Factors affecting formation of a wave.4 (a) How wind and fetch affect wave height?

(i) Fetch:The longer the fetch the smaller the velocity required to generate a wave of the same height. Hence a wave of 1m in height requires a wind speed of over 17m/s at 10 km but at a fetch of 100 km only a velocity of 8m/s.

Fetch allows a longer duration for wind to blow as waves are a product of friction produced by the wind.

Other factors which might affect the form of a wave is:Depth of sea:The depth of sea; wave height increase as depth of sea decreases. For example the wave is higher as it reaches the shore.

Shape of coastline:A Discordant coastline:On a discordant coastline, alternating layers of hard and soft rock are perpendicular to the coast. As waves approach this type of coastline wave is refracted, where erosional effect of the waves is concentrated on the headland, while wave energy is dispersed in the bays, hence deposition tends to occur in the bays.

A Concordant coasts:Concordant coasts have alternating layers of hard and soft rock that run parallel to the coast. The hard rock acts as a protective barrier to the softer rock behind it preventing erosion. If the hard rock is breached though, the softer rock is exposed and a cove can form (e.g., Lulworth Cove).

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(b) How does wave action contribute to both erosion and deposition on coasts?

High energy wave:

Wave action can be seen as occurring in high energy environments characterized by high and frequent waves due to long fetch and high winds. Here marine erosion will be dominant such as process of :

(i) Hydraulic action: the pressure of water hitting a cliff base and the sudden release of energy which breaks down incoherent rocks.

(ii) Wave quarrying: the entrapment of air in cracks, joints etc and decompression as water retreats.

(iii) Abrasion: the result of materials carried by waves striking cliff base giving rise to undercutting and notches.

(iv) Solution: Weak acids contained in sea water will dissolve some types of rock such as chalk or limestone.

Examples of coastal landforms produced by wave erosion are headland and wave-cut platform and formation of stack.

Headland and wave-cut platform:

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(ii) Stack and stump.

Low energy wave:

Conversely in low energy environments where there is a lower frequency of waves more swash and less backwash then waves can have constructive functions in building beaches and when associate with Longshore drift that can bring about other sedimentary landforms such as spits, bars, tombolo etc.

Features Spits Tombolo BarsWhat is it? A ridge of sand and

shingle projecting from the mainland

into the sea

A ridge of sand and shingle joining the

mainland to an island

A ridge of sand and shingle which has

joined two headlands, cutting

off a bayHow does it

form?When there is a break in the coastline and

drop in energy, longshore drift

deposited the material and build up spit.

Formed when spit continue to grow

outwards connecting the mainland to the offshore island just

like bridges

(a) form when a spit grows the whole way across a bay

(b) a sandbank develops offshore, parallel to the shore until it joins the mainland

Examples Hurst Castle Spit Isle of Portland Slapton sands

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Coastal zones:4. (a) Fig.2 shows a coastal zone with its tide and wave environments.

The wave activities occurring in the breaker, surf and swash zones:

The breaker zone or line is the portion of the nearshore region in which waves arriving from offshore become unstable and break.

The swash zone is the portion where the beach face is alternately covered by the run-up of the wave swash and then exposed by the backwash.

The surf zone is the portion of the nearshore between the breaker line and swash zone. The surf zone can have bore-like, breaking or broken waves propagating across it.

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Impact of these activities upon shape of backshore and nearshore zones:

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N03

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J’02 /J07

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J08

J09

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J’04

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N10

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J10

J11

N12

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