Measurement of longshore current with a Doppler radar velocimeter at the research pier HORS

We measured longshore current continuously with a radar Doppler velocimeter installed on the deck of the research pier HORS. The velocimeter emits K-band (24 GHz) radio waves obliquely to the seawater surface and collects their backscatters to determine the Doppler shift. The installation was mostly located in the surf zone. We verified the result of the surface velocity measurement with manual floater release measurements. They showed a linear correlation, with the direction matching well, and the magnitude being higher for the velocimeter, which measures water surface motion. We analyzed the relationship between longshore current speed and the so-called wave power P ( = H2T sin θ, H: wave height, T: wave period, θ: wave direction) and the longshore component of wind VWind by multivariate linear regression for a 29-month observation. The contribution of VWind was larger than P on longshore current speed, since the velocimeter is sensing the water surface motion. The bulk statistic shows that the absolute longshore current does not exceed 2 m/s, even during periods when the P and VWind were large. We conclude that the radar Doppler velocimeter is a robust instrument suitable for continuously measuring longshore currents.