Exponential distribution of wave-driven near-bed water speeds under short-crested waves: a case study in the eastern Gulf of Riga, the Baltic Sea

Loads exerted to the seabed by short-crested wind-seas with a wide directional spread have extensive spatio-temporal variability. We quantify this variability in terms of near-bed water speed using an array of nine high-resolution hydromast devices for recording pressure and water velocity in the range of 0.12-1 m/s mounted at a distance of 10 m from each other on a rigid rectangular frame of 20 x 20 m in approximately 4 m deep water and 700 m from the eastern shore of the Gulf of Riga near Skulte (Latvia) in August-September 2022. This array is complemented by an acoustic Doppler velocimeter (ADV). The average background current is very weak, approximately 0.003 m/s in the measurement location. The empirical distributions of velocity components are symmetric but greatly deviate from the expected Gaussian distribution. The empirical distributions of water speeds follow an exponential distribution rather than a Rayleigh or Forristall distribution. This shape of the distributions appears in the range of 0.2-0.7 m/s while the maximum speed reaches 1.22 m/s. The rate parameter (inverse scale parameter) varies almost by a factor of two in recordings by different devices. The recordings make it possible to identify wakes of vessels entering to or departing from the Port of Skulte.