Nonlinear tides and river flow of estuarine networks

Abstract

Tides are important for the mixing and transport of sediments, salinity and nutrients in most estuaries around the world. Therefore, a precise understanding of the tides in estuaries is needed to predict changes over time. This research focuses on hydrodynamics and specifically on the mechanisms that result in nonlinear tides in networks of interconnected channels, which are known as estuarine networks. To investigate this, a 2DV semi-analytical idealised model was developed that solves the hydrodynamics in estuarine networks. Here, two different networks were examined. First, the model was applied to an idealised three-channel network and subsequently the model was extended to the Rhine-Meuse estuarine network in the current state. Next, two scenarios are examined, studying the effect of respectively opening one of the Haringvliet sluices and 2-meters of sea-level rise on tides. The results demonstrate that the model has been successfully applied to both networks. A comparison with observations for the Rhine-Meuse network showed good agreement for the linear tides (M2). A fair agreement was found for the nonlinear tides (M4). Here, the divergence of excess mass due to Stokes drift and no-stress condition have been proven as the most dominant mechanisms for the internal generation of nonlinear tides in this system. In addition, the externally forced M4 tide was found to be a factor of 5-15 stronger than the internally generated M4 tide. Quantification of tidal asymmetry within the Rhine-Meuse has indicated that this system exhibits particularly flood dominant behaviour for both the water level and current velocity. Duration asymmetry showed strong flood dominance leading toward the middle part of the estuary, followed by a decrease in the river part. Even stronger asymmetry was found for the velocity asymmetry throughout almost the whole system. Mixed behaviour was found for the flood-to-ebb ratio, with still predominantly flood dominant behaviour in most of the channels. It was also discovered that velocity asymmetry increases with the depth of the channels and the flood-to-ebb ratio decreases with depth. This emphasizes the importance of a 2DV model. Concerning the two scenarios, the opening of one of the Haringvliet sluices demonstrated generally a slight increase in M2 and M4 elevation amplitudes and a combination of a slight increase/decrease for the current amplitudes, depending on the position in the network. The sea-level rise scenario revealed overall less impact on the tides than opening one of the Haringvliet sluices. Especially in the Haringvliet, an increase in M2 elevation amplitude was found for this scenario. The M4 elevation amplitude increased particularly in the Nieuwe Maas. The M2 current amplitude showed an overall minor increase in the network, whereas for the M4 current amplitude a combination of both minor increases/decreases was found.