Surges Control Salt Flux Variability in a Partially‐Mixed Estuary

Abstract Salt intrusion is a key parameter characterizing estuarine systems and assessing their response to engineering measures and climate change. Salt intrusion is highly variable as estuaries constantly adapt to changes in discharge, tidal range, and surges. This study is focused on the variations of the salt intrusion in a partially mixed estuary, the Weser Estuary, North Sea, and specifically on the influence of surges. Based on a realistic numerical model simulation of 1 year, we determined the patterns of salt flux contributions for different conditions. Salt intrusion is mostly controlled by a balance between barotropic effects and the tidal oscillatory salt flux. Discharge variations and water level fluctuations at the open boundary lead to various transient situations. The resulting short‐term variability is shown to be controlled by surge‐induced fluctuations. While surges induce upstream‐directed barotropic fluxes, they also increase the effectively dispersive fluxes, in the upstream parts of the estuary. It is shown how each surge can cause a net increase in salinity in the upper reaches. As surges frequently occur during the hydrological year, their cumulative effect can play a main role in the estuarine salt balance.