Author(s): Koen Blanckaert; Colin D. Rennie; Joris Heyman; Danxun Li
Linked Author(s): Colin Rennie, Koen Blanckaert
Keywords: Bedload transport; Acoustics; Three-dimensional acoustic Doppler velocity profiler (3DADVP); Digital videography; Particle velocimetry
Abstract: This paper reports a laboratory examination of bedload measurement using a threedimensional Acoustic Doppler Velocity Profiler (ADVP). In the past decade acoustic instrumentation has increasingly been used to measure sediment transport. For example, bias in acoustic Doppler current profiler (aDcp) bottom track velocity has been correlated to bedload transport rates. Further, range-gated acoustic backscatter intensity and/or attenuation have been used to estimate suspended sediment concentration in bins throughout a vertical profile. By combining concentration measurements with Doppler velocity estimates, it has been possible to measure suspended sediment flux. However, there are several reasons why attempts to measure bedload flux have been unsuccessful. A bedload layer is a complex granular surface consisting of discrete bedload particles moving at various speeds over a stable bed. Acoustic scatter from this layer tends to be saturated due to strong scattering from the stable bed, thus it is difficult to relate received intensity to bedload concentration. Furthermore, insonification of this layer leads to multiple scattering off particles moving at various velocities, which broadens the received frequency spectrum and renders Doppler velocity estimates imprecise. Lastly, aDcp measurements at the bed surface are hindered by separated beam configuration, and 3D acoustic velocity profilers are typically configured to obtain optimal measurements of flow properties in bins located further above the bed. In the present research we configured a laboratory ADVP to be most sensitive at the bed surface. Three experiments were performed in a flume with increasing sediment transport: 1) sporadic partial transport of fine sediments moving over a stable armour layer, 2) fine sediment dunes moving over a stable armour layer, 3) generalized transport including coarse particles from a broken armour layer. During these experiments data were collected with both the optimized ADVP and high-speed digital videography. In this paper the ADVP data are assessed for particle velocity in the first bins immediately above the stable bed. The measured bedload particle velocities are compared to estimates obtained from analysis of the digital videos.
Year: 2013