Salt Intrusion In The Pearl River Estuary

The Pearl River is the largest river system in South China and its river runoff ranks second in China only to the Changjiang River. The Pearl River includes Xijiang, Beijiang and Dongjiang River with Xijiang as the main stream. Characterized by "runoff comes from three rivers and goes through eight gates", the river runoff from the three rivers meets in the Pearl River Delta, run through a complicated river net system and finally gets into the South China Sea through eight gates. Since the Reform and Opening, the economy of the Pearl River Delta develops rapidly and the demand for the environmental resource also grows fast. The fresh water supply is in shortage especially in dry season when saltwater intrusion occurs. Supported by the Marine Special Program for Scientific Research on Public Causes (Program name:Research on the numerical prediction of salt tide in the PRE, Grant No.:200705019), this paper investigates the hydrodynamic process as well as the saltwater intrusion in the Pearl River Estuary. To this aim, a great amount of basic data was collected and processed such as hydrologic data and the bathymetry data of river network and estuary area.Based on the mechanism that salt intrusion is mostly affected by river discharge and tidal forcing, a statistical regression model of the daily averaged salinity, with tidal range and river discharge as variables, was established by processing the Pinggang's salinity and tidal level as well as the upstream river discharge data. Separating the different influence from short-timescale tidal range variation and long-timescale river discharge variation, a salinity-tidal range model and a salinity-discharge model could be obtained through stepwise regression analysis and then an integral salinity regression model was achieved by the combination operation. The model was improved by the means of tidal standardization as well as predict-revise operation. The approach in establishing salinity regression model is practical and of clear physical concept. However, due to the statistical nature, the model can reveal the quantity relations among salinity, tidal range and river discharge but it is not able to reveal the dynamics in them.To explain the dynamic mechanism of salt intrusion in the PRE, it is necessary to build a dynamical numerical model. Considering the bathymetry complexity of the PRE and the 3D feature of salt intrusion, a fully 3D salt intrusion numerical model was built in the PRE based on the unstructured model, FVCOM. The salt intrusion model considered relatively comprehensive dynamic factors such as river runoff, tide, wind, baroclinic effect and shelf circulation etc. and had a model domain covering the whole river network, estuary and the adjacent shallow shelf. The model was validated against tidal level, sectional water flux, section-averaged salinity, as well as current speed, direction and salinity, and validation results showed that the model is of an acceptable precision and could be used to simulate and investigate the hydrodynamic and salt intrusion processes.Based on the salt intrusion model, simulations of the hydrodynamics and salt intrusion in dry season condition were conducted and more sensitivity experiments against major dynamic factors were carried out. Then, tides, tidal current, circulation and salt intrusion in the PRE were analyzed and discussed according to results from those experiments.Cotidal charts of 8 major tidal constituents in the PRE and adjacent shallow shelf showed that the tide propagated from east to west in the shallow shelf with co-phase lines at southeast-northwest. It turned north when entering into the PRE with co-phase lines at west-east. In the shallow shelf, the Ki and O1 tidal component had relatively large amplitude while the M2 component dominated in the PRE. The shallow water tides were trivial. It was of a mixed tide pattern in the PRE indicated by a tidal form number ranging from 0.8-1.5. In the PRE the tidal range varied between 2.2m and 3.1m during the spring tide and between 0.6m and 1.1m during the neap tide.Tidal current varied at different area of the PRE, relatively weak in the adjacent sea and strong in the Lingdingyang Bay with a speed of 200cm/s at Humen outlet. In general, tidal current showed irregular semidiurnal variation but it turned to be irregular diurnal during the moderate tide after neap with flood duration far longer than ebb one.In the Lingdingyang Bay, the northeast wind drove a transverse circulation with westward flow at surface and eastward at bottom. In deep channels, the residual current varied irregularly that it showed gravitational circulation pattern during neap; this pattern became more obvious during moderate tide after neap; the seaward flow increased throughout the water column during the spring tide and reached its maximum during the moderate tide after spring. Due to the combination effect of river discharge, Coriolis forcing, salinity front and wind, a strong westward alongshore current was observed out of the Lingdingyang Bay at surface, which was greatly weakened at the bottom.As for the temporal and spatial variation, the salinity in the PRE changed irregularly similar to the tidal level and tidal current, that it was generally irregular semidiurnal and became irregular diurnal during the moderate tide after neap. In a spring-neap cycle, a salinity-tidal range phase difference was gradually emerged from west to east in the Lingdingyang Bay. As the river runoff was steered by the Coriolis forcing and northeast wind, the salinity contours lay along the coastlines, i.e. at NE-SW in the Lingdingyang and ENE-WSW out of the bay. In the Lingdingyang Bay, salinity increased eastward, the salt intrusion was more severe in deep channels, and the vertical stratification was most obvious in the moderate tide after neap. Salinity spatial distribution was sensitive to the wind condition and changed quite differently to different winds. The decrease and increase in river discharge would relieve and exaggerate the salt intrusion situation respectively, and the salt intrusion was more sensitive to the decrease than increase. Salinity was less sensitive to sea level rise in general except in local regions such as Modaomen area.In the Modaomen waterway, observed data showed that abnormal salt intrusion occurred there with an obvious salinity-tidal range phase difference, i.e. salinity peaks appeared mainly during the moderate tide after neap in a spring-neap cycle. Numerical simulations showed that the abnormal intrusion was closely related to the Hongwan waterway. Due to the combined effect of bathymetry and northeast wind, Hongwan waterway became an important channel transporting salt into the Modaomen waterway. Northeasterly wind could reinforce the salt intrusion in the Modaomen area however it was not the essential reason for the abnormal salinity variation. During the neap tide, the tidal force became weak while the wind force became relatively stronger and thus could generated a larger salt transportation through Hongwan waterway and finally resulted in the salinity maximum during the moderate tide after neap. The interactions among the Hongwan waterway, wind and tidal forcing were the dynamical mechanism of the abnormal salt intrusion in the Modaomen waterway.