Mechanisms of Salt Overspill at Estuarine Network Junctions Explained With an Idealized Model

Salt overspill, defined as the net salt transport from a channel of an estuarine network through a junction to another channel, can be a major contributor to salt intrusion. Here, an idealized subtidal model is constructed of a network consisting of one river channel and two sea channels, and used to investigate the sensitivity of overspill to different values of river discharge, tidal current, width, and depth of the channels. Two prototype systems are considered: the North and South Passage of the Yangtze Estuary and the Modaomen and Hongwan Channel of the Pearl River Estuary. Model results indicate that in both systems, increasing river discharge decreases the amount of salt overspill, except in the regime of weak river discharge in the Yangtze Estuary. Increasing the strength of the tidal current increases the overspill in the Yangtze Estuary, but it decreases the overspill in the Modaomen Estuary. Analysis of the model results shows that salt overspill is linearly related to the salinity difference at the upstream boundary of the two seaward channels, when they are considered as single channel estuaries. This salinity difference occurs because conditions in the channels are not identical, which results in different net water transports (causing export of salt), exchange flows, and horizontal diffusion (causing import of salt). An analytical expression is derived, which explains the dependency of salt overspill to the factors mentioned above.