Observational Study of Interactions Between River Discharges and Tides at the Outlets of Delta Distributaries

The hydrodynamics in delta distributaries has tremendous impact on sediment transport, water quality and deltaic ecosystems; yet many processes acting at the river-marine interface of a delta are not well understood. This contribution investigates the interplay between riverine runoff and tidal motions at the outlets of delta distributaries. We focus on the relationship between flow discharge and tidal range, the phase change between water level and velocity fluctuations, and the tidal-induced Eulerian residual currents. These processes are investigated by analysing water levels, velocities and discharges measured at the outlets of three main distributaries of the Pearl Rvier Delta, China.

Figure 1. Map of the Pearl River Delta and its distributaries and outlets.

Located in Guangdong Province at the northern reach of the South China Sea (SCS), the Pearl River Delta is one of the most complicated river networks and deltaic drainage systems in the world. The deltaic region is home to about 324 streams, and the density of channel length per unit area is 0.68~1.07 km/km². The Pearl River splits into three main distributaries, namely the West River, the North River and the East River (Figure 1). All the distributaries flow into the SCS through eight river outlets, i.e. Yamen, Hutiaomen, Jitimen, Modaomen, Hengmen, Hongqimen, Jiaomen and Humen (Figure 1). The tides in the Pearl River Estuary mainly come from the Pacific oceanic tidal propagation through the Luzon Strait with a mean tidal range from 1.0 to 1.7 m.

The field measurements of water levels, discharges, velocities at the eight outlets were conducted synchronously for five times, with three times in wet season during spring, moderate and neap tides, respectively, and once in normal season, once in dry season. During low flow conditions, the relationships between hourly discharge and tidal range can be expressed by linear regression equations, and the correlation coefficients are high, especially at the tide-dominated Humen and Yamen outlets. During very high flow regimes, the relationships between hourly discharge and tidal range fit the quadratic polynomial regression equations better with negative correlation during flood tide and positive correlation during ebb tide. The observations also reveal that during low flow regimes, river discharge increases tidal damping and decreases tidal velocity amplitudes; during high flow conditions, the effect of tides at the distributary outlets is surprisingly amplified with seaward Eulerian residual currents and an increase in tidal velocity amplitudes for which the phase lag change appears a major determinant.