Study on the impacts of floating seaweed on wind waves

Abstract

As human beings are becoming more and more aware of importance of the environment, resorts to clean energy have provoked great interest for the past decades. As a sort of clean power, wind has been widely considered as an ideal energy source to generate electricity. That is why the offshore wind farm plans have been drawn up, specifically in North Sea in Europe. Besides, in order to comply with marine spatial planning which aims to make use of marine resources sustainably, seaweed aquaculture will also be operated among wind farm zone under wind turbines, which provides the food necessity for mankind. The introduction of seaweed aquaculture may have influences on local hydrodynamics as seaweed attenuates waves, which may further affect the environment. This effect has not been understood. Therefore this study is conducted to investigate the impact of seaweed aquaculture on wind waves. As a relatively young sector, few studies have been found on the effect of floating vegetation on waves. Since there is no ready-to-use hydrodynamic model which can deal with floating seaweed, it is required to set up a numerical model and calibrate this model with experimental data. In this research the wave modelling program SWAN is used owing to the fact that there exists a vegetation model in SWAN. In this model the vegetation is schematised as stiff cylinders mounted at the bottom. The energy dissipation by vegetation is calculated as work carried out by the drag force induced by cylinders. However, the function of this vegetation model in SWAN is somewhat different from what is required for this study. • Firstly, the SWAN deals with bottom-mounted vegetation while in this research floating vegetation is addressed. • Secondly, in SWAN model stiff objects are considered while in current research the vegetation possesses flexibility. With flexibility the drag force will become smaller as the relative velocity between water particle and vegetation gets smaller. On the other hand, the inertial force may have an impact on the wave energy dissipation. Considering these two differences, corresponding measures need to be taken to meet the requirement of this study. To model the floating vegetation, the original vegetation model has been modified. In the beginning, the floating vegetation model is achieved by dividing the vegetation into two vertical layers and making the lower layer virtual. There is drawback in this model: only flatbed can be applied. For sensitivity analysis and calibration this model works well, but it is not applicable in the case study as non-flat seabed is implemented in the model setup. It leads to a second modification in the SWAN model. The method is to alter the interval of integration of dissipation term by vegetation, which results in further change in the source code. To account for the flexibility, the bulk drag coefficient is adjusted. This parameter accommodates the influence of flexibility of vegetation and uncertainty of other parameters related to the model. To test this floating vegetation model a sensitivity analysis is performed. Relevant parameters in this model are varied to see its individual influence on the wave dissipation. It has been observed that a larger wave height results in a higher dissipation rate. The opposite has been seen for wave period. Wave with a larger peak period gives a smaller wave height behind the vegetation field. It has been also demonstrated that larger vegetation length, larger vegetation diameter and larger vegetation density give rise to greater dissipation rates. In addition, spectrum analysis shows that the energy dissipation rate varies along the frequency domain with the greatest magnitude of energy dissipation observed at around the peak frequency. To calibrate this model the experimental data is used, obtained from flume test in Technology University of Braunschweig. The calibration is conducted by adjusting the bulk drag coefficient until that the floating vegetation model describes the experiment data. By this method a fitted drag coefficient is obtained for each wave condition. It has been verified that drag coefficient depends on both the wave condition and vegetation parameters. The relation between wave condition and vegetation characteristic is indicated by KC-number or Reynolds number. Through this calibration a relation between drag coefficient and Reynolds number or KC-number is obtained. With the calibrated model, a case study is conducted in the North Sea. The dominant wave data for the period 1991-2010 is used, as well as corresponding wind information. The vegetation parameters (vegetation density, vegetation width and vegetation length) are set according to local cultivation. With this information and the drag coefficient function obtained from the calibration stage, the drag coefficient for the field condition is obtained. Implementing all relevant input parameters in the model, the result shows that the dissipation is species specific. Also from the results it is shown that wave attenuation occurs mainly in the local scale. Behind the vegetation field waves develop due to the directional spreading. At the South boundary 85% of the wave height remains. Reflecting on the uncertainty associated with this study, such as measurement data, calibration of drag coefficient and parameterising the vegetation diameter in the field condition, it is still questionable whether seaweed aquaculture has a significant impact on the wind waves or not.