Tidal phase differences in multi-branch systems and their effect on salt intrusion

Abstract

The Hollandsche IJssel plays an important role in the freshwater provision of the province Zuid-Holland. Consequently, for Rijkswaterstaat it is key that salt intrusion is minimal in the Hollandsche IJssel. Recent studies noted that salt intrusion in the Hollandsche IJssel is limited due to a phase difference between tidal velocities in the main channel, the Nieuwe Maas, and the side channel, the Hollandsche IJssel. Earlier research investigated the impact of phase differences between branches and found it can lead to increased dispersion in the main channel, through a process known as tidal trapping. At the same time, this phase difference can prevent the saltiest water from entering the side channel, as was found at the Hollandsche IJssel. Because of this role, it is relevant to find out how this phase difference may be influenced by sea level rise, more extreme river discharges and particularly how it depends on the geometry of the main and the side channels. Especially the latter could help Rijkswaterstaat to minimize salt intrusion at locations relevant to freshwater intake, such as the Hollandsche IJssel.

The main objective of this thesis is to investigate how the geometry of the side and main channel influences the tidal phase difference between these two channels, and how this may impact the salt dispersion in the side channel. For this, an analytical model is developed describing harmonic wave propagation in multi-branch systems and this is used next to results from a 3D numerical model for the Rhine Meuse Delta (RMM3D). First, the influence of changes in geometry and forcing is systematically investigated for a network containing a single junction. This shows that the length and depth of the side channel are the most significant variables. The depth is one of the main variables impacting friction, which governs the type of wave which can form in the system. A decrease in friction allows a wave to transform into a standing wave pattern as the return wave becomes more important, while increased friction transforms it into a propagating wave. The length also controls the type of wave which can form as it determines the distance along which the friction can work. Additionally, the length also governs potential resonance in the side channel.

Next, the phase differences of the M2, M4 and M6 tide are determined for the junction with the Hollandsche IJssel in the Rhine Meuse Delta (RMD) based on the RMM3D model. The main tidal constituent regarding tidal trapping was found to be M2. However, this does not fully represent the time difference between flow reversal at the Hollandsche IJssel and the Nieuwe Maas, which was found to be around 75 minutes. Additionally, the phase difference at the Lek was investigated. For the M2 tide at the Hollandsche IJssel and Lek, a phase difference of 55⁰ and 31⁰ was found, respectively. These phase differences prevent salt intrusion in the respective side channels. The inflow of the side channels starts while the main channel still flows to the sea during the ebb. At this moment, the salt concentrations in the main channel have already returned to background levels...