COASTS
1. Introduction to the Coasts
British Isles are surrounded by coasts, and have a great variety of coastal geomorphology (the study of changing shape of the earth). The coast is the scene of constant change, sometimes in a very short time scale e.g in Feb 1953 floods, some cliffs were eroded back 10m.
Definition:
"The Coast is the boundary (interface) between the land and the sea"
Coast can be thought of as a system: it has inputs and outputs of energy.
Processes:
There are 3 processes which create, and modify the nature of this boundary:
- marine erosion
- transportation (longshore or littoral drift)
- deposition
2. Waves
Definition:
"waves are undulations of the water surface caused by winds blowing across the sea. They consist of orbital movements of water molecules which diminish with depth"
Main agents of change at the coastline are waves.
Wave energy = coastal erosion + sediment transport
When the wind blows over the surface of the ocean, surface waves are generated by the transfer of energy in the form of momentum from the air to the water: a ‘drag’ effect.
When the wind first begins to disturb the surface of the water, tiny capillary waves called ‘cat’s paws’ are formed. These quickly dissipate but while they exist they roughen the surface and increase the transfer of energy from the wind to the sea surface.
If there is no pattern to the wind, the surface becomes a chaotic state, called ‘sea’ or ‘chop’.
Over time, as the wind blows in the same direction, a swell will develop. Waves moving out away from a storm eventually organize themselves into a swell, and eventually, if they are not destroyed by interference (see later), they will reach the shore
WAVE TERMINOLOGY & CHARACTERISTICS
Waves are a form of energy. The energy of the wind is transferred to the water. The wind blows over the water and piles it up into waves.
· The stronger (faster) the wind, the bigger the waves.
· The longer distance over which the wind can blow (called 'fetch'), the bigger the waves.
· The more days a given wind blows (time), the bigger the waves – this can be shown on a wind rose diagram
Fetch:
The size of waves is partly determined by the area of open ocean over which the wind can blow e.g S. Coast of Oahu in Hawaii (19°N has a fetch which extends to Alaska)
Wavelength:
The average difference between successive wave crests or troughs.
Wave height:
Vertical distance between the wave trough and wave crest.
Wave steepness:
The ratio of wave height to wavelength. Powerful waves are high because they are high and have sort wavelengths.
Wave energy:
This is equal to the square of its height. So a wave 2m high has 4 times the energy of a wave 1m high. Wave power takes account of velocity, so it is H²xV
Wave velocity (celerity)
This can be expressed in direct terms, as m/sec¹ or in terms of wave period (time interval in seconds between successive waves)
The movement of water particles in a wave is in an orbit in deep water. As the wave approaches the coast, and the water gets shallower, the waves path becomes an ellipse (oval shape)
Remember: Waves don't actually move water from one place to another. Water in a wave moves up and down. The water moves up as a crest passes, and moves down as a trough passes. Each molecule of water draws a circle as one complete wave (crest to crest) passes.
As waves approach the beach, water depth decreases (it gets shallow!) and waves start to touch bottom where water depth is around 1/2 the wavelength.
The seafloor shallows as the waves approach shore, and eventually the waves touch bottom (they reach wave base). At this point we shift from "deep water" to "shallow water" (from the wave's perspective).
The waves begin to slow down (celerity decreases) due to friction and wave celerity now depends on water depth (remember, in deep water wave celerity depends on wavelength and period: C= L/T). The shallower the water, the more the waves slow down. Wavelengths shorten, but period remains the same.
The wave height increases and the trough flattens out. The wave gets so tall it can't support itself, and the water crashes over the top. This is called a breaker, and breakers form in an area called the surf zone.
The wave loses most of its energy by breaking (it actually gives off some light and heat), and the remaining energy causes the water to rush up the shore. It loses the rest of its energy to friction in this manner, then gravity pulls the water back out to sea. The surge onshore is called swash; the slump back to sea is called backwash. Swash and backwash occur in the swash zone.
So as the the waves come to shore from the sea they change from deep-water waves to shallow water waves at wave base, where water depth = 1/2 wavelength,
· they slow down (celerity decreases)
· wavelength decreases
· period stays the same
· height increases
· wave breaks and becomes
· swash, then backwash
There are 2 types of wave:
Surfing breakers: high energy waves: are steep and have short wavelengths
Surging breakers: low energy waves: shallow and have long wavelengths
Waves Interfere With each other
Waves generated by a storm march out away from the storm in all directions. A storm off the coast of Africa might generate a swell, and a storm off the coast of Spain the same day or a few days later will generate another swell. These two sets of waves will very likely have different wavelengths, different amplitudes, and different periods.
Longer wavelength waves travel faster than waves with shorter wavelengths.
When two wave trains meet,
· crests and troughs can add together to make a bigger wave
· crests meeting troughs will cancel each other out
These phenomena together are called interference. When waves interfere and a bigger wave results, we call it constructive interference. When waves interfere and cancel each other out, we call it destructive interference.
As waves approach the coast they are also refracted. They start to take on the shape of the coastline, which means that wave energy is concentrated at the headlands and dispersed in the bays.