Bedload transport measurements with impact plate geophones in two Austrian mountain streams (Fischbach and Ruetz): system calibration, grain size estimation, and environmental signal pick-up

Abstract

Abstract. 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.

Variability of Bed Load Transport During Six Summers of Continuous Measurements in Two Austrian Mountain Streams (Fischbach and Ruetz)

“…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 bed load grains during transport conditions. The Fischbach and Ruetz field sites are operated by the Tyrolean Water Power Company (TIWAG; Rickenmann & Fritschi, ; Turowski et al, ). They are located in partly glaciated catchments in the Tyrolean Alps (supporting information Figure S1).…”

“…A detailed analysis showed that there is pickup of geophone signal very probably due to vehicle traffic on the road (Fischbach) and paved parking lot (Ruetz) which are located very close to the measuring sill, resulting in implausible geophone impulse counts that could be clearly identified at smaller discharges. This analysis concluded that bed load transport is likely the dominant source of producing geophone impulses above a critical discharge Q c of about 3.5 m 3 s −1 at the Fischbach, and above a Q c of about 1.5 m 3 s −1 at the Ruetz (Rickenmann & Fritschi, ). A quantitative analysis comparing the mean impulses for all eight geophone sensors for periods in winter (16 October to 14 April) and in summer for flows with Q < Q c suggests that a large part of the geophone signal in summer during these flow conditions may be due to vehicle traffic (supporting information Table S1); however, traffic may considerably vary seasonally, particularly with the important winter tourism activity in the Ruetz catchment, complicating a detailed quantification of this effect.…”

“…The continuously monitored geophone signal was converted by the measuring software into the summary values impulses ( IMP ), maximum amplitudes ( MaxA ), and squared integrals (RMS values), and these values were stored in 15 min intervals for the period 2008–2013 (Rickenmann et al, ). In addition, for the period 2010–2013 so‐called amplitude histogram (AH) values were stored in 1 min intervals (Table ), which allowed to (roughly) estimate the grain size distribution (GSD) of the transported particles (Wyss et al, ; Rickenmann & Fritschi, ).…”

“…The metal wire mesh of the basket had a spacing of 10 mm. The analysis of these measurements is reported in Rickenmann & Fritschi () and in Rickenmann et al (). A total of 31 and 21 measurements from the Fischbach and from the Ruetz, respectively, resulted in the development of the following calibration relation (Rickenmann & Fritschi, ): where q b,p is the bed load transport rate per plate (kg 0.5 −1 m −1 s −1 ) for particles D > 10 mm, and IMPT is the impulse rate per plate (0.5 −1 m −1 s −1 ).…”

“…The focus of this paper is to examine the temporal variability of bed load transport in two mountain streams in Austria, Fischbach and Ruetz (Rickenmann & Fritschi, ). The continuous long‐term measurements with the Swiss plate geophone (SPG) system obtained during the period from 2008 to 2013 were analyzed with regard to the following objectives: (i) to examine how much time‐integrated bed load transport rates and flow variables can improve transport prediction; (ii) to identify periods with less variability in transport and to examine a possible relation to measured changes in lateral bed load transport activity and changes in bed load texture; and (iii) to identify daily and seasonal hysteresis patterns in bed load transport.…”