Bedload monitoring system
Developed by: VAW, ETH Zurich, Switzerland; Test Case partner: Limmatkraftwerke AG, Baden, Switzerland
Date: February 2019
Indirect bedload monitoring systems (BMS) allow the qualitative and quantitative assessment of bedload transport in watercourses (Gray et al. 2010). Applied at hydropower plants (HPP) or other hydraulic structures, these measurements can support the evaluation of bedload continuity across them. In the scope of FIThydro, a BMS was installed to monitor bedload transport in a vortex tube system at the case study HPP Schiffmühle (Rachelly et al. 2019).
The BMS installed at HPP Schiffmühle consists of two passive acoustic sensors, i.e. a geophone (GS-20DX manufactured by Geospace Technologies, Houston TX, USA) and an accelerometer (ICP352C03 manufactured by PCB Piezoelectronics, Depew NY, USA). The sensors do not directly measure bedload transport but register the impact plate's vibration, i.e. oscillations induced by the impingement of passing bedload particles. In the case study HPP Schiffmühle, the steel wall of the vortex tube is used as impact plate, and the sensors in a watertight housing are directly attached to it (Figure 1a and 3a). The vibration signal is sampled at a frequency of 51.2 kHz, transmitted, and then further processed (Figure 1b, c).
The BMS presented here is similar to the Swiss Plate Geophone System (SPGS) (Rickenmann et al. 2012), but includes an additional accelerometer sensor to expand the range of frequencies and hence the potentially detectable particle sizes compared to the SPGS.
The maximum amplitude recorded during a bedload transport event can be related to the maximum grain diameter. Additionally, the sum of impulse counts above a certain amplitude threshold can be connected to the transported bedload volume. Both relations are BMS setup- and site-dependent, and calibration is therefore required to correlate the recorded impact signals to known bedload transport rates, often obtained from traditional bedload sampling (Rickenmann et al. 2012). If possible, calibration in a laboratory flume as well as in the field setting is recommended (Gray et al. 2010, Rickenmann et al. 2014, Wyss et al. 2016a, Wyss et al. 2016b, Albayrak et al. 2017).
Within the scope of FIThydro, a BMS consisting of a geophone and accelerometer was installed on the vortex tube at HPP Schiffmühle, which diverts bedload from the headwater channel to the residual flow reach (Rachelly et al. 2019). The vortex tube consists of a steel tube embedded in the side weir, connecting the two parallel channels (Figure 2). A gate valve positioned in the side weir opens automatically when a predefined discharge is exceeded. The opening of the valve automatically triggers the BMS measurements.
In contrast to the SPGS, the steel tube is used as an impact plate for the BMS, and the sensors are mounted directly onto the outside of the steel tube (Figure 3). Therefore, laboratory calibration was not easily possible. Instead, the system was calibrated in the field by repeatedly dumping sediment samples of known grain size distribution and volume upstream of the vortex tube and subsequently flushing them to the residual flow reach. In addition, drop tests with single grains were performed when the vortex tube was not in operation. The single grain signals help understand the influence of grain size, grain form, drop height, and drop location on the signal's amplitude and frequency.
The first results of the presented BMS are promising, but the data analysis will be further refined and extended. Furthermore, a larger number of recorded flood events is necessary to check the plausibility of the results obtained so far. Overall, this study shows that the SPGS measurement principle can be extended to non-standardized impact plates such as steel vortex tubes and to the use of an additional accelerometer sensor, given that appropriate calibration measures are taken.
Relevant mitigation measures and test cases
The total costs for the geophone and accelerometer sensors amount to approx. 900-1'400 €. The costs for the field computer, the analog-digital-converter, and the 3G modem are approx. 5'300-6'200 €. Depending on the site conditions and setup, additional costs must be considered for the installation, data transmission, and calibration.
- Albayrak, I., Müller-Hagmann, M., Boes, R.M. (2017). Calibration of Swiss Plate Geophone System for bedload monitoring in a sediment bypass tunnel. In Proc. 2nd Intl. Workshop on Sediment Bypass Tunnels (Sumi, T., ed.), paper FP16, Kyoto University, Kyoto, Japan. https://doi.org/10.3929/ethz-b-000185296
- Gray, J.R., Laronne, J.B., Marr, J.D.G. (2010). Bedload-surrogate Monitoring Technologies, US Geological Survey Scientific Investigations Report 2010-5091. US Geological Survey: Reston VA. https://pubs.usgs.gov/sir/2010/5091/
- Rachelly, C., Albayrak, I., Boes, R.M., Weitbrecht, V. (2019). Bed-Load Diversion with a Vortex Tube System. In E-proceedings of the 38th IAHR World Congress, Panama City, Panama, 5900-5909. https://www.research-collection.ethz.ch/handle/20.500.11850/370865
- Rickenmann, D., Turowski, J.M., Fritschi, B., Klaiber, A., Ludwig, A. (2012). Bedload transport measurements at the Erlenbach stream with geophones and automated basket samplers. Earth Surface Processes and Landforms, 37, 1000-1011. https://doi.org/10.1002/esp.3225
- Rickenmann, D., Turowski, J.M., Fritschi, B., Wyss, C., Laronne, J., Barzilai, R., Reid, I., Kreisler, A., Aigner, J., Seitz, H., Habersack, H. (2014). Bedload transport measurements with impact plate geophones: comparison of sensor calibration in different gravel-bed streams. Earth Surface Processes and Landforms, 39, 928-942. https://doi.org/10.1002/esp.3499
- Wyss, C.R., Rickenmann, D., Fritschi, B., Turowski, J.M, Weitbrecht, V., Boes, R.M. (2016a). Laboratory flume experiments with the Swiss plate geophone bed load monitoring system: 1. Impulse counts and particle size identification. Water Resources Research, 52, 7744-7759. https://doi.org/10.1002/2015WR018555
- Wyss, C.R., Rickenmann, D., Fritschi, B., Turowski, J.M, Weitbrecht, V., Boes, R.M. (2016b). Laboratory flume experiments with the Swiss plate geophone bed load monitoring system: 2. Application to field sites with direct bed load samples. Water Resources Research, 52, 7760-7778. https://doi.org/10.1002/2016WR019283