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Long-term bedload monitoring in steep streams

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High waters in mountain torrents often sweep away large quantities of sediment and/or trigger debris flows. Measuring bedload transport in natural torrents is important, as it enables us to better understand the processes involved and to develop methods for calculating the risks.

 

When a certain water volume (discharge threshold) is surpassed, the flow of a mountain torrent can set in motion stones and sand from the stream bed. The more water flowing and the steeper the channel, the greater the bedload transport. The quantity of bedload that can be transported depends on the structure of the stream bed and how much rock and earth ends up in the torrent as a result of landslides and bank erosion.

Years' worth of bedload transport measurements indicate that after extreme flood events or the passage of a debris flow, larger bedload quantities can be mobilised and displaced than after longer "quiet" periods.

A major part of the research on bedload transport is conducted at WSL's hydrological test sites in Alptal (canton of Schwyz), especially in the Erlenbach and Vogelbach streams. Our hydrological research in Alptal began back in 1967; the bedload measuring stations at the Vogelbach and Erlenbach were installed in 1982 and 1994. Both here and at other streams, we continuously measure bedload transport using geophone sensors (formerly: hydrophone sensors).

 

This is how it works: a geophone is a device that measures vibrations. It is secured to the underside of a steel plate, which is built into the solid channel bed. When a bedload particle rolls over the plate, it triggers vibrations that are registered by the sensor. These signals are saved electronically and geophone impulses are counted in a simplified way to constitute a measure of bedload transport. To quantify the bedload transport, we have to calibrate this system with independent bedload measurements (e.g. in a retention basin).

In parallel, we have recently installed automatic basket samplers in the Erlenbach, for example, which collect bedload for a certain period of time. These allow us to more accurately determine how much bedload there is at what time and how large the grain is, and to calibrate this with the geophone measurements.

In 2019, we carried out further bed load measurements for calibration using catch nets, among others in Albula (GR), where we also installed a bed load measuring system in 2015. The aim is to link the signals of geophones with certain amounts of bedload.

 

The object: bedload basket sampler

In the Alptal (central Switzerland) WSL operates a sediment transport observatory. This videclip illustrates the automatic baskets used to sample sediment transported in the Erlanbach mountain stream.

 

Aims

  • To compile long-term sediment transport data series in small catchments.
  • To calibrate and develop the acoustic bedload measuring method (geophones).
  • To measure bedload transport in mountain torrents under different boundary conditions (geology, soil, vegetation, climate).
  • To obtain basic knowledge necessary for the characterisation of torrents, the planning of protection structures, and the hazard assessment for endangered areas.
  • To compare bedload transport behaviour of different torrents and mountain streams.
  • To constrain the influence of vegetation and geology on the variability of transport rates.
  • To describe single-grain transport as a function of hydraulic parameters, channel morphology and sediment availability.
  • To determine the most important factors influencing sediment yield in steep streams and to develop predictive methods.

Scientific methods

  • Measuring bedload transport rates with geophone sensors.
  • Measuring yearly sediment yields in retention basins.
  • Tracer methods.
  • Surveying channel morphology and characteristic parameters.
  • Derivation of predictive methods.
 

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