SNF Project: Sediment transport measurements with geophone sensor
Many formulas derived from a large range of different approaches have
been created to predict sediment transport. In contrast to lowland gravel-bed
rivers, a small number of studies have tried to predict sediment transport in
torrents (stream gradients higher than about 5%). Bedload dynamics prediction
in steep headwater channels is challenging mainly due to the interaction with
active hillslope processes, the wide range of sediment sizes, discharge
variance in time, the alternating bed roughness and the presence of steps and
pools.
To increase our understanding in bedload transport processes in steep
streams, it is necessary to monitor it as accurate as possible. We can classify bedload monitoring methods in two main
branches: direct and indirect methods. Typical direct methods include trapping
sediment (i.e. retention basin), collecting moving particles (i.e. Helley-Smith
samplers) and using tracer particles (i.e. Radio-frequency identification RFID).
The promising advantage of indirect sensors (active and passive) is that they can provide high-resolution
and continuous measurements of the transport intensity. The signal
characteristics from these sensors can be related to the bedload. The
registered signal depends firstly on the sensor’s type and sensitivity and
secondly on the in-situ hydraulic conditions and the transported grain
properties which mean that the calibration of these systems is necessary in
order to obtain absolute bedload transport rates.
|
|
Figure 1: Automatic basket sampler and retention basin at the Erlenbach steep stream.
|
Figure 2: The particles are transported over the steel plates equipped with geophone sensors before they are catched by the basket samplers.
|
The Erlenbach stream in Alptal (figure 1 and 2) is one of the Swiss Federal
Research Institute WSL catchments were Bedload transport observations are
available since 1982.
Objectives
In 1999 the piezoelectric bedload impact sensors (PBIS)
were replaced by geophone sensors (fsampling = 10'000 [Hz]). Previous studies have demonstrated that the
signal created by the kinetic impacts of bedload material registered by the geophone over an impact plate is a possible way
to quantify bedload transport (The number of impulses per unit time being
proportional to the total bedload volume of an event). The device calibration clearly
depends on site specific variables such as the grain distribution, material
density and flow characteristics.
|
During a sediment transport event, gravel particles are transported over
the steel plate. These particles, depending on their characteristics and on the
hydraulic conditions, will roll, slide or saltate. When a particle collides against the plate, the shock is transmitted to the
geophone, which is continuously recording the elastic deformation of the plate.
|
Figure 3: Schematic representation of the Swiss-type geophone-plate system.
|
The project aims to identify the main factors influencing the response
of the geophone sensor and to determine which aspects of calibration can be
generalized in un-calibrated field sites. Further on, the possibility to
extract grain size information from the geophone measurements will be explored (acoustic
signal analysis).
|
Figure 4: When the voltage exceeds a threshold value (cyan dashed line) an impulse is registered. The number of registered impulses are well correlated (R-squared = 0.91) with the total transported bedload in the Erlenbach (Rickenmann et. al 2012). The figure above illustrates both the raw signal (unfiltered and filtered to avoid low frequency noise) and the spectrogram (raw signal) of a flume experiment where 2 x 10 grains of D = 52 [mm] are transported with a mean flow velocity of Vm= 3 [m/s] over the geophone's plate.
|
Approach and methods
|
We will perform systematic flume experiments with bedload material from 5
streams (Erlenbach CH, Riedbach CH, Fischbach AU, Ruetz AU and Eshtemoa
IL)
where geophone measurements data is available. The geophone-plate system
used in the laboratory will be exactly the same
that is found in the field (impact steel plate dimensions and geophone
sensor). The emphasis of the lab experiments will be on the reproduction
of flow
conditions similar to prototype flow conditions.
The flume experiments will be carried
out using both uniform grain size classes and mixtures of different grain size
classes to identify the relevant factors responsible for the variation of the
different site calibrations. The laboratory setup includes the installation of
a camera over a light table at the exit of the flume to determine instantaneous
transport rates by tracking the particles.
We will apply the insights obtained from the flume experiments to the five studied streams.
The possibility to extract grain size information is also being hereby explored.
A detailed frequency analysis is also planned in order to identify the natural
frequency of the system.
|
Figure 5: Schematic illustration of experimental set up for the laboratory experiments at the VAW-ETHZ. The bedload transported over the steel plate will come from different streams where field measurements with geophone sensors and field calibration data are available.
|
|
|
|
Project cooperation
- Volker Weitbrecht (VAW, ETH Zürich)
- Robert Boes (VAW, ETH Zürich)
Project team
- Carlos R. Wyss
- Jens M. Turowski
- Dieter Rickenmann
Project link
Contact
