## 1 Introduction  
Water circulation induced by local wind effects can contribute to the water exchange between an estuary and the adjacent open sea [1]. Seminal investigations presented by [PERSON] [2] described how wind stress applied at the surface of a basin of variable depth sets up a circulation pattern characterized by coastal currents in the direction of the wind, with return flow occurring over the deeper regions. [PERSON] et al. [3] showed that the observed transverse partition of the subtidal circulation is consistent with that driven by local wind in a channel with lateral depth variations. [PERSON] et al. [4] showed how the impact of the wind-induced coastal sea level at the entrance of an inlet exerts a huge impact on the subtidal variability within the interior of the bay. The influence of wind can also have effects on salinity structure or resuspension events in shallow estuaries [5; 6; 7; 8]. These studies indicate that spatial scales and water depth determine the flow response to wind forcing. Moreover, [PERSON] et al. [9] determined that both river and wind-driven flow patterns change as a function of water depth in a shallow estuary (below 4 m). In this case, for wind-driven flows the critical parameter is the degree of stratification in the lower bay. However, the particular role that plays the geometry and the bathymetry in wind response in a small-scale bay is still not clear. An illustrative contribution presented by [PERSON] and [PERSON] [10] reflects the role of the bathymetry in the water contribution for largerand deeper basins than that mentioned in this study. Numerical experiments reveal a wind-induced water circulation dominated by an axially symmetric transverse structure due to laterally varying bathymetry. So, the investigation at very small-scales (order of few km) and shallow depths (few meters) still presents challenges that may be addressed by complementing observational and numerical efforts. The short time scales expected due to the shallowness, the role that plays the geometry (for instance the mouth width) and the link between bathymetry and water circulation are challenging questions to describe properly small-scale bays under micro-tidal conditions.  
Contributions focusing on coastal bays and estuaries conclude that they are regions with complex hydrodynamics due to multiple forcing [7; 9; 11]. In some cases, wind stress competes with baroclinicity to determine the hydrodynamics. However, the astronomic tide [12], seiches [13], water run-off, the bathymetry or the effect of rotation [10] are variables that may not be neglected in the analysis of the local wind influence on this type of water bodies. In these cases, the combination on observational and numerical efforts have allowed obtaining substantial new knowledge on water circulation and termohaline structure.  
This paper investigates the relationship between wind and hydrodynamics of small-scale, shallow and micro-tidal estuary. The research combines \"in situ\" data using the case study of Fangar Bay, located in the Ebro delta, NW Mediterranean Sea, and the implementation of numerical experiments in idealized geometries. These simulations should figure out the impact of the bathymetry on wind-driven water circulation. The complexity of the flow observed have suggested to focus, at this first stage, on the effect of the bathymetry in the wind-driven circulation. Fangar Bay presents problems of water renewal, high water temperatures, anoxia and permanence of pollutants in the water. All these problems affect the oyster and mussel crops present in the bay that need a sustained water renewal for their growth and development [14]. Several intensive field campaigns have been carried out to characterize the bay hydrodynamics as a first step for a subsequent water renewal analysis. In this sense, Fangar Bay is a good example to investigate the hydrodynamics of small-scale and shallow scale bays under micro-tidal conditions.