Hydrodynamic and ecological impacts of land reclamation in semi-enclosed bay

Abstract

Many coastal areas worldwide have been reclaimed to meet the increasing land demand. Land reclamation would not only directly influence the hydrodynamics and ecosystems in estuarine regions, but may also alter the potential impacts of other anthropogenic activities such as sea-level rise. Semi-enclosed bay is often surrounded by coastal cities and subjected to land reclamations. Understanding the effects of land reclamation on the hydrodynamics and ecosystem of a semi-enclosed bay is therefore of significance. This thesis aims to (1) evaluate the hydrodynamic impacts of reclamation in different areas of a semi-enclosed bay, (2) investigate the impacts of sea-level rise on hydrodynamics before and after reclamations, (3) assess the ecological impacts of reclamation on coastal ecosystem dynamics. The Deep Bay, Pearl River Estuary, China is used as a case study as reclamations have been gradually taken place in bay mouth and head areas during the past few decades, and sea-level rise is also a potential future stress. First, a three-dimensional numerical model was used for the Deep bay to simulate hydrodynamics and to perform scenario experiments. The model was validated with field observations of tidal elevation, current velocity and salinity, and the simulation results agreed well with field observations spatially and temporally. Scenario experiments were first designed to investigate the hydrodynamic impacts of reclamation in different areas. The reclamations in bay mouth and head presented different hydrodynamic impacts but both could substantially alter the current field, tidal energy flux, and salt transport processes. Idealized semi-enclosed bay models were then generated with bay mouth/head reclamations in various scales, whose results presented similar hydrodynamic impacts as that in the Deep Bay. This suggested that the results and insights from the Deep Bay could be generic and therefore applicable to other semi-enclosed bays. Furthermore, scenario experiments were designed to evaluate the hydrodynamic changes with future sea-level rise before and after reclamations at the bay head, where reclamation would introduce more impacts. Simulation results indicated that the hydrodynamic impacts of sea-level rise were considerably altered by the reclamation, and the impacts became more complicated and dynamic. With 0.5 m of sea-level rise, the salinity level at the bay head increased by 5.4 and 4.7 psu, respectively, after and before reclamation, as reclamation substantially altered the salt transport process in bay. The aquatic environment of the bay might face more ecological stress with the combined effects of sea-level rise and reclamation. Finally, an ecological model was generated to simulate the dynamics of tidal marsh ecosystem with temporally varying environmental factors, and was then employed to model the ecological impacts of reclamation in different areas of the bay. The biomass of benthic infauna at the tidal marsh of Deep Bay was used as the ecological indicator. Among 29 multidisciplinary factors, two hydrological indicators (i.e. water age and salinity variation) and one meteorological indicator (i.e. total sunny period) showed governing effects and could be used to simulate the biomass dynamics, which were validated with field observations in 1994 and 2002-2008. The results of reclamation scenarios suggested that the reclamation ended in 2007 could reduce the benthic infauna biomass by 20% and might be an important reason behind the 28% benthic infauna loss observed in the field.