Abstract
Acoustic signals can be used in various ways for suspended sediment monitoring. One possibility which lends itself particularly well in the context of hydropower plants (HPPs), is to use installations for acoustic discharge measurement (ADM). Such installations already exist at waterways of many HPPs. Similar to certain turbidimeters, the attenuation of the forward scattered signal travelling through the water-sediment mixture is correlated with suspended sediment concentration (SSC). This correlation can be based on reference SSCs, e.g. from gravimetric analyses of bottle samples. Without the need of additional sensors and practically maintenance-free, this method is used successfully in the HPP Fieschertal to warn the HPP operator of high SSC to prevent excessive turbine abrasion.Acoustic methods and systems that allow for estimating both SSC and particle size distribution (PSD) are under development. The simultaneous determination of SSC and PSD is not possible using a single frequency. Therefore, multi-frequency approaches are investigated for generally scattered signals. When backscattered signals are used, a stronger frequency dependency can be exploited. However, the reliable simultaneous determination of particle size (and distribution) and concentration is still a major challenge due to a low signal-to-noise ratio and an ill- posed problem of estimating concentration and size from recorded signals. The optimal setup configuration (angles, frequencies) for such a system is not unique and further investigations are recommended.
Highlights
In various high- and medium head hydropower plants (HPP) worldwide, e.g. in the Alps, the Andes and the Himalayas, fine sediment particles contained in the water poses at least temporarily a problem in terms of damaging turbines and other equipment due to abrasion
In the Fieschertal field study it was confirmed that the single-frequency acoustic method based on the acoustic discharge measurement (ADM) installation allows to measure suspended sediment concentration (SSC) up to 15 g/l of mainly medium silt
The correlation between the attenuation of the acoustic signals and the SSC was established based on gravimetrical analysis of bottle samples
Summary
In various high- and medium head hydropower plants (HPP) worldwide, e.g. in the Alps, the Andes and the Himalayas, fine sediment particles (silt and fine sand) contained in the water poses at least temporarily a problem in terms of damaging turbines and other equipment due to abrasion. SSC can be determined by weighing of the water-sediment samples and the dried residues after filtration or evaporation of the water from water samples This gravimetrical method is still the reference. For on-line SSC monitoring, two techniques are widely used: optical backscatter and optical transmission. Forward scattering can be exploited in installations for acoustic discharge measurement (ADM) based on the acoustic transit time (ATT) method. Such installations exist in many HPPs and can be used – after a calibration – to estimate SSC or PSD, but not both [5]. SSC monitoring exploiting the attenuation of forward scattered signals as can be measured with ADM installations is presented. In a final section the opportunities for a simultaneous estimation of SSC and PSD by acoustic methods are presented
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