University of Twente Student Theses


Modelling ship waves for overtopping assessment purposes

Kampherbeek, D. (2020) Modelling ship waves for overtopping assessment purposes.

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Abstract:In the Netherlands, inland waterways are an important part of the transport infrastructure. On the Waal alone, 120140 million tonnes of freight gets transported annually. Every ship sailing on these waterways causes waves. Along low lying quays and dikes, overtopping by ship waves can pose hazards for pedestrians and vehicles. Although a lot of effort has been spent on quantifying the effects of ship waves since 1949, there is no accurate way of estimating overtopping by ship waves. With models for overtopping being available, the problem lies in the availability of a model that can estimate shipinduced wave conditions at the bank. The analytical methods that are available for this purpose are limited in their accuracy and validity. This thesis aims to clarify whether the nonhydrostatic, nonlinear shallow water flow model SWASH is a suitable tool for modelling shipinduced wave conditions at the bank for the purpose of overtopping. Based on earlier research, there are indications that SWASH should be able to model both primary and secondary components of the ship wave. Up till now, the short, secondary ship waves were the limiting factor of comparable models. In SWASH the dispersion of secondary waves should be accurately represented. To test this hypothesis, three main steps are undertaken with as aim to find out how SWASH performs when modelling shipinduced waves for overtopping. The first step is the implementation of the pressure field method in SWASH. In the pressure field method, a ship is represented as a timevarying atmospheric pressure field. The timevarying pressure field mimics the sailing ship. Implementing the pressure field method in SWASH proved to be possible. With a suitable numerical scheme, it was shown that the model can simulate a ship passage without crashing due to numerical instability. This is a proof of concept for simulating ship passages in SWASH. The spinup effects can be separated from the actual wave signal by launching the ship first, and then accelerating it. For testing the implementation of the pressure field method, model settings were varied. The generated wave signal proved to be sensitive to the horizontal resolution of the computational grid. Important wave overtopping parameters like bottom roughness and turbulence only have a small influence on the ship wave signal. The biggest limitations for application of the model are the required computational effort and the numerical instability. The second step in this research is the validation of the model to measurements and comparison of SWASH to existing analytical methods. In a towing tank, SWASH can reproduce primary components of the wave signal, but it overestimates the secondary wave height. The simulations of real passages in the Port of Rotterdam and the Nauw van Bath show that SWASH can model the wave signal in complex geometries. The uncertainties in the measurements and simulations make it hard to draw a quantitative conclusion about the accuracy of SWASH. When comparing estimated wave characteristics with conventional methods, SWASH outperforms both Dutch and German guidelines. The third step in this research is a step towards extending the shipwave model to include overtopping. Here, it was shown that with the full grid at a resolution useful for overtopping, the calculation time becomes unworkable. Options for grid refinement are local refinement around the overtopping area or splitting the model into a shipwave generation part and an overtopping part. In this study, stability issues prevented SWASH from simulating the overtopping caused by a ship passage. Overall, SWASH is a promising tool for estimating shipinduced wave conditions. The model has proven to be able to generate both primary and secondary ship waves. Wave signals and components can be estimated more accurately than with other methods. For the purpose of overtopping, SWASH can be used to generate the wave signal that serves as input for an overtopping model. To use SWASH in a standardized engineering methodology, further study on the certainty and sensitivity in the outcomes of the wave signals modelled by SWASH is necessary to increase the reliability of the model to levels acceptable for engineering applications. For this kind of study, measurements on the shipinduced surface excursion and flow velocities at the banks would be a useful addition.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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