Muriwai Coastal Geographic Environment

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Muriwai Coastal Geographic Environment

• A Study of Natural Processes

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Muriwai

• INTRODUCTION
• We define the Muriwai Coastal Geographic
  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.

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Muriwai

• The erosional features found at Muriwai Coastal
  Geographic Environment include -
• The stack (Motutara Island)
• Shore Platform (Fishermans Rock)
• The Blowhole/s
• The Cave
• The depositional features found at Muriwai
  Coastal Geographic Environment include-
• The beach and associated sand spit (Rangitira
  Beach)
• The dunes
• The River Delta
• The Cliffs
• The Offshore sand bar

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Muriwai

• On a large scale the West Coast of New Zealand is
  subjected to 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 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
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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

• 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 gt1METRE)
• MAINLY SUMMER WAVES STRONG BACKWASH
• DESTRUCTIVE WAVES LONGER WAVELENGTH
• HIGH ENERGY WAVES LESS FREQUENT PERIOD
• ENERGY SPREAD OVER ENERGY CONCENTRATED

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Muriwai Description

• "The shoreline is an asymmetrically 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
  shoreline-retrogradation 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.

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Muriwai

• 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 well-defined 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.Fluctuation
  s 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.

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Muriwai

• 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.