Coastal physical processes md. yousuf gazi

Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
COASTAL PROCESSES

▪ The offshore
▪ Nearshore
▪ Foreshore
▪ Backshore
▪ Beach
Coastal profile

COASTAL PROCESSES
▪ Coastal processes are the set of mechanisms that operate along a coastline, bringing about various
combinations of erosion and deposition.
▪ The energy due to tide, waves, wind and currents is constantly working in the coastal zone.
Dissipation of energy (due to tide, wind, waves and current) is often provided by the beaches,
mudflats, marshes and mangroves.
 Sediment transport
 Currents
 Denudation
 Deposition
 Erosion
 Flooding
 Diffraction
 Refraction

▪ Changes in water level
▪ Tides
▪ Waves
▪ Currents
▪ Stream outflow
❑ Processes responsible for initiating sediment Transportation:
 Sediment transport

❑ Another nearshore dynamical system is littoral transport - the movement of sediments
in the nearshore zone by waves and currents.
❑ Littoral transport is divided into two general classes:
▪ Longshore transport: Transport parallel to the shore.
▪ Onshore-offshore transport: Transport perpendicular to the shore.

 It is the movement of sediments most often sand, along a coast parallel to its shoreline - a zig-zag
movement of sediment along the beach .
 The largest beach sediment is found where the process begins, updrift, and the smallest, most
easily moved, downdrift.
Longshore drift:

 Where waves are strong, the coast will be eroded and sediment carried away and where they are
weak sediment will be deposited.
 Longshore drift can have undesirable effects for humans, such as beach erosion. To prevent this, sea
defences such as groynes or gabions may be employed.

Swash and Backwash
▪ Swash – the rush of water up a beach after a wave breaks (powerful)
▪ Backwash – the returning water to the sea (less powerful)
▪ Constructive Waves – Break gently, far from shore and swash pushes material into shore, very
weak backwash
▪ Destructive Waves – Break very close to shore, very powerful backwash

Modes of Sediment transport:
Sediments are transported as:
 Bed load transport
The bed load is the part of the total load that
is more or less in contact with the bed
during the transport. It primarily includes
grains that roll, slide or bounce along the
bed.

➢ Suspended load transport:
It is the part of the total load that is moving in suspension without continuous contact with the bed.
▪ The determination of the rate of suspended load transport is straightforward by comparison with
measurement of the rate of bedload transport.
➢ Wash load transport:
▪ It consists of very fine particles transported in water.

Modes of Sediment transport

❑ Coastal Processes:
▪ Accretion - natural or artificial deposition of sediment in a particular location
▪ Beach nourishment - the restoration of a beach by the mechanical placement of sand on the beach
for recreational and/ or shore protection purposes.
▪ Cross shore transport - the displacement of sediment perpendicular to the shore
▪ Erosion - the removal of sediment from a particular location by the action of wind or water.

Accretion
Beach nourishment

Cross shore transport Erosion

▪ Groin - Shore perpendicular structure, installed singly or as a field of groins, designed to trap sand from
the littoral drift system .
Groin

▪ Longshore Bar - offshore ridge or mound of sand, gravel or other loose material running parallel to the
Shore which is submerged (at least at high tide) and located a short distance from the shore.

▪ Tombolo - the combination of an offshore rock or island which is connected to the beach by a sand spit.

▪ Sand spit - low tongue of land or a relatively long, narrow shoal extending from the land.

▪ Gabions- Boulders and rocks are wired into mesh cages and usually placed in front of areas vulnerable
to heavy to moderate erosion.
▪ Onshore transport - movement of sediment or water toward the shore.

❑ Processes of Erosion:
▪ Hydraulic action – power of water (waves)
▪ Abrasion – Rocks/sand thrown against the coast
▪ Compressed Air – trapped in small crevices in the rock
by waves, this causes mini “explosions” which eventually
shatter the rock
▪ Attrition – Rocks wearing off each other
▪ Solution – Salts in seawater dissolving rock (sedimentary
rock)
▪ Wave refraction – waves bending on contact with the
coast, concentrating it’s effects in certain areas

Wave refraction

Landforms of Erosion
▪ Cliffs and Wave Cut Platforms (Cliffs of Moher)
▪ Bays and Headlands (Dublin Bay, Bray Head)
▪ Sea Caves (Cliffs of Moher)
▪ Sea Arches (Old Head of Kinsale)
▪ Sea Stacks (Dun Briste, Co. Mayo)
▪ Sea Stumps (Cliffs of Moher)

▪ Cliffs and Wave Cut Platforms (Cliffs of Moher) ▪ Bays and Headlands (Dublin Bay, Bray Head)

▪ Sea Caves (Cliffs of Moher) ▪ Sea Arches (Old Head of Kinsale)

▪ Sea Stumps (Cliffs of Moher)▪ Sea Stacks (Dun Briste, Co. Mayo)

❑ Process of deposition:
This requires three basic conditions:
▪ Gently sloping beaches to reduce wave energy
▪ Shelter from strong winds/currents (e.g. in a Bay)
▪ Constructive waves
For Transport, the main process is Longshore Drift, also known as Littoral drift

Coastal Deposition:
▪ Whatever gets eroded from one area on the coast eventually gets deposited in another area – nature
always balances the books.
▪ The rate of erosion also affects the rate of deposition – an area with mostly hard rock (e.g. Granite)
will erode slowly and thus have little material for deposition.
▪ Coastal deposition features (e.g. beaches) often have important economic benefits such as tourism

Beaches:
▪ Beaches are not only formed from sand – mud, shingle and stones can also form beaches – however
these beaches are unattractive for tourism.
For a beach to form you need-
• A sheltered area to trap sediment
• Longshore drift to transport material to the beach
• Constructive waves to deposit beach material between the high and low tide marks

Muddy Beach Shingle Beach

Coastal physical processes md. yousuf gazi

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