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Item type:Publication, Access status: without files , Experiments on uni-directional and nonlinear wave group shoalingWe report an experimental study addressing the characteristic hydrodynamic transformations of unstable wave groups as well as JONSWAP wave fields propagating from deep-water towards shallow regions. Long zones of linearly rising floor levels have been installed to mimic a simplified coastal morphology. The breather surface elevation evolution data show a very good agreement with the depth-adapted nonlinear Schrödinger-type model. Our results suggest that the residual effects of four-wave resonance interactions in the shallower regions is relevant for the nonlinear group propagation over steep bottom topography slopes. When considering broadband JONSWAP processes, the model fails in predicting the evolution of kurtosis, independently of the values of significant wave height adopted. Nonetheless, consistency with the data is achieved when narrowing the spectrum of the JONSWAP wave field. Our study emphasizes the strong potential of weakly nonlinear frameworks in modeling complex wave shoaling problems and wave transformations in coastal zones.Publicationtype: Journal Article49 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Modelling dynamics of the estuarine turbidity maximum and local net deposition(2019-04-01); ; Net deposition in estuaries is often linked to the estuarine turbidity maximum zones, in which fine, cohesive sediments accumulate due to residual transport by the estuarine circulation and tidal asymmetries. Sediments deposit in fairways or harbours, which creates high maintenance dredging costs and the need for better prediction of dredging hotspots with process-based numerical models. In this paper, a new efficient modelling approach is presented which enables the simulation of the ETM formation, its seasonal dynamics and the local sedimentation. A 3D baroclinic large-scale estuary model with a characteristic sediment fraction with simplified sediment transport properties is used with realistic boundary conditions, but without initial sediment distribution. This approach is referred to as supply-limited, regarding the ETM formation by residual transport. A dynamic equilibrium between residual sediment import from the open boundaries, accumulation and local sedimentation establishes in the model. This is achieved by combining the large-scale supply-limited model with an extended bed exchange formulation (2-Layer-Concept). A model of the Weser estuary is used as case study to reproduce and analyse the ETM formation and the resulting sedimentation simulated with this approach. The results are compared with the equivalent sediment concentration of turbidity measurements and dredging volumes.Publicationtype: Journal Article196 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Long-term statistics of potentially hazardous sea states in the North Sea 1958–2014(2018-11-01); ; Significant wave height and mean wave period are two of the most commonly used parameters to describe wave climate, wave climate variability, and their potential long-term changes. While these parameters are generally useful to characterize the distribution of waves within a given sea state, they provide less information about potentially high-risk situations. Over the recent years, a number of criteria were suggested that are considered to better characterize high-risk situations and which could bear a potential for the development of safety warning indices. Based on a multi-decadal high-resolution wind-wave hindcast, a climatology of such parameters is developed for the North Sea covering the years 1958–2014. More specifically, average conditions, inter-annual variability and long-term changes for unusually steep, rapidly developing and crossing sea states are considered. Generally, there are pronounced spatial variations in the frequency of such sea states, while over time, there is some seasonal and inter-annual variability but no substantial long-term trend could be identified.Publicationtype: Journal Article195 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Simulation and detection of tsunami signatures in ocean surface currents measured by HF radar(2011-06-13); ; ; ; High-frequency (HF) surface wave radars provide the unique capability to continuously monitor the coastal environment far beyond the range of conventional microwave radars. Bragg-resonant backscattering by ocean waves with half the electromagnetic radar wavelength allows ocean surface currents to be measured at distances up to 200 km. When a tsunami propagates from the deep ocean to shallow water, a specific ocean current signature is generated throughout the water column. Due to the long range of an HF radar, it is possible to detect this current signature at the shelf edge. When the shelf edge is about 100 km in front of the coastline, the radar can detect the tsunami about 45 min before it hits the coast, leaving enough time to issue an early warning. As up to now no HF radar measurements of an approaching tsunami exist, a simulation study has been done to fix parameters like the required spatial resolution or the maximum coherent integration time allowed. The simulation involves several steps, starting with the Hamburg Shelf Ocean Model (HAMSOM) which is used to estimate the tsunami-induced current velocity at 1 km spatial resolution and 1 s time step. This ocean current signal is then superimposed to modelled and measured HF radar backscatter signals using a new modulation technique. After applying conventional HF radar signal processing techniques, the surface current maps contain the rapidly changing tsunami-induced current features, which can be compared to the HAMSOM data. The specific radial tsunami current signatures can clearly be observed in these maps, if appropriate spatial and temporal resolution is used. Based on the entropy of the ocean current maps, a tsunami detection algorithm is described which can be used to issue an automated tsunami warning message.Publicationtype: Journal Article22