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Innovative mooring in the port of the future : scale model testing of the Shore Tension system
Moored vessels often experience low-frequency vessel motions when moored in a port due to wave excitation. Under such conditions the loading and offloading of vessels may be hampered when these movements become too large. Innovative mooring techniques can be used for reducing issues with excessive motions of moored vessels in waves. Considering applying such techniques as part of the design of mooring facilities and ports is expected to make different approaches to port or mooring facility designs possible. Such techniques, like the ShoreTension (ST) system, are already applied successfully worldwide in ports, however the application and performance limits of such systems under extreme conditions are not well known. This paper describes the results of a research project using physical scale modelling to systematically verify and extend the applicability and performance limits of innovative mooring systems. It resulted in a solid validation database for validating numerical models. The knowledge developed in this research will benefit developers of mooring facilities (including ports) to significantly reduce costs by limiting the need for structures providing shelter from waves. Furthermore, this may also help lowering the impact of port infrastructure on the coastal system when using less invasive infrastructure.
Proceedings of the 39th International Conference on Ocean, Offshore and Arctic Engineering - OMAE2020 (June 28-July 3, 2020, Fort Lauderdale, USA)
Projected 21st century changes in extreme wind-wave events
We describe an innovative approach to estimate global changes in extreme wave conditions by 2100, as a result of projected climate change. We generate a synthetic dataset from an ensemble of wave models forced by independent climate simulation winds, enhancing statistical confidence associated with projected changes in extreme wave conditions. Under two IPCC representative greenhouse gas emission scenarios (RCP4.5 and RCP8.5), we find that the magnitude of a 1 in 100-year significant wave height (Hs) event increases by 5 to 15% over the Southern Ocean by the end of the 21st century, compared to the 1979–2005 period. The North Atlantic shows a decrease at low to mid latitudes (≈5 to 15%) and an increase at high latitudes (≈10%). The extreme significant wave height in the North Pacific increases at high latitudes by 5 to 10%. The ensemble approach used here allows statistical confidence in projected changes of extremes.
Stability of berm type breakwater with cube blocks in the lower slope and berm
In this study, the stability of cube armour units in the lower slope and berm of rubble mound breakwaters with a berm was investigated for irregular and regular placement methods. Cubes are simple and cheap in production. However, the design conditions for the upper slope and lower slope may differ depending on the water level of the dominant storm conditions. For the upper slope, relatively higher water levels are dominant. The berm has a reducing effect on the wave loading on the upper slope and therefore, it may be relevant to use cubes in the lower slope, in combination with rock in the upper slope. In this study, a regular placement technique named “the double pyramid placement method” was applied for cubes placed on the lower slope and the berm. This placement technique was compared with the irregular placement technique by performing experiments in a wave flume. As a result of this investigation, the regular placement method was found to be superior for the stability of cubes in the lower slope and berm of rubble mound breakwaters with a berm.
Influence of oblique wave attack on wave overtopping at smooth and rough dikes with a berm
Oblique wave attack can significantly reduce the amount of wave overtopping at coastal structures compared to perpendicular wave attack. Guidelines exist to estimate the influence of oblique waves on wave overtopping. Guidelines to estimate the influence of a berm in the seaward slope are also available. These guidelines for the reducing effects of oblique waves and the reducing effects of a berm are frequently applied in combination, although limited data is available for this combination of reducing effects. To develop a method that can account for the reducing effects of oblique waves and a berm in the seaward slope of a dike, physical model tests have been performed in a wave basin on structures with smooth and rough slopes. Based on these data, wave overtopping guidelines for breaking waves on dikes have been validated and one of the guidelines has been extended to take the influence of oblique waves, berms and roughness into account.
Numerical modelling of the migration direction of tidal sand waves over sand banks
Tidal sand waves are large-scale bed forms found in shallow sandy seas, which show a migration of several meters per year. Field data from the Dutch part of the North Sea revealed a migration pattern causing bidirectional migration of sand waves over a sand bank, resulting in sand wave migration uphill from both sides of the sand bank. In order to understand the physical mechanisms behind this migration behaviour, we study the inclusion of a sand bank under a sand wave field using the numerical model Delft3D. First, the schematized model set-up (i.e. schematized bathymetry and simple tidal forcing) showed that the alteration of the tidal flow by a sand bank resulted in tide-averaged horizontal flow towards the top of the sand bank, causing the bidirectional migration of sand waves over the sand bank. Second, a case study was performed in which a more complex model set-up was used (bathymetry and tidal forcing identical to a study site in the North Sea where an offshore wind farm is developed). Here, the model results revealed migration directions comparable to field observations. The results open opportunities to explore modelling migration patterns of sand wave fields for offshore wind farm development in areas with complex bathymetric environments.
On the stability of rock armored rubble mound structures
Estimation of the required armor size is a major task in the design of coastal structures under wave loading such as breakwaters and revetments. Several semi-empirical formulas have been developed for this purpose. However, these formulas are often either limited to certain water depth conditions or do not incorporate the permeability of the structure in an appropriate way. The main objectives of this study are to (a) develop a unified physically sound formula for the estimation of the required rock size in all relevant water depth conditions and (b) to relate the effects of the permeability of the structure directly to physical parameters. To achieve these, first a comprehensive data base of deep and shallow water experiments within the design conditions was built. Then physical reasoning along with a robust data mining approach, i.e. M5 model tree, were used to develop formulas for armor stability. In the stability formula, wave characteristics such as the significant wave height and spectral energy mean period (Tm-1,0) are invoked while the permeability is incorporated using the ratio between the size of the core material and the armor stones. Accuracy metrics such as discrepancy ratio and scatter index indicated high performance of the model in different conditions. Finally, a probabilistic formula and some guidelines are provided for practicing engineers.
Measurements of hydrodynamics, sediment, morphology and benthos on Ameland ebb-tidal delta and lower shoreface
A large-scale field campaign was carried out on the ebb-tidal delta (ETD) of Ameland Inlet, a basin of the Wadden Sea in the Netherlands, as well as on three transects along the Dutch lower shoreface. The data have been obtained over the years 2017–2018. The most intensive campaign at the ETD of Ameland Inlet was in September 2017. With this campaign, as part of KustGenese2.0 (Coastal Genesis 2.0) and SEAWAD, we aim to gain new knowledge on the processes driving sediment transport and benthic species distribution in such a dynamic environment. These new insights will ultimately help the development of optimal strategies to nourish the Dutch coastal zone in order to prevent coastal erosion and keep up with sea level rise. The dataset obtained from the field campaign consists of (i) single- and multi-beam bathymetry; (ii) pressure, water velocity, wave statistics, turbidity, conductivity, temperature, and bedform morphology on the shoal; (iii) pressure and velocity at six back-barrier locations; (iv) bed composition and macrobenthic species from box cores and vibrocores; (v) discharge measurements through the inlet; (vi) depth and velocity from X-band radar; and (vii) meteorological data. The combination of all these measurements at the same time makes this dataset unique and enables us to investigate the interactions between sediment transport, hydrodynamics, morphology and the benthic ecosystem in more detail. The data provide opportunities to calibrate numerical models to a high level of detail. Furthermore, the open-source datasets can be used for system comparison studies.
Detecting floating plastics using a high-frequency X-band radar : a feasibility study on unconventional monitoring techniques for the detection of plastics
Integrale analyse morfologische effecten bodemdaling door gaswinning Ameland-Oost
De voorliggende rapportage beschouwt het conceptuele model over de werking van het morfologische systeem rond Ameland-Oost. Het is tot stand gekomen op verzoek van de Begeleidingscommissie “Monitoring Bodemdaling Ameland” naar aanleiding van de integrale monitoringsrapportage van 2017 (NAM e.a. 2017). Deze studie geeft een overzicht van de fysische principes die gelden voor het hele morfologische systeem rond de gaswinningslocatie Ameland-Oost en de samenhang tussen de ontwikkelingen van het morfologische systeem. Het doel van de rapportage is het inzichtelijk maken van verbanden tussen effecten van gaswinning op morfologie tussen de deelgebieden van het systeem zoals de Noordzeekust, het zeegatsysteem, de wadplaten en de kwelders. De directe en mogelijke indirecte effecten van bodemdaling op de morfologische ontwikkeling van het integrale systeem en zijn deelgebieden zijn uiteengezet in dit rapport. Daarnaast zijn monitoringsgegevens vanaf de start van de monitoring in 1989 tot en met 2019 samengevat en wordt er besproken in hoeverre de metingen aansluiten bij het systeembegrip.