Mechanisms of salt intrusion in estuarine channels and networks

Coastal areas are worldwide densely populated. However, a danger that threatens the economy and public health is the occurrence of salt intrusion. Fundamental knowledge of the properties of this phenomenon is essential to secure the freshwater resources of coastal regions. This thesis aims to improve the knowledge of salt intrusion in estuarine channels and networks. To this end, idealised models are developed and applied. These models are simplified such that they only contain physics which are essential to study the considered problem setting. After an introduction in Chapter 1 focusses the second chapter on the variability in salt intrusion induced by changes in river discharge. We find that the response time of salt to an increase in river discharge is mostly determined by the size of the discharge peak and the change in salt intrusion length. The response time to a decrease in river discharge is determined by the discharge after the peak, but the change in salt intrusion length is not important. Chapter 3 shows that, if the effects of shipping locks are taken into account with a simple parametrisation, a model that is developed for open estuaries is also applicable to canals which are closed off from the sea with shipping locks. The amount of salt in a canal is stronger determined by the discharge through the canal than the salinity of the adjacent sea. We suggest a new formulation of the salt dispersion coefficient for canals. In Chapter 4 we consider the salt overspill between channels, when an estuary consists of multiple channels. The direction and strength of the salt transport between the channels is quantified. We explain that earlier studies found different directions of salt transport between two channels. We present an equation to compute what the direction and strength of the salt transport between the considered channels is. Chapter 5 focusses on properties of salt transport by so-called exchange flow and tidal flow. In different estuaries (the Delaware, Guadalquivir and Loire estuaries), these components contribute in a different ratio to the total salt transport. We find that the relative contribution of the tidal flow increases for shallower systems, systems with a stronger tide, and with weaker salinity gradients. The presence of salt transport by tidal flow decrease salt transport by exchange flow, but the presence of the latter increases the salt transport by tidal flow. Chapter 6 present properties of salt intrusion in complex channel networks, with the Rhine-Meuse Delta as a case study. The contribution from salt transport by the tidal flow in this network is large around junctions, which is caused by phase differences of the tidal flow around these points. Channels which are closed off at one side show different behaviour than adjacent channels. Changes of channels depths can affect the salt intrusion locally but also in other parts of the network. Conclusions are found in Chapter 7. Here, also ideas are provided for further research, concerning refinement and expansion of the model physics and their broader application.