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.
Highlights
The hydrodynamic properties of the bottom boundary layer are important to evaluate physical processes such as vertical mixing and sediment transport
The effects of friction between the currents and seabed are perceivable in this part of the water column, resulting in marked vertical velocity gradients and creating important environmental factors for the distribution of organisms across the sediment-water interface
The ocean boundary layer generated by tidal currents can extend several meters above the bottom or across the entire water column in shallow seas
Summary
The hydrodynamic properties of the bottom boundary layer are important to evaluate physical processes such as vertical mixing and sediment transport. The effects of friction between the currents and seabed are perceivable in this part of the water column, resulting in marked vertical velocity gradients and creating important environmental factors for the distribution of organisms across the sediment-water interface. Las propiedades hidrodinámicas de la capa límite del fondo son importantes para evaluar procesos físicos tales como la mezcla vertical y el transporte de sedimentos. En esta región de la columna de agua se manifiestan los efectos de la fricción entre la corriente y el lecho, dando lugar a marcados gradientes verticales de velocidad y creando factores ambientales importantes para la distribución de organismos de la interfase agua-sedimento.
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