CHANGES OF LOW WATER LEVEL IN THE CHANNEL NETWORK SYSTEM OF THE PEARL RIVER DELTA CAUSED BY SAND MINING

摘要

The channel network system of the Pearl River delta is one of the most complex river systems in the world, where human activities have caused significant changes in the low water level of the main waterways. The Pearl River network system is characterized by its abundant water discharge, low sediment concentration and stable banks. In the last 20 years the large-scale sand mining from riverbed in the Pearl River delta has a crucial impact on the fluvial processes in the channel network system, which overshadowed the influence of any natural agent. In this paper, the sand mining activities since the 1980s have been summarized based on survey data, and the total volume of sand mining in the channel network system was evaluated. The future demand for sand mining, and the consequent changes of river channel geometry and hydrodynamic conditions have been predicted using a numerical model based on the fuzzy system theory. The impact of human activities on the future trend of low water level change are predicted using a numerical model for water flow in the entire river channel network system, with a 2D water flow and sediment numerical model applied for the key reaches of North River and East River. The model is able to predict the effect of sand mining and retrospective scour on the low water level changes. The Pearl River delta channel network system is affected by both the river flows and the tidal currents, and its reaches are divided into 3 types accordingly, which are: 1) the reaches affected by the tidal currents all year round; 2) the reaches affected seasonally by the tidal currents; 3) the reaches affected only by river flows, i.e., without influences from the tidal currents. Calculation methods are developed for different types of reaches to find the design low water level.