No abstract

For the flow and deposition behaviour of debris flows, phenomena like particle sorting, levee formation and the development of roll waves are expected to be important processes. However, these processes are not well understood and hardly implemented in modelling approaches. In this study, we focus on the development of roll waves and derive advanced criteria separating stable and instable flow regimes for three debris flow models. These criteria are expressed using critical Froude numbers. Each of these simple flow models reflect different sources of flow resistance: laminar-viscous stress (Bingham type), dispersive stress due to particle collision (Bagnold type), and a model combining turbulent and dispersive stresses. Subsequently, we compare the predictions from these models with results from laboratory experiments with grain-fluid mixtures in a straight flume and with observations from a debris flow monitoring site at the Lattenbach creek in Austria. The experimental flows match with a turbulent flow model including particle collisions. For the natural flows the connection between models and observations is not clear due to limited field data. The results of our study contribute to an improved determination of critical flow conditions and provide data for model testing.

Abstract. Floods including intensive bedload transport represent a severe hazard to the often densely populated alluvial fans of small Alpine watersheds. In order to minimize the risk of future inundation, an existing bedload deposition area on the fan upstream of the village Vorderberg in southern Austria is planned for reconstruction. The suggested concept for protection measures includes dividing the area into three similar sections of reduced slope. The three sections are to be separated by a block ramp. To test this concept and to optimize the sedimentation process, an analysis was performed by using both a physical scale model (1:30) and a numerical simulation tool (SETRAC). Four configurations for the section-outlet were tested based on three flood scenarios. The results support the general protection concept and suggest a minimum construction configuration, including a woody debris filter. Employing a physical scale model for analysing small watershed processes is rarely found in literature. This contribution represents an applied study and provides quantitative information on bedload deposition and outflow from a deposition area. We test a novel simulation tool for bedload transport on the steep slopes against the measurements in the laboratory and show that the combination of physical and numerical modelling is a valuable tool to evaluate the efficiency of planned measures for torrent hazard mitigation.

Debris-flow events often comprise a sequence of surges, sometimes termed “roll waves.” The reason for this surging behavior is still a matter of debate. Explanations include the growth of hydraulic instabilities, periodic sediment deposition and release, or grain size sorting. High-resolution field measurements together with triggering rainfall characteristics are rare. We present results for 3 years of monitoring debris-flow events at Lattenbach Creek in the western part of Austria. The monitoring system includes a weather station in the headwaters of the creek, radar sensors for measuring flow depth at different locations along the channel, as well as a two-dimensional rotational laser sensor installed over a fixed cross section that yields a three-dimensional surface model of the passing debris-flow event. We find that the debris flows at Lattenbach Creek were all triggered by rainstorms of less than 2 hours and exhibited surges for each observed event. The velocities of the surges were up to twice as high as the front velocity. Often, the first surges that included boulders and woody debris had the highest flow depth and discharge and showed an irregular geometry. The shape of the surges in the second half of the flow, which carried smaller grain sizes and less woody debris, were rather regular and showed a striking geometric similarity, but still high velocities. The results of our monitoring efforts aim to improve our understanding of the surging behavior of debris flows and provide data for model testing for the scientific community.

No abstract