Bottom boundary layer properties in the Upper Gulf of California derived from velocity profiles

Abstract

The vertical structure of currents in the Upper Gulf of California was studied using velocity profiles observed at a site in 25 m water depth during one fortnightly cycle, in order to determine the development of a logarithmic bottom layer and to estimate parameters derived from the law of the wall. The velocity data included two neap-tide periods in which gravity currents have been shown to develop. The currents were dominated by tidal forcing, and were oriented along-gulf, with the main axis towards 323.8º. Spring tide velocity amplitude was 0.5–0.9 m s–1, decreasing to less than 0.30 m s–1 during neap tides. A logarithmic layer structure was observed within the lower half of the water column during spring tides. This layer was less than 5 m thick during neap tides, and was not observed under low velocity conditions, around the times of current reversals. The seabed shear stress values were typical of macrotidal environments, reaching ±2.5 Pa under spring tide conditions and decreasing by a factor of 2–3 during neap tides. Mean values of the bottom drag coefficient and seabed roughness parameter were 10–2 and 0.05 m, respectively. These relatively high values were attributed to the influence of near-bed stratification. During the two neaptide periods, two gravity current events were observed within 4–5 m from the seabed, reaching 0.30 m s–1, intensified by the ebb tidal flow. The near-bed velocity profiles were markedly modified during these events that persisted for about three days.