This document discusses dune formation and examples of eroded and vegetated dunes at Muriwai Beach in New Zealand. It provides details on: - How dunes are formed by wind transporting sand and depositing it in sheltered areas behind obstacles. - Examples of eroded dunes at Muriwai Beach that have been exposed by erosion of dunes in front of them. - The role of vegetation like pingao sedge in trapping sand and building/stabilizing dunes at Muriwai Beach. - How the interaction between aeolian sand transport processes and vegetative growth shapes the dunes at Muriwai Beach.

1. IGCSE GEOGRAPHY
EXTRA WORK
DUNE FORMATION. EXAMPLES OF
ERODED DUNES AND DUNE
VEGETATION. CASE STUDY:
MURIWAI BEACH, NEW ZEALAND

2. A dune is a mound of sand
formed by the wind, usually
along the beach or in a desert.
Dunes form when wind blows
sand into a sheltered area
behind an obstacle.
Dunes grow as grains of sand
accumulate.
A dunes windward side is the
side where the wind is blowing
and pushing material up.
Beach sand dunes at Muriwai, New Zealand

3. Beaches and fore dunes (the dunes closest to the sea) are in a
constant state of change in response to waves and wind.

4. Fore dunes are formed
when vegetation traps
wind-blown sand.

5. the front face of a fore dune
is eroded when storm waves
crash onto the dunes and
wash away plants and sand.

6. The dunes form again as
vegetation is re-established
on an exposed site and
begins to trap sand.

7. Sand dunes are sometimes found at the top of beaches, above
the high tide mark.
For dunes to form there must be a supply of sand, a means of
transporting it, and somewhere for it to be deposited more
quickly than it is eroded.
At low tide, deposits of sand in the inter-tidal zone (between the
high and low tide marks) may be exposed and start to dry.
The dry grains of sand can then be transported by the wind (by
saltation). If the wind blows towards the land, sand will be
transported up the beach and beyond the high tide mark.

8. When the dried sand reaches the top of the beach it can be
trapped by debris such as driftwood, dead seaweed or rocks and
pebbles as in this photograph.
If the sand is not eroded again it may become colonised by small
plants, or trap other windblown debris, increasing its size and
thus trapping even more sand.

9. As the dunes grow in size they
are able to catch more of the
sand blown across them by the
wind. In the photograph the
wind is blowing sand from the
right to the left, but the
middle part of the picture is
sheltered by grass growing
behind and to the right of the
image.
This causes the wind speed to lessen and the sand grains are
deposited. The smooth area of lighter sand in the centre of the
picture is the sand deposited during the day on which the
photograph was taken.

10. In this picture you can see a well
established sand dune lying about
100m above the top of the present
beach.
The dune forms part of several
parallel rows of dunes that trap
sand blown off the beach.
The thick covering of vegetation
preserves the dune in two ways.
Firstly the roots bind the sand
together, and secondly the above
ground vegetation traps particles
of sand as they are blown over the
surface.

11. This is an old dune that has, due
to erosion of the dunes infront of
it, become exposed to the sea. The
front of the dune shows a debris
pile consisting of sand that has
fallen from the face during the
last couple of days.
The sea now reaches this dune at
the highest tides and erosion will
be rapid. Note how the root
systems of the grasses are
exposed along the top of the
dune, showing how they penetrate
the sand and bind it together.

12. This is an another old dune that is now being
eroded by the sea at high tide. The pebbles and
flat sand at the base of the dune show where the
last tide stopped. The collapsed sand beyond
them is a result of waves undercutting the base
of the dune, weakening it and it being unable to
support the weight of the sand above it. At the
base of a dune there are no plants or roots to
bind the sand grains together so the sand is
easily eroded by the wind, water and even
animals crossing over it. Ten years ago this dune
was in the middle of a wide belt of similar dunes
over 100m from the high tide mark, but due to
rapid erosion there are now very few dunes left
at this location.

13. Muriwai Coastal dunes such as the ones at Muriwai are different
from other coastal landforms as they are formed by wind rather
than water movement.
However it is the interaction of Aeolian sand transport and
Vegetative growth that makes the dunes different from desert
dunes.

14. Constructive wave action and tidal processes provide an
abundant supply of dry sediment to Muriwai's coast.
Aeolian transport moves the sediment to form the dunes.
These are known as suspension, saltation and surface creep.

15. Tidal action and the sun allow sediment to dry in the intertidal
range (3m at Muriwai) meaning the sediment is much lighter
thus being able to be transported by saltation or surface
creep.
Saltation moves sediment by applying a shear velocity of
5m/s or above to a protruding grain. (Muriwai's wind velocity
is 6m/s).
In order for the grain to be moved, it must be at or less than
the critical size.
As grains move they collide with other grains, sometimes
causing the grains to eject into the airflow.

17. Muriwai's sand is iron rich which means the grains are finer,
so travel at a greater velocity.
Saltation hop lengths are 12-15 times the height of the grains
bounce.
It is because of this that Muriwai's strong Westerly winds are
capable of transporting dried sand from the intertidal zone to
the foredune.

18. The interaction of Aeolian sand transport and vegetative
growth form Muriwai's dunes.
Vegetation plays an important part in the formation and
stabilization of coastal sand dunes, as it traps and binds sand
with its root systems.
Dune vegetation can also reduce the extent of recession and
erosion produced by storms.
Native grasses can tolerate a hostile environment which
include high winds, salt spray, sand blast, covering by sand
and water.

19. Pingao is a golden sand
sedge which is thick and
has rope like stems that
trail across the dunes.
It binds to the sand and
helps to build and hold
the dunes.
It is the interaction of
these grasses and aeolian
deposition that helps to
form and bind Muriwai's
sand dunes which give
them their shape.

20. The Dune profiles are as follows:
• At Muriwai, vegetation was visible at the top of each dune.
• Both Spinifex and Pingao were present on the dunes and it
was this presence that meant the structural integrity of the
dune could be ensured, as the root systems that were in
place with the grasses would hold the sand and allow the
steep gradient of the dune to be maintained without
crumbling.
• As saltating sediments deposit on top of the vegetation, the
vegetation grows seeking light.
• Therefore there is an increase in vegetation growth and
therefore an increase in dune size.

22. It is this structure that makes Muriwai's dunes the shape that
they are.
Because of Muriwai's strong westerly winds (average 6m/s), a
large amount of dry sediment such as iron sand, is
transported and deposited on top of the vegetation.
The stronger winds transport more sediment to the dune and
thus build the dunes higher and higher.

23. Like all beaches, Muriwai's profile changes from winter to
summer. This is largely due to the weather at different times of
the year. In winter, waves at Muriwai adopt steeper, more
destructive characteristics which have a strong backwash and
weak swash. it is this high energy action that erodes material off
the beach which contributes to its flat profile.
Winter brings steeper beaches with no berm and a longshore bar.
Currently destructive waves dominate over constructive waves,
meaning the beach is in an erosive phase.
As a result the beach narrows and is cut back toward the dunes.
With more wet sediment present, saltation does not occur as
much meaning dunes experience destructive forces rather than
constructive, especially in stormy conditions.

25. In summer Muriwai's waves tend to be flatter and more
constructive.
This is because they have a strong swash and a weak backwash.
These waves are known as constructive waves as they introduce a
lot of sediment rather than remove it.
In summer we have flatter beaches with a berm and no longshore
bar.
The weather in summer allows sediment to dry in the intertidal
range, meaning that Aeolian transport is more likely.