Effects of fine grains in suspension on fluid viscosity and debris flow mobility

Debris flows are high speed saturated mass movements, which are controlled by gravity and shear processes. The flow matrix consists of water and granular material, ranging in size from clays to boulders. Particle diameters below 63 µm, the “fines”, are able to remain in suspension for the flow duration owing to their small settling velocities. Therefore, water and fines are often considered as a single, fluid, phase in the literature. This assumption means the fluid phase properties are governed by fines concentration and microstructure, and the fluid shear state. During propagation downslope, the debris flow matrix shears as a sequence of contractions and dilations. This process causes a reduction, or enlargement, of the voids between large grains, and allows flow of the fluid phase out of, or into, this space. The influence of increased fluid viscosity caused by fines on these processes and its impact on the macro-scale outcomes is largely under-researched. This study undertakes tests in a small-scale flume to physically model idealised debris flows with increasing viscosity. To achieve this, a uniform coarse granular material is replaced with increasing percentages of fines, a kaolinite clay. To identify the contribution of viscous properties of the fluid phase on the model, each fluid phase composition is independently tested for its rheological properties. The majority are observed to be non-Newtonian and shear-thinning in behaviour.