Debris flows are a mass displacement process, usually occurring in mountainous regions, in which a mixture of solids and water runs down the valley in a wave-like motion. The flow behaviour depends chiefly on the proportion of solids and liquid, but is also equally dependent on the composition of the solid matter. If the solids are composed only of fine-grained material, a sludge-type of run-off behaviour is observed. If there are only coarse-grained components, the run-off is more drum-like or caterpillar-like and is known as 'granular flow'. Such debris flows can cause extensive damage with heavy financial consequences or even loss of life.
Within the scope of this thesis, two aspects of debrisflows were examined in physical model tests - namely the flow and the erosion behaviour, both along the (steep) transit stretch between the initiation zone and deposit area. The use of a near natural material mix to achieve most natural conditions formed the basic principle and essential difference to most laboratory examinations. Debris flows in Switzerland are mostly of granular material composition, the examinations therefore were restricted to this type of material.
Two materials were used in the experiments, whereby most trials were carried out with material from a debris flow torrent. Besides this, the initial conditions such as slope angle, surge volumes and water content of the start mixture were systematically altered. Using various measuring instruments, the run-off parameters were recorded, and also the flume bed topography was measured. Some 170 experiments were undertaken and analysed.
The four basic models frequently recommended for debris flows – laminar or turbulent flow, Bingham-laminar behaviour and also a grain-shear model – display limits in their application with regard to flow behaviour. It appears that laminar flow is more often applied in cases of high solids concentration, whereby questions remain unanswered regarding the flow coefficient (in this case the viscosity) or the transferability to natural events, since reliable information regarding viscosity of natural material mixtures is missing. The same is also true for the Bingham-laminar model, although for interpretation of the results further assumptions have to be made for the shear stress which is additionally present in the flow coefficient. Although in the grain-shear model the solids concentration of the mixture is explicitly present in the coefficient, there is a further dependency over the whole concentration area. The concentration dependency is apparently more prominent in grain mixes than in uniform materials; the internal losses caused by grain contacts become greater in ratio to the increase in concentration. As expected, even with a turbulent model there is a clear concentration dependency of the flow coefficient. Based on a dimensionless Chézy approach and supplemented by a concentration term, a simple new approach to run-off velocity estimation for debris flows is postulated.
Conclusions pertaining to erosion performance based on the experimental data are less clearcut. The application of a natural material with wide grain-size distribution already causes a certain scatter of data, since no homogenous conditions can be guaranteed in the restricted space of a laboratory flume. In such coarse loose material any representative measurements are difficult to achieve. A relationship is established between the erosion volume of a debris flow surge and a 'bed stress term' that incorporates surge volume and density, as well as inclination of the watercourse. In comparing the normalised relationship with the results of Russian field experiments on prototype scale, on the one hand the same tendencies are indicated, but there are also limits visible regarding scaling of surge volumes, and the duration of the discharges.
As well as new findings on flow and erosion behaviour, new questions have also emerged. The question arises about the flow relationship and how far the concentration dependency itself is dependent on the material composition. What is the influence of more fines or the absence of coarse particles? Concerning erosion performance, similar studies in future will call for even greater efforts with controlled conditions at the riverbed. This includes assembly and irrigation as well as supervision of moisture distribution within the river bed. Additional procedures, however, must be developed for this, which will also achieve correct results when used in (coarse) loose material.