The role of estuarine convergence on the salinity distribution and the estuary response to short river discharge pulses

Abstract

Estuarine behaviour is primarily controlled by the interaction of river discharge and tidal mixing. Transient increases in river discharge, which occur naturally during the wet season or can be managed in regulated basins, alter estuarine dynamics and reduce salt intrusion. Two characteristic times have been identified in the response to these river pulses: the adjustment time (the time required to adapt to the peak discharge) and the recovery time (the time required to return to steady state conditions). Previous work has analysed the effect of peak discharge and pulse duration in the adjustment process, focusing on long discharges (i.e. longer than the intrinsic estuarine timescale) and straight channels. This paper presents an idealised 3-dimensional model (Delft3D) to assess the effect of estuarine convergence on the response to short river pulses in tide dominated, well-mixed estuaries. The similarity between the tidal period, the estuarine timescale and the duration of the pulse led to the introduction of the time between tidal and river conditions as an additional parameter in the analysis. The results show that salt intrusion in convergent estuaries have a lower sensitivity to variations in river discharge compared to the behaviour of prismatic estuaries. Conversely, the influence of convergence on changes in salt intrusion due to the increased river discharge gains relevance in convergent estuaries. Seaward transport of salt is significantly enhanced by freshwater releases that occur between early and mid ebb, especially during spring tides. Increasing convergence reduces the asymmetry of the estuarine response, shortens the recovery time, and shifts the onset of maximum salt displacement and the minimum adjustment time. The results are particularly relevant for water management and salt intrusion control in regulated basins with limited resources, as the volume of fresh water used to reduce salt intrusion can be optimised considering the timing of the pulse release, which depends on the tidal conditions at the estuary.