Estuarine Sediment Dynamics Influenced by Successive Typhoons: Turbidity Maximum Zone Response and Mechanisms in the Pearl River Estuary

Abstract

Successive typhoons, defined as two or more typhoons passing within a few days, drive cumulative atmospheric-oceanic responses; however, their effects on estuarine sediment dynamics remain understudied. In particular, for the turbidity maximum zone (TMZ)—a critical “source-sink” area for sediment transport in estuaries—the mechanisms of “destruction” and “reconstruction” during successive typhoons are still unclear. Therefore, the MIKE 21/3 coupled model was used to investigate sediment transport processes during successive typhoons Hato and Pakhar in the Pearl River Estuary. The results show that hydrodynamic conditions recovered within 50 hr, shorter than the 4-day typhoon interval—but Hato-induced sediment resuspension persisted for 10 days, resulting in the seaward sediment transport flux (STF) caused by the subsequent, weaker Pakhar being higher than that of Hato. During Hato, wind-induced resuspension became the main source of sediment in the TMZ, tripling its spatial extent through erosion (“disruption”). The subsequent Pakhar compressed the TMZ longitudinally by 7.8 km and thickened it vertically by 4.4 m through density anchoring at the salt-wedge interface (“rebuilding”). Strong winds were the main driver of STF through tidal pumping and shear effects, contributing more than 60%. The differential dynamics of “rapid hydrologic response and recovery” versus “delayed sediment response and recovery” triggered by successive typhoons enable weaker subsequent typhoons to significantly alter depositional mechanisms through the residual suspended sediment effect. This necessitates a reconceptualization of the timescale and intensity thresholds of successive typhoons affecting estuarine sediments.