Advanced Ocean Modelling: Using Open-Source Software

84 3 Basics of Nonhydrostatic Modellingprecipitation-related inflows. Inverse estuaries are characterised by an outflow ofhypersaline bottom water and inflow of ambient seawater in the surface layer.3.23.2 Classification of Estuaries According to OriginAccording to their geological characteristics, estuaries can be classified as:• drowned river valleys or coastal-plain estuaries• bar-built estuaries or lagoons• fjords• tectonically caused estuariesCoastal-plain estuaries and drowned river valleys were formed by the gradualrise of sea level after the last glacial period, 10,000 years ago. Lagoons are similarto coastal-plain estuaries, but these are usually situated parallel to the coastline.Many lagoons have narrow outlets and minimum freshwater inflow, often creatinghypersaline conditions.Fjords are long, narrow inlets with steep sides, created in a valley carved byglacial activity. Most fjords are deeper than the adjacent sea with depths of 1,000–2,000 m. Fjords generally have a sill at their mouth which limits the exchange withthe ambient sea, but in many cases creates extreme tidal flows. For instance, Saltstraumenin Norway involves water currents of up to 10 m/s in speed and is thereforeoften described as the world’s strongest tidal current.Tectonic estuaries include those created by landslides, faulting and volcanic eruptions.San Francisco Bay in the USA is an example.3.23.3 The Dynamics of Positive EstuariesContinental runoff is the principle driver of flows in positive estuaries. This runoffis associated with a barotropic pressure gradient directed toward the sea aiming topush low-salinity surface water seaward. The density interface between the layers,however, gives rise to baroclinic pressure gradients that support a bottom inflow ofambient seawater into the estuary (see Fig. 3.50). The degree of mixing betweenthe layers, facilitated by oscillatory tidal flows, modifies the strength of this inflow,which can supply oxygen to the benthic estuarine life.3.23.4 Brief Overview of TidesTides appear in the sea as regular oscillations of the sea level on predominantlysemi-diurnal (12 hrs) and diurnal (24 hrs) periods. Obviously, these variations arecaused by convergence and divergence of oscillatory lateral flow. High tide is the