So friends welcome to this lecture series
of geomorphology and today we will discuss about this Dune Classification. So if we recollect the last class when we
are talking about this formation of dune or in Aeolian environment we found that the dunes
can form depends upon the wind speed, depending upon the sand supply and depending upon the
vegetation cover. Second thing that to form a dune we must have
a sand body more than this critical size. That is to 4 to 6 meter spread and sand supply
can be continuous so that a dune can form and to form a dune we need more wind speed
as compared to formation of a ripple. So as the wind speed increases the whole upper
layer of the sand that means this saltating layers and some parts of the upper layer they
try to make a shearing action with the sand body. As a result, the whole upper layer moves. And at the windward side we will find a gentle
slope and where this upper side or this lee side we will get a steep slope where this
brink point it is called brink point where this sand slides down and forming the dune
face and it is stepper in nature. So this way dunes are formed. Though dunes migrate from one place to another
place, but its internal structure remain constant. So the whole dune can migrate dune may deform
so it may divide into separate parts and a size of the dune may changes, the shape may
changes, but its internal structure remains as it is. So this is all about this characteristics
of dune. So in this class we are going to discuss about
the dune classification, how the dune look like and how this dunes they form from one
dune to another dune and how this wind velocity and directions, sand supply and vegetation. They affect the size of the dune, they affect
the type of dune involved and what the dune can say about the past climatic changes. So first of all dune classification we will
deal with the star dunes. So here if you see this 2 photographs this
surface appearance of this 2 photographs it looks like a star that means it is radiating
all sides from a center point and the radiating ridge it is sharp ridge and a long ridge which
is indicating the wind direction. So that means if you see there are more than
that means there are many ridges, sharp and long ridges they are merging at a point or
radiating at a point that means it is indicating a variable windblown direction. From different sides the wind is blowing and
merging here and finally it is forming a star dune. So as per this dune migration is concerned
this type of dune they do not migrate or it is very little migration occurs instead it
increases its size vertically. So with more and more sand addition with more
sand supply and this wind velocity or wind speed increase of wind speed its size or its
height wise it increases in size. So star dunes maybe of several hundred meters
in height and several kilometers in diameter with radiating sinuous ridge culminating at
the common crest. So this is a dune having this radiating system,
this is dune having this radiating system. So here it is culminating and the wind direction
will be from all sides, all side wind merge at this point and forming the star dune. They accumulate under affective wind that
blow from several different directions. At various seasons, they may show slip faces
of various sides of their multiple radiating arms. So this is important to note here that so
though we have a star dune, but at various seasons they may show slip face of various
sides. For example, suppose this is the star dune
and this is a arm and in this particular case suppose this is the slip face similarly suppose
this is the slip face, but that does not mean the slip face will same for the whole season. So in different seasons different windblown
direction changes that is why the slip face changes its direction. They seem to grow in height rather than migrates
as though they are at some focus of depositional wind. So that means more and more sand supply, more
and more increase of wind speed, they try to grow in height rather than migrating. Then second type of classification or second
class of the dune is the longitudinal dune. Longitudinal dune that means as the name says
it elongated in nature and elongation the direction of elongation is parallel to this
windblown direction. So it may be huge landform extending hundreds
of kilometers in a length and kilometer are more in width and several hundreds of meter
in height. So now you see it is extending kilometers
several hundreds of kilometers even if the entire Sahara entire Kalahari can extend one
dune can extend all through its lengths. So they are especially well developed in the
heart of the trade wind deserts where wind is either form a constant direction or varies
seasonally that means we want to say here the wind should blow in a particular direction
for a long time. So that is why if you see here these dunes
mostly they are confined in the trade wind deserts. So parallel to this trade wind the dunes are
aligned. Longitudinal dunes covers approximately 30%
of the total area of the Aeolian deposition. So that total area is covered 30% that means
all through this length of this deserts this type of dune can continue. As many of them are now relict have led to
much speculation about their origin. So now you see whatever these dune we are
looking at nowadays that is the longitudinal dunes nowadays very rarely we are getting
the fresh longitudinal dune that means at the present geological condition at present
atmospheric circulation do not allow this formation of this longitudinal dunes. So whatever the dunes nowadays we are studying
and getting this characteristics of this and that. So this is all those characteristics are based
on the study of the ancient dunes. This gentlemen Bagnold in 1941 suggested that
they were built by seasonal shifting of winds that preferentially elongated one arm of a
large barchan dune until it becomes wavy, elongated, longitudinal dunes. So barchan dune what is barchans dune we will
talk about after few minutes, but here if you see this gentlemen is a pioneer worker
in this arid region geomorphology. He found that these elongations it is a part
of the large dune that is barchans and this 2 arms of the barchans they are elongated
in a particular direction and the wind direction, but later on this was modified to certain
extent. Livingstone 1986 and Tsoar in 1989 demonstrated
that longitudinal dunes extend parallel to the dominant wind direction, but that other
effective wind at an angle to the dominant flow. Change direction as they cross the dune crest
to blow nearly parallel to the dune crest. So their observation is that longitudinal
dunes they are forming parallel to the wind direction and if from other directions the
wind is hitting this dune. So after crossing the dune face it becomes
parallel to this elongation direction. So finally more or less we can say this longitudinal
dunes are nothing. They are this large long dunes, they are formed
during this consistency of windblown direction for a particular time for more than that means
for a long time and they are parallel to this windblown directions. Longitudinal dunes are found in area of nearly
devoid of vegetation because you see when you are talking a longitudinal dune of hundreds
of thousands of kilometers so that means up to that kilometer, up to that distance if
we want to distribute this sand in a linear fashion that means that should devoid of any
obstacle. So that means those area having vegetations
they will not allow to form such undisturbed dune for a long distance. So that is why this nearly that means areas
nearly vegetation that is lack of any vegetation covers they are more suitable for the formation
of this longitudinal dunes where winds are persistent for many months that means a long
duration of the wind blow. For a persistence the consistent direction
will be there for long duration the wind will continue for a particular direction and sand
supply is irregular. That means that 3 classification, 3 criteria
which are mainly responsible for the dune formation applies here that means it is nearly
devoid of vegetation that vegetation should be not there and persistent wind for many
months then sand supply is irregular if this conditions satisfy then we will get a longitudinal
dune of hundreds of kilometers length and kilometers of wide. The dune ridges are separated by gravel armored
reg or hammada we know that the gravel dominated desert is called reg and the sand dominant
desert is called ergs. So here this longitudinal dunes, this elongated
sand body they are separated from each other by hammada. Hammada means barren rock and by the reg,
reg means this boulders. So that means these are isolated one elongated,
isolated. If you see this figure this is an longitudinal
dune. This is a longitudinal dune and it is separate
this side this should be hammada either it is hammada or reg will be there either hammada
or reg will be there. So that means I want to say these are isolated
dunes for long distance of kilometers length, hundreds of kilometer length or kilometers
of width form in the area constituting persistent wind direction, irregular sand supply, less
vegetation cover. So this is the conclusion about this. Then another class of this dune is called
transverse dunes. Here transverse means it is perpendicular
to this windblown direction. So that means in the longitudinal case these
dunes were formed parallel to the windblown direction, but here this is classified this
dunes they form perpendicular to the windblown direction. For example, if you see in this 2 figures. Here you see this is the dune axis, this is
dune axis these are dune axis. And you see this is the windblown direction
that means the sand waves, the dunes they form perpendicular to the windblown direction
similarly it is seen also here. They are associated with massive quantities
of sand and relatively ineffective wind. They are common along coastlines and on zones
of erodible sandy bedrock or alluvium. So that means here you see these dunes they
are different from the longitudinal dunes based on this windblown direction. Based on the sand supply. Here if you see we are getting massive quantities
of sand, but here in the longitudinal dunes irregular sand supply. Wind direction is parallel, wind direction
is perpendicular and they are forming in this area lack of vegetation, but here they are
forming in this area having vegetation likely the coastal plain, coastal plains will be
having vegetations. So that means we will have coastal plain or
coast line and those zones of erodible sandy bedrock are alluvium that means those alluviums
which are reworked by this wind this glacial outwash plains this other this ephemeral streams
of a braided nature. So those areas this alluvial sediments they
were reworked by wind action and form this ridges, form this alluvium ridges, form this
windblown ridges or the dunes. So those are mostly the transverse in nature. So these are responsible for massive sand
supply. Their crest are ridges roughly perpendicular
to the effective wind and sinuous or fish scale outlines or breakup into fields of isolated
barchans or longitudinal dunes. It is very interesting to discuss here. You see once we are getting a transverse dune
here is the windblown direction, but this transverse dune may break into different types
of dune. For example if you see here this side if I
elongate it, this may elongate in this way, this horn may elongate in this way. So that means it will create a barchan or
this will be finally it is converted to a longitudinal dune, it will convert to a longitudinal
dune. So that is why it is breakup into fields of
isolated barchan or longitudinal dunes that can form. The barchanoid ridge is a common transitional
type between this transverse dune and barchan. Barchan dunes is the another type of the transverse
dune and it is the most prominent and most popular one also. Barchan dunes are classic Aeolian landforms. Here you see this most popular dune whenever
we talk about or think about this Aeolian system and dune formation always a picture
comes in our mind that dunes would be like this, this. So in that case this barchan comes in our
mind. So this is the most popular type of dune. If you see here their crescentic form consists
of gently inclined windward slope and a steep lee side around which the horns or cusps of
the dune project downwind, making this slip face concave to the downwind direction. If you see here in this figure this is the
upwind direction and this is downwind direction the wind is blowing here. And here this is gently sloping body and this
side is the steep sloping one and this is one horn, this is another horn. Similarly, here you see this is gentle slope
and it is steep slope and it is lee side and this is stoss side. So here you see this is the windblown direction. This is called horn and this side there will
be free flown and finally we will get some loose sand and this sand relatively compact
due to this shearing motion. And finally this making a slip face concave
to the downwind direction. If you see the slip face it is concave and
this concavity it is towards the downwind direction. So this is a peculiar kind of dune and most
popular dune in the Aeolian system. Barchans are isolated dunes that migrate freely
across rock or gravel desert plains usually downwind from some other dune forms. So that means the barchans can be divided
into different other type of dunes. So based on this wind based on the sand supply
or so. For example, you see if this is the windblown
direction and we are creating a barchan here. This is the windblown direction and creating
a barchan here. With time you see one limb or one horn is
elongated and finally it may convert to a longitudinal dunes also. Similarly, here it may convert to a longitudinal
dune. Here you see there are several subdivisions
are occurring here. So that means barchans are isolated dune they
migrate freely across the rock or gravel desert plain usually downwind form some other dune
form. So that means they in the downwind they can
be divided into different types of other types of dunes. Although they have a nearly constant sand
mass, they are large enough to maintain themselves as they migrate and represent a remarkable
balance between accumulation, transportation and erosion. Now see here we have we are migrating the
system. So this is the downwind direction and from
dune here to here, here to here that means though we are migrating very less loss of
its mass. So that means from here to here the total
mass near total mass is transported from here transported again here transported. So that means they migrate and represent a
remarkable balance between accumulation, transportation and erosion. So that means the whole system, the whole
mass remains near about constant during its migrations. Then comes to another type of dune that is
called parabolic dune. So in the other types of dune where we are
talking about this longitudinal dunes, we have transverse dune this barchans or transverse
barchan is part of this transverse dune. So in that case a barchan in this transverse
dune we were hindering to certain extent by this vegetation matter or of some obstacles. And here this parabolic dune it is another
example where some of this vegetal material are there, some of the obstacles are there
which are resisting the sand to move completely and forming a parabolic dune and irregular
blowout dune it has to be noticed here this is blowout dune. Blowout means if you remember our earlier
classes when we are talking about the wind erosion. There is a terminology blowout we are using
that means removing the material, removing the material from one place to another place
and this is blowout we say when this deflation the deflation hollow is up to this water table
or up to this root zone of this plant that is called blowout. So that means this blowout dune itself says
the material which is involved in the formation of the dune is coming from this blowout that
is from removal, from the deflation. So this material is deposited here and it
removed from here so this is called blowout dune or it is the parabolic dune. So this parabolic term it is used for its
size, for its shape that means by shape if you are looking from the top surface its shape
itself indicate it is looking like a parabola. So that is why it is called parabolic dune. There characteristics of partially stabilized
sandy terranes that develops blowout partially stabilized. Partially stabilized that means sand dune
stabilization it is a environmental problem or the sustainable development when we are
talking about the geomorphology in sustainable development. So in that case we have to stabilize the dune
otherwise it will migrate, it will enclose the locality, it will enclose our day-to-day
activities. So that is why we have to stabilize it. So there are many process of stabilization
either it is and the most prominent process is by plantation. So that means either naturally or artificially. If some vegetative material are allowed to
grow on this dune surface that means it will hinder the wind to blowout this material freely
that is why this wind or the dune will be stabilized. So this parabolic dune they are characteristics
features of this area where we have partially stabilized the Aeolian system. That means there characteristics partially
stabilized sandy terranes that develops blowout. The crescentic shape of a parabolic dune has
a very different orientation from that of the barchan dunes likely to confuse for the
beginners. Here the difference is that in this, this
is a parabolic dune, this is a parabolic dune. Here the wind direction is from this direction
to this. You see this is the wind direction and your
parabolic dune is looking like this. But had it been a barchan this if this is
the wind direction the barchan will be look like this. So that means it is opposite directions and
this opposite directions it distinguishes itself from the barchan, but at the beginners
we are working very recently we have started working they may not properly distinguish
it so should not be there confusion that this is the barchan and this is the parabolic dune. On the lee side of an elongated blowout a
dune may accumulate it windward slope rising from blowout and slip face convex towards
the plan view. If you see here we have this is the region
where blowout occurs and finally the sand is moving and creating a parabolic dune. Here this is the steep slope where slip face
occurs this is the steep slope and this side is the gentle slope. And this blowout material due to this presence
of vegetation this is not able to move completely and they are arranged according to this orientation
of this vegetation and this is the elongated side, this is the elongated branch of this
dune and it is forming a parabolic shape. A parabolic dune may becomes elongated to
form a hairpin dune if you see here this is a parabolic dune and wind blowing in this
particular directions. And gradually this arms of this parabolic
dunes getting longer and longer and this side will be the steep slope face and it is looking
like a hairpin. So that is why it is called also hairpin dune. So the parabolic dune may becomes elongated
like a hairpin by downwind migration of blowout until it is split into 2 minor longitudinal
ridges parallel to the long axis of the blowout. Now you see here if I allowing again and again
wind to blow finally this will be elongate this will grow its elongation this will grow
its elongation and this sand will be again divided up to this and from this and finally
this will appear like this that means 2 longitudinal dune will form. So that means I want to say either it is barchan
it is of transverse dune, it is of parabolic dune with time and with more and more windblown
direction for duration of wind blow that will convert it to different types of dunes. These dunes are usually much longer and narrower
than barchan dunes and are always associated with central blowout. This is the difference between this two. If you see here this is barchan and this is
parabolic dune. Here it is always associated with blowout,
but here we may not need a blowout to form a dune, but as it is blowout is associated
with and vegetation is associated with. So this is creating a dune of elongated nature. However, it is creating a dune of more crescentic
nature. Sand dune on coasts except in desert commonly
are parabolic and blowout dunes. So desert we do not have vegetations or much
vegetation. So sand dunes on coast except in deserts,
desert is the exception commonly are parabolic and blowout dunes because in coast we have
vegetations because even though this shorezone may provide a copious supply of sand and onshore
wind maybe an effective transport medium, but the water table likely plays a major role
here. So the water table once it reaches up to the
water table or there will be capillary water zone. So that means it will provide a strength to
sand to be intact in this position. So it will prevent the sand particles to move. That is why the water table likely to be near
to the surface which limits the depth of which deflation can work. So that is why either it is and second thing
that once the water table is near to the surface it will promote the vegetation growth. So that is why there will be blowout. And that is why this sand dunes of coast they
are commonly of parabolic and blowout dunes, but in arid regions total complete arid regions
like the desert so there are less chance of getting this vegetation cover I think this
water table near to the surface. So that is why mostly this parabolic dunes
or the blowout dunes they are confined in the coastal regions. Further coastal vegetation quickly colonize
and stabilize dunes as they migrate inland. This is also important topic here or important
points here. One is our water table another is the vegetation. So both plays important roles equally important
role for blowout to occur and once blowout is there that means we are going to create
parabolic dune. This two gentlemen 1993 defined dry, wet and
stabilized Aeolian system which are distinguished by relative role of the water table and surface
stabilizing factors such as vegetation as the controlling sand movement. One is dry dune that means here free movement. If you see this photographs frequent change
in the dune position, frequent change in sand supply it is dry than wet if you see this
is the playas near to this playas we have wet conditions. So that means relative stabilized movement
is there and this is totally stabilized that means the whole system is stabilized here. So this total the sand surface it is covered
by vegetations so this is dry dune, this is wet dune, this is stabilized dune. Coastal sand dunes commonly include a large
portion of the biogenetic calcareous sand and relatively cement as eolianite. So here if you see in the coastal region if
you move you will such type of porous very porous and permeable materials are there then
there consolidated to semiconsolidated that is called eolianite and in the eolianite the
most abundant constituent is the calcareous material which biogenic origin. Because we have organism, we have remnants
of organism near to the coast either it is plant material or it is maybe animal material. So this whole system is consolidated together
and somewhat odd smell you will feel it there. So this is the eolianite. And this is Whalebacks and Zibar dunes where
longitudinal dunes have migrated downwind. The coarser sand is left behind as a prominent
low rolling ridge and coarse sand lacking slip face. This whalebacks or zibar dunes attached to
stability of longitudinal dune in form of facing over long period. So these are the huge dunes whaleback dune
it is the huge dunes are there and it is showing the rolling topography if you see here very
gentle rolling topography is there. This is one type of dune. So either by large longitudinal dunes migrated
a long distance downwind or longitudinal dunes repeatedly followed this same track across
the desert the in order to leave this type of large residual lag. So that means I want to say either this is
the way through which the longitudinal dune have passed for a long time or a repeatedly
same path has been followed by many longitudinal dunes. So that is why this lags are remaining in
a linear fashions. So that is undulating fixed sand sheet so
this undulating fixed sand sheets are there, separated by this type of gravel beds. Larger area of semi arid steppe and prairies
especially in the middle latitude interiors of the large continents are there underlain
by sandy sediments have a sparse grass cover and have intermittent rain and winter snow
and those regions are mostly responsible for this type of formation of this type of topography. And local areas of shallow deflation basins
develop alternating with subdued parabolic dune forms. Winds are not sufficient effective to sufficiently
effective to create dunes with active slip face, but in combination of deflation and
deposition makes a landscape of undulating hills. So that means these are the wind actions through
which different shape of this dunes we are getting, different size of this dune we are
getting, different structures we are getting within the dunes. And if you summarize it here there are different
types of dunes depending upon the wind directions, depending upon the sand supply, but irrespective
of its nature after migration also the internal structure of the dune remains constant. And here I can conclude that we have this
wind 100% wind and we have vegetation 100%, we have sand supply 100%. In this triangle if you see this portion is
occupied by longitudinal dunes. This is transverse dune, this is parabolic
dune and here no dune. No dunes means we have vegetation 100%, we
have wind less wind or zero wind or less than 50% wind. So in that case we are getting no dunes. So even if sand supply will be there but due
to vegetation, due to less wind we cannot create a dune there. So to form a dune we should have we must have
that sufficient wind speed so and sufficient area should be there and critical more than
the critical area should be there and from which the sand sheet will move and create
a dune. So I think we should stop here. Thank you very much.