The predicted Ruapehu crater lake lahar: Maximising the scientific gain

Executive Summary

The major obstacle to accurate modelling of travel times, inundation levels, and the
physical impacts of lahars is the lack of knowledge of the internal properties. In
particular, their typically high sediment concentration makes analogies with hydraulic
governing equations difficult to justify.

To address this knowledge gap, we are developing and testing a range of instruments
and geophysical monitoring techniques to measure the internal properties of moving
lahars. In this EQC-funded program, geotechnical and hydrological pore-pressure and
earth-pressure (load-cell) sensors are being customised in attempt to weight lahars to
determine sediment concentrations in active flows. This effort is part of a wider approach
within GNS Science and Massey University with support from FRST and the RSNZ
Marsden fund.

Considerable challenges surround equipment suitability (and survivability) in high-energy
mass-flows such as lahars. Although laboratory tests under controlled conditions have
demonstrated that the concept works, field trials at Semeru volcano, Indonesia, and
Ruapehu, New Zealand have met mixed success.

1. Sub-bed pore pressure transducers, both cabled and non-cabled have proved the
most successful, as they suffer least from thermal disequilibrium.
2. Non-cabled load-cells are proving problematic due to leakage and loss of
calibration as a result of the customisation process, and thermal disequilibrium
due to temperature variations. Cabled load-cells should be immune to both issues
due to factory-finish and calibration, and thermal stability as a result of deep
burial. However, installation is more problematic.
However, a great deal has been learnt about the realities of measuring active lahars over
the course of the program, and these data will make future quantitative studies of lahars
more accurate.