Muriwai Coastal Geographic Environment

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Muriwai Coastal Geographic
Environment
A Study of Natural Processes
1
Muriwai
• INTRODUCTION
• We define the Muriwai Coastal Geographic
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•
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Environment as being south from the stack (Motutara
Island) to the river (Okiritoto Stream) 2.5 km north.
Offshore to the outer limits of surf and approximately
500 m inland to the road east of the golf course and
giving access to the beach beside Okiritoto Stream.
The interacting natural processes operating at Muriwai
are:
Coastal erosion including wave erosion and refraction
and sand saltation.
Coastal transportation including longshore/beach drift,
longshore currents and littoral drift.
Coastal deposition including dune formation, spit and
bar formation, beach formation.
2
Muriwai
• The erosional features found at Muriwai Coastal
Geographic Environment include: 1. The stack (Motutara Island)
2. Shore Platform (Fisherman’s Rock)
3. The Blowhole/s
4. The Cave
5. The depositional features found at Muriwai Coastal
Geographic Environment include:6. The beach and associated sand spit (Rangitira Beach)
7. The dunes
8. The River Delta
9. The Cliffs
10.The Offshore sand bar
3
Muriwai
• On a large scale the West Coast of New Zealand is subjected to
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prevailing S.W. winds which are strongest during the winter
season (about June to September).
The longshore current that occurs around New Zealand tends
to move in a northerly direction on both coasts due to the
polar sea from Antarctica meeting the warmer Tasman Sea and
Pacific Ocean. This is called the sub-antarctic convergence.
Waves tend to be a westerly swell that varies in height and
therefore energy seasonally. They approach Muriwai from a
southwesterly direction as the beach faces toward this
direction (S.W.) This means that longshore (or beach) drift
tends to move in a northerly direction. The combination of
both longshore currents and beach drift is called littoral drift.
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Muriwai
What are the elements and interactions involved in each
process?
• At Muriwai Coastal Geographic environment, (MCGE) there are three
main processes operating. Coastal erosion, coastal transportation, and
coastal depostition. Each of these also has various sub-processes.
• The elements and interactions within the process of Coastal erosion are
quite varied. The main elements however are things such as the wind,
the waves, the sand, the rock type, the vegetation, sun, humans,
landforms and tides. However the interactions that occur within these
elements are the important thing. One of the biggest interactions that
occurs is the interaction between the landforms and the tides and tidal
processes.
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Muriwai
• What causes coastal
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erosion?
We know that periods
of erosion coincide
with storms and large
waves, so the obvious
answer to the
question is BIG
WAVES
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Muriwai
• Waves At Muriwai
Waves at the Shore Platform
Waves at the Beach
7
Muriwai
• Types of Waves
Breaking Wave Types
Beach Slope
Wave Steepness
Spilling
Shallow
Steep
Plunging
Shallow to intermediate
Less steep
Collapsing
Intermediate to steep
Intermediate
Surging
Steep
Shallow
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Muriwai
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CHARACTERISTICS OF WAVES
The character of any beach is determined by the type of waves that break on the beach, however
the wave types themselves are determined by the shape of the sea bed.At any one time beaches
are either in a CONSTRUCTIVE period or a DESTRUCTIVE period,
either moving material away from the backshore to form a LONG SHORE BAR or moving material
on to the backshore to form a BERM.
TASK:- Using the following labels construct a table with two columns .
One describing CONSTRUCTIVE and the other describing DESTRUCTIVE features. Include a
diagram.
FLAT GENTLE WAVES
SHORTER WAVELENGTH
MORE FREQUENT WAVE
STRONG SWASH
PERIOD
LOW ENERGY WAVES
OCCUR ON FLAT BEACHES
(6-8WAVES PER MIN)
LOW WAVE HEIGHT
MAINLY WINTER WAVES
STEEP WAVES
CONSTRUCTIVE WAVES
WEAK SWASH
STRONG BACKWASH
LONGSHORE BAR FORMS
BERM FORMS
OCCUR ON STEEP BEACHES
(10-14WAVES PER MIN)
(HEIGHT >1METRE)
MAINLY SUMMER WAVES
STRONG BACKWASH
DESTRUCTIVE WAVES
LONGER WAVELENGTH
HIGH ENERGY WAVES
LESS FREQUENT PERIOD
ENERGY SPREAD OVER
ENERGY CONCENTRATED
A LARGE AREA
ON SMALL BEACH AREA
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Muriwai Description
• "The shoreline is an asymmetrically
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concave line which at first is more or
less straight for 32 km and then bulges
seawards, later to be recurved to the
east in the sand hook at Kaipara South
Head. This cuspate shape is the
result of contrasting processes even
now affecting the shorelineretrogradation along the straight
portion and progradation at the bulge.
The straight shoreline trends thirty
degrees west of north and is therefore
not at right angles to a wind blowing
from the south-west. As a result,
transverse dunes behind the beach lie
at an angle of ten to fourteen degrees
with the shoreline and the sand moves
along the coast to a small degree as
well as inland.
10
Muriwai
•
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A foredune backing the steep wide beach is
continuous from Muriwai township along the entire
length of the shoreline rarely being more than 10 m
high and 20 m wide, and is broken only by the
mouth of Okiritoto Stream.
Many large blowouts have been developed by the
removal of loose sand from the seaward face of the
foredune, producing large marram grass hummocks
with short tail dunes. Immediately to leeward of
the foredune and parallel to the beach is a welldefined shallow eddy hollow, downwind from which
the loose sand rises again as long tongues gaining
height inland. "Much of the detritus forming the
dunes was brought to the West Coast by the
Waikato River from an ultimate source in the
Central Volcanic Plateau.Fluctuations in the quantity
of material carried to the coast by the Waikato
River must have affected the rate of progradation in
the dune area north of Muriwai. However, it is
doubtful if such fluctuations in the supply of
sediment were sufficient to induce alternating
periods of extensive deposition and erosion.
11
Muriwai
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An added impetus to progradation of the
coast would have been supplied by the
recession of sea level that is known to have
occurred in Recent time. Progradation and
sand dune formation concurrent with this
path of sea level undoubtedly added great
quantities of sand to the Muriwai foreland,
and therefore could account for the
development of any one of the dune belts.
Nevertheless such a sequence of events fails
to explain the intervention of two erosion
periods. It seems most probable that the
main course of dune destruction and in part,
of dune formation, has been the climate
cycle. Periods of calm climatic conditions
many years in length are known to alternate
with similar periods of generally stormy
weather.
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