COASTAL PROCESSES Background information

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COASTAL PROCESSES

Background information

The coastal processes include all natural processes that operate in a region covered by coastal waters and shore

The importance of coastal zone is the availability of the following resources:

i) Sources of minerals, oil, coal, and aggregates for construction

ii) In shore food (fish, seaweed, crustacea, etc): Coastal waters are the most productive and bio-diverse areas of the seas almost 90% of the global fish catch comes from coastal waters

iii) Major recreational environments and tourism

iv) Areas suitable for land reclamation, farming, settlements and shipyards. The coastal lands are intensely populated with about 60% of the world’s human population lives within 100 km of the coast

Geomorphic agencies

The agencies operating at the coastal zone are:

i) Tides

ii) Waves

iii) Currents

iv) Storms and tsunamis

Tides

Tides are result of gravitational pull of the moon and the sun on the earth. Both land masses and oceans experience pull effect. However, water responds quickly and at greater magnitude than the landmasses. This gravitational pull causes periodic rises and falls of the sea level. This phenomenon occurs twice daily. The rise in the sea level causes increase in vertical height of water, a condition that favours waves to attach the coastal areas.

When the sun, moon and earth are aligned, a phenomenon that took place at new and full moon periods, the coastal area experiences SPRING TIDES. When the moon and the sun are at right angle with the earth, the coastal area experiences NEAP TIDES. The pull is more during springtides due to the effect of the moon and the sun that reinforce each other as compared to when there are neap tides where they oppose each other. Spring and neap tides are experienced after every two weeks, which is approximately 14¾ days.

Waves

Waves are normally generated by wind (Fig.4.2). They are propagated in the direction of the wind. The waves increase in height (size) with wind strength and duration. During storm, wave heights at the coast may reach up 15m.

Waves are of two types:

i) Sea waves – locally generated by wind

ii) Swell waves: Lower and longer waves that travel far from the area of their generation. When travelling longer period waves absorb those with shorter period thus producing regular sinusoidal waves called swell waves. Swell waves are capable of travelling thousands of km across the ocean without losing much energy.

The waves are responsible for erosion and formation of beach features by adding and removing sediments from the swash zone. The waves are also responsible for generating coastal currents that drift the sediment along the beach. At shallow water, the speed of water at the bottom is high, as such sand and shingles are moved followed with coarse sediments.

Normally the waves break as approaching the shoreline. Once the waves break, energy is released as turbulent swash runs up to the beach carrying sediments. The water running back is called backwash. The volume of backwash is always less than swash water. This is due to part of the backwash water percolates in porous beach sediments. The work done by backwash and swash is to adjust the beach slope until it is in equilibrium with the forces acting on it.

Alternating periods of storms and calms cause beach slope to change repeatedly. Some of the waves near the beach are refracted in the direction of the shoreline. The currents produced by the lateral movement of water are known as longshore currents. These currents are responsible for moving beach sediment diagonally to the shoreline. The movement of sediment is known as longshore drift, which is distinct from the beach drift that operates on land at the beach. The beach drift is the lateral movement of sediments on a beach when the angles of swash and backwash differ. The combined movement of sediment via longshore drift and beach drift is known as littoral drift. The sediment transportation direction is therefore largely governed by the direction of dominant waves. The longshore currents play fundamental role in influencing local patterns of erosion and deposition of sediments along the coast.

The energy (E) brought by waves at the beach is a function of wave height (h) (Fig. 4.2) and water density (r).

E = ¹/₈ rgh² Where g is the acceleration due to gravity

The deep water waves with the height to length ratio exceeding 0.025 are likely to be destructive whereas those with smaller ratios are constructive. The plunging breakers form little swash but create high backwash and are therefore likely to be destructive.

Breaker types:

i) Spilling