SHALLOW-WATER WAVES OBSERVED BY A VHF GROUNDWAVE DOPPLER RADAR

This paper presents original data obtained with a VHF, narrow beam, gr ound-wave Doppler radar over a coastal area of small depth (about 2 m) . When meteorological conditions are such that waves of significant he ight greater than 0.7 m are incoming from off-shore, Doppler spectra o f the radar echoes from this area are quite different from those gener ally obtained in deep water. Besides the narrow lines due to the influ ence of freely propagating waves at the wavenumber k(b)=2m-1 selected by the Bragg backscattering mechanism, a wideband continuum centred at a Doppler frequency corresponding to the velocity of the dominant wav es in the shallow water is also observed. The conditions of occurrence and the properties of this continuum are presented and discussed. It is suggested that it is related to bound waves advected by the dominan t waves.