The movement of sand under the action of waves and tidal currents is of considerable importance to hydrographers, engineers and geologists because it may lead to sediment accumulation, bank and channel migration and structural or pipeline scour.

Although a range of predictive sediment transport equations exist, direct determination of sediment transport rates in the marine environment is presently limited to low-frequency measurements with traps, tracers or transmissometers. This chapter described a non-intrusive probe which utilizes optical fibres to direct a modulated infra-red light beam over a short seawater path length. Interruption of the beam by moving sand grains is recorded and the transport rate is estimated. The instrument has the advantage of being able to resolve the movement of individual grains preliminary trials in Carmarthen Bay are described.

INTRODUCTION
Formulae Which Need Not Exist

Sediment transport-and we shall use the phrase here to refer to the movement of loose sand and gravel under the action of waves and tidal currents-has been studied for practical reasons for at least 20 centuries; documents show that the Greeks were concerned with a problem of harbor siltation in the Mediterranean (Graf, 1971). More recently, other have addressed the so-called two-phase problem; ranging from geographers working on landforms in ancient strata, oceanographers interested bedform in ancient strata, oceanographers interested in bed roughness and drag, as well as a wide variety of engineers concerned still with siltation, but now also scour, channel maintenance, coastal erosion, winning aggregates and related problems. However, very few of these groups would wish to be concerned with the process of sediment transport itself. They are generally concerned with the result of sediment transport, for example the infilling of a dredged channel by tidal flows, and need an understanding of the process only to link the known hydraulics with the expected change in the hydrography of the channel. In an ideal world, the knowledge of the flow would lead directly to a prediction of the hydrography. Such a solution is, of course, presently not available, although recent developments in the application of non-linear systems theory to morphodynamic models may improve the postion (e.g. Hardisty, 1986a).

Existing Formulae

Presently, the problem is tackled by substituting some characteristic of the flow and of the bed sediment into one of the sediment transport formulae, and inte grating the resulting rate over a chosen time interval to determine the hydrographic change. A large number of these formulae, and integrating the resulting rate 26 which have been applied, with varying degree of success, to the marine environment.

Research Objectives

The Marine Morphodynamics Unit at RHBNC has been examining these formulae with a view to improving their predictive capability by achieving a more fundamental grasp of the physics of the processes involved. It appears that, largely because sediment transport does reflect the inherent turbulence of the forcing flow, the group of formulae which originated from the work of Einstein (1950) includes a probabilistic element. An alternative group of formulae appears to presume a purely determine relationship.

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