Bedload transport is difficult to measure since sediment movement takes place within a few grain diameters of the sea bed. As a result, field studies usually focus on grain and flow parameters governing average transport rates in order to derive and test expressions for bedload transport. Few in-situ techniques have been adjusted to estimate average transport processes: 1) Particle tracking method which offers a solution to investigate transport pathways of a variety of sediment over wide temporal and spatial scales. Black et al. (2007) provides a comprehensive review of the method and highlights its technical limitations, 2) Ripple progression monitoring using a camera based technique (Kachel and Sternberg, 1971), or an echo-sounder based technique (Bell and Thorne, 1997). It can only be applied where hydraulic and sedimentary conditions allow ripple formation, and 3) Bedload traps, which according to Dyer (1986), are of very variable efficiency due to the difficulties of restricting the sampling to the movable layer.
The above methods yield averaged bedload transport hence providing poor resolution of individual or collective grain motion in time and space. On the other hand, two techniques establish a detailed link between boundary layer turbulence and sediment mixture dynamics. These are: 1) Self generating noise measurements due to particles hitting against each other as they move (Thorne et al., 1983/1984). Threshold of movement, size of the moving grain and instantaneous transport rate can be determined. This method is most suitable for coarse grains but presents problems in calibrating the acoustic signal, and 2) Video observations. Williams (1990) used this technique to observe gravel transport. He determined individual transport velocities and distances for 1680 particles whilst he managed to investigate bedload response to momentarily high bed shear stresses.
This paper describes a new field device able to characterise multimodal sediment response to turbulent fluctuation in terms of mode of transport, instantaneous transport rate, threshold of movement, individual grain velocity, transport thickness, sorting processes and armouring.
Apparatus and deployment
DySPI is associated with several optical and acoustic sensors on a benthic tetrapod. SPI is a remote sensing technique for mapping surficial sediment properties along with observing and quantifying animal sediment interactions in aquatic systems (Rhoads and Young, 1970). A remotely operated camera is used to obtain profile photographs of the sediment-water interface. DySPI enlarge the scope of SPI by allowing bedload processes to be studied with video imagery. Indeed, the sediment-water interface remains undisturbed during the penetration and the main flow is not modified during video acquisition.
An inverted periscope is placed at the middle of a half hull-shaped walking beam. The periscope consists of an optical mirror mounted at a 45°angle into a box with 2 Plexiglas face plates. The Plexiglas box is filled with clear water and sugar to prevent corrosion of the mirror and to obtain the same light diffraction as the sea water. A high definition digital video camera (resolution of 1080x1980, 50 half frames per second) is housed on the rotation axis on top of the mirror. The field of view is centred in order to see both the sediment vertical section reflected by the mirror and the sea floor directly. It is noted that the finest grain size to consider is determined by the camcorder resolution. Because video frames are taken through the periscope box, turbidity of the ambient water does not affect image quality. Light is provided alternatively by a spotlight to illuminate the entire area of interest and by a light pencil to see a specific volume of water. The periscope is horizontally sliced through the sediment thanks to a motorised winch that ensures the penetration is slow enough to minimise disturbance of the sediment-water interface. The penetration depth can be adjusted to a maximum of 10 cm width inside the sedimentary layers. A drag anchor is mounted on the DySPI frame to ensure it is trimmed right in the current direction when it descends through the water column. Thus, the periscope vertical face plate is parallel to the current without any disturbance from obstacles upstream.
Two OBS, a particle-sizer, two ADV and an ADCP are mounted at different heights of the tetrapod in order to monitor boundary layer characteristics simultaneously with video observations.
Discussion and conclusions
White (1998) highlighted that despite the plethora of available methods, it is still not possible to make detailed or accurate field measurements of bedload, suspension of mixed sizes, or suspension very close to the seabed. The device presented herein, with the addition of appropriate image processing (Keshavarzy and Ball, 1999), provides a way to investigate in details these transport processes at the sediment water interface. Threshold of movement, size of the moving grain, instantaneous transport rate, transport thickness and sorting processes can be studied in addition to usual SPI parameters (Kennedy, 2006). Furthermore, if DySPI is moored at a bidirectional current location, study of sediment structure reorganization following turn of tide is made accessible.
This new apparatus enables in-situ sediment processes investigation in a large range of hydro-sedimentary conditions which are monitored as accurately as during flume experiments.
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