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Validation of X-Band radar derived hydrodynamic phenomena:

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Abstract

The coastal area is a dynamic environment that suffers from human interventions and climate change. This requires extensive research and monitoring of the coastal areas. Measurement systems play a key role in the monitoring. Current velocities are for example often measured to investigate local hydrodynamics. At the moment the ADCP is often used to measure current velocities. The disadvantage of this measurement system is the lack of spatial information. New innovative measurement systems are developed to overcome this disadvantage. The X-Band radar is an example of such a measurement system. The X-Band radar can be used to survey water surfaces resulting in images with wave information. These images can be used to derive current, depth and wave information. The main advantage of the X-Band radar is the spatial extend. The radar images have a range over several kilometres. The aim of this study is to validate X-Band derived current velocities. SeaDarQ is an algorithm that can be used to process the X-Band radar images and derive depth, current and wave information. The technique used in the SeaDarQ algorithm was developed by (Young et al., 1985). A 3D Fourier Transformation is used to gain the wave information from the images. Subsequently the linear dispersion relation, which describes the fixed relation between wavenumber and wave frequency as a function of the depth and the current, is fitted through the obtained wave information to derive the depth and the current. The algorithm is analysed to find the valid depth for the derived currents. The analysis has revealed that the current is fitted using the short waves obtained from the images. The active depth of a wave depends on the ratio between the wave length and water depth. A long wave is active over a larger part of the water column than a short wave. The consequence of using the short waves for the current fit is therefore that the derived for the top part of the water column. A case study is done to validate the current output. A X-Band radar is installed near the Sand Motor, which is a large scale nourishment at the Dutch coast at ter Heijde. The velocity output from point of the SeaDarQ output field is compared with ADCP measurements during different metocean conditions. It can be concluded from the case study that the X-Band derived velocities are indeed valid for the top part of the water column. The measurement period includes calm periods as well as a severe storm. Wind from sea and from land is observed including wind speeds between zero and twenty meter per second. The wave height varied between 28 cm and 5 meters. The data comparison revealed that SeaDarQ is not able to derive current velocities with wind speeds lower than 2 m/s or heavy rain conditions. All other circumstances resulted in accurate velocity output. Also during stratified conditions, resulting from a fresh plume passing by, the SeaDarQ output is accurate. SeaDarQ can cope with the broad variety of conditions due to the current fitting procedure in which only the short waves are included. The short waves are less easily affected by the bottom than long waves due to their small active depth. The short waves follow therefor the linear dispersion relation very well. The same reasoning holds for the stratified conditions. The stratification does not affect the short waves that much due to the small active depth. The short waves follow the linear dispersion relation therefore very well during these conditions. This new measurement system can derive accurate current velocities and can therefor become a powerful measurement system in the future.

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