INTRODUCTORY
OCEANOGRAPHYINTRODUCTORY
OCEANOGRAPHY
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In the first part of this lesson we will define some terms and differentiate between erosional and depositional coasts.
If you spend any time at the coast you notice that the beach itself is in a dynamic state, constantly being shaped by ocean waves and a sea level that rises and falls with the tides (all of which I call the "dynamic zone"). The zone between the lowest tide and the highest elevations on the continent that are affected by storm waves (including their spray) is called the shore, and the shoreline is defined as the water's edge, which moves up and down the shore with the tide. The region from the landward limit of the shore inland, as far as you find features that are related to marine processes, is called the coast, and the demarcation line between the terrestrial and marine flora and fauna is called the coastline.
Some coasts are shaped primarily by non-marine processes and are called:
The
coastal region that includes the coastline, the shoreline, and the
shallow water seaward of the shore as shown in Fig. 12.15 and in a
different view on the right (the description of the various divisions
of the coastal region given in the textbook is basically correct, but
Fig. 12.15 is confusing, so use the figure above to follow the
discussion below, and get a better graphic of the divisions). Note
that the shore is subdivided into the backshore and the
foreshore at the shoreline. The foreshore (also called the
littoral or intertidal zone) is that part of the shore between
the high-tide and low-tide shorelines that is submerged and uncovered
repeatedly by the tides. The region between the low tide shoreline
and the low-tide breaker line is called the nearshore, and
everything seaward of the low-tide breaker line is called the
offshore. The beach is the wave-worked sediments that are
moved along the shore.
Coasts
that are shaped primarily by erosion are called wave erosion coasts,
and very often the landward limit of the shore is marked by a cliff
(e.g., Big Sur in California, or much of the coast of Maine). Here
the coastline would run along the cliff connecting points that mark
the upper extent of the effective wave action (for a high cliff, this
may be part way up the cliff, but for low cliffs it may be at the top
or just landward of the cliff).
Often these coastlines are irregular with headlands and bays. Given enough time, the headlands (being attacked on both sides by breaking waves) will progressively erode to produce sea caves, sea arches and sea stacks (as shown above in Fig. 12.12 and below in photos of the Australian Coastline provided courtesy of Ms. Susan Bader, a former student in this class). In addition, waves may also cut a bench that underlies the sand of the beach and shore regimes.
There are three things that determine the amount of erosion:
The erosion of coastlines and erosion of sediments being carried to the shoreline by rivers produce large amounts of sediments that must be distributed along the continental margin. Near-shore currents may distribute some of this sediment to the edge of the shelf break and onto the slope, but most of the sediments are distributed in the surf zone by waves. Waves that approach a shoreline at an angle and then break in the surf zone will produce a longshore current and longshore drift as described next.
When these wave breaks, the water continues to rush up the beach (at the same angle as the waves) as swash, but when it stops it runs back down the beach along the greatest slope as backwash. This swash/backwash motion will result in a zigzag movement of the water with a net direction that is the same as the approaching waves and which will produce a longshore current. Much more important for understanding marine depoitional coasts, however, is that this wave action will pick up sediment which will be moved by the longshore currents -- the movement of sand by these currents is called longshore drift.
All
of the features of a marine depositional coast are created by this
longshore drift. As shown in Fig. 12.21 on the right, these include a
sand spit, where sediments are
deposited from a point of land along the direction of the longshore
drift, a bay mouth bar
(which, given time may become a bay
mouth barrier when the spit extends all the way
across a bay), the Tombolo, when
a spit extends between an island and the land, and a
barrier island, when sediments
are deposited parallel with the shoreline.
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