DYNAMIC OF COASTAL SYSTEM

1. Basic Concept

What is Dynamic of Coastal System

Dynamics of Coastal Systems is about the dynamic interaction between
water motion and seabed topography, which affects the natural
response of coastal systems to change in external conditions and to
human interventions — from the scale of sea bed ripples up to the scale
of entire barrier and delta systems. The book highlights major concepts
developed during the past 50 years for the description of current-
topography, tide-topography and wave-topography interactions. It
provides simple analytical tools and models for diagnosing and
predicting coastal response to change, with references to a great
variety of coastal systems around the world. These concepts and tools
are crucial for sustainable management of beaches, deltas and coastal
wetlands. Coastal areas are an interface between the lithosphere and
hydrosphere. Coastal ecosystems are good habitats for innumerable
amount of flora and fauna. Coastal areas are unique and typical land
masses bordered by the seas and oceans.

2 . Coastal zones

Coastal zones are very sensitive zones. The oceans have a great impact on the

Earth and its climate. Coastal zones are not static but dynamic environments. They involve

transformation of mass and energy through waves and currents. Shorelines are always

subjected to both continental and oceanic processes. Waves, tides and currents are very

powerful geomorphic agents. The erosional and depositional work of the sea waves can

create many spectacular landscapes along the borders of the continents. Studying the

coastal landforms are interesting aspects in geomorphology

3. Coastal Morphodynamic

Coastal morphodynamics is the study of coastal geomorphology and its change

under the influence of waves, tides, etc. It is important to understand coastal

morphodynamics as natural coastal geomorphology such as beaches provides

substantial protection and this needs to be monitored and evaluated.

Further, there is increasing interest in soft as opposed to hard approaches to coastal

engineering that are inspired by natural geomorphic systems.

This module introduces the students to:

 The concepts of coastal morphodynamics in a range of settings and regimes,

with an emphasis on beaches;

 Methods for analysing and predicting coastal change; and

 ‘Soft’ approaches to coastal engineering and management.

This includes consideration of basic sediment and morphological theory, analytical

and numerical models, and relevant contemporary issues, such as shoreline

management planning and impacts and responses to sea-level rise.

Having successfully completed this module, you will be able to demonstrate knowledge

and understanding of:

 Key concepts in coastal morphodynamics for sandy and muddy coasts, cliffs

and shingle beaches

 Standard numerical models for longshore and cross-shore evolution and

sediment budget methods

 The morphodynamics of soft coastal engineering solutions

 COASTAL CURRENTS

1. Basic Concept

What is Coastal Currents?

Coastal currents are intricately tied to winds, waves, and

land formations. Winds that blow along the shoreline—
longshore winds—affect waves and, therefore, currents.
Before one can understand any type of surface current,
one must understand how wind and waves operate.

2. Coastal Currents

Wave

Wave height is affected by wind speed, wind duration (or how long the wind

blows), and fetch, which is the distance over water that the wind blows in a single

direction. If wind speed is slow, only small waves result, regardless of wind

duration or fetch. If the wind speed is great but it only blows for a few minutes, no

large waves will result even if the wind speed is strong and fetch is unlimited.

Also, if strong winds blow for a long period of time but over a short fetch, no large

waves form. Large waves occur only when all three factors combine (Duxbury, et

al, 2002.)

As wind-driven waves approach the shore, friction between the sea floor and the

water causes the water to form increasingly steep angles. Waves that become

too steep and unstable are termed “breakers” or “breaking waves.”

The highest surface part of a wave is called the crest, and the lowest part is
the trough. The vertical distance between the crest and the trough is the
wave height. The horizontal distance between two adjacent crests or troughs
is known as the wavelength.

Longshore Currents

The speed at which waves approach the shore depends on sea floor and

shoreline features and the depth of the water. As a wave moves toward the

beach, different segments of the wave encounter the beach before others, which

slows these segments down. As a result, the wave tends to bend and conform to

the general shape of the coastline. Also, waves do not typically reach the beach

perfectly parallel to the shoreline. Rather, they arrive at a slight angle, called the

“angle of wave approach.”

 When a wave reaches a beach or coastline, it releases a burst of energy that

generates a current, which runs parallel to the shoreline. This type of current is

called a “longshore current.”

Longshore drift can be very destructive to manmade structures.

3. Features of Longshore Drift

A result of longshore drift and longshore currents sediment is trandsferred

alomg the coast floowing the direction of the current. This leads to the development of

features shown in the diagram below.

 Features of a drift aligned depositional beaches

Spits are long narrow ridges of sand and shingle which project from the

coastline into the sea. The formation of a spit begins due to a change in the direction of

the coastline, where a low energy zone is found. This can also be at the mouth of the

estuary. The main source of material building up a spit is from long shore drift and

current, which brings material from further down the coast.

Where there is a break in the coastline and a slight drop in energy, long shore

drift will deposit material at a faster rate than it can be removed and gradually a ridge is

built up, projecting outwards into the sea - this continues to grow by the process of long

shore drift and the deposition of material. A change in prevailing wind direction often

causes the end of spits to become hooked (also known as a recurved lateral). On the

spit itself, sand dunes often form and salt-loving vegetation colonises. Water becomes

trapped behind the spit, creating a low energy zone, as the water begins to stagnate,

mud and marshland often begins to colonise behind the spit; spits may continue to

grow until deposition can no longer occur, for example due to increased depth, or the
spit begins to cross the mouth of a river and the water removes the material faster than

it can deposited - preventing further build up. These marshland are called salt marshes

Bars forms in a similar way to spits, as longshore drift transports sediment and shingle

down the beach it deposits it low energy zones, such as bays. At a bay the bar, if

continiued to be fed by sediment will extend across the bay cutting off a lagoon behind.

In some area, bars extend to join the mainland to an island. This forms a sediment ridge

called a tombolo;tombolos. Cuspate forelands can be described as triangular

beaches. They form due to logshore drift meoveing sediment in opposing directions.

The two sets of storm waves build up a series of ridges, each protecting the material

behind it, creating the triangular feature. Cuspate forelands form due to the positioning

of the coast and theirorientation to incoming tides and prevailing winds.

Features of longshore drift SPIT
Features of longshore drift BARS
Features of longshore drift TOMBOLOS
4. EROSIONAL LANDFORMS

Landforms of coastal regions are classified into two major

groups as erosional landforms and depositional landforms.

The notable erosional landforms of the coasts are:

a) Sea cliffs
b) Sea caves
c) Sea Arches
d) Sea stacks
e) Wave-cut notches
f) Wave-built terraces.

The most widespread landforms of erosional coasts are sea cliffs.

Wave erosion undercuts steep shorelines creating coastal cliffs.

A sea cliff is a vertical precipice created by waves crashing directly on a

steeply inclined slope. These very steep to vertical bedrock cliffs range
from only a few metres high to hundreds of metres above sea level.

Their vertical nature is the result of wave-induced erosion near sea

level and the subsequent collapse of rocks at higher elevation.

Hydraulic action, abrasion, and chemical solution all work to cut a notch at
the high water level near the base of the cliff.

Constant undercutting and erosion causes the cliffs to retreat landward.

Sea caves form along lines of weakness in cohesive but well-jointed
bedrock. Sea caves are prominent headlands where wave refraction
attacks the shore.

A sea arch forms when sea caves merge from opposite sides of a
headland. If the arch collapses, a pillar of rock remains behind as a sea
stack.
Seaward of the retreating cliffs, wave erosion forms a broad erosional
platform called a wave-cut bench or wave-cut platform. After the constant
grinding and battering, eroded material is transported to adjacent bays to
become beaches or seaward coming to rest as a wave-built terrace.

5. DEPOSITIONAL LANDFORMS

Eroded sediments along the coasts are transported as drifts.

Longshore Drift is one important mechanism. Longshore Drift are powerful

geomorphic agents.

They can erode, transport and deposit coastal sediments.

Longshore drift erodes and deposits sand masses continuously along the
beach.

The sand that is removed from one point along the beach is replaced by
sand eroded from some other zones.

Longshore drift consists of the transportation of sediments like clay, silt,

sand and shingle. The drift happens along a coast at an angle to the
shoreline. This is mainly dependent on the prevailing wind direction, swash
and backwash.

This process occurs in the littoral zone, and in or close to the surf zone.
The process is also known as longshore transport or littoral drift.

Littoral transport is the term used for the transport of non-cohesive

sediments, i.e. mainly sand, along the foreshore and the shoreface due
to the action of the breaking waves and the longshore current.

Tides:

Tides are routine coastal processes.

Nearly all marine coastlines experience the rhythmic rise and fall of sea
level called tides. The daily oscillation in ocean level is a product of the
gravitational attraction of the Moon and Sun on Earth's oceans and it
varies in degree worldwide. Tidal action is an important force behind
coastal erosion and deposition as the shoreline migrates landward and
seaward.

Tidal Currents are responsible for mechanical sorting of sediments under

the water. During a high tide water moves landward as a flood current.
During low tide water recedes seaward as an ebb current.

The notable depositional coastal landforms are:

a) Beaches
 b) Spits and bars
c) Tombolo
d) Barrier islands
e) Mud Flats.

Beaches:

A beach is an area of sediment accumulation. They are exposed to wave

action along the coast.

Beaches morphology changes from season to season.

There are two basic beach types.

One is called as dissipative beach and the other one is called as reflective
beach.
Together with the intermediate types, there are six major microtidal beach
types.
The reflective beach occurs when conditions are calm and/or the sediment
is coarse.

There is no surf zone. The waves flow upon the reflective beaches. It
reflects a major part of the incoming wave.

When bigger waves cut back a beach and spread out its sediments to form
a surf zone, the reflective beaches create a series of intermediate types.

When wave action is very strong and/or sediment particle size is fine, the
dissipative beach type is created.
This type has a flat and maximally eroded beach.

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The sediments are stored in a broad surf zone that may have multiple
sandbanks parallel to the beach.

The intermediate types are characterized by high temporal variability, sand

storage both on the beach and in the surf zone and sandbanks and
troughs.

Beaches are classified into three categories as high, low and moderate
energy beaches.

Normally, high energy conditions prevail during summer months. The wave
heights are normally expected to increase after the onset of monsoons.
These produce significant changes in the beach morphology.

Spits and bars

A sand spit is one of the most common coastal landforms. A sand spit is
a linear accumulation of sediment that is attached to land at one end.
Sand carried parallel to shore by longshore drift may eventually extend
across a bay or between headlands especially where water is relatively
calm. Spits are typically elongated, narrow features built to several
meters high above sea level by the action of wind and waves.

Spits often form when wave energy decreases as a result of wave

refraction in a bay. When the wave energy is dissipated, it will cause the
sediment to accumulate, due to the loss of ability to transport the
sediments by water.

The term bar refers to a long narrow sand embankment formed by wave
action. Littoral drift from an island may form a tombolo, which is a sand
bar connecting the island with the mainland.

Spits can extend across the mouth of a bay, but wave action is usually
strong enough to wash sand out to sea or be deposited in the
embayment. They may curve into the bay or stretch across connecting to
the other side as a baymouth bar. When the bay is closed off by a bar it
becomes a lagoon.
Simple spits consist of narrow finger of sand with a single dune
ridge that elongates in the downdrift direction.
Double spits can form if drift transports sand in two directions across and
inlet, or if a baymouth barrier is cut by a tidal channel. Wave refraction at
the end of a spit will transport sand to form a recurved spit.

Complex spits form when a plentiful supply of sediment is transported by

both the ocean and bay currents. Multiple lines of dunes can be formed
by wind transport of sand across the spit.

Tombolo:

A tombolo is a depositional landform in which an island is attached to the

mainland by a narrow piece of land such as a spit or bar. Tombolos are
formed by wave refraction.

Coastlines paralleled by offshore narrow strips of sand dunes, salt

marshes and beaches are known as barrier islands. A variety of barrier-
related features could be seen along the shoreline.

Barrier spits and islands:

Barriers that connect headlands together along the outer reaches of

an embayment are called baymouth barriers.

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Barrier spits are beaches that are attached at one end to their source of
sediment.

Capes are barrier islands that project into the open sea to form a right
angle shoreline. These are generally large features that are exposed to
wave attack on each side, but one side is accreting while the other is
eroding. This produces a distinctive series of truncated dune ridges.

Mud flats:

Mud flats are formed due to the action of tidal currents. They contain
silt and clays. They are exposed during low tides and are covered
during high tides. In some of the exposed mud flats, after a full retreat
of a sea level, plants grow in these mudflats forming salt marshes.

In addition to these some other features are also located in the coastal
areas:
Estuaries :

An estuary is a coastal wetland where freshwater from runoff of a river

and saltwater from tides of the seas and oceans mix together. Most of
the large rivers in the world do not empty their waters abruptly into the
seas. They merge with the sea in a transitional basin-like area near
their mouths called as an estuary.
 ACARA I

A. FEATURES OF LONGSHORE DRIFT

Focus : Coastal Features

Longshore drift and Longshore current

Spit, Bar, Tombolo

Focus Questions :
 How to know features of longshore drift

 How to know Formation of Spit, Bar and Tombolo

Focus Objectives :
 Students to describe and explain coastal features.

 Student will know Formation of Spit, Bar and Tombolo

MaterialS :
 Hardware ( Laptop,)

 Software

Activity Students :

1. Learning activity for what students know about Sonar Video..

2. Show this video: https://www.youtube.com/watch?v=wTcaFYeUR10
SONAR | Class 9th Science Video Lecture

3. Show this video https://www.youtube.com/watch?v=ZTEjt_m2RLc

How Sonar Works

4. https://www.youtube.com/watch?v=_ww2PflbWD8

Mapping the sea floor with the multibeam echosounder and the
Moving Vessel Profiler

5. This is a very short, silent and voice video that shows sonar mapping
of the oceans.

6. Explain the simple ocean floor profile below and label. Have students
draw and label along with you.