Mapping Geomorphological Environments Phần 5 ppsx

when the Aeolian energy exceeds the cohesion of the sand layer or the resistance of vegetation, large parts of the dune can possibly a beach consists of homogeneous and fine sediments, i

The development and stabilisation

of a dune depend on vegetation which entraps the sand and reduces the activity of aeolian energy After the development and stabilisation of the dune, its migration towards the land’s interior may possibly follow,

as its seaward side might be eroded depositing its material along the shoreline when the Aeolian energy exceeds the cohesion of the sand layer or the resistance of vegetation, large parts of the dune can possibly

a beach consists of homogeneous and fine sediments, it is related to

an environment of low and constant energy; however, if the material

is heterogeneous, it is related

to an environment with a highly variable dynamic The presence of roundstones possibly indicates the beach’s supply by a torrent Such material is often found in average energy environments Material like sand or clay could have originated from a deep submarine basin,

between 5-12ο, while the lee side

has a higher inclination which can

reach up to 20ο to 30ο

Regarding their shape, it also has

high variety, from very simple to

composite forms Thus, there are

crescent, longitudinal, matterhorn

dunes and also dunes which are

created by very complicated

combinations

Dunes can develop in every subaerial

environment where loose material,

of sand grain size, are exposed to the

wind’s activity and can easily migrate

and accumulate in large masses

every obstacle on the ground, such

as protrusions, ditches, the presence

of vegetation etc, contribute to this

accumulation Furthermore, the

presence of humidity may stabilize

the sand accumulation, beginning

the creation of a dune

It is a fact that the coastal dunes

cover a very small area compared

to the vast areas of dunes which are

generated in the deserts Coastal

dunes are generated where a broad

sandy coast exists; this is usually

characterised by a great tidal width

and rarely by rocky and steep coasts

Many sand dunes originate from

older geological eras when sea level

was much lower than it is today

The sand is transported towards

the land in a bouncing way, during

which the movement of grains is

accelerated after they are raised by

the wind The wind speed is slower

close to the ground and faster as

we rise, since the ground’s friction

is reduced when a cloud of sand

passes over an uneven surface, its

speed is reduced due to the loss of

energy This results to the deposition

of transported material and leads in

the creation of a new dune

Aberdeen-UK (by A Vassilopoulos, N evelpidou)

Mapping Geomorphological environments

slope, while relatively high dynamic pressures occur when waves of a vertical front plunge on the cliff with their top and their trough simultaneously

CUSPATE FORELANDS

Cuspate forelands may represent a high variety of landforms, but most, when they are fully formed, have a more or less triangular shape, with the base of their triangle on the coastline and the apex advancing towards the sea Their formation is usually related to wave refraction

on a neighbouring submarine ridge and to bilateral sediment supply

In areas with convergence of the coastal transported sediments, where the sediments are provided

by both sides of the gradually forming land extension, the created cuspate forelands have more intense seaward development

since it can be transported over

great distances by sea waves and

currents, or it could have originated

from river mouths (i.e deltas)

COASTAL SLOPE

Steep coastal slopes

which occur as a result

of marine erosion The

slopes’ resistance to

erosion is a function of

the wave energy and of

the cohesion of the

rocks which constitute the slopes

one could mention that the erosional

role of sea waves is double, as it

does not only erode the base of the

slope’s front but also removes the

weathering products from its base

when the weathering products are

accumulated at the slope’s front

base, its retreat will possibly slow

down or stop, since they protect it

from the incident waves At a slope

front two types of pressures are

exerted; the one is related to the

weight of the incident sea mass

(static pressure) and the other one

depends on the wave type, which is

a function of the waves’ dimension

and of the inclination of the slope

front (dynamic pressure) we have

the lowest dynamic pressures when

the waves are fully reflected or when

they break before they reach the

Sporades-Greece (by A Vassilopoulos,

N evelpidou) Samos-Greece (by C Centeri)

Coastal environments

FLOODED FLUVIAL

VALLEYS - RIA

BEACHES

These belong to primary coastlines,

and owe their formation to sea

impact on a landform created by

terrestrial factors They are identified

by the shallow waters of the valleys’

sunken rivermouths which cut the

land serrately, presenting a rich and

complex horizontal dissection when

not interrupted by some natural

barrier, their axes dip towards the

sea The characteristic types of Ria

coasts are the dendritic type, the

shape of which resembles an oak

leaf and is due to the fluvial erosion

of horizontal layers of homogeneous

material, and the network type

which is due to the fluvial erosion of

tilted layers of different hardness

INCLINATION OF

THE COASTLINE

The distance from the

beach to the relatively flat area

which comes after the beach front,

is a parameter which influences the

inclination of the coastline, which is

measured in degrees or % If this

distance is small, then the inclination

of the coastline is high and the beach

is qualified as steep; however, if it

is big, then the coastline is qualified

as of gentle inclination The rock

beaches are usually distinguished

by steep inclinations, while the sand

beaches by more gentle inclinations

LONG SHORE CURRENT

A very powerful

coast-al current, due to which sediment is transported along the shore

LONGSHORE BARS AND BEACH RIDGES

Landforms which look like sand rumples, but are bigger and have lower normal gradation They are usually created in shallow epicontinental environments or on the shelf borders, by the activity of waves and coastal currents, and are found individually or in groups The longshore or sand bars are classified

as longitudinal or transversal in relation to the predominant coastal current or the coastline

Longitudinal longshore bars, are found in river mouths, in funnel-shaped river bays, in straits and also in creeks, where the tidal effect

is observed As far as transversal

Aigina-Greece (by A Vassilopoulos, N evelpidou)

Kavala-Greece (by Keramoti municipality)

Mapping Geomorphological environments

Kefalonia-Greece (by K Pavlopoulos)

period or in warm and humid areas, where the development of dunes is not possible Since the sandy beach ridges are directly connected with fossil beaches, they sometimes contain significant accumulations

of heavy minerals of great financial value

MARINE TERRACES

The continuous wave activity in the coastal zone generates a typical coast profile, which consists of a sea slope and a submarine terrace The sea slope begins as a low form, whose height is increased towards the inland, while it remains as a submarine platform in its base The material which comes from the slope’s weathering is transported by the bottom currents and deposited off the edge of the rocky terrace, resulting to the creation of a terrace generated strictly by the waves’ activity Along some beaches, the coastal currents are so powerful, that the sediment coming from inland erosion is carried away, so that the only remaining landform

is the platform generated by sea erosion If the sea level remains stable for a long time or if the rise

of the sea level happens extremely slowly, the sea cliff will be quite far

ridges are concerned, the

crescent-shaped longshore bars, which

are found in river mouths and in

tidal creek channels, are typical

examples

Longshore bars are formed of sand

that moves parallel to the coastline

Particularly during low tide, they

may be uncovered and exposed

to atmospheric activity often

several longshore bars are formed,

in one or more series, which are

arrayed parallel to each other, and

at different depths in relation to

the sea surface The term beach

ridges is used to describe a series

of longitudinal and parallel ridges,

consisting mainly of sand, shells

and roundstones , varying in width

from a few centimetres to a few

meters andat intervals of 25-500 m

They are usually located behind the

contemporary beach The ones that

are found in deltaic environments,

appear concentrated on a muddy

substratum and are known by the

term cheniers every ridge indicates

the position of the paleo-coastline

Usually, the beach ridges are, as far

as their construction is concerned,

the natural evolution of some coastal

landforms, such as the longshore

bars or the cuspate forelands Many

researchers consider that most

of the beach ridges which can be

found today were formed after the

stabilisation of the sea level at the

current levels, that is in the last

4.500-6.000 years approximately,

a period which coincides with

the Flandrian transgression The

sandy beach ridges may possibly

contain also a percentage of

transferred aeolian material This

however rarely occurs in places

where successive ridges have been

developed within a very short time

Kavala-Greece (by Keramoti municipality)

Coastal environments

MARMITE

Round ditch created by the turbulent move-ment of the roundstones which are transported by waves or by turbu-lent currents

NOTCH

Formations located on rocky coasts They are located in places where the sea surface meets the land and are created due to processes of friction, solution or biological factors Since during the last years sea level is rising, their presence above sea level indicates tectonically active areas, where the land is rising Therefore,

by studying the sea fauna in these notches, we collect characteristic data for periods of constant rise , for the rising rate and for the earthquake risk of the studied area

inland, while the terrace resulting

from the wave activity will be quite

expanded If the evolution period is

shorter, this wave cut terrace will be

less wide The changes in sea water

volume are not the only causes that

contribute to coastline changes

If the sea level rises and reaches

a new level where it can remain

constant for quite a time period, a

new sea slope and a new platform

will be created Thus, every period

of sea level stabilisation is followed

by the creation of a sea slope and a

platform If the sea level is lowered,

its former levels will become

apparent through a succession of

terraces The topographically higher

terrace, having sustained weathering

and erosion for a longer time period,

tends to become indistinct and is

usually represented by an increase

of inclinations at the locations where

the older sea slopes existed The

terraces that were formed when the

sea level was on lower levels than

it is today, have been flooded when

the sea reached its current level

As the rising sea has covered these

terraces, they will have suffered

considerable erosion by waves and

will have been partially covered with

material

Samos-Greece (by C Centeri) Limbe-Cameroun (by K Pavlopoulos)Mapping Geomorphological environments

SAND BEACH

A beach which consists

of fine-grained material, the size of which varies from 50μm

up to 2mm

SEA ARCH

The sea arch is a natural opening at the front of

a coastal slope, and is created due

to marine processes of erosion Arches are developed in areas with a lithological and tectonic status which allows the creation of coastal caves by wave activity Their creation is similar to that of coastal caves Two caves that are created

on both sides of a cape may meet after a long time span, first forming

a tunnel, and finally an arch as the erosion progresses The central part of the arch’s roof, is known as the “keystone” and it supports the entire structure The architectonic structure of an arch reflects the hosting lithology The arch’s shape may be arcuate or rectangular,

RETREATING BEACH

The exact opposite of

the advancing beach;

if the beach consists of loose

sediments, the erosion factor clearly

depends on the dynamic of waves

and on their ability to transport

material During the beach’s retreat

entire zones of beach ridges or even

dunes can move As in the case of

the beach’s advance, during the

retreat, the basic formation factors

are time, energy, sediment supply,

the change of sea level and the

development of vegetation Time

guarantees a complete dynamic

counterbalance after a change in

one of the factors energy, in the

form of sea currents, is increased

during intense weather conditions

and accelerates the retreat The

decrease of sediment supply in

areas where tidal currents exist,

also leads to the aggravation of the

retreat The existence of vegetation

in the dunes decreases the erosion

rate during a temporary retreat

ROCK MUSHROOM

A landform characterised

by the attenuation of its

base due to humidity and aeolian

TOMBOLO

A Tombolo is a landform which is formed when

a cuspate foreland connects the coastline with an islet, rocky or sandy The term Tombolo initially originated from Italy and was referring to one or more sandy tongue-shaped formations which were connected to the land It is a quite usual landform along flooded coastlines that are in their youth or

at the beginning of their maturity In the areas where a double Tombolo

is formed, a lagoon between the two landforms is created, which is gradually filled with material and thus a broad, flat mound is formed Gibraltar is a typical example of a double Tombolo The world’s greater Tombolo is considered to be the one that formerly connected Ceylon (Sri Lanca) to India along the Palk Strait, which is known by the name Adams Bridge The particular landform was destroyed during a small scale change of the sea level which has taken place many thousands of years ago and which remains today

as a series of islets

LAGOON LANDFORMS LAGOON

A basin of longitudinal shape which is located

submarine or not and the height of

their opening may reach up to tens

of meters above the basic level Sea

arches are considered as ephemeral

landforms of differential erosion and

exist only for a few decades or cen

turies

STACK

Rocks of pyramidal

shape that protrude

in the sea They are created when

the slope retreats, leaving erosion

residues at the sea The sides of

stacks are generally steep and

vertical, a fact which indicates that

the erosion has taken place at wave

height and not below the sea surface

The term stack comes from the word

stakkur, in the Scandinavian dialect

of the Faeroe islands, where the

particular landforms are very often

found in front of high, rocky beaches

often, in the foreign bibliography,

the terms pillars, chimney, rock

column, skerries, needles etc., are

of sediment and water between the two environments

along the coastline, very close to

it, and is separated from the sea

by island barriers Usually it is

developed diagonally to the estuary

of one or more torrents; the calm

waters behind the island barrier

are an ideal environment for the

hydrophilic vegetation The water

and the sediments of these areas

are usually very dark coloured, even

black in some cases This is due to

the presence of much decomposing

organic material

MOUTH OR ORIFICE

A natural opening of

the lagoon towards the

sea, which facilitates the exchange

Naxos-Greece (by A Vassilopoulos, N

evelpidou)

Samos-Greece (by A Vassilopoulos, N

evelpidou)

Coastal environments

osterseen lake (Upper Bavaria-Germany) (by o Bender)

Chapter 4

lacustrine environments

prevailing climatic conditions in the area Lake water originates directly from precipitates, from water springs, from runoff water, or even from the sea

Lakes, although they are open systems regarding material and energy, they are examined and analysed as independent systems and are characterised by special physical, chemical and biological parameters linked to their degree of isolation and the geographic position

of each lake

History of the existence of lakes

All lakes have limited life duration and in general follow a disappearance course In humid areas their disappearance begins after the erosion of their barrier, the outflow of its water and the deposition of sediments and organic material on deltas or on basis deposits During their short history, their chemical composition

Lakes-Introduction

In the second half of the 20th

century, the famous Swedish

limnologist Forel has defined lake

as a mass of stagnant water located

in a trough of the ground and has

no direct contact with the sea It

can be geologically considered as a

temporary water mass, appearing

or disappearing within a short time

period

Today, lakes are located everywhere

on our planet However, they are

more frequent in higher geographic

latitudes, and in mountain areas

They are principally common

in glacial and periglacial areas,

especially where the alteration,

from glacial geoenvironments into

more warm and humid ones, was

quite recent, and also along rivers

with low inclinations and broad

valleys, where they connect to other

branches

Lake water can be either fresh or

salty This depends mainly on the

lacustrine processes

Salty lake (Marsh) at Samos Island (Greece) (by C Centeri).

Mapping Geomorphological environments

limnologists dealing with one group of lakes should consider the conditions that lead to their formation

Hutchison, taking these positions, based a classification on the origin

of lakes, which is presented below, simplified

Lakes of tectonic origin

In this case the basin may have been formed in one of the following ways:

I By gentle movements of the crust This category includes:(i) Residual sea basins which have been isolated due to continental movements e.g the Caspian Sea (ii) Lakes created because of sea level rise i.e Lake okeechobee, Florida (iii) Lakes located in areas with mild inclination that may eventually lead to the runoff inversion, e.g Lake Kioga, eastern Africa (iv) Lakes having

does not significant change In arid

areas, lakes disappear due to higher

evaporation and to deposition of

material transferred by wind and

water Due to evaporation, many

lakes in arid areas are gradually

made saline, even if the initial lake

was a fresh water lake

Classification of lakes

A lake can be formed by one or

more factors Various specialists

have classified lakes in different

ways, for example a classification

based on the conditions that may

have possibly created the basins,

and have termed them creative,

destructive or retarding other

scientists have classified lakes

on the basis that they are formed

within troughs consisting of bedrock,

in basins formed by natural or

artificial barriers, or are organic

lakes Both systems can possibly

be criticised because they exclude

natural, territorial groupings The

A group of lakes which originates from ice melting These lakes are characterised

by an extraordinary diversity of hydrological and chemical properties osterseen (Upper Bavaria, Germany) (by o Bender).

Lacustrine environments