Improvements to the detection and analysis of external surges in the North Sea

External surges are a key component of extreme water levels in the North Sea. Caused by low-pressure cells over the North Atlantic and amplified at the continental shelf, they can drive water-level changes of more than 1m at the British, Dutch and German coasts. This work describes an improved and semi-automated method to detect external surges in sea surface time histories. The method is used to analyse tide gauge and meteorological records from 1995 to 2020 and to supplement an existing dataset of external surges, which is used in the determination of design heights of coastal protection facilities. Furthermore, external surges are analysed with regard to their annual and decadal variability, corresponding weather conditions, and their interaction with storm surges in the North Sea. A total of 33% of the 101 external surges occur within close succession of each other, leading to the definition of serial external surges, in which one or more external surges follow less than 72h after the previous external surge. These serial events tend to occur more often during wind-induced storm surges. Moreover, the co-occurrence with a storm surge increases the height of an external surge by 15% on average, highlighting the importance of the consideration of combined events in coastal protection strategies. The improved dataset and knowledge about serial external surges extend the available basis for coastal protection in the North Sea region

Tools for the Improvement of the Efficiency and Sustainability of Shore Nourishments – Results of the research project STENCIL

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Hydrodynamic Loading of Dutch terraced houses due to flood actions using Computational Fluid Dynamics

Whenever a region - especially the coastal area - is affected by flood risk, it is essential for residential and life protection to gain knowledge about the mechanisms leading to the collapse of houses. There are empirical mortality functions to predict the mortality of population in the Netherlands directly affected by flooding, but due to probable changes in building quality it is necessary to investigate the fragility of the current building stock. For a detailed structural research containing collapse mechanisms, the resulting loads on buildings in case of flooding – depending on flow velocity, water height, building orientation, width, height etc. – should be known. In current research, several physical experiments have been conducted in order to gain information about hydrodynamic loads on houses in flood actions. The present project aims to set up a numerical model for assessing the flow-induced pressure loads on residences. The model is based on physical experiments already conducted in model scale: a dam break wave is generated, impacting on a model residence of typical Dutch dimensions; then the flow-induced pressure loads are determined. The focus is on the quasi-steady flow part after the initial wave impact. For moderate computational load Reynolds-averaged equations are used for the numerical model. The generated flow conditions and load magnitudes are compared to physical results in order to validate the numerical model. It can be shown that the results generated during the quasi-steady flow part conform with physical results largely. Appearing discrepancies may result from model constraints regarding strongly mixed interface regions of air and water. Finally, possible further model applications are demonstrated: the effect of urban density (realised by blockage ratio variation) on the resulting load is investigated; in addition, the experiment is scaled to prototype scale. Generally, the numerical model serves as a useful tool for load estimation induced by the quasi-steady part of a dam break wave. The model can be used to investigate further modifications; a large range of input variables like e.g. flow conditions, residence geometries, residence arrangements can be assessed to gain information about main interrelations or specific scenarios.Civil Engineering and GeosciencesHydraulic EngineeringAdditional project thesi

Wave Run-Up on Mortar-Grouted Riprap Revetments

The wave run-up height is a crucial design parameter that determines the crest height of a sea dike and is used for estimating the number of overtopping waves. Therefore, a reduction of the wave run-up height is generally aspired in the design of dikes, which can be achieved by mortar-grouted riprap revetments (MGRR). Although MGRRs are widely utilized revetments along the German North Sea coast, no investigations into the wave run-up height on this revetment type are available to date. Full-scale hydraulic model tests were hence conducted to investigate wave run-up heights on partially grouted and fully grouted MGRRs. The wave run-up was determined using 2D-LIDAR measurements, which were validated by video data. Partially grouted MGRRs, due to their roughness, porosity, and permeability, reduce wave run-up heights from 21% to 28%, and fully grouted MGRRs due to their roughness reduce wave run-up heights from 12% to 14% compared to smooth impermeable revetments. Influence factors have been determined for four widely used revetment configurations, which can now be used for design purposes. A comparison and subsequent discussion about the representation of the physics of wave run-up by different parameters is carried out with the results presented