Why are local variations found in the processes operating at Muriwai?

Why are local variations found in the processes operating at

Muriwai?

Muriw

ai Beach

Otakamiro Point

OP

Muriw

ai Beach

MAP 1Wave Processes

The prevailing winds at Muriwai are south-westerly. Generally, winds are 16kph or more. They have blown over a long fetch, so the swell can be high.

The prevailing winds at Muriwai are south-westerly. Generally, winds are 16kph or more. They have blown over a long fetch, so the swell can be high.

Muriw

ai Beach

Otakamiro Point

Around the headland (O.P.) the waves are destructive – they are eroding.

The prevailing winds at Muriwai are south-westerly. Generally, winds are 16kph or more. They have blown over a long fetch, so the swell can be high.

Around the headland (O.P.) the waves are destructive – they are eroding.

Muriw

ai Beach

Otakamiro Point

This is due to wave refraction… the shape of O.P. bends the waves, focussing their energy into the sides of O.P. This has resulted in a large cave to the N of O.P and an arch to the S. There is also a large stack and a wave-cut platform at the tip of O.P.

The prevailing winds at Muriwai are south-westerly. Generally, winds are 16kph or more. They have blown over a long fetch, so the swell can be high.

Around the headland (O.P.) the waves are destructive – they are eroding.

This is due to wave refraction… the shape of O.P. bends the waves, focussing their energy into the sides of O.P. This has resulted in a large cave to the N of O.P and an arch to the S. There is also a large stack and a wave-cut platform at the tip of O.P.

Otakamiro Point

Muriw

ai Beach

Most days of the year the waves approaching the beach are constructive, i.e less than 11 waves breaking per minute. Therefore, swash is more powerful than backwash, so sand is deposited to form beach.

The prevailing winds at Muriwai are south-westerly. Generally, winds are 16kph or more. They have blown over a long fetch, so the swell can be high.

Around the headland (O.P.) the waves are destructive – they are eroding.

This is due to wave refraction… the shape of O.P. bends the waves, focussing their energy into the sides of O.P. This has resulted in a large cave to the N of O.P and an arch to the S. There is also a large stack and a wave-cut platform at the tip of O.P.

Otakamiro Point

Muriw

ai Beach

Most days of the year the waves approaching the beach are constructive, i.e less than 11 waves breaking per minute. Therefore, swash is more powerful than backwash, so sand is deposited to form beach.

Okiritoto Stream

DESTRUCTIVE WAVES OP

Muriw

ai Beach

DESTRUCTIVE WAVES

CONSTRUCTIVE WAVES

OP

Muriw

ai Beach

What are the reasons for these differences?

Wave Processes

The main reason is because of the relief/geomorphology (shape of the land):

The headland is a highland coast – the land was uplifted millions of years ago and it is formed of more resistant rock, it hasn’t been eroded very quickly. It has maintained its shape and wave refraction and wave erosion occur around it.

The beach is a lowland coast made up of softer rock, so over time the wide gently sloping beach at Muriwai has formed.

Put this as a heading after your 1st map

OP

Muriw

ai Beach

MAP 2Longshore Drift (LSD)

Maori Bay

LSD is the process of transportation of beach material. At MCE, this black sand which originated from the volcanic central plateau of NZ

Along the beach the prevailing winds are south-westerly, therefore waves normally approach the beach from the SW.

In the surf zone the breaking waves (swash) transport the sand up the beach on a diagonal (NE)

The backwash carries the sand straight down the beach. Therefore LSD occurs uniformly in a Northerly direction towards Kaipara Heads

At the headland and Maori Bay the prevailing winds are still S-Westerly. However, the processes around the headland (wave reflection, refraction and rip current formation do not allow LSD to operate uniformly. So LSD does not occur in the northerly direction predicted.

Muriw

ai Beach

OPLSD is the process of transportation of beach material. At MCE, this is commonly black sand which originated from the volcanic central plateau of NZ.

Along the beach the prevailing winds are south-westerly, therefore waves normally approach the beach from the SW.

In the surf zone the breaking waves (swash) transport the sand up the beach on a diagonal (NE)

The backwash carries the sand straight down the beach. Therefore LSD occurs uniformly in a Northerly direction towards Kaipara Heads

At the headland and Maori Bay the prevailing winds are still S-Westerly. However, the processes around the headland (wave reflection, refraction and rip current formation) do not allow LSD to operate uniformly. So LSD does not occur in the northerly direction predicted.

Uniform LSD

OP

Muriw

ai Beach

LSD in various directions OP

Muriw

ai Beach

Uniform LSD

Maori Bay

What are the reasons for these differences?

Longshore Drift

The difference is again due to the geomorphology of the MCE:

The beach is long and straight so wave processes can operate without interruption.

Around the headland reflection, refraction and rips cause many currents in the water which cause LSD to occur locally around the headland in various directions.

Put this as a heading after your 2nd map

Maori Bay

Muriw

ai Beach

MAP 3Sand Dune Formation

BEHIND MURIWAI BEACH THERE IS A LARGE SYSTEM OF WELL-ESTABLISHED SAND DUNES.

At Muriwai Beach the tidal range is about 2m. At low tide this exposes up to 300m of the beach, as the beach has a gentle gradient.

The exposed sand dries and, as the winds at Muriwai are frequently 16kph or more, saltation takes place. Large amounts of sand are blown towards the back of the beach.

Vegetation, such as spinifex grass traps the sand and over time large dunes have formed.

At Maori Bay the beach is narrower and steeper. Therefore, at low tide less than 100m of beach is exposed. Less sand dries. Although winds are 16kph or more, less saltation takes place.

AT MAORI BAY THERE IS ONLY A SMALL AREA OF DUNES ON THE NORTHERN END OF THE BEACH.

Maori Bay

Muriw

ai Beach

BEHIND MURIWAI BEACH THERE IS A LARGE SYSTEM OF WELL-ESTABLISHED SAND DUNES.

AT MAORI BAY THERE IS ONLY A SMALL AREA OF DUNES ON THE NORTHERN END OF THE BEACH.

At Maori Bay the beach is narrower and steeper. Therefore, at low tide less than 100m of beach is exposed. Less sand dries. Although winds are 16kph or more, less saltation takes place.

At Muriwai Beach the tidal range is about 2m. At low tide this exposes up to 300m of the beach, as the beach has a gentle gradient.

The exposed sand dries and, as the winds at Muriwai are frequently 16kph or more, saltation takes place. Large amounts of sand are blown towards the back of the beach.

Vegetation, such as spinifex grass traps the sand and over time large dunes have formed.

What are the reasons for these differences?

Sand Dune Formation

Maori Bay is surrounded by cliffs, so there is limited space for dune development.

In addition there is less saltation occurring at Maori Bay, therefore there is less sand available for dune formation.

Put this as a heading after your 3 rd map