Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially to well-mixed main estuary

Abstract. Salt intrusion in estuaries has been exacerbated by climate change and human activities. Previous studies have primarily focused on salt intrusion in the mainstem of estuaries, whereas those in sub-estuaries (those branch off their main estuaries) have received less attention. During an extended La Niña event from 2021 to 2022, a sub-estuary (the East River estuary alongside the Pearl River Estuary, China, experienced severe salt intrusion, posing a threat to the freshwater supply in the surrounding area. Observations revealed that maximum salinities in the main estuary typically preceded spring tides, exhibiting significant asymmetry in salinity rise and fall over a fortnightly timescale. In contrast, in the upstream region of the sub-estuary, the variation of salinity was in phase with that of the tidal range, and salinity rise and fall exhibited more symmetrical.

Inspired by these observations, we employed idealized numerical models and analytical solutions to investigate the underlying physics behind these behaviors. It was discovered that under normal dry condition (with a river discharge of 1500 m³ s^-1 at the head of the main estuary), the river-tide interaction and change in horizontal dispersion accounted for the in-phase relationship between the salinity and tidal range in the upstream region of the sub-estuary. Under extremely dry conditions (i.e., a river discharge of 500 m³ s^-1 at the head of the main estuary), salinity variations kept pace with those of the tidal range from the middle to upstream regions of the sub-estuary. The variation of salinity in the main estuary, along with those of salt dispersion and freshwater influx inside the sub-estuary collectively influenced salinity variation in the well-mixed sub-estuary. These findings have important implications for water resource management and salt intrusion prevention in the catchment area.