Estimation of riverine sediment fate and transport timescales in a wide estuary with multiple sources

Sediment retention time in estuaries plays a significant role in the morphological evolution and estuarine ecosystem. In this study, we used a well calibrated numerical model of the Pearl River Estuary (PRE) and examined its sediment budget, transport processes, and transport timescales as regards to sediment discharge from four outlets. The sediment transport timescales are characterized by sediment age, which means the time elapsed since sediment particles enter into the estuary. The wide PRE (with a Kelvin number = 1.4-7.8) allows river plumes to develop inside the estuary, which is a controlling factor on sediment dispersion. Sediments from the estuary head are initially dispersed downstream when flushed from the outlets. However, a large portion of them are carried to the low-salinity region owing to the development of freshwater plume. These sediments therefore have a larger sediment age than the remainder, which are transported by energetic tidal currents via the high-salinity region. Most riverine sediments from the lateral outlets are constrained in the low-salinity region on the West Shoal. The sediments deposit near the mouths of the lateral outlets because of weak dynamics on the shoal. An additional time is therefore needed for the sediments from the lateral outlets to be transported outside of the estuary. Because of the sediment transport pattern in the estuary, a large portion (58%) of the total riverine sediments ultimately accumulates on the West Shoal. The sediment settling velocity greatly affects the sediment age which ranges between 10 days for sediment with a settling velocity of 0.05 mm/s to approximately 110 days for sediment with a settling velocity of 0.2 mm/s. These findings regarding the fate and transport timescales of riverine sediments have implications for ecosystem managements.

Automated summary

Sediment retention time in estuaries is crucial for morphological evolution and ecosystem health. This study used a numerical model to analyze sediment transport and found that sediment age is influenced by river plumes and tidal currents. Most riverine sediments accumulate on the West Shoal due to sediment transport patterns, while sediments from lateral outlets deposit near the shoal mouths, requiring additional time to be transported out of the estuary.