Optimization of gate design in the Afsluitdijk based on dynamic wave impact

Abstract

The Afsluitdijk will undergo major redevelopment. After more than 80 years, the structure is ready for an upgrade. One of the differences which face the engineers of today compared to the original designers is climate change as well as the strict flood safety standards of the 21st century. Climate change is the lead cause of the sea level rise, which results in several issues at the Afsluitdijk. First, higher water levels result in larger waves causing more damage and secondly the discharge complexes will have difficulty in the natural discharge of the IJsselmeer. These discharge complexes consist of several locks side by side with each two gates; A northern and a southern gate. When the water level is too high in the IJsselmeer the gates will open during low tide and the water naturally drains into the Waddenzee. However, with sea level rise the water level difference is less and the time gap is also shortened creating a shortage in drained volume. The discharge complexes at Den Oever and Kornwerderzand are therefore redesigned with extra locks and pumping stations. The scope of this thesis concerns the new designs of the northern gates at Den Oever in the original discharge sluices. The concrete structure of the sluice will not be altered and is not part of the scope. The flood safety has become a lot more complex in the last 80 years. According to the safety standard, the Afsluitdijk should be able to withstand a storm with a return period of 10.000 years, which is the highest in the country except for a dike segment close to a nuclear power plant. The result is that the gates have to withstand waves of. However, it was found by Hofland (2015) that these largest waves with the highest water levels were not governing. Waves with water levels lower than an overhanging structure were much more violent. This overhanging structure is now redundant, and it is decided to be removed for the new gates. However, it is still interesting how these impulsive forces can be calculated and predicted. In order to find an alternative to model tests when encountering such a problem with an overhanging structure, a numerical method has been developed in this thesis. The method is based on the notion from Wood and Peregrine (1996), which directly calculates pressure impulse field. Their method is analytical and with the numerical alternative given in this thesis it has been made easier to adapt the boundaries and include gaps in the structure. The method is compared to the results of the model test executed by Hofland (2015). A problem arises when these impulsive wave forces impact a relatively thin plated structure such as the gate. The dynamic response of the gate can cause an amplification in the expected displacement and stress. The surrounding fluid creates an extra complexity to the problem. The current engineering practice to analyze the dynamic response in such a situation is based on a simplified quasi-static method by Kolkman (2007). This simplified method is based on a Dynamic Amplification Factor (DAF) of a single degree of freedom system incorporated with an extra hydrodynamic mass. A semi-analytical method derived by Tieleman (2015), that includes fluid-structure interaction has more potential to find a more realistic solution than the current engineering practice. The semi-analytical method still depends on a homogeneous isotropic thin-plate, but with the coupling of a finite element model (SCIA) the complexity of the design is theoretically limitless. This coupling is developed and numerically compared with the gate at the discharge sluice complex at Den Oever in this thesis.
The semi-analytical method allows for a complete dynamic calculation in a relatively short time. This gives the opportunity to perform multiple calculations back-to-back. This has been exploited in this thesis by the development of a parametric model. This parametric model is based on the gate design in the Afsluitdijk and can find the most optimal designs regarding thickness and placement of the several elements in the structure by utilizing an optimization scheme. The optimization is based on a predefined post-analysis, which in this case concerns equivalent stress and stability. However, the possibilities are endless with possible addition many more failure mechanisms like fatigue. The model itself can be applied in a preliminary design phase where it gives the engineer more options while still incorporating the dynamic effect. It also helps to locate the more governing parameters, which is often difficult in a dynamic problem.