Salinity intrusion and tidal propagation in estuarine channel networks with different channel lengths. An idealised model study using Delft3D

Abstract

Estuaries are regions that form in the narrow boundary zone between the sea and the river. Consequently, they show a large variety of processes, including river flow, tidal processes and wave action. A very important characteristic of estuaries is that salt from the sea often intrudes several tens of kilometres upstream. This is becoming an increasingly large problem, particularly in low-lying, urbanised estuaries. In the past decades, a lot of research has been conducted with regards to salinity intrusion in estuaries. However, these studies mostly focused on single-channel estuaries, whereas many estuaries consist of multiple channels.

This master thesis investigates the effect of different channel lengths on salt intrusion and tidal dynamics in an idealised multi-channel estuary with limited stratification. An existing depth-averaged (2DH) Delft3D model is extended to a 10-layer (3D) Delft3D model and calibrated for the new setup. The results indicate that with increasing length differences, tidal energy from the shorter channel is transferred to the longer channel. This affects the water level amplitudes and water level gradients in the shorter channel, causing the freshwater discharge to be concentrated in the shorter channel. As a result, salt can easily intrude upstream through the longer channel, causing vast quantities of salt to be transported upstream. The larger the length differences, the more pronounced this effect gets. The final result is a drastic increase in salt intrusion length for larger differences in channel length.