Debris flows : interior velocity profiles.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Debris flows are an increasingly potential hazard in steep mountain valleys. Obtaining understanding of this phenomena is made difficult by the highly complex interaction between fluid and solid particles of a wide range of sizes. Debris flows are an interaction of both fluid and geotechnical engineering disciplines. They have a highly mobile, unsaturated coarse front, a highly dynamic body and a fluidised tail. The major defining features are the segregation of the flow and the ability to move the largest fraction along the free surface in a conveyer like system, creating damming and surging phenomena. The majority of historical research has been done on highly concentrated fluid flows and the flow rheologies reflect this. Field and laboratory investigation is limited by the opaque nature of the material.
The main aim was to identify the influence of boundary effects, moisture content and slope on the micro-mechanical behaviour of debris flows. This was conducted by an innovative combination of planar laser-induced fluorescence (PLIF) and particle imaging velocimetry (PIV). The laser plane was used to illuminate an interior plane for recording a series of high speed images from both the interior and exterior of laboratory debris flow experiments. By substituting real opaque materials for transparent solids and fluids that are optically matched, the laser light was able to be captured from the interior of the flow. The resulting images were of black particles in a white illuminated fluid. This combination of pattern was ideal for particle imaging velocimetry (PIV) which was able to calculate the velocity and displacement of sections of the flow. This methodology was used to look directly within the flow while giving non-invasive data on the behaviour of the solid particles and fluid. The resulting outputs were able to give velocity with depth and time to help analyse the spatial and temporal behaviour.
Measures of run-out, deposit shape, deposit particle size distributions, spot-heights and height-over-time plots demonstrated the PLIF material behaved in a manner consistent with field observations of debris flows. There was a high level of variability that is intrinsic to the complex nature making debris flows difficult to statistical analyse. Instantaneous velocity plots at various locations in the flow demonstrated evolving behaviour that was more significant than the changes in slope and moisture content. A slip velocity in the front of the flow was the most observable difference between exterior and interior images. Another difference identified was the slowing of the flow at the boundary wall. Large particles were seen to accumulate smaller particles, creating damming structures and in some cases small surges.
The methodology was well tested and provided quality insights into the interior mechanics of the flows in a mainly qualitative fashion that supported many theories that had not been observed directly. The findings of this research shows how complex and dynamic debris flows are and how hazardous and unpredictable the velocity and surging behaviour can be. The observations obtained through PLIF and PIV are a step toward applying more effective rheologies and theories to these unique flows.