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Long-term bedload monitoring
During storm rainfall many mountain torrents feature sediment transport or debris flows. Accurate measurements of bedload transport in natural channels are important to improve process understanding and predictive methods.
If a minimal discharge is exceeded in the channel the shear stress at the bed is large enough to mobilise sediment grains and bedload transport commences. Increasing discharge or higher channel slopes eases mobilisation and sediment transport. In addition to the morphology of the bed, the supply of material from hillslopes through landslides and bank erosion is an important factor in determining the availability of transportable material. Measurements spanning long time periods indicate that after extreme flood events or after the passage of a debris flow larger volumes of sediment may be mobilised and displaced than after more “quiet” periods.
An important part of the investigations on bedload transport is based on the measurements from the hydrological test sites in the Alptal (Canton SZ), especially from the Erlenbach and the Vogelbach. There, and in other streams continuous measurements of transport rates are obtained with geophone sensors (formerly: hydrophone sensors). The geophone is mounted under a steel plate, which is placed flush into a stable part of the channel bed. As a bedload particle rolls over the plate, it triggers oscillations that are registered by the sensor. Up to now the signal has been recorded in a simplified way and sensor impulses have been counted. For a quantitative analysis, the system needs to be calibrated by independent measurement (obtained for example from a retention basin). The site at the Erlenbach will shortly be upgraded with automatic basket samplers to allow a better temporal comparison of geophone signals with transport rates and grain size distribution.
Rickenmann D, Turowski JM, Fritschi B, Klaiber A, Ludwig A (2012) Bedload transport measurements at the Erlenbach stream with geophones and automated basket samplers. Earth Surface Processes and Landforms 37:1000-1011
Badoux A, Turowski JM, Mao L, Mathys N, Rickenmann D (2012) Rainfall intensity–duration thresholds for bedload transport initiation in small Alpine watersheds. Natural Hazards and Earth System Science 12(10): 3091-3108
Turowski JM, Yager EM, Badoux A, Rickenmann D, Molnar P (2009) The impact of exceptional events on erosion, bedload transport and channel stability in a step-pool channel. Earth Surface Processes and Landforms 34(12): 1661-1673