Gao Shu

The use of grain size trends in marine sediment dynamics: A review

Spatial changes in grain size parameters (i.e. grain size trends) contain information on sediment transport patterns. An analytical procedure has been proposed to transform the grain size trends into an image of trend vectors, which may represent net sediment transport pathways. A fundamental assumption for such an approach is that the frequency of occurrence of the trend adopted is much higher in the transport direction, than in any of other directions. Preliminary studies show agreement between this assumption and observations. However, further investigations into the physical processes and mechanisms for the formation of grain size trends are required to improve the technique, including flume experiments and numerical modeling. Moreover, attention should be paid to the trends associated with finegrained sediment, for the method of grain size trend analysis is so far designed for coarse-grained material only. The processes of flocculation during settling and the wash-load property must be considered. Appropriate interpretation of grain size data will improve our understanding of the physics of granular materials.

A PRELIMINARY STUDY ON SUSPENDED SEDIMENT CONCENTRATION MEASUREMENTS USING AN ADCP MOUNTED ON A MOVING VESSEL

A DR300 Broad Band ADCP mounted on a vessel moving at a speed of 2–3 m/s was used to measure the profile of suspended sediment concentrations (SSCs) at the entrance to Jiaozhou Bay, Shandong Peninsula, where the water is characterized by low SSCs. The echo intensity data produced by the ADCP were regressed against the SSCs derived using the filtration method. The results show that the calibrated relationship can be used to calculate the SSC, with a relative error of 30%. Therefore, it is feasible to measure the SSC (even if the concentration is low) using the ADCP mounted on a moving vessel. Compared with OBS, ABS and other instruments for SSC measurements, the ADCP represents a potentially powerful tool to retrieve SSC data in continental shelf waters.

Equilibrium coastal profiles: I. Review and synthesis

Applicability of the coastal equilibrium concept depends upon proof of the existence of equilibrium. The present study demonstrates that, on the basis of the sediment continuity equation, three types of equilibrium are possible. Type I equilibrium requires that instantaneous sediment transport rates in both longshore and cross-shore directions are small, representing a final stage of erosion in response to natural processes. Type II equilibrium is reached if there are no variations in the net sediment transport rate in the longshore directions (i.e. zero cross-shore sediment transport). Such a situation occurs if the coastline is straight and there are no alongshore variations in hydrodynamic (i.e. wave and tidal) conditions. Type III equilibrium occurs when there are variations in longshore transport rates but the magnitude of instantaneous transport rate in the longshore direction is small compared with that in the cross-shore directions. In this case, the coastal profile is characterised by parallel advancement or retreat. Disequilibrium occurs if these conditions are not satisfied. Hence, prior to the selection of methods to determine the equilibrium coastal profile and the response time, the type of equilibrium must be identified.

EQUILIBRIUM COASTAL PROFILES:II. EVIDENCE FROM EOF ANALYSIS

The validity of the concept of coastal equilibrium depends upon a proof of the existence of equilibrium. In addition, the methods of calculating the actual equilibrium profile and the response time are required. This study evaluates equilibrium conditions using evidence from EOF (empirical orthogonal function) analysis of the coastal profile data (bed slopes and associated elevations) obtained from Poole and Christchurch Bays, southern England. The preliminary results show that the largest eigenvalue is much greater than the other eigenvalues and the temporally-related eigenfunction is stationary; this observation implies the existence of equilibrium. Further, the spatially-related eigenfunction associated with the largest eigenvalue is used to calculate the actual beach profile at equilibrium. Although the temporal eigenfunction may fluctuate, it recovers rapidly, indicating that the response time is short. However, further studies are required to determine the response time accurately.

OBSERVATION OF A SHALLOW WATER INTERNAL OSCILLATION EVENT

At a shallow water station (6 m in depth), an internal oscillation event which consisted of one or two wave-like features, with a period of 3 h and a height of 1.5 m, was observed. The velocities within the water column were modified by the event during the flood phase of the tide; a multilayered velocity structure and intense shear were generated. Further investigations are required to understand fully the mechanism for the formation of such an event.