The coastal system: G3.1.Ai

The coast is a SYSTEM. It has INPUTS, PROCESSES AND OUTPUTS of energy. It is an OPEN SYSTEM. That means that it has inputs from outside, like sediment

brought down by rivers and outputs to other systems such as sand dunes which migrate inland.

The coastal system is COMPLEX, many of the factors inter-relate and it is also very DYNAMIC, in that it is constantly changing, sometimes within a single day.

Coastal systems vary primarily in response to wave intensity and tidal currents

breaking waves (and resultant currents) provide most of the systemic energy

several additional factors also influence coastal processes and landforms

original geology of the coastline

relative 'erodibility' of regional bedrock

sea level variations (global and local tectonic or glacio-eustatic changes)

coastal systems typically characterized as erosional or depositional

erosional or depositional nature of any coastline varies with the systemic energy

Inputs:

The inputs of the coastal system are categorised in to 4 different sections these are Marine – processes that beginning in the ocean, Land – processes that’s occur on land these often occur on the beach or near them on cliffs, Atmosphere – the processes that naturally occur in the air around us, People – these are processes that happen due to man’s influence on the area.

Also theses 4 sections can be categorised into further points:

In marine the processes are the waves and tides. In Land the processes there are rock types and structures and Tectonics. Atmospheric the processes are weathering and climate, the change of climate and

Solar energy In People the processes are Human activities, Economics, Recreation/tourism and

sea defences.

Processes of the coastal system can be categorised into 3 categories

1.Weathering & Mass Movement

1. Freeze thaw2. -Salt-weathering3. -Carbonation

2.Erosion

1. -Hydraulic Action2. -Abrasion/Corrosion3. -Solution

3.Transportation

1. -Sediment cells2. -Longshore Drift3. -Deposition

The coastal system outputs are sediment that has been washed out to sea or deposited further along the coast line.

The outputs consist of two main categories, which are:

Beaches are made up of:

Cusps – which are there arc patterns of sediment on the coast line. Berms- the shingle ridge often found towards the back of the beach.Runnels – These are depressions in the sand created by small tide with low water movement. Ripples are created under the water. Berms are formed as sand/shingleis slowly moved up a beach by successive incoming tides - they are more common on shingle beaches, whilst ridges and runnels are found more on sandy beaches. Both form 'crests'.Long shore troughs- are elongated depression extending parallel to the shoreline and any long shore bars that are present, often representing the low point in the profile between successive bars.Cliffs – Cliffs are formed through a combination of erosion and weathering.Soft rocks e.g. clay, erodes easily to create gentle sloping cliffs.Hard rock e.g. chalk, erodes slowly because it is more resistant, it creates steep cliffsWave cut platforms – These are flat surfaces which are left behind when a cliff is eroded. The sea erodes the bottom of the cliff forming a wave- cut notch. The notch then increases causing the cliff to collapse. The backwash keeps on eroding the cliff forming a wave- cut platform.Stacks – Weak areas in rock are eroded to form caves. If the roof is weakened along a major joint by hydraulic pressure and the roof collapses to form a blow hole. Side of the headland may eventually join to Caves on the opposite Caves on the other side of the headland may eventually join to form an arch. When an arch Collapses it forms a stack.

Spits – spits form when the coast suddenly changes direction at the mouth of a river. Long shore drift continues to deposit material across the river mouth, leaving sand and shingle sticking out into the sea. (see depositional features)

Salt marshes –form in areas of sheltered water, as silt and mud are deposited by the river or the tide, mudflats develop. The mudflats are colonised by vegetation that can survive the high salt levels and long periods of submergence by the tides.

Sand dunes- are formed when sand deposited by long shore drift is moved up the beach by the wind. Sand trapped by driftwood is colonised by plants and grasses E.G Marram grass. The vegetation stabilises and encourages more sand to accumulate there, forming embryo dunes

Beach Zones:

A beach can be divided into different zones, and the activities that occur on a beach can be placed within particular areas (as shown on the diagram above). The most important function of a beach is to act as a barrier between waves and the

coast. Foreshore and nearshore:

The foreshore is located closest to the backshore and it is here, due to the breaking of waves that sediment transport may take place. Much of the energy of waves is reduced (dissipated). The nearshore performs similar functions, but usually only at low tide. Activities in this area include, recreation, quarrying, and coastal defences.

Offshore: There is limited direct sediment movement

here as tidal currents are more important than wave action. Sewage outfalls, oil extraction and fishing may occur.

Berms are formed as sand/shingleis slowly moved up a beach by successive incoming tides - they are more common on shingle beaches, whilst ridges and runnels are found more on sandy beaches. Both form 'crests'.

Waves G3.1.Bi The extent to which the shape of a beach or coast is altered depends largely on the action of waves upon it. Waves can be gentle and infrequent or larger, more frequent and more powerful.

The formation of waves and their size and shape is a result of the exchange of energy from wind blowing over the sea. The longer the wind blows for, and the greater the distance it blows over, the larger the waves that result, and the greater their energy. Other factors include:

Wind strength.

Time wind blows for.

Distance (fetch).

In the UK, the direction of maximum fetch is from the South West (for example, if you stand at Lands End, your nearest land mass is the USA) this is why the Cornish Coastline can experience huge high-energy waves.

There are two types of wave: constructive and destructive.

Constructive waves: these are depositional waves which drop there load on the beach. This is because the SWASH IS STRONGER THAN THE BACKWASH. These are depositional waves as they lead to sediment build up, and are most common where a large fetch exists. They tend to have a low gradient, a larger swash than backwash, low energy and an elliptical orbit. The wave period is long, with 6-8 waves breaking in a minute.

Destructive Waves: These are the type of waves that cause erosion to the beach and cliff. This is because the BACKWASH IS STRONGER THAN THE SWASH. This means that beach material is taken away from the beach easily. These act as agents of erosion, because backwash is greater than swash. They are most common where fetch is short, have a mainly circular orbit, a steep gradient, and 'plunge' onto the beach. The wave period is short, with 12-14 waves breaking per minute.

Wave fetch: The distance of open water over which a wave has passed. Maximum fetch is the distance from one coastline to the next landmass, it often coincides with prevailing wind direction (South West in the UK).

It is very rare for waves to approach a regular uniform coastline, as most have a variety of bays, beaches and headlands.

Because of these features, the depth of water around a coast varies and as a wave approaches a coast its progress is modified due to friction from the seabed, halting the motion of waves. This refracts the waves due to the velocity of the wave changing.

As waves approach a coast they are refracted so that their energy is concentrated around headlands but reduced around bays. Waves then tend to approach coastline parallel to it, and their energy decreases as water depth decreases.

Wave fetch: The distance of open water over which a wave has passed. Maximum fetch is the distance from one coastline to the next landmass, it often coincides with prevailing wind direction (South West in the UK).

Wave crest: Highest point of a wave.

Wave trough: Lowest point of a wave.

Wave height: Distance between trough and crest.

Wave length: Distance between one crest/trough and the next.

Swash: Water movement up a beach.

Backwash: Water movement down a beach.

Tides G3.1.Ci

As well as being affected by the moon, tidal range can also be affected by proximity to the equator this is due to the amount of gravitational pull being highest at this point.

Tidal range can be categorised into 3 categories:

1. Macrotidal tidal range is 4m plus2. Mesotidal tidal range it 2m-4m3. Microtidal tidal range is less than 2m

Tides are important influences at the coast. For instance:

If there is small tidal range the power of the waves (erosion) is concentrated on a relatively narrow section of cliff.

If there is a large tidal range then vast tracts of sand can be exposed during a low tide, dried out and blown up the beach to create sand dunes.

Tidal currents are important in moving sediment to and from the beach.

Case Study: G3.1.Di

The Bristol Channel

Due to the narrowness of the channel when the tide changes the a sudden rise and influx of water it sends a tidal wall of water upriver, this wave is called the Severn Bore (1m high water moves upstream at 30km per hour) The Tidal range of the Bristol channel is: 13m

The river Severn has the third highest tidal range in the world, only the bay of Fundy (north America) and Ungava bay ( Hudson straits) are bigger.The tidal range on the Severn can be as much as 15m (49ft), this combined to the 'funnel' shaped estuary causes the incoming tide to create great a bore. The front of the oncoming surge of water creates a visible wave, varying in height. From around the Noose at Fretherne the bore can be seen forming right up to Maisemore weir, around 20miles of river where the can be seen forming, dissipating, reforming sometimes miles at a time. As the river narrows around Minsterworth the bore has less area to spread over thus causing larger tides to have an increase in wave height.There are many factors contributing to a 'good bore' a SW wind gives a good driving force behind the bore, increasing its speed and power. Bores can be between 8-12mph on the river Severn. Less fresh water flowing in the river allows the tidal bore to travel further

inland, maintaining more of its speed and power. Low air pressure is also a key factor as this allows the movement of water more freely, thus assisting the bore in its travel.