Measurement and analysis of sea waves near a reflective structure
Methods and equipment for the measurement of ocean waves were reviewed and their suitability assessed for the aim of this project: field measurement of sea waves near a reflective coastal structure such as a breakwater. None was found to be suitable. The functional and performance objectives are set out for a new system. The evolution of the final design, based on an array of pressure sensors, is described. The whole system is intended to be deployed on the sea-bed. It is fully self contained and independent of shore based services. Located away from the surf zone it is well placed to survive storm conditions and unauthorised interference. Theoretical methods for the re-construction of surface elevation records from measured sub-surface pressures, and the experimental findings of other workers, are presented. Available methods of estimating the wave directional spectrum from a spatial array of surface elevation records are reviewed, and the most appropriate one implemented. The system has given extensive service at a number of coastal defence sites. The results of subsequent analysis of selected data sets are presented in detail. They show the pronounced nodal structure in amplitude expected in the presence of wave reflection, clearly demonstrating that a single point measurement is likely to give misleading estimates of incident wave height. For near-calm to moderate, shore-normal incident wave conditions the results were found to agree with theoretical predictions both of wave height as a function of distance offshore, and of the structure's frequency-dependent reflection coefficient. For rougher conditions, in which both theoretical and physical models are less applicable, the results agreed with visual observations.