Author(s): D. Weber; D. Rickenmann
Linked Author(s):
Keywords: Debris flow; Physical modelling; Erosion; Rheology; Velocity; Flow resistance; Soil stability
Abstract: Physical model tests are performed to study the debris flows and erosion processes along torrent channels. The main goal is to identify the erosion mechanisms and the most important parameters influencing them. It is observed that the water content C w not only influences the flow behaviour of the debris flow wave but also the stability of the torrent bed. Simple stability tests show that the angle of stabilityE s stays more or less the same with increasing saturation up to a critical water content C w, CR. A further increase of Cw results in a sharp reduction of the stability angle. For slope angles E> E s, the material starts to flow for water contents C w > C w, CR, whereas it slips down the as a whole in dry state and at low C w values. For several field materials as well as for the lab material C w, CR is in the range of 8-13% by weight. Considering the flow resistance of the experimental debris flows it is found that the Manning’s n varies only in a small range for mixtures with Cw larger than the void ratio, and that these n values are similar to the n value for water pulses. Considering field data on granular type debris flows, the mean velocity can also be described with the Manning' s equation, the n values being about 4 times larger than for the laboratory flows. Although a part of this difference can be explained by the requirement for Froude similarity, more research is needed to scale laboratory observations to prototype conditions. It is observed that the erosion of the torrent bed consists of at least three mechanisms. First, direct erosion of the torrent due to the body of the debris flow wave takes place. This process then leads to a vertical deepening of the torrent and to an undercutting of the sideslopes of the riverbed. If vertical erosion is big enough, slope failure of the banks occurs. At steeper slopes, the sediment transport capacity of the tail parts of wet debris flows is large enough to entrain the eroded material. To better examine the process of direct erosion, further experiments are planed in a narrow flume where the riverbed is represented by a flat layer of material only.
Year: 1999