Hong Kong - Guangdong Joint Development of a Pearl River Delta Water Quality Model
Hong Kong is situated at the mouth of the Pearl River Estuary and we have witnessed the most rapid economic development in the Pearl River Delta (PRD) (Figure 1) in the last two decades. Nonetheless, pollutions arising from the interconnected developments and human activities have inevitably posed increasing pressure on the water environment.
In August 2000, the Hong Kong SAR and Guangdong Provincial Governments jointly established the PRD Water Quality Protection Special Panel (the Special Panel) under the Hong Kong-Guangdong Joint Working Group on Sustainable Development and Environmental Protection (the Joint Working Group) to promote exchange and cooperation on the protection of water environment in the Pearl River Estuary region.
Water Environment in the Pearl River Estuary Region
Effective management and protection of the water environment in the Pearl River Estuary is no easy task due to its complicated natural settings and wide spatial coverage. The estuary water body comprises two main components: the river network and the coastal waters. The flow dynamics and interactions between the two water bodies are extremely complex.
First of all, fresh water gathered in the upstream catchments will pass through the complicated river network in the PRD where flows are distributed to the smaller distributaries and eventually discharged into the estuary via the 8 river outlets, namely Humen, Jiaomen, Hongqimen (Li), Hengmen, Modaomen, Jitimen, Hutiaomen and Yamen. The discharge varies significantly from less than 4,000 m3/s in the dry season to 28,000 m3/s in the wet season. During a flood event, the peak discharge can exceed 40,000 m3/s.
The estuary is under the tidal influence of the coastal waters. As the estuary topography is relatively flat, the tidal motion can propagate upstream into the river network and bring in saline marine water. Adding to the complexity is the strong stratification in the estuary and coastal water column, especially during the wet season, due to the large density difference between fresh river discharges and marine waters, as well as surface heating.
Figure 1 Coverage of the Pearl River Delta
To strengthen the scientific basis for water quality management and to enhance cooperation in the region, the Special Panel resolved in end 2000 to jointly develop a computer water quality model for the Pearl River Estuary (the PRD Model). The primary objective is to construct a computer model that is capable of simulating the complicated flow distributions and water quality processes in both the river network and the coastal region.
In the past, models were either specifically designed and built for the study of river networks (river models), or solely developed for the study of estuarine and coastal waters (estuary models). These models adopt utterly different computation schemes and modelling approaches due to the very different nature of the river and estuary systems, and are used independently to simulate their respective areas. But in reality, the river network, the estuary and the coastal region are inseparable units interacting dynamically with one another.
To provide a holistic understanding of the highly complex estuarine environment, a 1-dimensional river model and a 3-dimensional estuary model were built and joined together numerically to concurrently simulate the hydrodynamics and water quality in the river network and the coastal region. This approach required the development of an advanced flexible coupling mechanism between the two totally different computation schemes, which was a pioneer attempt in the PRD.
South China Institute of Environmental Sciences, an experienced Mainland water quality modelling specialist, was hired as the project management consultant to provide expert advices on model development and responsible for data collection and work coordination.
The actual model development work was conducted by Deltares, a worldwide acclaimed water quality modelling specialist. Two of their internationally renowned software, namely SOBEK (a 1-dimensional river model) and Delft3D (a 3-dimensional estuary model) were employed as the fundamental building blocks of the PRD Model.
Figure 2 Schematic layout of the PRD Model which is composed of a 1D river model (SOBEK) and a 3D estuary model (Delft3D)
The coverage of the river model domain extends along Beijiang to Shijiao and Yagang in the north, along Dongjiang to Boluo and Qilinzui in the east, and along Xijiang and Tanjiang to Gaoyao and Shizui respectively in the west. On the other hand, the coverage of the estuary model domain extends to Zhenhai Bay in the west, to Daya Bay in the east, and covering approximately the 75m depth contour in the off-shore direction (Figure 2).
In water quality simulations, both the river and estuary models take into account the transport of pollutants and include an extensive library of inter-related source and sink terms to represent different water quality processes. More than 30 water quality parameters in the water column and the bottom sediment, e.g. salinity, temperature, suspended solids, dissolved oxygen (DO), biochemical oxygen demand (BOD) and nutrients, together with the relevant physical and biochemical processes, e.g. water re-aeration, settling of suspended solids, decay of organic matters, algal growth and nitrification, are included in the PRD Model.
A panel of modelling experts was formed in 2007 to evaluate the model. They reckoned that the study methodology and modelling approach adopted were the state-of-the-art and the overall design was reasonable and realistic. The model performance met high international standards with strong scientific basis and yet practical and flexible in operation. The PRD model developed is on par with the best in the world in terms of its spatial coverage, amount of water quality parameters considered, dynamic coupling of the 1-dimensional river and 3-dimensional estuary water quality models and visualization of modelling results.
The PRD model developed is an important milestone and provides the two governments a platform to strengthen cross-boundary collaboration on regional water quality protection.
The Governments of Guangdong and HKSAR have begun discussion on the next stage of cooperation to use the PRD model to assess the load carrying capacity of the estuary under different water quality objectives, which will lay the foundation for the development of regional water quality management plans for the Pearl River Estuary.
To illustrate the capability of the PRD model, we have prepared two simulations below using a 3-dimensional visualization tool to show the pollutant dispersion process in the Pearl River Network and the Estuary due to a hypothetical continuous point source discharge within the river network.
Example 1: Wet Season Scenario.
Example 2: Dry Season Scenario.
Hong Kong Environmental Protection Department