Shenzhen River is on the border between Hong Kong and Shenzhen. It flows southwest towards Deep Bay in the Pearl River Estuary in south China. In the past, Shenzhen River was narrow, meandering and only capable of conveying floods of return period 2 to 5 years. To alleviate flooding hazard, Hong Kong and Shenzhen governments have carried out the Shenzhen River Regulation Project jointly since 1995. The river downstream has been trained to cater for flooding of 50-year return period. However, unexpectedly, the trained Shenzhen River has been seriously silted by 3 m of sediment 5 years after the completion of river training. This diminishes the effectiveness of river training on enhancing flood discharge capacity.

In this paper, we present a study on the impact of river training on Shenzhen River hydraulics by an integrated three-dimensional numerical hydrodynamic model. The models are verified against field data and show good agreement with the observed hydrodynamics; wetting and drying of inner Deep Bay tidal flats is successfully simulated. The model simulations show that river training has drastically altered the hydraulics. Tidal propagation in the river has changed from a damped tidal oscillation to a more standing wave character with less amplitude damping. Tidal current is reduced by about 50%. Vertical salinity stratification and gravitational circulations are thus formed within the trained river in the dry season. The bottom upstream residual current in the two-layered residual circulation strongly suggests the input of sediment from Deep Bay and trapping in the river.