Experimental study on rheologic behaviour of debris flow material

In order to describe the flow behaviour of debris flows often rheologic models are used. This study introduces two novel facilities to determine rheologic parameters of different models for debris flow material mixtures containing grain sizes larger than to be measured in standard viscometers. The diameter of the vertically rotating flume (VRF) is 2.5 m, the rectangular cross section has a width of 0.45 m, and the maximum rotational speed is around 30 revolutions per minute, corresponding to a mean speed of the examined mixture of ∼4 m/s. From the measured flow parameters total boundary shear stress and corresponding shear rate of the flowing mixture are derived. The ball measuring system (BMS) consists of a sphere that is dragged at specific speeds across a sample of 0.5 l volume with the help of a small sphere holder. Accordingly torques due to drag exerted on the sphere and its holder as well as corresponding speeds are measured and transformed into values of shear stress and shear rate based on the method of Metzner and Otto. Material taken from fresh debris flow deposits in Eastern Switzerland have been investigated with both facilities. We present results from experiments involving mixtures with different sediment concentrations and with grain sizes up to 5 mm. Although estimated from completely different approaches the rheologic parameters of the independent measurements are generally in reasonable agreement.