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Muriwai Coastal Muriwai Coastal Geographic EnvironmentGeographic Environment

A Study of Natural ProcessesA Study of Natural Processes

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• INTRODUCTIONINTRODUCTION• We define the Muriwai Coastal Geographic We define the Muriwai Coastal Geographic

Environment as being south from the stack Environment as being south from the stack (Motutara Island) to the river (Okiritoto Stream) (Motutara Island) to the river (Okiritoto Stream) 2.5 km north. Offshore to the outer limits of surf 2.5 km north. Offshore to the outer limits of surf and approximately 500 m inland to the road east and approximately 500 m inland to the road east of the golf course and giving access to the beach of the golf course and giving access to the beach beside Okiritoto Stream.beside Okiritoto Stream.

• The interacting natural processes operating at The interacting natural processes operating at Muriwai are: Muriwai are:

• Coastal erosion including wave erosion and Coastal erosion including wave erosion and refraction and sand saltation.refraction and sand saltation.

• Coastal transportation including longshore/beach Coastal transportation including longshore/beach drift, longshore currents and littoral drift.drift, longshore currents and littoral drift.

• Coastal deposition including dune formation, spit Coastal deposition including dune formation, spit and bar formation, beach formation.and bar formation, beach formation.

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• The erosional features found at Muriwai Coastal The erosional features found at Muriwai Coastal Geographic Environment include: -Geographic Environment include: -

1.1. The stack (Motutara Island)The stack (Motutara Island)2.2. Shore Platform (Fisherman’s Rock)Shore Platform (Fisherman’s Rock)3.3. The Blowhole/sThe Blowhole/s4.4. The CaveThe Cave5.5. The depositional features found at Muriwai Coastal The depositional features found at Muriwai Coastal

Geographic Environment include:-Geographic Environment include:-6.6. The beach and associated sand spit (Rangitira The beach and associated sand spit (Rangitira

Beach)Beach)7.7. The dunesThe dunes8.8. The River DeltaThe River Delta9.9. The CliffsThe Cliffs10.10.The Offshore sand bar The Offshore sand bar

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• On a large scale the West Coast of New Zealand is On a large scale the West Coast of New Zealand is subjected to prevailing S.W. winds which are strongest subjected to prevailing S.W. winds which are strongest during the winter season (about June to September). during the winter season (about June to September).

• The longshore current that occurs around New Zealand The longshore current that occurs around New Zealand tends to move in a northerly direction on both coasts tends to move in a northerly direction on both coasts due to the polar sea from Antarctica meeting the due to the polar sea from Antarctica meeting the warmer Tasman Sea and Pacific Ocean. This is called warmer Tasman Sea and Pacific Ocean. This is called the sub-antarctic convergence.the sub-antarctic convergence.

• Waves tend to be a westerly swell that varies in height Waves tend to be a westerly swell that varies in height and therefore energy seasonally. They approach and therefore energy seasonally. They approach Muriwai from a southwesterly direction as the beach Muriwai from a southwesterly direction as the beach faces toward this direction (S.W.) This means that faces toward this direction (S.W.) This means that longshore (or beach) drift tends to move in a northerly longshore (or beach) drift tends to move in a northerly direction. The combination of both longshore currents direction. The combination of both longshore currents and beach drift is called littoral drift.and beach drift is called littoral drift.

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What are the elements and interactions involved in each What are the elements and interactions involved in each process?process?

• At Muriwai Coastal Geographic environment, (MCGE) there are At Muriwai Coastal Geographic environment, (MCGE) there are three main processes operating. Coastal erosion, coastal three main processes operating. Coastal erosion, coastal transportation, and coastal depostition. Each of these also has transportation, and coastal depostition. Each of these also has various sub-processes. various sub-processes.

• The elements and interactions within the process of Coastal The elements and interactions within the process of Coastal erosion are quite varied. The main elements however are things erosion are quite varied. The main elements however are things such as the wind, the waves, the sand, the rock type, the such as the wind, the waves, the sand, the rock type, the vegetation, sun, humans, landforms and tides. However the vegetation, sun, humans, landforms and tides. However the interactions that occur within these elements are the important interactions that occur within these elements are the important thing. One of the biggest interactions that occurs is the thing. One of the biggest interactions that occurs is the interaction between the landforms and the tides and tidal interaction between the landforms and the tides and tidal processes.processes.

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• What causes coastal What causes coastal erosion?erosion?

• We know that We know that periods of erosion periods of erosion coincide with storms coincide with storms and large waves, so and large waves, so the obvious answer the obvious answer to the question is to the question is BIG WAVESBIG WAVES

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• Waves At MuriwaiWaves At Muriwai

Waves at the Shore Platform

Waves at the Beach

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• Types of WavesTypes of WavesBreaking Wave TypesBreaking Wave Types Beach SlopeBeach Slope Wave SteepnessWave Steepness

SpillingSpilling ShallowShallow SteepSteep

PlungingPlunging Shallow to intermediateShallow to intermediate Less steepLess steep

CollapsingCollapsing Intermediate to steepIntermediate to steep IntermediateIntermediate

SurgingSurging SteepSteep ShallowShallow

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MuriwaiMuriwai• CHARACTERISTICS OF WAVESCHARACTERISTICS OF WAVES• The character of any beach is determined by the type of waves that break on the beach, however the 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 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 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 either moving material away from the backshore to form a LONG SHORE BAR or moving material on to the backshore to form a BERM.to the backshore to form a BERM.

• TASK:- Using the following labels construct a table with two columns . TASK:- Using the following labels construct a table with two columns . • One describing CONSTRUCTIVE and the other describing DESTRUCTIVE features. Include a diagram.One describing CONSTRUCTIVE and the other describing DESTRUCTIVE features. Include a diagram.

FLAT GENTLE WAVESFLAT GENTLE WAVES SHORTER WAVELENGTHSHORTER WAVELENGTHMORE FREQUENT WAVE MORE FREQUENT WAVE STRONG SWASHSTRONG SWASHPERIODPERIODLOW ENERGY WAVESLOW ENERGY WAVES OCCUR ON FLAT BEACHESOCCUR ON FLAT BEACHES(6-8WAVES PER MIN)(6-8WAVES PER MIN) LOW WAVE HEIGHTLOW WAVE HEIGHTMAINLY WINTER WAVESMAINLY WINTER WAVES STEEP WAVESSTEEP WAVESCONSTRUCTIVE WAVESCONSTRUCTIVE WAVES WEAK SWASHWEAK SWASHSTRONG BACKWASHSTRONG BACKWASH LONGSHORE BAR FORMSLONGSHORE BAR FORMSBERM FORMSBERM FORMS OCCUR ON STEEP BEACHESOCCUR ON STEEP BEACHES(10-14WAVES PER MIN)(10-14WAVES PER MIN) (HEIGHT >1METRE)(HEIGHT >1METRE)MAINLY SUMMER WAVESMAINLY SUMMER WAVES STRONG BACKWASHSTRONG BACKWASHDESTRUCTIVE WAVESDESTRUCTIVE WAVES LONGER WAVELENGTHLONGER WAVELENGTHHIGH ENERGY WAVESHIGH ENERGY WAVES LESS FREQUENT PERIODLESS FREQUENT PERIODENERGY SPREAD OVERENERGY SPREAD OVER ENERGY CONCENTRATED ENERGY CONCENTRATED A LARGE AREAA LARGE AREA ON SMALL BEACH AREAON SMALL BEACH AREA

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Muriwai DescriptionMuriwai Description

• "The shoreline is an asymmetrically "The shoreline is an asymmetrically concave line which at first is more concave line which at first is more or less straight for 32 km and then or less straight for 32 km and then bulges seawards, later to be bulges seawards, later to be recurved to the east in the sand recurved to the east in the sand hook at Kaipara South Head. This hook at Kaipara South Head. This cuspate shape is the result of cuspate shape is the result of contrasting processes even now contrasting processes even now affecting the shoreline-affecting the shoreline-retrogradation along the straight retrogradation along the straight portion and progradation at the portion and progradation at the bulge. bulge.

• The straight shoreline trends thirty The straight shoreline trends thirty degrees west of north and is degrees west of north and is therefore not at right angles to a therefore not at right angles to a wind blowing from the south-west. wind blowing from the south-west. As a result, transverse dunes As a result, transverse dunes behind the beach lie at an angle of behind the beach lie at an angle of ten to fourteen degrees with the ten to fourteen degrees with the shoreline and the sand moves shoreline and the sand moves along the coast to a small degree along the coast to a small degree as well as inland. as well as inland.

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• A foredune backing the steep wide beach is A foredune backing the steep wide beach is continuous from Muriwai township along the continuous from Muriwai township along the entire length of the shoreline rarely being entire length of the shoreline rarely being more than 10 m high and 20 m wide, and is more than 10 m high and 20 m wide, and is broken only by the mouth of Okiritoto broken only by the mouth of Okiritoto Stream. Stream.

• Many large blowouts have been developed by Many large blowouts have been developed by the removal of loose sand from the seaward the removal of loose sand from the seaward face of the foredune, producing large marram face of the foredune, producing large marram grass hummocks with short tail dunes. grass hummocks with short tail dunes. Immediately to leeward of the foredune and Immediately to leeward of the foredune and parallel to the beach is a well-defined shallow parallel to the beach is a well-defined shallow eddy hollow, downwind from which the loose eddy hollow, downwind from which the loose sand rises again as long tongues gaining sand rises again as long tongues gaining height inland. "Much of the detritus forming height inland. "Much of the detritus forming the dunes was brought to the West Coast by the dunes was brought to the West Coast by the Waikato River from an ultimate source in the Waikato River from an ultimate source in the Central Volcanic Plateau.Fluctuations in the Central Volcanic Plateau.Fluctuations in the quantity of material carried to the coast the quantity of material carried to the coast by the Waikato River must have affected the by the Waikato River must have affected the rate of progradation in the dune area north of rate of progradation in the dune area north of Muriwai. However, it is doubtful if such Muriwai. However, it is doubtful if such fluctuations in the supply of sediment were fluctuations in the supply of sediment were sufficient to induce alternating periods of sufficient to induce alternating periods of extensive deposition and erosion. extensive deposition and erosion.

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• An added impetus to progradation of An added impetus to progradation of the coast would have been supplied by the coast would have been supplied by the recession of sea level that is the recession of sea level that is known to have occurred in Recent known to have occurred in Recent time. Progradation and sand dune time. Progradation and sand dune formation concurrent with this path of formation concurrent with this path of sea level undoubtedly added great sea level undoubtedly added great quantities of sand to the Muriwai quantities of sand to the Muriwai foreland, and therefore could account foreland, and therefore could account for the development of any one of the for the development of any one of the dune belts. dune belts.

• Nevertheless such a sequence of Nevertheless such a sequence of events fails to explain the intervention events fails to explain the intervention of two erosion periods. It seems most of two erosion periods. It seems most probable that the main course of dune probable that the main course of dune destruction and in part, of dune destruction and in part, of dune formation, has been the climate cycle. formation, has been the climate cycle. Periods of calm climatic conditions Periods of calm climatic conditions many years in length are known to many years in length are known to alternate with similar periods of alternate with similar periods of generally stormy weather.generally stormy weather.