Sediment exchange at the sediment-water interface in Western Harbour (Hong Kong) and its effect on sediment plume dispersion

Abstract

Several large scale infrastructure developments, such as Shenzhen-Deep Bay Western Corridor, Disneyland Phase II in Penny's Bay and Central-Wanchai reclamation Phase III, have been constructing in western Hong Kong waters over the years. The impact of sediment plumes induced by these works has raised environmental concerns. In this study, a three dimensional hydrodynamic and cohesive sediment transport model, developed by Wang (2006), has been adopted to simulate the sediment plume dispersion caused by sand dumping in western Hong Kong waters. An accurate simulation of sediment transportation requires the incorporation of sediment transport properties, such as critical shear stress for deposition, critical shear stress for erosion, settling velocity and erosion rate. Thus, one of the foci of this study is the investigation of sediment exchange characteristics at the sediment-water interface in western Hong Kong waters. The sediment properties, which are both basic properties and transport properties, of Deep Bay and Penny's Bay have been studied and sediment transport property experiments conducted in a Lid-driven Elongated Annular Flume (LEAF). Various initial suspended sediment concentrations (SSC) and salinity levels were used in the experiments. The decline of SSC was observed, in order to investigate the critical shear stress for deposition. The settling velocity was obtained by measuring the deposition rate. Sediment samples for testing erosion behavior were prepared by putting small disc shaped containers in a high concentrated suspension for 60 hours. The process allowed sediment to settle and consolidate naturally in the containers. All erosion tests were finished within one day to ensure the same sample consolidation state. A continuous decline of SSC level was observed in most of the deposition tests. This unique behavior is thought to be the result of the presence of multiple classes of sediment. This implies that there is no one representative critical shear stress for the sediment. The equation for deposition rate, which is a function of effective deposition velocity and sediment concentration, was adopted owing to the absence of one representative value of critical shear stress. The effective deposition velocity of Deep Bay mud is 0.0034 mm/s with a salinity of 0 psu. The value is 0.0041 mm/s with a salinity of 12.5 psu. The velocity for Penny's Bay sediment is 0.0192 mm/s with a salinity of 31 psu. The critical shear stress for erosion of Deep Bay mud with a salinity of 0 psu is 0.36 Pa. The one with a salinity of 12.5 psu is 0.50 Pa. The value for Penny's Bay sediment with a salinity of 31 psu is 0.07 Pa. The values of the empirical constant, a, for the erosion rate equation, i.e. Eq. (4.3) are 0.0246 kg/m2/s, 0.073 kg/m2/s, and 0.0009 kg/m2/s for null and 12.5 psu salinity tests for Deep Bay mud and 31 psu for Penny's Bay sediment, respectively. The sediment transport properties acquired in the laboratory were utilized in Wang's model to simulate sediment dispersion in western Hong Kong waters. The model simulated flow velocity, salinity and SSC under four different wave scenarios: (1) no wave, (2) south-easterly waves, (3) southerly waves and (4) south-westerly waves, in both spring and neap tides in the wet season. The highest flow velocity, which reached 1.5 m/s in the surface layer, was mainly found in Ma Wan Channel in all simulated cases and in all three layers due to its bottleneck-like topography. The salinity level in the region ranged from 2 to 30 psu was found in the waters at the northern side of Lantau Island, Ma Wan Channel and Rambler Channel. The high salinity level ranging from 32 to 34 psu or above was found in Victoria Harbour and East Lamma Channel, mainly in the bottom layer. The results also show that seawater entered Western Harbour mainly through Victoria Harbour and East Lamma Channel along the seabed. The highest SSC level was raised by natural plumes reached 200 mg/L, and occurred in the waters at the northern side of Lantau Island, Ma Wan Channel and Victoria Harbour. The lowest SSC level was found at the east coast of Lantau Island and in East Lamma Channel. SSC level was kept at 0 to 40 mg/L and 0 to 20 mg/L, respectively. Sediment dumping cases at two dumping locations were conducted to assess the most significant effect on SSC level. One was at the southern end of Ma Wan Channel (Location 1) which is inside a sand dumping area. The other one was at a point in the waters near the east coast of Lantau Island (Location 2). The sediment plume with the highest SSC level was that caused by dumping sediment at Location 2 during a neap tide flood without waves. SSC level of the formed plume was ranged from 50 to 50000 mg/L with area of 1.5 km2. The SSC level and the size of the plume were maintained for thirty minutes. The results provide valuable information for decision makers in the selection of the appropriate times and locations for critical cases of sediment dumping.