Sand-spit accumulations at the mouths of the eastern Cantabrian estuaries: The example of the Oka estuary (Urdaibai Biosphere Reserve)

The majority of the eastern Cantabrian estuaries occur on a coast with waves approaching dominantly from a NW direction. However, at their mouths sand-spit accumulations occur on their eastern margin. These sedimentary features have extended westwards, although the main littoral transport path on this coast is dominantly eastwards. The origin, morphology and development of these sand accumulations can be considered unusual, because they do not show the expected sedimentary response. Through a multidisciplinary approach (surface sediment analysis, morphodynamic, oceanographic and meteorological monitoring, and mathematical modelling) developed during the period May 2005–April 2006 in the Oka estuary, it has been possible to establish a genetic, hydrodynamic and morphodynamic sedimentary model for these estuaries. The main factors to determine the origin of these sedimentary features are wave intensity and direction, wave refraction and diffraction processes, spatial orientation and morphology of estuarine mouths, tidal range and the intensity and direction of tidal currents. On this coast, but elsewhere due to the influence of headlands, usually situated in the western margin of the estuarine mouths, wave diffraction and a variation in energy occur along the approaching waves. Consequently, wave set-up is forced in the eastern side of the estuarine mouths. Because of this induced wave pressure gradient an energy-compensation is needed and thus wave induced currents towards southwest originate. The action of the tidal currents is also important. In this paper a cyclical sedimentary transport pattern for the nearshore zone around the mouths of these estuaries is described. This process is taking place under tidal influence that induces wave set-up and breaking processes and reinforces also the sedimentary transport towards the estuarine inlet. The combination of all these hydrodynamic and sedimentary processes has produced these “anomalously-oriented” sedimentary features.