Hydrodynamic response in a microtidal and shallow bay under energetic wind and seiche episodes

Abstract

In this contribution we investigate the hydrodynamic response in a micro-tidal and shallow semienclosed domain. We chose a set of observations which include currents, hydrography and meteorological data obtained in Alfacs Bay (NW Mediterranean Sea). Short-term response to energetic winds events was found in the hydrography and water velocity observations, sometimes inverting the estuarine circulation or developing one-layered flow. In comparison to previous investigations in Alfacs Bay, we observed that water current variability, and also maximum velocities, were directly related to the development of surface standing waves (i.e. seiches). Mixing mechanisms versus buoyancy sources are studied through potential energy anomaly equation, proving the leading freshwater contribution to stratification, enhanced by heat fluxes in summer. On the other hand, mixing is directly related to winds, mainly in winter and early spring when both buoyancy forces are lower. We also study turbulent bottom mixing by seiches through observations, dimensionless relations and numerical modelling. Seiche induced mixing is suggested as an eventual mechanism that may break the stratification within the Bay under special circumstances.

Towards Cost-Effective Operational Monitoring Systems for Complex Waters: Analyzing Small-Scale Coastal Processes with Optical Transmissometry

“…The vertical profiles of temperature and salinity showed a stratified water column, with a fresher and warmer surface layer and an underlying cooler and saltier water layer (Fig 6b and 6c). The pycnocline was located at ~3.5m depth, consistent with previous studies [22, 32–33]. The averaged temperature and salinity gradients between surface and bottom were of ΔT = 1.33°C and ΔS = 1.84, with maximal differences of 2.1°C and 2.7, respectively (found in the bay mouth).…”

“…The strongest winds blew from SW from 5 pm to 10 pm. The observed wind pattern responded to the typical land breeze characterized by weak nocturnal winds (<2 m·s -1 ) blowing from land, that reverses the direction and increases the intensity during the daytime (sea breeze) [30, 33]. A similar behavior was observed in the surface current velocities, indicating that the water circulation at the sampling station was driven by the prevailing wind (Fig 10c).…”

“…The freshwater inputs induce vertical stratification, while only during strong wind events the water column is vertically mixed [21]. Heat fluxes in the ocean-atmosphere boundary layer in summer periods contribute in addition to stratifying the water column [33]. …”

Modeling circulation patterns induced by spatial cross-shore wind variability in a small-size coastal embayment

“…The bay has been defined as a salt-wedge estuary (Camp and Delgado 1987) Fig.2. In general, the water column used to be stratified due to the freshwater discharge, but well-129 mixed conditions are more common during winter and mostly related to the hydrodynamic response 130 to wind forcing (Llebot et al, 2013) and occasionally to seiches (Cerralbo et al, 2015a). …”

Use of a hydrodynamic model for the management of the water renovation in a coastal system

“…The bay has been defined as a salt-wedge estuary with an almost year-round stable stratification, alternated with well-mixed periods directly related to strong wind (Camp and Delgado, 1987) or seiche events (Cerralbo et al, 2015). Solé et al(2009) found that drainage discharges were the main factor controlling the observed stratification.…”