Spatial distribution of heavy metals in Hong Kong's marine sediments and their human impacts: a GIS-based chemometric approach.

A review of open channel turbulence, focusing especially on certain features stemming from the presence of the free surface and the bed of a river. Part one presents the statistical theory of turbulence; Part two addresses the coherent structures in open-channel flows and boundary layers.

Turbulence in Open-Channel Flows

Trace metals and organochlorines in the bamboo shark Chiloscyllium plagiosum from the southern waters of Hong Kong, China.

Co-occurrence correlations of heavy metals in sediments revealed using network analysis.

An open channel with an emergent vegetation patch: predicting the longitudinal profiles of velocities based on exponential decay

Free-surface flows over patchy vegetation are common in aquatic environments. In this study, the hydrodynamics of free-surface flow in a rectangular channel with a bed of rigid vegetation-like cylinders occupying half of the channel bed was investigated and interpreted by means of laboratory experiments and numerical simulations. The channel configurations have low width-to-depth aspect ratio (1.235 and 2.153). Experimental results show that the adjustment length for the flow to be fully developed through the vegetation patch in the present study is shorter than observed for large-aspect-ratio channels in other studies. Outside the lateral edge of the vegetation patch, negative velocity gradient (
 $$\partial \overline{u}/\partial z < 0$$
 ) and a local velocity maximum are observed in the vertical profile of the longitudinal velocity in the near-bed region, corresponding to the negative Reynolds stress (
 $$- \overline{{u^{\prime}w^{\prime}}} < 0$$
 ) at the same location. Assuming coherent vortices to be the dominant factor influencing the mean flow field, an improved Spalart–Allmaras turbulence model is developed. The model improvement is based on an enhanced turbulence length scale accounting for coherent vortices due to the effect of the porous vegetation canopy and channel bed. This particular flow characteristic is more profound in the case of high vegetation density due to the stronger momentum exchange of horizontal coherent vortices. Numerical simulations confirmed the local maximum velocity and negative gradient in the velocity profile due to the presence of vegetation and bed friction. This in turn supports the physical interpretation of the flow processes in the partly obstructed channel with vegetation patch. In addition, the vertical profile of the longitudinal velocity can also be explained by the vertical behavior of the horizontal coherent vortices based on a theoretical argument.

Characteristics of flow structure of free-surface flow in a partly obstructed open channel with vegetation patch

A wave propagation model based on the wave action conservation theory was developed and applied to the modelling of wave characteristics and the interaction of wave and current in the Pearl River Estuary, by coupling with a three-dimensional hydrodynamics model. The modelling results show that waves propagating from the open sea would be attenuated significantly because of sheltering by islands and shoaling. Waves incoming from the south can propagate further upstream. The tidal current generally increases the wave heights and the effect of the ebbing flow on waves is significant.

Modelling of the wave-current interaction in the Pearl River Estuary

Sediment cores, suspended particles and overlying water were collected in Deep Bay, Hong Kong. Enrichment of Zn in surface sediments in the landward direction and the decreasing of exchangeable Cd, Ni and Zn in sediment from the inner bay to the outer bay indicated the influence of anthropogenic pollutants discharged from the riparian runoffs.

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Wing-hong Onyx Wai

Papers

Characteristics of flow structure of free-surface flow in a partly obstructed open channel with vegetation patch

Modelling of the wave-current interaction in the Pearl River Estuary

Characteristics of metal enrichment in Deep Bay, Hong Kong