Debris flows in New Zealand Alpine Catchments
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This research aims to improve our knowledge of debris flow occurrence and behaviour in New Zealand. Detailed field data collected in four debris flow prone areas in New Zealand are presented and compared. The travel distance of these events is then modelled with an empiricalstatistical model, UBCDflow, and an analytical, “equivalent fluid” continuum model DAN-W. While field studies are useful, they are often not linked to the underlying mechanics of debris flow motion or compared with the behavior of small scale flows due to the inherent complexity and unknown boundary conditions in field scale flows. Physical modelling simplifies the situation and allows boundary conditions to be controlled. The second part of this research uses physical modelling, including a series of novel debris flow tests in a geotechnical centrifuge, to compare and contrast flow behaviour and mechanics of laboratory and field scale flows. The debris flows events investigated in the field were categorized into hillslope, torrent, or intermediate-type events. Hillslope events were less channelized and progressively deposited on high slope angles. Consequently, high friction coefficients were needed to model their mobility. Torrent flows entrained more material than hillslope flows and deposited on lower angle slopes in response to unconfinement on the debris flow fan. Friction coefficients back-calculated for torrent events were lower than for the hillslope flows, but still larger than most of the friction coefficients given for large, channelized, debris flow events in the literature. Intermediate events were similar to hillslope events in terms of deposition angle and best-fit friction coefficients, but were very confined. Both UBCDflow and DAN-W were found to be useful decision support tools, but the capability of each model was limited. Greater modelling capability was gained by using the volume change behaviour predicted by UBCDflow in DAN-W, as DAN-W simulates flow heights and velocities, but does not predict the depth of erosion. In the second part of the research, a geotechnical centrifuge is used to model debris flow processes in a larger acceleration field than earth’s gravity. While centrifuges have been used to model a variety of processes in other geotechnical problems, debris flows are a relatively new phenomenon to be tested on a centrifuge. The centrifuge was successful in increasing the frictional properties of flow, but viscous forces were still the dominant form of shear stress with the materials used. Markedly different flow behaviour of tests using different pore-fluid rheologies suggested that the dominant mechanism of shear resistance may have changed between confined, downslope movement and unconfined runout. The results also showed that in geotechnical centrifuge testing, the viscosity of the pore fluid scales with the g-level, N. This research is an important step in developing centrifuge testing as an accepted method of modelling debris flow processes. Finally, a brief comparison of friction slopes between small-scale 1-g flume tests and field scale flows suggests that 1-g flume experiments are able to model the mobility of field scale flows if the soil used is well-graded and the pore-fluid is not too viscous. This research shows that the the ability of laboratory scale flows to model large scale processes may not be as limited as previously suggested by some investigators.