Most existing methods for separation of two-dimensional (long-crested) waves into incident and reflected components are based on linear wave theory. Recently, a new method for separation of incident and reflected nonlinear regular waves was presented including separation of bound and free superharmonics. The present paper extends this method to irregular waves. Irregular waves are much more complicated to separate because bound components are caused by interaction of many different frequencies, thus, some simplifications are needed. The presented nonlinear separation method is based on narrowband approximation. Second-order wave theory is used to demonstrate that errors for more broad-banded spectra are acceptable. Moreover, for highly nonlinear waves, amplitude dispersion occurs and is included by a simplified amplitude dispersion correction factor. Both assumptions are evaluated based on numerical and physical model data. The overall conclusion is that existing reflection separation methods are reliable only for linear and mildly nonlinear nonbreaking irregular waves, whereas the present method seems reliable for the entire interval from linear to highly nonlinear nonbreaking irregular waves. The present method is shown to be an efficient and practical approximation for an unsolved theoretical problem in the analysis of waves in physical models.

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Generation of high-quality waves is essential when making numerical or physically model tests. When using a wavemaker theory outside the validity area, spurious waves are generated. In order to investigate the validity of different wave generation methods, new model test results are presented where linear and nonlinear wave generation theories are tested on regular and irregular waves. A simple modification to the second-order wavemaker theory is presented, which significantly reduces the generation of spurious waves when used outside its range of applicability. For highly nonlinear regular waves, only the ad-hoc unified wave generation based on stream function wave theory was found acceptable. For irregular waves, similar conclusions are drawn, but the modified second-order wavemaker method is more relevant. This is because the ad-hoc unified generation method for irregular waves requires the wave kinematics to be calculated by a numerical model, which might be quite time-consuming. Finally, a table is presented with the range of applicability for each wavemaker method for regular and irregular waves.

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In the present paper, the performance of active absorption systems based on nearfield surface elevation measurements is studied. Based on linear wavemaker theory the performance of such systems can easily be calculated for linear waves. A recent study demonstrated that bound superharmonics in regular waves is also well absorbed by such system and has a re-reflection similar to a linear component. However, the performance of active absorption systems on nonlinear irregular waves has never been studied. In the present paper the absorption of bound sub and superharmonics in bichromatic and irregular waves is examined based on new model tests. The conclusion is that also in irregular waves the bound harmonics are well absorbed by the studied active absorption system.