Factors Influencing Coastal Processes


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Factors Influencing Coastal Processes

In this essay I aim to describe and explain factors affecting coastal
processes. I will focus and explore how waves, tides, winds and mass
movement processes can change the form of the coasts within our
lifetime. The three key questions I will focus on are:

* What are the energy and sediment inputs into the coastal system?

* What are the processes that erode coasts?

* How is sediment transported and deposited?

I will conclude by describing and explaining factors influencing
coastal processes and how they make up the South Dorset coast.

The littoral zone is a narrow zone between high and low water zones
where energy and sediment concentrate. The sediments on the coast are
sand, shingle and clay. The river, cliffs and sea are sources of
sediment inputs into the coastal system. (Fig. 1)

(Fig 1. Sediment inputs into the coastal system.)

The energy inputs include wind, waves and tidal currents. The wind
produces sand dunes on beaches with shallow offshore gradients; these
produce ecosystems, which help with plants that cope with lack of
nutrients and water etc. Sand dunes help absorb the pounding of high
waves and reduce over wash flooding in storms. Below is a diagram of
Studland Dunes and beach, showing the different parts of the beach (Fig.
2)

Fig. 2 Studland dunes and beach

[IMAGE]

Winds create waves that ripple across the surface of lakes and seas
until they break on the shallowing bottom and crash into the shore. In
many areas, prevailing winds produce waves that consistently approach
the coast at oblique angles. Even the slightest angle between the land
and the waves will create currents that transport sediment along the
shore. These longshore currents are a primary agent of coastal
movement; they are a major cause of sand migration along barrier and
mainland beaches. (Fig 3)

Fig 3. Fistral Beach, Newquay, Cornwall UK

[IMAGE]

Tides ebb and flood in response to the gravitational attraction of the

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moon and sun; exceptional high and low tides occur each month when the
sun and moon are aligned. Tides help determine where the waves break
-- low on the beach at low tide, high on the beach at high tide --
and, therefore, where sand is deposited and removed. Rip tides, or
undertow, occur along most beaches and can move significant amounts of
sand offshore. Where the waves hit the shore at an angle, they set up
longshore currents that can move vast quantities of sand and silt.

(Fig 4. Tidal currents)

Coastal environments are always changing as a result of the action of
the waves. Waves are formed by the wind blowing across the surface of
the water. The power of the wave increases as the strength of the wind
and the distance over which the wave has built up (known as fetch)
increases.

When waves break, water runs up a beach, forming the swash and back
toe beach as the backwash.

Constructive waves are lower waves with a strong swash and weaker
backwash that build up beaches. Destructive waves have a stronger
backwash and erode material from the coast. There are four main ways
in which waves cause erosion:

1. Abrasion/Corrasion occurs when the waves throw sand, shingle and
pebbles against cliffs, grinding and breaking away the rock surface.

2. Corrosion happens when the acids in seawater dissolve rocks.

3. Hydraulic action results in the breakdown of rocks when waves
breaking against the cliffs trap air in cracks causing pressure to
build up. When the waves retreat the pressure is released breaking off
fragments of rock.

4. Attrition occurs when pebbles and stones carried by the waves are
broken up into smaller fragments as they are thrown against each
other.

These processes are quite simple but the patterns of erosion are
complex as they are influenced by these four factors:

1. Rock type- its resistance to wave action and sub aerial processes
and its solubility

2. Rock structure- the division between joints, bedding planes and
faults

3. Beach character- the size of the beach varies within seasons, it
may shrink if there is a reduction in erosion and sediment supply. The
wider the beach, the more protective it is from erosion.

4. Fetch/Currents- the fetch of dominant and prevailing winds and the
influence of tidal and long shore currents.

The development of a dune begins with weathering of parent rock.
Boulders and stones are ground up into sand by destructive waves and
water movements. The resulting sand is deposited on a shore by
constructive waves. The swash of the wave tends to push material up
the shore and the backwash tends to wash it back again. If there are a
lot of waves (high frequency) per unit time backwash will tend to meet
the swash of the next wave. This will spoil the forward motion, so
less material will be pushed up the beach. The backwash will continue
unhindered and there will be a net loss of material from the beach.
Hence, the term destructive waves. The difference between destructive
and constructive waves is their frequency.

(Fig 5. Longshore Drift)

[IMAGE]

Longshore drift is generated by wave and current action. Water and
sediment are transported in a zigzag pattern as waves and currents
approach the shoreline at an angle. This zigzag pattern is the
mechanism for sediment transport as the backwash immediately follows
the swash to the ocean. This energy created from the breaking waves
allows for weak currents to carry large amounts of coarse-grained
sediment down the shoreline. These weak currents, known as longshore
currents, carry water and sediment parallel to the shoreline toward
the down drift end of the beach. (Fig 5)


Conclusion
----------

In this essay, I have found out that winds, waves, tides and mass
movement are the factors that influence coastal processes; this
therefore changes the form of the south Dorset coast, areas such as
Lyme Regis, Charmouth, West Bay, and Furzy Cliff, which have been
affected mostly by erosion caused the factors mentioned above. The
coastline in South Dorset has been divided into littoral cells on the
basis of sediment pathways. Erosion is active along the majority of
the coast and most noticeable when human developments have taken place
on unstable softer rocks. I also found out that deposition occurs
where the sea's energy for transportation is reduced and where an
obstacle hinders the free movement of LSD material. A long groyne,
such as one in Hengistbury Head, Dorset is an obstacle that causes
sediment to be deposited, therefore causing the build up of the beach
at south Dorset. Sediment moves southeast due to the dominant
influence of LSD on the south Dorset coast, this is mainly on the Lyme
Bay littoral zone. On the Weymouth Bay and Lulworth Coastal littoral
zone; LSD moves material from west to east, whereas the sediment
movement is east to west because the ebb tidal currents are greater
than the tidal current and LSD.


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