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Coasts and Waves
The coast is the area of interaction between the sea and the land resulting in the
formation of different landforms. It extends from the deepest point to which waves
transport materials to the highest level reached by sea on land.
Coastal Zones
In-shore Areas
Backshore - This is the area which extends from the high tide shoreline towards the
land.
Foreshore - This is the area between the high tide shoreline and the low tide shoreline.
Nearshore - This is the zone between the foreshore zone and the offshore zone.
Offshore - The area beyond the point where waves cease to impact the seabed and in
which activity is limited to the deposition of sediments. It is the last zone going seaward.
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Off-Shore Areas
Continental Shelf - The portion of a continent (land mass) that is submerged under an
area of relatively shallow water.
Continental Slope - The slope between the outer edge of the continental shelf and the
ocean floor.
Why live near the coast?
The temperature is cooler
The land is more flat, easier for buildings
The land has alluvial soil, it is more fertile
The ease of recreational activities taking place
It is more convenient for fishermen.
Waves
A wave is a disturbance on the surface of the sea/ocean created by the increased
friction as the wind blows over it. Waves possess energy and acts as an agents of
erosion. Similar to other agents of erosion (wind, water, ice) coastal waves erode,
transports and deposits. Waves break when they enter shallow water.
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➜ N.B: Waves may also be formed when an earthquake shakes the sea bed. These
waves are called tsunamis.
Features of a Wave
Other Vocabulary
Crest
Swash
The highest point of a wave
The lowest point of a wave
This is when surf runs up the beach.
The swash takes a cloud of sand
particles and even small stones up
the beach with it.
Wave Height
Backwash
The distance between a crest and a
trough
This is when the water starts to flow
back down the beach. It drags
Wave Length
sediment from the beach back into
the water.
Trough
The distance between two
successive crests of the wave
Wave Speed
How fast or slow the wave is moving
at a given period
Wave Period
The time taken for a wave to travel
through one wavelength
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Low Wave Frequency
This is when a small amount of
waves are breaking every minute.
Example, 6-8 wpm
High Wave Frequency
This is when a large amount of
waves are breaking every minute.
Example, 10-14 wpm
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The strength of a wave is determined by three factors:
Wind speed/velocity
The period of time during which the wind is blowing
The fetch (the maximum distance of open water over which the wind can blow)
N.B: The longer the fetch the larger the wave, and the faster the wind speed the
larger the wave.
Types of Waves
There are two types of waves:
Constructive Waves
These are waves that build the coastline/landforms (eg, beach)
Destructive Waves
These are waves that build/carve the coastline/landforms by eroding them (eg, cliff)
N.B: With a constructive wave, the swash is stronger than the backwash— they
deposit material unto the coast. With a destructive wave, the backwash is
stronger than the swash— they erode the coast.
Difference between Constructive Waves and Destructive Waves
Constructive Waves
Destructive Waves
Usually low/small
Usually high/large
Has a long wavelength (up to 100 cm)
Has a short wave length (perhaps 20 cm)
Has a low frequency (6 to 8 per minute)
Has a high frequency (10 to 14 per minute)
Spills over gently when breaking and is not very
erosive
Plunges over when breaking with strong
erosive force
Weak backwash which allows for the deposition
of sediments
Powerful backwash which which can move a
lot of sediments don the beach
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Common on gently sloping coast
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Common on steep coastal slope
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