rain, the model also takes into account such factors as evaporation and the rate of drainage from the soil.

"In tests of the model for Wellington, it indicated that the highest probability of slips occurred over a three-day period in July. For the same period, reports indicate that the Wellington City Council cleared over 70 slips throughout the city -- an unusually high figure."

Crozier believes that when the model has been further refined it could be useful to local authorities in knowing whether to prepare for slips when rain is predicted, and to judge whether conditions are suitable for ground excavation and other land development operations.

"It isn't simply a case of heavy rain automatically producing slips," he says. "Sometimes you get heavy rain and almost no landslips, while at other times you get a lot of slips after quite a moderate fall of rain. We have found that there seems to be a critical threshold at which slips suddenly start to occur, and our model can tell us how close we are to that threshold at any time. It depends very much on how much moisture has accumulated in the soil over a period of 10 days or so beforehand, and this is determined by how much rain fell during that time and the rates of drainage and evaporation that have been able to occur."

The high landslip risk detected during the 1996 winter was the result of persistent rain over a period, with little opportunity for drying out. This brought the soil to saturation levels close to the critical threshold. Graduate students Thomas Glade and Nick Preston are now carrying out work in the Wairarapa and Hawke's Bay to further refine the thresholds used in the model and extend its ability to make predictions in different regions throughout the country.

Glade, a PhD student from Germany, is looking at how the model can be applied to areas outside Wellington and what adjustments are needed to take into account the effect of different rock types and other terrain factors.

Nick Preston, a Masters student, is studying whether an area which has undergone exceptional slipping in a short span of time becomes more or less susceptible to further slipping in the immediate future. The slip prediction project is part of the Geography Department's on-going programme on land and water sustainability.

Crozier is also using his model to look at what effect climatic change could have on land stability. Global warming is expected to bring increased rainfall; the question is to what extent the extra rain -- estimated at 15% by many researchers -- would be offset by warmer temperatures that should increase the rate of evaporation of ground water.

Bernard Carpinter is a journalist at Victoria University.