Long-term bedload monitoring
|
|
Measuring station in the Erlenbach. The geophone
sensors are mounted under the steel plates in the check dam above the
retention basin.
|
|
Discharge
and geophone measurements for two flood events in the Erlenbach.
|
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.
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.
Links
Participants
- Dieter Rickenmann
- Bruno Fritschi
- Jens Turowski
- Alexandre Badoux
- Manuel Nitsche
- Johannes Schneider
- Karl Steiner
Contact
