Dynamics of particles

Oceans 2025 Theme 8, work package 8.5b

Sediment dynamics control morphology (LeaCoast2), water quality and (biogenic) particle transports. Modelling entails parameterising small-scale processes of sediment transport. The following developments aim for simultaneous co-located profiles of near-bed turbulence and intra-wave flow, for suspended particle concentration and size, to resolve the wave boundary layer and variation of suspended sediments over a rippled bed; to distinguish suspended load, sheet flow and bed-load; for long duration to include storms.

Simultaneous, co-located profiles of near-bed flow and turbulence are measured by the present POL 3-axis coherent Doppler (CDVP). However, averaged data (initially sampled at ~ 500Hz) are noisy above 2 Hz, limiting intra-wave temporal resolution; the spatial resolution, 0.04 m, is useful but not sufficient in the wave boundary layer. We will further develop the CDVP, for good signal / noise ratios up to resolutions 8 Hz and 0.01m, and for autonomy in the sea. To measure profiles of suspended particle size and concentration, we will add another acoustic transducer at a different frequency. Testing is in the EU-FP6 project Hydralab III.

3-D Coherent Doppler geometry and record

Three estimates of particle size are given by POL's 3-frequency acoustic backscatter system (ABS). However, inversion for mean particle size and concentration requires a priori knowledge of the relative size distribution in suspension. We plan a 6-frequency ABS, for 15 particle-size estimates and some evidence of size distribution. To profile the wave boundary layer and near-bed transport, we will also aim for finer resolution ~ 0.0025 m. For particle size and shape (especially small, cohesives), we will also develop multi-frequency optical backscatter (OBS). OBS will enable estimates of settling velocity and flocculation (via its fractal effect on OBS). We will also use photography/video through collaboration with a university group (Plymouth, Bangor).

Waves over a rippled bed generate vortices, which may be a prime cause of sediment suspension. Process studies in sediment dynamics need a transducer array or multi-beam system, to image suspension over the length of a ripple.

Intra-wave intra-ripple acoustic measurements of sediment entrainment above a vortex ripple. The arrows within the frames show the magnitude and direction of the flow

Bed-load is poorly measured despite normally being dominant in sand transport when the bed is flat, as often under large waves in shallow water. Outside the swash zone, we aim to use acoustics. Building on POL's experience, we propose a 5-10 MHz 1-axis acoustic Doppler system, wavelength 0.3-0.15 mm, with spatial resolution O(1 mm). This should estimate bed-load flux directly from the Doppler shift (for velocity) and signal amplitude (for concentration).

To span storms and periods between, we need long-term continuous measurements by ADV, ADCP, ripple-profiler etc. To install such instruments in inter-tidal areas, POL will develop a modular framework, cabled power and data transmission. The system may be used in the Dee or adjacent Coastal Observatory (with Web display), a representative area for mobile sediments; cable laying by boat from Hilbre Island may be feasible along a protected, short route with permissions.