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How Natural Processes Operate at Coastal Geographic Environment.

Natural Processes are actions or events that have natural causes, which result in natural events. The three main coastal environment processes that operate at Muriwai are Coastal Erosion, Coastal Transportation and Coastal Deposition.

The elements that interact to produce natural processes are wind, waves and tides. Each phenomenon at Muriwai s coastal geographic environment has been produced by interaction.

Coastal Erosion is a process at Muriwai that gradually wears away the rock particles of the earth s surface, transporting them to another location. There are many types of processes that cause erosion at Muriwai such as wave erosion, wind erosion and wave refraction.

Thousands of years ago when sea levels dropped over years at the Southern end of Muriwai, the sedimentary rock and sandstone was exposed to the air. Rock from volcanic activity mixed with the sedimentary rock; this is called Breccia a mixture of all rock. An example of this is at Maori Bay.

Coastal Erosion operates at different rates and different times. Limestone rock is eroded slower than sedimentary rock. The cliff at Muriwai made of sedimentary rock was eroded back to expose Fisherman s Rock – the shore platform which, made of limestone tended to erode back slower than the cliff.

The types of wave erosion that caused this are –

Hydraulic Action, when waves hit the cliff, air is forced into cracks, and then as the wave retreats this air expands explosively. Over time the cracks enlarge, weakening the base of the cliff causing erosion.

Attrition is the breakdown of rock particles when they hit Otakamiro point and each other causing the base of the headland to erode.

Chemical Erosion/Corrosion occurs due to the content of limestone in the rockface of Otakamiro point. The seawater combined with the limestone produces a weak chemical solution, which erodes the base of the cliff and produces a pitted effect.

Chemical Weathering is when water weakens the structure of the rock and Mechanical Weathering is where water seeps into the rock face causing fragments of rock to break off.

These types of erosion have caused the formation of several phenomena at Muriwai.

Motutara Island (stack) was produced by the formation of two caves on either side of the headland forming an arch and the roof slowly eroding away due to vertical erosion.

Fisherman s Rock was produced by cliff retreat when undercutting stones are thrust at the cliff by wave action.

The blowhole was formed by vertical, hydraulic and mechanical erosion causing the cracks and fissures to enlarge over time to produce this hole.

Types of wave erosion operating on the headland have produced the cave.

Wave Refraction is an important process as it influences wave erosion on Otakamiro headland and on the beach at Muriwai (refer to Diagram 1). Wave Transportation (Longshore Drift) varies spatially due to wave refraction. It is the process by which waves alter their course as they interact in shallow water with the seabed along the coastline. This process produces wave erosion, which focuses its energy on Otakamiro Point.

Diagram 1

Constructive, Destructive and Dissipative are the types of waves that operate at Muriwai that cause erosion.

Dissipative waves are the waves usually found at Muriwai they lose their energy before they reach the shore as they usually break on the offshore bar. Constructive waves are long waves with little height that leave material at the top of the beach, building it up.

Destructive waves are erosive and carry material seawards.

Wind Erosion is the driving mechanism for Coastal Transportation and Coastal Deposition. Strong winds have a better ability to cause erosion at Muriwai s Coastal Geographic Environment. Wind erosion is most effective on well-drained landforms such as sand dunes. Loose particles are more easily eroded than wet heavy soil, sand and rock.

Coastal Transportation is the movement of sediment and sand by waves and wind. Drifts, currents, sea level changes and the wave types also control it.

There are two main types of Coastal Transportation theses are Wave Transportation and Wind Transportation.

Longshore Drift is a transportational process, which determines the shape of Muriwai.

Longshore drift occurs as a result of wave action. Driven by the prevailing southwest winds the wave (swash) hits the beach and moves sediment sideways up the shore and the backwash returns the sediment at right angles. The following waves repeat this process and as a result the sediment gradually moves along the shore in a sawtooth motion called Longshore Drift (refer to Diagram 2).

Longshore Drift is the main way that sediment is moved from Otakamiro Headland to create the beach, leaving more erosion to take place on Otakamiro Point. As this is a natural process it disrupts human activities and may occur to the point where Muriwai becomes destroyed.

Littoral drift is when sedimentary material is moved along the shoreline under the influence of waves and currents

Diagram 2

Wind can move material by three processes these are saltation, surface creep and suspension.

Saltation is when fine to coarse sand is picked up briefly and the sand particles bounce along. This process created the sand dunes at Muriwai. The prevailing southwest winds will build up the dunes and travel inland.

Surface Creep occurs when landing sand particles remove the larger and heavier particles, pushing them forward.

Suspension is the picking up of sand by wind. This is when sand is airborne and then deposited anywhere.

Coastal Deposition is the third main natural process occurring at Muriwai s coastal geographic environment.

It is the process of sediment being deposited to form natural features.

This is when the rock fragments from Otakamiro Headland are ripped away by waves, broken down by attrition and transported along the coast where they are deposited as beaches and sand dunes. The movement of the material is called Longshore Drift; the direction of the deposit depends on the direction of the winds.

Titomagnetite sand (black sand) was deposited at Muriwai when it was bought from the south by Longshore Drift.

Coastal Erosion, Coastal Transportation and Coastal Deposition are natural processes that have occurred at Muriwai s coastal geographic environment. These processes outlined have formed such phenomena as Motutara Island (stack), Otakamiro Point (headland/cliff), Fisherman s Rock (shore-platform), cave and the blowhole.

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