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4.b. Unintentional change Mangawhai Pakiri - Coggle Diagram
4.b. Unintentional change
Mangawhai Pakiri
Economic development
High quality holocene sand occurs in nearshore zone
essential mineral resource
Wide range of uses essential in modern economy
Construction
Concrete making
Glass manufacture
Beach replenishment
located 50km North from Auckland
Tourism centred on Auckland's outstanding coastal amenities
2.3 million visitors in 2015
Convenient for New Zealands largest and economically most dynamic metropolitan region
Population over 1.5 million
Accounts for 1/3 of New Zealand's total population
Accounts for 35% of GDP
Unintentional impacts
beaches starved of sand which become wider and flatter and less effective at absorbing waves
The storage if sand sediment has diminished
Rate of output (extraction) > rate of input (dynamic equilibrium)
Unsustainable
Foredune ridges are undercut by wave action
Higher wave energy erodes beach landforms (dunes and spits become vulnerable)
Higher energy waves as waves are carrying less sediment so not wasting as much energy
Increases erosion and flooding
Effects of impacts
Nearshore sand dredging on the 20km long coastline between Mangawhai and Pakiri operated for over 70 years
Between 1994-2004, 165,000 m3/ year was extracted
2005 mining stoped in Mangawhai but it still continues in Pakiri
Current stage of extraction are 75,000 m3/ year until 2020
Large proportion of this sand for replenishment of Aucklands beaches
Coastal sediment budget is a closed system
outputs not replaced by inputs
non renewable resource
Extraction rates at Pakiri exceed inputs by a factor of 5
deposited during Holocene (9000 yrs)
As a result movements of sand between the major stores have diminished
Store on the sea bed (up to 2km offshore)
Store on beach
Deplete the total sand supply
Store in dunes
Consequences
Erosion of spits has lead to loss of vegetation
more susceptible to wind and wave erosion
High energy waves with little sediment threaten beaches and can impact tourism
A breach at the Mangawhai spit has led to the sedimentation of the Mangawhai harbour
The shallow area has made it harder for ships to travel across
Sedimentation also threatens flooding
Due to climate change there is increased risks to storms and the coastline will have little natural defence
coastal retreat is already evident and the sand mining is the main cause
coastal management has been put in place to mitigate the impacts
e.g. groynes constructed to absorb wave energy - only on the spit
dreading reduce to 75,000 m3/ yr