Bedload Transport Measurements in Mountain Streams with the Swiss Plate Geophone System: Towards a General Calibration Procedure for Fractional Transport
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2022
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Doctoral Thesis
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Abstract
Flood events across Europe in the summer of 2021 have illustrated the threat of flood-related hazards like bedload transport to human life and infrastructure, especially in small and steep mountainous catchments. Predicting bedload transport in higher gradient streams still represents a considerable challenge because of its large spatio-temporal variability. To cope with these uncertainties, intensive efforts were made to develop surrogate bedload monitoring technologies. The present thesis focuses on one such system, the Swiss plate geophone (SPG), which has been deployed and calibrated in numerous steep water courses, mainly in the Alps.
In a first stage, we conducted field calibration measurements at three Swiss field sites recently equipped with a SPG system which are located at the Albula, the Navisence, and the Avançon de Nant streams. There, direct bedload samples were collected with a net sampler to calibrate the signal recorded by the geophones. Earlier studies have shown that the calibration relationships obtained from such campaigns can vary substantially between different monitoring stations, likely due to site-specific factors such as the flow velocity and the bed roughness.
In a second stage, we attempted to improve our understanding of these disparities by performing full-scale controlled flume experiments at an outdoor flume facility. The mentioned field sites were replicated one after another in a 24 m-long flume on the basis of their morphological and hydraulic characteristics, and a part of the bedload samples collected in the field served as test material. Even though these flume experiments could not accurately reproduce field-based calibration relationships, they enabled us to relate variations in the SPG signal response to changes in the grain-size distribution of bedload mixtures. Furthermore, we showed that a geophone plate detects vibrations from impacts occurring either on a neighboring plate or on the surrounding concrete sill, which introduces a significant site-specific bias to field calibration relationships.
Recent studies have reported combinations of amplitude and frequency information to infer particle sizes and improve the detectability of bedload particles using various surrogate monitoring techniques. Motivated by these findings, we developed in a last stage two amplitude-frequency-based methods aiming to identify these extraneous impacts and reduce their effect on site-specific calibration relationships. Using a field calibration dataset extended with measurements from the Erlenbach site, we showed that including frequency information in the calibration of the SPG signal results in more homogeneous signal responses across all sites and significantly improves the accuracy of fractional sediment flux and grain-size estimates. Thus, it was possible to develop a single general calibration relationship capable of providing fairly accurate bedload transport estimates at all the four investigated field sites. Optimizing the calibration procedure for each field site separately, resulted in a further increase of the accuracy of the predicted bedload transport rates.
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ETH Zurich
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03798 - Kirchner, James W. (emeritus) / Kirchner, James W. (emeritus)