COASTAL PROCESSES & MANAGEMENT

TOPIC 2

INTRODUCTION TO COASTS

What are coasts?

• Coasts are narrow contact zones between the land and sea which are constantly changing (dynamic)

due to the effects of land, air and marine processes.

• Marine processes refer mainly to the effects that waves have on coasts.

Waves are created by the transfer of energy from the wind blowing over the surface of the sea

(except for a tsunami which is the result of plate tectonics). The largest waves are a result of very

strong winds which blow for lengthy periods and cross large expanses of water.

Fetch – A fetch is the maximum distance of water over which wind can blow.

WAVE CHARACTERISTICS

• The energy acquired by waves depends on 3 factors:

1. Wind velocity

2. The period of time during which the wind has blown

3. Length of the fetch

Wave terminology

• Crest: highest point of a wave

• Trough: lowest point of a wave

• Plunge line: the point at which the wave breaks

• Swash: the body of foaming water which rushes up a beach after a wave breaks

• Backwash: the body of water returning back down to sea after the swash

WAVE MOTION

• Water particles within waves move in a circular orbit. This means that each particle tends to move

vertically up and down.

• Therefore it is only the shape of the wave and its energy which is transferred horizontally towards

the coast.

• When the wave reaches shallow water, due to the friction of the sea-bed, the motion of the wave

becomes more elliptical and does not remain in a circular orbit.

• The top of the wave would be unaffected by the friction so it becomes higher and steeper until it

breaks.

TYPES OF WAVES

• CONSTRUCTIVE WAVES

• Form when the fetch distance is long.

• Usually small, flat, have a long wave length and have low

frequency.

• Have limited energy which is used by the swash to

transport material up the beach.

• DESTRUCTIVE WAVES

• Form when fetch distance is shorter.

• Usually large, steep, have a short wave length and have a

high frequency.

• Have much more energy which is used by the backwash

material back down the beach.

EROSION LANDFORMS

• THERE ARE 4 WAYS HOW WAVES CAN ERODE LAND:

1. ABRASION: (aka wave-cut platform) caused by large steep waves breaking at the foot of a

cliff.

2. ATTRITION: caused when waves cause rocks and boulders on the beach to bump into each

other and break up into smaller particles.

3. CORROSION: (aka solution) caused when salts and other acids in seawater slowly dissolve a

cliff.

4. HYDRAULIC PRESSURE: when the force of the waves compresses the air present in the cracks

in a cliff.

COASTAL FEATURES CAUSED BY EROSION

HEADLANDS AND BAYS

• In areas of alternating resistant and less resistant rock

• Initially, the less resistant rock experiences the most

erosion and develops into a bay, leaving the more

resistant out crops as headlands.

• Later, the headlands receive the highest energy waves

and so become more vulnerable to erosion than the

sheltered bay.

• The bay starts to experience low energy waves which

lead to the accumulation of sand.

COASTAL FEATURES CAUSED BY EROSION

WAVE-CUT PLATFORM & CLIFF RECESSION

• Wave-cut platforms start with the process of abrasion.

when high energy waves start to under-cut the foot of

the cliff to form a wave-cut notch.

• Continuation of this process leads the unsupported

cliff above to collapse – aka – cliff recession.

• A wave-cut platform is the gentle sloping expanse of

rock marking the foot of the retreating cliff (exposed

at low tide/ covered at high tide).

COASTAL FEATURES CAUSED BY EROSION

CAVES, ARCHES, STACKS & STUMPS

• Where cliffs are of resistant rock, wave action

attacks any lines of weakness such as joints or

faults.

• Caves are formed when the waves under-cut

part of the cliff.

• Caves may be widened and deepened until

the sea cuts through to form an arch.

• The foot of the arch will continue to be eroded

until it is too weak and collapses forming a

stack (isolation of part of the cliff).

• By time, when the stack collapses, it will

become a stump.

EROSION LANDFORMS OF CLIFFS IN THE MALTESE

ISLANDS

Blue Grotto – Malta

CominoElephant’s Head Rock – Malta

Azure Window – Gozo

WAVE TRANSPORT

LONGSHORE DRIFT

• Most often, waves are not parallel to the shore but arrive at a slight angle. Wave angles are

determined by wind direction, the configuration of the coastline, refraction of waves at headlands, etc.

• When waves break, the swash carries

material up the beach at the angle

which it had approached the beach.

• However, the backwash returns this

material straight down at right

angles to the water due to gravity.

COASTAL FEATURES CAUSED BY DEPOSITION

• When sand or shingle which are being transported along the coast by longshore drift reach a sheltered

area, they may be temporarily deposited and in turn forming a beach.

SPITS: long, narrow accumulations of sand/shingle with one end joined to the mainland and the other end

projecting out to sea or extending part way across a river estuary.

COASTAL FEATURES CAUSED BY DEPOSITION

TOMBOLOS: beaches which extend outwards to join with

an offshore island.

BARS: if a spit develops in a bay into which no major

river flows, it may be able to build across that bay, linking

the two headlands to form a bar.

COASTAL FEATURES CAUSED BY DEPOSITION

SAND DUNES: dynamic landforms which depend on the

interrelationship between mineral content (sand) and

vegetation.

• Sand dunes are mounts, hills or ridges of sand that lie

behind the part of the beach affected by tides. They are

formed over many years when windblown sand is trapped

by beach grass or other stationary objects.

• Dune grasses anchor the dunes with their roots, holding

them temporarily in place, while their leaves trap sand

promoting dune expansion.

• Without vegetation, wind and waves regularly change the

form and location of dunes.

• Dunes are not permanent structures.

COASTAL MANAGEMENT

Why do we need to manage the coasts?

1. Defence against natural events such as

flooding and erosion.

2. Human demands that include settlement,

economic activities and recreational use.

Coastal management is a means by which coastal

erosion is limited as much as possible by the use of

various measures.

COASTAL MANAGEMENT MEASURES

SEA WALLS: vertical or near vertical shore-parallel

structures designed to prevent upland erosion and

storm surge flooding.

Generally massive concrete structures emplaced along

a considerable stretch of shoreline at urban beaches.

COASTAL MANAGEMENT MEASURES

GROYNES: shore protection structures built

perpendicular to the shoreline of the coast, over the

beach to reduce longshore drift and trap sediments.

• They trap sediments from longshore drift so that the

coast behind the sand layer is protected from

erosion.

• Their effectiveness depends on their extension into

the river or sea.

COASTAL MANAGEMENT MEASURES

BOULDER BARRIER: barriers made from large

boulders which prevent the high energy waves from

eroding the coasts.

This allows for the boulders to take the hit from the

waves.

COASTAL MANAGEMENT MEASURES

BEACH NOURISHMENT: the supply of sand to the

shore to increase the recreational value and/or to

secure the beach against shore erosion by feeding

sand on the beach.