In this study , in a bid to discriminate the effect of sediment and bubbles on backscatter signal , a simple predictive model for air bubble entrainment is tested that relies on a linear regression between backscatter and normalized height above the bed.
Effect of wave bubbles on ADCP backscatter in two shallow lakes
IJsselmeer and Markermeer are two adjacent shallow lakes located in the central part of The Netherlands, with typical depths between 2 and 6 meters. Wind-waves acting on the lake's surface re-suspend bed sediment, exerting a strong impact on the distribution of pollutants, nutrients and, ultimately, the biota of the lakes. Rijkswaterstaat has equipped a number of observation poles in the lakes, accommodating various sensors for environmental monitoring. Each pole contains, among others, an upward looking acoustic Doppler current profiler (ADCP), a wave measuring device and turbidimeters near the bottom and near the surface. The turbidimeters are optical backscatter devices that require regular maintenance, and yield single-point measurements. ADCPs require less maintenance, are more robust and less intrusive, and collect flow velocity profile data with collocated turbidity information from acoustic backscatter intensity.
In this study, approximately one month of data from six poles are analysed to determine whether ADCPs can potentially be used to collect vertical profiles of turbidity. Surface waves entrain air bubbles, which affect the acoustic backscatter signal of the ADCPs used to observe suspended sediment. A simple predictive model for air bubble entrainment is tested that relies on a linear regression between backscatter and normalized height above the bed. For poles in shallow areas, the intercept in the linear regression, interpreted as an indicator for the sediment contribution, shows a high correlation with wave height. This can be explained by the quick response of suspended sediment concentration to wave activity in shallow water, as a result of re-suspension. For the poles in relatively shallow water, slopes increase with relative wave height, whereas the opposite is true for poles in deeper water. This may be explained from a total depth dependence of the relative contributions of sediment and air bubbles to backscattering. Wave height is established to be a decisive factor in controlling the variation of acoustic backscatter in the two lake areas, irrespective of the weather conditions. Discriminating between the effects of suspended sediment and air bubbles is hampered by the relatively low concentrations of fine sediment in suspension. Variations over depth are small, which limits the possibility to exploit the fact that wave bubbles are entrained from the surface, whereas sediment is resuspended from the bed.