Chris Phillips

Determining rheological parameters of debris flow material

Abstract A 2.0 m diameter steel 30° inverted cone-and-plate viscometer/rheometer was designed, constructed, and used to test the behaviour of coarse-grained debris flow materials. A 1: 5 scale model machine was also constructed and used to test the internal flow dynamics of the viscometer/rheometer and to obtain results for fluids, grain-fluid mixtures, and debris flow fines. For fluids and grain-fluid mixes, our results were similar to those obtained earlier using standard viscometric systems. Derived rheological parameters for debris flow materials and clay slurries agreed well with those determined from calculation, direct measurement, and field observation. Apparent viscosities were shear-rate dependent, extremely sensitive to water content, and as high as 6000 Pa·s. Instantaneous stresses within shearing debris flow material varied over a wide range. Debris flow materials with bimodal grain-size distributions had a dilatant plastic rheology. Those with a low content of coarse material and unimodal grain-size distribution, or exceptionally high fines content, had a plastic or viscoplastic rheology.

A comparison of earthflow movement mechanisms on forested and grassed slopes, Raukumara Peninsula, North Island, New Zealand

Abstract Surface movement rates on forested earthflows are 2–3 orders of magnitude less than those on grassed earthflows. Subsurface deformation results largely from extension flow on grassed earthflows and from compression flow on forested earthflows. A rafting mechanism in which blocks of roots from individual trees interact with those of neighbouring trees to retard surface movement is used to explain deformation profiles of forested earthflows. The rheology of earthflow materials is changed by the presence of tree roots.

Internal deformation of a fast-moving earthflow, Raukumara Peninsula, New Zealand

Abstract Sub-surface deformation of a fast-moving earthflow was studied in the East Coast Region of the North Island, New Zealand, for a period of 2 years. Tiltmeter profiles indicated that earthflow materials were subjected to a variety of movement mechanisms, including internal deformation by gravity-shearing flow, extension flow, compression flow, rotation over curved slpes, rotation of material beneath the earthflor ∗ , and sliding. Overall, internal deformation accounted for less than 25% of the total surface movement, the remaining 75% being the result of sliding movement along the basal shear plane. Gravity-shearing flow occurs along the basal shear plane, which is thought not to be more than a few centimetres thick. Micro-topography (features on a scale of 1–10 m) largely affected tilting behaviour (internal deformation) of fastmoving earthflows. In the longitudinal direction, tiltmeter profiles on curved slopes were monoclinal and rotation was the principal mechanism causing tilt. On concave slopes the tiltmeter profile had a forward (downslope)-convex shape, with compression being the principal deformation mechanism. On even (planar) slopes. extension flow (creep) predominated, with the profile showing little evidence of rotation. However, minimal deformation (tilting) was likely to be caused by a combination of either sub-earthflow rotation, extension flow, or compression flow. In the lateral direction, topographic influences produced considerable deformation, particularly on concave slopes as a result of compression flow. There was little or no lateral deformation on even slopes.

Surface movement in an earthflow complex, Raukumara Peninsula, New Zealand

Abstract The surface movement of an unstable earthflow complex was studied over a 10-year period using a network of pegs inserted to a depth of 1 m. The mean and maximum surface movement velocities in the transport zones of the two study earthflows ranged from 0.2–0.4 m and 1.7–2.8 m/month respectively. Maximum velocities for individual pegs ranged up to 3 m/month. The movement data indicated that the earthflows were approximately in a steady state for the 10-year duration of the study. Application of Iversons theory for the kinematics of unsteady, non-uniform landslide movement indicated that the earthflows had Pe values in the range of 0.1 to 2.0 (Pe is the dimensionless parameter known as the landslide Peclet number). These values indicated that the earthflows showeda blend of plastic and viscous behaviour and that diffusion might be more significant than kinematic-wave propagation in transmitting disturbances.