The utilization of Delft3D model to relate the coastal hydrodynamics and water quality with the illustration of Yong River Estuary in the southeast of China

Abstract

Coastal rivers and estuarine systems currently face growing pressure brought on by population increase and speedy economic development, which threatens the deterioration of the water quality and water ecological environment. This study takes the example of the Yong River Estuary, which is located on the eastern coast of China and south of Hangzhou Bay, to investigate the relationship between hydrodynamics and water quality response in the coastal water. Based on the hydrodynamics and water quality characteristics of a 26-kilometre-long river network, a hybrid model is established using the open-source Delft3D model. We used the bathymetry data at a 1:10,000 scale from China Navy Hydrographic maps published by China Navigation Publications Press, water level changes data from Ningbo gauging stations, and water quality data (DO, COD, BOD5, NH4+, PO43-) during 2020 and 2021.

The proposed model has two major components including hydrodynamics modelling and water quality simulation. It reconstructs the three-dimensional temporal-spatial variations of key water quality indices in the river reach and coastal waters. Six different sampling points were chosen from the river and the nearby sea area to monitor the water quality during the period from January 2020 to December 2021. The performance of the dynamic water quality model was evaluated using the coefficient of determination (R2), the Root-Mean-Square Error (RMSE), and the Nash-Sutcliffe Efficiency (NSE) values for both the calibration and the validation periods. The hydrodynamic model provides reasonable validation of water levels and tides in the Yong River and the sea area near the estuary and provides stable flow field conditions for the water quality model. The results of the water quality model simulations are generally consistent with the measured values, providing a basis for further research into the distribution of water quality parameter concentrations in the estuary.

The results show that the hydrodynamic periodicity of coastal rivers and ocean tides has a significant effect on the distribution of water quality parameter concentrations. The flow direction and velocity of the Yong River Estuary are influenced by irregular semi-diurnal tides, and the hydrodynamic circulation in the estuary varies with the tidal conditions. The hydrodynamic circulation in the estuary can influence the transport of pollutants and the concentration distribution of water quality parameters in the estuary area changes its position in the estuary with the tide. Seasonal differences will have an impact on the tidal conditions and freshwater inflow as well, which will subsequently have an impact on the hydrodynamic circulation and the values and distribution of water quality parameter concentrations. This study provides a scientific basis for research on coastal rivers and estuarine systems. It gives support to the management of the water environment and the environmentally friendly development of the coastal waters. Although the approach is used for the Yong River Estuary in eastern China, the methodology is also applicable to other coastal areas that are affected by coastal river inflow and tidal action.

Date of Award	Mar 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Meili Feng (Supervisor) & Fangfang Zhu (Supervisor)