Validation of areal wave and current measurements based on X-band radar

Abstract

Coastal waters are characterised by complex wave fields that are influenced by inhomogeneous bathymetries, and changing tidal- and wind-induced currents. Understanding these interactions is important for coastal engineering and environmental management. Remote sensing techniques, such as radar for flow field data collection, increase the amount of available information. Unlike in-situ techniques (e.g. buoys, or moored current meters), remote sensing can provide continuous observation of several parameters (waves, currents and bathymetry) simultaneously across a wide area. The wave and current monitoring system, WaMoS II, is a remote sensing system using standard nautical X-Band radars generally used for navigation and ship traffic control. Nautical radars are designed to monitor the sea surface continuously over a relative large area (~ 10 km2) with high spatial (~7.5 m) and temporal resolution (~2 s). Under various conditions, signatures of the sea surface itself become visible in the near range (less than 3 nautical miles) of such radar images. These signatures include spatial and temporal information of the sea surface waves (wind/sea and swell), currents and in shallow water also about the local water depth. In recent years, development has focussed on retrieving current and wave data at that high resolution on an operational basis (Hessner et al., 2007 [1]; Hessner and Bell, 2009, Hessner et al. 2014). In this paper, a brief introduction will be given to the high resolution current and water depth measurement principles of WaMoS II. WaMoS II current and wave data will be shown from the Sizewell test site, where a system has been installed since September 2013. This site is located on the East coast of England, an area of coastline which has been intensively studied over many decades. The hydrodynamics of this area are characterised by strong prevailing tidal currents with current magnitudes between 1-1.5m/s and a strongly bi-directional wave climate. The WaMoS II system at this site, operates in connection with a Kelvin Hughes (Manta Digital) with a horizontally polarized 8 ft antenna and a radar repetition rate of 1.34s. The antenna is mounted 66m above sea level. The area of radar observation ranges from 150m to 4000m, off shore (0-180° relative to N). The system delivers standard sea state measurements with an update rate of 2 min and high resolution current and depth information in a range up to 4 km with an update rate of 20 minutes and a spatial resolution of about 180m. The observation area is characterised by a straight North-Southward aligned coastline and an offshore sandbank with varying water depth between 5-15m. The localised effect of the bank on the wave and flow fields is thought to have significant impact at the shoreline. The complex hydrodynamics and spatially varying currents would be impossible to monitor at the appropriate scale with point measurements only. Here typical tidal states and sea state conditions will be discussed. For the validation of the WaMoS II wave and current measurements, reference point measurements acquired by ADCP sensors at 3 different locations in the radar view field were used. Data acquired within the working range of WaMoS II (Hs > 0.5m, wind speed > 5m/s) show an excellent agreement with reference data but also spatial wave and current variation due to the bathymetry. During the observation period the prevailing wave direction was East North East, but periods of South-East waves were also observed. The radar data revealed the complexity of the wave and flow field with respect to the interaction between tides, waves and bathymetry.