The Swiss plate geophone system is a bedload surrogate measuring technique that has been installed in more than 20 streams, primarily in the European Alps. Here we report about calibration measurements performed in two mountain streams in Austria. The Fischbach and Ruetz gravel-bed streams are characterized by important runoff and bedload transport during the snowmelt season. A total of 31 (Fischbach) and 21 (Ruetz) direct bedload samples were obtained during a 6-year period. Using the number of geophone impulses and total transported bedload mass for each measurement to derive a calibration function results in a strong linear relation for the Fischbach, whereas there is only a poor linear calibration relation for the Ruetz measurements. Instead, using geophone impulse rates and bedload transport rates indicates that two power law relations best represent the Fischbach data, depending on transport intensity; for lower transport intensities, the same power law relation is also in reasonable agreement with the Ruetz data. These results are compared with data and findings from other field sites and flume studies. We further show that the observed coarsening of the grain size distribution with increasing bedload flux can be qualitatively reproduced from the geophone signal, when using the impulse counts along with amplitude information. Finally, we discuss implausible geophone impulse counts that were recorded during periods with smaller discharges without any bedload transport, and that are likely caused by vehicle movement very near to the measuring sites.
In the past decade or so, an increasing number of studies have been undertaken on bedload surrogate acoustic measuring techniques which were tested both in flume experiments and in field settings. A review of such indirect bedload transport measuring techniques was recently published by Rickenmann (2017a, b). Examples of measuring systems include the Japanese pipe microphone (Mizuyama et al., 2010a, b; Uchida et al., 2013; Goto et al., 2014), the Swiss plate geophone (Rickenmann and Fritschi, 2010; Rickenmann et al., 2012, 2014), other impact plate systems (Krein et al., 2008, 2016; Møen et al., 2010; Reid et al., 2007; Beylich and Laute, 2014; Taskiris et al., 2014), and hydrophones, i.e. underwater microphones (Barton et al., 2010; Camenen et al., 2012; Rigby et al., 2015). It is well known that bedload transport rates often show very large variability for given flow conditions (Gomez, 1991; Leopold and Emmett, 1997; Ryan and Dixon, 2008; Recking, 2010), and that prediction of (mean) bedload transport rates is still very challenging, particularly for steep and coarse-bedded streams (Bathurst et al., 1987; Nitsche et al., 2011; Schneider et al., 2015, 2016). For such conditions, direct bedload transport measurements are typically difficult to obtain, or may be impossible to make during high-flow conditions (Gray et al., 2010). In contrast, indirect bedload transport measuring methods have the advantage of providing continuous monitoring data both in time and over cross sections, even during difficult flow conditions, and are therefore expected to increase our understanding of bedload transport.
A fair number of these measuring techniques have been successfully calibrated for total bedload flux, which generally requires contemporaneous direct bedload transport measurements in the field (Thorne, 1985, 1986; Voulgaris et al., 1995; Rickenmann and McArdell, 2007, 2008; Mizuyama et al., 2010b; Rickenmann et al., 2014; Mao et al., 2016; Habersack et al., 2017; Kreisler et al., 2017). Essentially, linear or power law relations were established between a simple metric characterizing the acoustic signal and bedload mass. In some studies further calibration relations were established to identify particle size, either based on signal amplitude (Mao et al., 2016; Wyss et al., 2016a) and/or on characteristic frequency of that part of the signal which is associated with a single impact of a particle (e.g. for impact plate systems; Wyss et al., 2016b) or by determining a characteristic frequency for an entire grain size mixture (for the hydrophone system; Barrière et al., 2015a). A few of the acoustic measuring techniques were used to determine bedload transport by grain size classes (Mao et al., 2016; Wyss et al., 2016a). Finally, some studies examined to what extent findings from flume experiments can be quantitatively transferred and applied to field sites for which independent, direct calibration measurements exist (Mao et al., 2016; Wyss et al., 2016b, c).
In this study we report on calibration measurements of the Swiss plate geophone (SPG) system in two mountain streams in Austria. The Fischbach and Ruetz gravel-bed streams are characterized by important runoff and bedload transport during the snowmelt season. During a 6-year period, 31 (Fischbach) and 21 (Ruetz) direct bedload samples were obtained in the two streams, respectively. The objectives of this paper are (i) to present and discuss different ways of analysing the geophone calibration measurements, also in comparison with data and findings from other field sites and flume studies, (ii) to show that the observed coarsening of the grain size distribution with increasing bedload flux can be qualitatively reproduced from the geophone signal, and (iii) to discuss implausible geophone impulse counts that were recorded during periods with small discharge and without any bedload transport, and that are probably associated with close-by vehicle movement.
The first indirect bedload transport measurements using impact plates were made in the Erlenbach from 1986 to 1999 using a piezoelectric crystal as sensor, with the aim of continuously monitoring the intensity of bedload transport and its relation to stream discharge (Bänziger and Burch, 1990; Rickenmann, 1994, 1997; Hegg et al., 2006; Rickenmann and McArdell, 2007). A geophone sensor was used at the Erlenbach and at all other field sites that were set up in the year 2000 and later (Rickenmann and Fritschi, 2010). In the meantime, the SPG system has been installed in more than 20 streams primarily in central Europe (Rickenmann, 2017b). An array of steel plates is typically installed flush with the surface of a sill or check dam, a location where there is only a small chance for (substantial) deposition of bedload grains during transport conditions.
Location of the Fischbach and Ruetz mountain stream catchments in the Stubai Alps of Tyrol in western Austria. The measuring
sites are indicated with a green pentagon, and the catchment boundaries are marked with a gray line. (Source of topographic map:
Abteilung Geoinformation, Amt der Tiroler Landesregierung;
Monitoring sites equipped with a Swiss plate geophone system and a flow gauging station.
Schematic stream cross section at the geophone measuring site in both Fischbach and Ruetz. The steel–concrete pillar is located downstream of sensor plate 5. The sill with the steel plates is inclined towards the left bank to improve the resolution of the flow gauge measurements at low discharges. On the banks, the dotted horizontal line indicates the paved local road on the river's right side at the Fischbach, and the two dashed horizontal lines indicate the gravelled parking lot on both river sides at the Ruetz.
The Fischbach and Ruetz field sites were installed by the Tyrolean Hydropower Company (TIWAG). They are located in partly glaciated
catchments in the Tyrolean Alps (Fig. 1), at elevations of 1544
At both sites, the concrete sill is located 4
At each of the two sites, a streamlined metal pillar was installed 0.5
Grain size distribution (GSD) of the surface bed material upstream of the measuring sites. The GSD was measured on 3
October 2012 at the Fischbach and on 4 October 2012 at the Ruetz. The line-by-number samples included observations for grain sizes
The calibration measurements used here were obtained by TIWAG in both streams during the summer months of 2008–2013 using the basket
bedload sampler. A total of 31 measurements from the Fischbach and 21 measurements from the Ruetz were used in this analysis
(Table 1). The maximum sample mass caught in the sampler was 518
Catchment and channel characteristics at the field sites
and range of typical parameters for the conditions during the geophone
calibration measurements. The
The sampling duration of the calibration measurements was essentially selected according to bedload transport intensity. For very high bedload transport rates, the sampler may be quickly filled; ideally, sampling should be stopped before the basket is full (to avoid scouring of previously caught particles and to reduce uncertainty about the exact filling time). For very small bedload transport rates, the total sampled mass may be relatively small for a fixed sampling duration; if only few particles travel over the steel plate, the variability of the signal response is larger, due to random factors influencing the signal response (e.g. different transport modes and impact locations) that only tend to average out for larger numbers of particles that moved over the plate (see also beginning of Discussion section below). Therefore we used generally longer sampling durations for lower transport rates.
Threshold values of the signal amplitude
Coefficients, exponents and statistical properties for the calibration relations according to Eqs. (1)–(5). All calibration
relations refer to bedload mass with
The bedload impact shocks on the steel plate are transmitted to the geophone sensor and, thereby, an electrical potential is
produced. The standard geophone sensor uses a magnet in a coil as an inductive element. The magnet picks up the vibrations of the steel
plate and induces a current in the coil which is proportional to the velocity of the magnet. Whenever the voltage exceeds a preselected
threshold amplitude value,
At most of the field sites with SPG measurements, several signal summary values were routinely stored in the past. The most often used summary values for calibration purposes are the summed impulse counts, IMP. These values were found to correlate reasonably well with bedload mass or volume transported (Rickenmann and McArdell, 2007, 2008; Rickenmann et al., 2012, 2014). Another useful summary value is maximum amplitude, MaxA, that may be determined for different recording intervals. During calibration measurements, all summary values were typically stored in 1 s intervals. During normal flow monitoring, the recording interval for the summary values at the Fischbach and Ruetz was 15 min. (At other SPG measurement sites operated by WSL this recording interval is typically 1 min.)
Using the so-called amplitude histograms (AHs), Wyss et al. (2016, 2014) demonstrated for the SPG measurements at the Erlenbach (Swiss
Prealps) that absolute bedload masses for each grain size class could be successfully calculated for both the calibration and
validation data obtained with the moving basket samplers. The continuous recording of AH data was also implemented at the Fischbach and
Ruetz measuring sites, with a recording interval of 1 min. At these sites, impulses were determined separately for 17 amplitude
classes as listed in Table 2. For the analysis in this study, for each amplitude threshold value
The following calibration relations and calibration coefficients were
determined using the transported bedload mass,
Fischbach: geophone calibration relationships for grains with
Ruetz: geophone calibration relationships for grains with
For the Fischbach, the calibration relations in the form of Eqs. (1) and (2) show a rather high correlation coefficient (Fig. 5,
Table 3), which is also characteristic for similar calibration relations determined for the Erlenbach (Rickenmann et al., 2012,
2014). For the Ruetz, the calibration relations in the form of Eqs. (1) and (2) are less well defined (Fig. 6, Table 3). Due to the
inclusion of four additional calibration measurements obtained in 2012 and 2013, the correlation coefficient for the Ruetz is lower
than in an earlier analysis that used only 17 measurements from the period 2008 to 2011 (Rickenmann et al., 2014). This level of
correlation is similar to calibration measurements obtained for the Navisence stream in Switzerland (Wyss et al., 2016c) for which most
measured bedload masses were smaller than 20
Linear calibration coefficient
Characteristic grain size
Linear calibration coefficient
Unit bedload transport rate
Fischbach:
Systematic flume experiments were performed for different grain size classes to investigate the dependence of a linear calibration
coefficient, defined as
In Fig. 10, the regression relation for higher impulse rates was derived based on 14 calibration measurements from the Fischbach with
Ruetz:
The amplitude histograms (AH data) for each calibration measurement were used to estimate grain size distributions (GSDs) for the basket
sampler measurements, which were then compared with the sieve analyses of the bedload samples. For the analysis of the AH data, the
lowest class with impulses for
For the bedload samples from both Fischbach and Ruetz a general coarsening trend of the GSD with increasing
unit bedload transport rate
For the Fischbach (Fig. 11) it is noted that only two calibration samples were available for the class Fi1, and these had the two smallest
bedload masses (with 19 and 8
Fischbach: arithmetic mean of geophone impulses per 15
Ruetz: arithmetic mean of geophone impulses per 15
Both measuring stations are situated at a relatively high elevation, and the stream catchments include mountain peaks with elevations
above 3000
For the Fischbach and the discharge classes smaller than 3
For the Ruetz and the discharge classes smaller than 1.0
To further investigate the potential source of the implausible geophone recordings, we classified the measured IMP
Turowski et al. (2011) analysed the start and end of bedload transport in gravel-bed streams, including geophone measurements from the
Fischbach and Ruetz for the years 2008 and 2009. They determined discharge values at the start (
For a system such as the Swiss plate geophone it is known that the signal response depends on factors such as grain size, fluid or particle velocity, particle shape and mode of transport (i.e. sliding, rolling, saltating), and impact angle and impact location on the steel plate (e.g. Wyss et al., 2016b; Rickenmann, 2017b). For a given stream we may assume that the most of these factors vary within a given range, and the linear calibration coefficients primarily vary with flow conditions. Therefore, we expect that the mean signal response from a given particle size travelling over the plate becomes more stable the larger is the total number of particles that have been transported over the plate. This is the main reason why we have primarily considered the summed geophone summary values in the past (e.g. Rickenmann et al., 2012, 2014). Calibration measurements from various sites confirmed the expectation that random factors influencing the signal response tend to be more averaged out for longer integration periods (Rickenmann and McArdell, 2007, 2008; Rickenmann et al., 2012, 2014; Wyss et al., 2016a, c).
Comparison of geophone calibration data from eight different stream sites. Unit bedload transport rate
However, it may also be interesting to consider calibration relations for example between bedload rates and impulse rates. If a linear
calibration relation in the form of Eq. (1) is generally valid, a division of
The estimated bedload mass per sample using the method in Wyss et al. (2016a) developed for the Erlenbach,
For extreme flow conditions and very high bedload transport rates, there may be some limitations to extrapolating calibration relations
for the SPG system from the typical range of conditions investigated so far. Using the same steel impact plates, we had installed
piezoelectric bedload impact sensors in an earlier study to make bedload measurements at a water intake of the Pitzbach mountain
stream in Austria during two summer periods (Rickenmann and McArdell, 2008). Impulses were counted in a similar way as for the Swiss
plate geophone system. At the Tyrolean weir a total of 12 steel plates with sensors were installed, with a natural gravel-bed surface
upstream of the sill of 6
We used the AH data recorded during the calibration measurements at the Fischbach and Ruetz to estimate the transported bedload mass
for each calibration measurement,
Comparison of yearly bedload (YBL, in t) calculated with two different calibration relations, for the year 2010 and for different ranges of
To illustrate the uncertainty associated with using different calibration relations, we determined the yearly bedload (YBL) for
2010, which represents the year with the largest peak discharges and the largest YBL values (Table 4) for the period
2008–2013. For both streams, the YBL values are larger when using Eqs. (4) and (5) as compared to using Eq. (1); this is not
surprising when comparing the linear with the power law calibration relations in Fig. 10. The power law calibration relations result in
a 66 % higher YBL for the Fischbach and in a 85 % higher YBL for the Ruetz, if only plausible IMP values
for discharges larger than
Based on the analysis of the GSD of all bedload samples we found on average a coarsening of the GSD with increasing bedload transport intensity (Figs. 8 and 11). However, GSDs from individual bedload samples are quite variable within given classes of bedload transport rates. The same is true if GSDs of the bedload samples are analysed in terms of changing discharge. The bedload samples were taken too randomly in time and too infrequently over the 6-year study period as to allow examining whether there is any hysteresis trend for daily discharge cycles or over the entire summer season. In a follow-up study, possible hysteresis trends were investigated based on the continuous geophone data which were converted into bedload fluxes using Eqs. (4) and (5), and the related findings are discussed in a forthcoming paper.
Hydrophones (underwater microphones) have been used to monitor bedload transport both in riverine and in coastal environments (e.g. Thorne, 1990; Camenen et al., 2012; Basset et al., 2013). The objective of using such a system is to record self-generated noise produced by collisions of moving bedload particles against each other or against the bed. The application of this bedload surrogate measuring system can be impaired by other sources of noise, which may be caused by vessel traffic, marine seismic exploration, or underwater military operations. If the main interest is in the acoustic signal due to bedload transport, discounting for other sources of noise may be challenging and will also depend for example on the spatial distance and the dominant frequencies of the different acoustic sources (Hildebrand, 2009; Etter, 2012; Basset et al., 2013).
For the application of impact plates with acoustic sensors installed in
a streambed there is very little experience with
non-bedload-transport-related sources of noise that may compromise their
usefulness. We have shown in Sect. 3.3 that road traffic is a likely source
of environmental noise producing a similarly strong signal at the SPG system
to low-intensity bedload transport during periods with moderate discharges.
This observation was made for our two study streams Fischbach and Ruetz,
where in both cases the stream bed runs very close to roads, which are
located only about half the stream-width away from the edge of the bed. We
have checked the impulse counts recorded for SPG systems installed at
mountain streams in Switzerland, particularly for low-flow periods during
wintertime. There were generally very few impulses recorded at these sites,
indicating that road traffic is not an important source of noise. At these
sites roads with regular traffic are situated clearly farther away from the
channel profile than at the two Austrian sites of this study: at the
Navisence stream in Zinal (Ancey et al., 2015) about 45
At the Riedbach stream in Switzerland the geophone measuring site is situated
at a water intake at an elevation of 1800
The Fischbach and Ruetz gravel-bed streams are characterized by important runoff and bedload transport during the snowmelt season. As a bedload surrogate measuring technique, the Swiss plate geophone (SPG) system has been installed in 2007 in both streams. During the 6-year period 2008–2013, 31 (Fischbach) and 21 (Ruetz) direct bedload samples were obtained in the two streams, and these measurements were analysed to obtain calibration relations for the SPG system at the two sites.
As applied at many other SPG sites in the past, we first established
calibration relations using total transported bedload mass and the number of
geophone impulses. A second way of analysing the geophone calibration
measurements consisted in using bedload transport rates and geophone impulse
rates. For the Fischbach the second approach resulted in two power law
calibration relations, with different coefficients and exponents for small
and large transport rates. The exponent was smaller than one for small
transport rates, and larger than one for larger transport rates. For the
Ruetz data with essentially only lower transport intensities, the power law
relation derived from the Fischbach is also in reasonable agreement with the
Ruetz calibration measurements. The non-linear power law calibration
relations are in qualitative agreement with the observed coarsening of the
bedload with increasing transport rates. According to findings from flume
studies the signal response per unit bedload mass increases for small grains
up to a grain size of approximately
40
Amplitude information from the geophone signal was recorded in minute intervals at the Fischbach and Ruetz by summing impulse counts separately for different amplitude classes (so-called AH data). Since signal amplitude correlates with grain size at several SPG sites (Wyss et al., 2016a, b, c), this information was used to estimate the grain size distribution for the bedload samples from the Fischbach and Ruetz. It was found that the observed coarsening of the grain size distribution with increasing bedload flux could be qualitatively reproduced from the geophone signal using the AH data.
For smaller discharges at the Fischbach and Ruetz, in particular during the wintertime, it was found that many implausible geophone impulse counts were recorded. Both SPG measuring sites are situated very close to local roads with regular traffic. The roads are only about half the stream width away from the steel plates, and we therefore identified vehicle traffic as a likely source for the implausible geophone impulses. This is indirectly supported by a comparison with other SPG sites in Switzerland. At most of these sites only very few implausible geophone impulse counts were recorded in the past, which is probably due to the fact that the local roads are farther away from the steel plates, generally at least about twice the stream width.
The data cannot be made publicly available for the time being since they are used by the Tyrolean Hydropower Company TIWAG, the owner and provider of the data, in an ongoing hydropower project authorization procedure.
Information about the grain size distribution of the transported bedload over a Swiss geophone plate can be determined using the number of impulses per amplitude class (called amplitude histogram method). Amplitude histograms (AH data) can be interpreted as a statistical distribution of the signal's amplitude over a given time interval. Using the number of bedload particles per unit mass, absolute bedload masses for each grain size class were calculated for the Erlenbach stream in Switzerland.
For
BF was reponsible for the concept and installation of the SPG system at the Fischbach and Ruetz. He had suggested to record the AH data as a memory efficient way to extract grain-size-relevant information from the raw geophone signal. DR was responsible for the analysis and wrote the paper. Support of colleagues for figure preparation is acknowledged below.
The authors declare that they have no conflict of interest.
This article is part of the special issue “From process to signal – advancing environmental seismology”. It is a result of the EGU Galileo conference, Ohlstadt, Germany, 6–9 June 2017.
We are grateful to the Tyrolean Hydropower Company (TIWAG) for having performed the geophone calibration measurements in the Fischbach and Ruetz streams and for providing these data and the continuous geophone measurements to WSL for further analysis. The study was supported by SNF grants 200021_124634 and 200021_137681. We thank Nicloas Steeb, Philipp von Arx, and Thomas Weninger for help with the preparation of some figures; Thomas Weninger also performed grain size analyses of the streambed surface. Edited by: Michael Dietze Reviewed by: two anonymous referees