Risk assessment of dredging processes in Pearl River estuary - Hong Kong waters

Abstract

Dredging is considered an effective way to generate land reclamation materials in coastal areas. However, the subsequent sediment plumes can raise massive environ­mental problems, such as blocking sunlight, affecting the growth of seagrass and aquatic populations, causing re-suspension of contaminants absorbed on sediment particles, etc. The understanding of the quantification of pollution brought by dredging activities is limited. This study stimulated by the “Lantau Tomorrow” reclamation project proposed by the Hong Kong government, aims to suggest an innovative risk assessment framework regarding dredging activities with the help of computational fluid mechanics (CFD) and machine learning. Two main contri­butions have been made in this study: 1) the main mechanisms of generation of macro-vortices have been described in detail within the estuarine circulations in Pearl River Estuary (PRE) and Hong Kong (HK) waters, 2) a novel risk assessment framework has been demonstrated and then applied to two case studies regarding the dredging impact on rare animals and the estimation of the re-suspension of persistent contaminants. The Finite-Volume Coastal Ocean Model (FVCOM) has been implemented to in­vestigate estuarine circulations and sediment transport. The computational domain centered on the HK waters and covered the whole PRE, extending to the South China Sea (SCS). Tide, wind, air pressure, and Pearl River discharge were taken into account as external forcing. The model has been well validated by both observed tidal elevations and velocity time series. The first mechanism of the generation of macro-vortices was identified in the in­teraction of tidal and wind-induced currents with the islands and headlands. A classification of these coherent structures was suggested based on their typical evo­lution to distinguish between tidal vortices and wake vortices using non-dimensional parameters. The spatial distribution, seasonality, and probability density functions of their geometrical characteristics were presented and discussed in detail. The results suggested that tidal currents played a dominant role in most of the PRE and the HK waters, generating for the entire year a high and constant number of macro-vortices that occupied most of the north channels of HK. The second mechanism was about the dynamic generation of macro-vortices in the estuarine compound channel. The Lantau Channel, lying in the middle of the PRE, had the velocity magnitudes in channels significantly larger than that of floodplains, resulting in high-velocity gradients and generation of macro-vortices at the depth discontinuity. The ratio of the main channel depth of the Lantau Channel to flood­plain depth (Rh) was calculated to characterize the channel flow. This dynamic change in Rh greatly influenced the generation of macro-vortices. Transverse dis­persive stresses were calculated to evaluate the longitudinal momentum transfer in the lateral direction. It was found that the dispersive stresses played an important role in the redistribution of momentum in addition to barotropic and baroclinic transport. The risk assessment framework was formulated based on the analysis of sediment transport simulations and the vulnerability index. To avoid applying climate con­ditions of periods selected arbitrarily, 100 representative met-ocean scenarios were generated by the K-means clustering analysis based on 42 years of historical me­teorological data. The dredging scenarios were enriched by considering 1 dredging location, 2 dredging methods (hydraulic and mechanical dredging), and 3 working modes. 600 sediment transport simulations were conducted regarding each met-ocean condition and dredging scenario. Five criteria have been defined to evalu­ate the hazard based on suspended sediment concentration (SSC) signals, namely the occurrence, event counting, duration, sustainability, and significance of hazard, among which the significance parameter provides the most comprehensive informa­tion. The risk values were obtained by multiplying the hazard with the vulnerability index, which was the summation of environmental and socioeconomic index. Results showed that sediment transport patterns were significantly affected by estuarine con­vection. The west side of PRE-HK waters was at a higher risk than the east side considering all possible climate conditions. The comparisons of risk among dredging methods and working modes provide the decision-makers with quantified references regarding the risk of five criteria when designing the dredging operations. The re­lationship between risk and wind was also quantified using a novel interpretation method, namely the risk rose, showing that dredging generated a lower risk during the summer monsoon, i.e. south-southwest wind. Based on the above risk assessment framework, two case studies have been con­ducted: 1) the impact of dredging locations on the habitats of Indo-Pacific hump­back dolphin (Sousa chinensis), 2) the estimation of re-suspension of E-waste con­taminants – liquid crystal monomers (LCMs) brought by dredging plumes. 500 sediment transport simulations were conducted and used in both two case studies considering 100 met-ocean scenarios, 5 dredging locations around Lantau Island, 1 dredging method, and 1 working mode. In the first case study, the vulnerability in­dex referred to the sighting frequency of the humpback dolphins in PRE-HK waters. Results showed that dredging on the east side of Lantau Island caused minimum risks on the dolphins. The macro-vortices around the north channels of HK played a promoting role in convecting the sediment particles, whereas the north coastlines of Lantau Island shield the sediment plumes from spreading. In the second case study, the risk definition was modified to adapt to the risk categories (low, medium, high, very high) of LCMs on benthic organisms. The Environmental Burden (EB) was defined as the integral of SSC over time, shaping the places with high SSC or long residence time or both. The distribution of the most probable risk level along with its probability was presented. Dredging upstream of Lantau Island generated more severe re-suspension of LCMs than downstream, and had very high risk with a probability of reaching 100% on the east coast of PRE.