Multi-beam sonar: visualizing the flow field
Background:
This project seeks to test and apply new multibeam echosounding technology for the 3D quantification of coherent flow structures in large alluvial channels.
Coherent flow structures, generated by fluid-fluid (e.g. combining flows) and/or fluid-form (e.g. over bedforms) interactions are critical in determining sediment transport both insuspension and as bedload. Thus the quantification of their velocity, magnitude and 3D form is clearly vital to better predicting and modeling sediment transport.
The movie on the right shows the potential of this approach with the multibeam swath clearly identifying the movement of suspended sand sediment over the bed.
Recent laboratory work has used the holistic flow mapping ability of particle tracking to assess the 3D form of dune-related macroturbulence, its effect on surrounding flow field and links todune formation. Studies of the mechanics of such turbulence have necessarilycome from laboratory studies: it has been impossible to quantify the 3D structure of such vortices in the field. However, recent developments in
multibeam echo sounding technology now permit, for the first time, an opportunity to investigate the form and sediment transport characteristics of
coherent flow structures in large alluvial channels. Here we seek to assess the
feasibility of this approach and to use this methodology within an ongoing
investigation of one of the world’s largest alluvial channels.
Our project partners RESON (www.reson.com) have developed a new multibeam sonar system, enabling quantification of the
backscatter signal within the entire water column at up to 40 Hz. This enables
high frequency imaging of flow morphology and suspended sediment in the entire
swath width (~7x flow depth). This has the potential to revolutionise the study
of both fluvial & shallow marine environments. Given the above, this
proposal has two aims: (a) to assess the quality of the flow imaging and
suspended sediment data that can be provided, and (b) to utilise the MBS to explore
the three-dimensional evolution of coherent flow structures associated with
migrating dune bedforms and along a large channel confluence shear layer in the
vicinity of the Paraná-Paraguay junction.
(O1) Assessment of the Reson SeaBat 8101 and 7k MBS systems. This objective aims to test the Reson MBS
system for establishing a linkage between MBS backscatter and at-a-point
sediment concentration. MBS systems have been proven in the quantification of
3D bed morphology (Fig. 1b) and the new potential offered here is for
simultaneous spatial mapping of suspended sediment concentration at a high
frequency, in order to quantify spatio-temporal evolution of the suspended
sediment signal of coherent flow structures. We will achieve this by matching
the return signal with in-line real-time monitoring of the flow using a
LISST-100 particle size analyser owned by SVF Kostaschuk. Building on
NER/A/S/2001/00445, this will have significant benefits for the whole research
community in assessing the viability of this approach.
(O2) Testing the linkage between 3D bed morphology, flow structures and
sediment suspension, focusing on sand dunes and junction shear layer dynamics.
The system will be used to investigate the relationship between suspended
sediment fluxes, the 3D form of coherent flow structures and the associated bed
morphology. For the first time, the new system will allow quantification of
instantaneous suspended sediment fluxes that will be calibrated by the LISST.
By using the boat at a fixed point, we will quantify the development and
migration of these dunes in relation to high frequency data on flow structure
and suspended sediment transfer. In the same way, we will quantify the
relationship between shear, suspended sediment transfer and measured bed
morphology in the junction scour and beneath the shear layer. As this system
generates instantaneous 3D data on coherent flow structures that transport
suspended sediment, it will be possible to quantify the advection and diffusion
of suspended sediment fluxes within the water column, in relation to position
within the junction region.
(O3) To couple the 3D data on suspended sediment transfer to measurements acquired using an Acoustic Doppler
profiler. Results from O2 will be related to flow velocities using ADCP
profiling. We have demonstrated that these two techniques can be used
simultaneously from our survey vessel and this will allow unification of data
on changing 3D bed morphology (MBS), coherent flow structure (ADCP) and
suspended sediment flux (MBS/LISST). This will provide a unique field
quantification of flow and suspended sediment dynamics of coherent flow
structures associated with large dunes and confluence shear layers.
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