Geophysical investigation into the internal dynamics of moving lahars : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand

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Lahars and other mass flows are highly hazardous phenomena that can pose great risk to areas in their path. Due to their often unpredictable onsets, scientific observations are limited. In addition, the erosive capabilities of a lahar mean that the most commonly used monitoring and sampling methods, such as load cells and bedload traps, are often damaged early in the flow. The cost of repair and maintenance of these instrumentation prohibits comprehensive coverage of each channel that might be at risk from lahars. The development of seismic sensors as an alternative monitoring method could prove effective as they do not require contact with a flow and are therefore less at risk from damage. The complex behaviour of a lahar can be witnessed in the geophysical record of its passage which, in combination with more traditional monitoring methods, can be used to record the detailed evolution of a flow. The three-dimensional analysis of seismometer recordings can provide an approximation of the frontal velocity that may differ from maximum velocity estimates made using super-elevation calculations. Comparisons of the seismic records of different mass flow types illustrate that it is possible to differentiate between them. Frequency analysis allows for the distinction of the flow mechanisms, particle interactions, and dominant rheology of a lahar. Low frequencies are more indicative of bedload frictional motion, while higher frequencies reflect the collisional impacts of particles, either between themselves or with the substrate. Detailed records of a flow at a single site provide a comprehensive understanding of the temporal variations that occur within the duration of a lahar, while comparative analyses of numerous sites along a channel highlight its downstream evolution. While initial onset signals can be recorded at local-to-source sites, they are attenuated too quickly to be observed further downstream. The records at proximal sites can, however, reflect the stages, or packets, involved during the main bulk of lahar initiation. At more distal sites, observations show that a lahar transitions to a [minimal] 4-phase behaviour. This consists of a frontal bow wave of ambient streamwater that increases in volume with distance from source, and immediately precedes the lahar proper. The following phases are defined by variations in sediment concentration, velocity, stage, and, in the case of Crater Lake-originating lahars, water chemistry. The understanding of the variable behaviour possible during a lahar, as well as the identification of the specific flow type recorded, is fundamental to modelling approximations of flow volumes, sediment concentrations, likely inundation areas, and probable damage by the flow. It is essential for the development of future warning systems that the variations that can occur within a single lahar are better understood, as lahars represent a serious threat to the slopes of many volcanoes worldwide.
Code and data files available with hard copy of the thesis in the library.
Lahar, Mathematical models, Geophysics, Earth science