Set-Up of a Process-based Model to Investigate the Outer Weser Estuary Development

Abstract

This graduation project investigates the main processes that are influencing the morphological development of the Outer Weser estuary by setting up a process-based model (Delft 3D) and applying it by means of simulating multiple scenarios, each driven by a different forcing. The aim of this project is to gain insight into the causes for the development of a two channel system in the Outer Weser and into the origin of the related alternation pattern of a dominant channel. Due to the construction of training walls and groynes at the end of the 19th century, the described characteristics disappeared and thus, they are a historical phenomenon but yet today not fully understood. However, to gain insight into the natural development in the Outer Weser estuary is beneficial for current and future engineering projects.
For the investigation a schematic model is generated in which only essential processes are considered and simplified. By applying a flat bed as initial bathymetry, using simplified boundary conditions, multiplying the morphological changes with a morphological factor and using relatively coarse grid, the model is able to predict long term morphodynamics. In the next step scenarios are composed by considering the most relevant processes that could be responsible for the generation of two channels and for the alternation, leading to six scenarios. These scenarios are compared to a base case.
In order to investigate the morphologic equilibrium, the development of two channels and their cyclic behavior, a number of methods are applied. These include time depended hypsometries, cumulative bed load transport, visual inspection and a special method for characterization of channel forms / geometries in a cross-section.
The six scenarios cover the influences of the Kelvin wave, the Coriolis effect, wind waves, extreme river discharge, the absence of river discharge and an increased tidal range. All simulations have a period of ten years of hydromechanics which in combination with the applied morphological factor of 400 results in 4000 years of morphodynamics. The scenarios as well as the reference simulation reach a morphologic equilibrium within the first fourth of the simulation time. Afterwards, all simulations reveal the development of two channels with different dimensions and locations with respect to the reference simulation: While the scenarios excluding the Kelvin wave and river discharge result in a stronger development of the western channel, the scenario excluding Coriolis and the scenario including wave action lead to a more dominant eastern channel. The simulation with extreme discharge and the simulation with an increased tidal range do not favor one of the two channels. For the alternation, the developed evaluation method indicates an alternation between western dominance and equal dominance for the no Kelvin wave and no river discharge scenarios. For the no Coriolis scenario the alternation is observed between eastern dominance and equal dominance. No alternation is found for the extreme river discharge and wave scenarios, while the only scenario indicating an alternation between eastern and western dominance is the scenario with an increased tidal range. For the scenarios in which an alternation is indicated the alternation period is determined and compared to the reference case. This shows a shorter alternation period for the no river discharge scenario and a similar alternation period for the no Kelvin wave and no Coriolis simulation, while the alternation period is raised in the increased tidal range scenario.
The conclusions are that the development of the two channel system is mainly caused by the tides and the basin geometry, since it evolves even when the Kelvin wave, Coriolis, the river discharge and waves are excluded. Furthermore, it is reasoned that the alternation pattern and period are dependent on the dominance of the tides and the depth of the two channels, due to a shortening of the alternation period in the scenario without river discharge (stronger domination of the tides) and an increased alternation period in the scenario containing an amplified tidal range (deeper channels). Additionally, it has been reasoned that the alternation might be introduced by the shape and geometry of the Weser estuary.