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Scientists have set up a radar on Mt Ruapehu to measure the depth of flows when the next big lahar spills from the Crater Lake towards Tangiwai.
A lahar could theoretically happen this month.
Researchers from GNS Science and Massey University will determine the velocity, volume and sediment load as it rips through the Whangaehu Valley.
Latest measurements show the lake level is 3.1m above the hard rock rim. There is a 4.5m "dam" of soft volcanic ash, tephra, on top of the rock rim, and it is this that is expected to give way.
If the water reaches the top of the tephra dam, it will spill 1.5 million cubic metres into the Whangaehu River, and the lahar would be twice the size of the one on Christmas Eve 1953 that caused the Tangiwai disaster in which 151 people on an overnight train died.
- NZPA
