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3 Basics of Nonhydrostatic Modelling
precipitation-related inflows. Inverse estuaries are characterised by an outflow of
hypersaline bottom water and inflow of ambient seawater in the surface layer.
3.23.2 Classification of Estuaries According to Origin
According to their geological characteristics, estuaries can be classified as:
• drowned river valleys or coastal-plain estuaries
• bar-built estuaries or lagoons
• fjords
• tectonically caused estuaries
Coastal-plain estuaries and drowned river valleys were formed by the gradual
rise of sea level after the last glacial period, 10,000 years ago. Lagoons are similar
to coastal-plain estuaries, but these are usually situated parallel to the coastline.
Many lagoons have narrow outlets and minimum freshwater inflow, often creating
hypersaline conditions.
Fjords are long, narrow inlets with steep sides, created in a valley carved by
glacial 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 with
the ambient sea, but in many cases creates extreme tidal flows. For instance, Saltstraumen in Norway involves water currents of up to 10 m/s in speed and is therefore
often 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 Estuaries
Continental runoff is the principle driver of flows in positive estuaries. This runoff
is associated with a barotropic pressure gradient directed toward the sea aiming to
push low-salinity surface water seaward. The density interface between the layers,
however, gives rise to baroclinic pressure gradients that support a bottom inflow of
ambient seawater into the estuary (see Fig. 3.50). The degree of mixing between
the 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 Tides
Tides appear in the sea as regular oscillations of the sea level on predominantly
semi-diurnal (12 hrs) and diurnal (24 hrs) periods. Obviously, these variations are
caused by convergence and divergence of oscillatory lateral flow. High tide is the
3 Basics of Nonhydrostatic Modelling
precipitation-related inflows. Inverse estuaries are characterised by an outflow of
hypersaline bottom water and inflow of ambient seawater in the surface layer.
3.23.2 Classification of Estuaries According to Origin
According to their geological characteristics, estuaries can be classified as:
• drowned river valleys or coastal-plain estuaries
• bar-built estuaries or lagoons
• fjords
• tectonically caused estuaries
Coastal-plain estuaries and drowned river valleys were formed by the gradual
rise of sea level after the last glacial period, 10,000 years ago. Lagoons are similar
to coastal-plain estuaries, but these are usually situated parallel to the coastline.
Many lagoons have narrow outlets and minimum freshwater inflow, often creating
hypersaline conditions.
Fjords are long, narrow inlets with steep sides, created in a valley carved by
glacial 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 with
the ambient sea, but in many cases creates extreme tidal flows. For instance, Saltstraumen in Norway involves water currents of up to 10 m/s in speed and is therefore
often 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 Estuaries
Continental runoff is the principle driver of flows in positive estuaries. This runoff
is associated with a barotropic pressure gradient directed toward the sea aiming to
push low-salinity surface water seaward. The density interface between the layers,
however, gives rise to baroclinic pressure gradients that support a bottom inflow of
ambient seawater into the estuary (see Fig. 3.50). The degree of mixing between
the 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 Tides
Tides appear in the sea as regular oscillations of the sea level on predominantly
semi-diurnal (12 hrs) and diurnal (24 hrs) periods. Obviously, these variations are
caused by convergence and divergence of oscillatory lateral flow. High tide is the