Geography Reference
In-Depth Information
wind and a large supply of sand. Onshore winds (see Chapter 1)
capable of sediment transport must occur for a significant amount
of time. Dunes often develop just above the spring high tide line
where litter such as seaweed and wood collect. This litter starts to
trap sand blowing around and promotes small dunes to form. Once
these start then they can enlarge, especially when plants grow
within them allowing a greater height of sand to be trapped around
the plants. Dunes can grow relatively quickly under the right con-
ditions reaching 2 metres high after just five years.
Just off the coast in wave-dominated environments there can be
formation of barrier landforms including barrier islands and lagoons.
These extremely dynamic landforms can be found along about an
eighth of the world's coastline and there are many famous exam-
ples such as those along the Florida Atlantic coast or the coast of
The Netherlands. Barriers help buffer inland areas from storm wave
energy and represent a large accumulation of onshore moving sand
bars. They form long strings of parallel island chains punctuated by
tidal inlets that allow the transfer of water and sediments between
the open sea and the lagoons behind the barrier. Some barriers are
aligned to the swash direction whereas others are aligned to the
prevailing direction of long shore currents. For example, a spit is a
narrow accumulation of sand or gravel, with one end attached to
the mainland and the other projecting into the sea or across the
mouth of an estuary or a bay. Spits grow in the long shore drift
direction and can only exist where there is a regular supply of
sediment.
Estuaries are river mouth locations where sedimentary deposits
from both river and sea sources create landforms. Sea levels rose
around river mouths as ice melted following the last glacial period
and this stabilised around 6,000 years ago. Infilling of estuaries
occurred as sediments from land and sea could inundate the new
deep waters at the river mouth. Estuaries have three broad zones;
an upper, middle and outer part. River processes dominate the
upper estuary while marine processes dominate at the outer estuary.
The middle section is mixed. The same volume of sea water leaves
the estuary during the falling tide as enters during the rising tide.
However, the duration and strength of the rising and falling tide
tends to be different. Estuaries or channel sections that display a
flooding tide that is faster and stronger than the ebbing tide are said
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