4.b - UNINTENTIONAL - MANGAWHAI-PAKIRI Flashcards
how was sandmining along the mangawhai-pakiri coastline unintentional?
intentional action
UNINTENTIONAL impact on coastline
why is sand in demand for industry?
essential mineral resource in a modern economy
used for construction, concrete making, glass manufacture and beach replenishment
very valuable -> NZ$25/m2
Auckland is 1/3 of the NZ population and 35% of its GDP.
with this comes construction to support Auckland growing economically
large proportion of sand is used for replenishing Auckland’s tourist beaches (2.3 mill foreign visitors in 2015)
what is the sand type in Pakiri?
Holocene, deposited at the end of the last Ice Age
clean, doesnt contain iron
non-renewable resource
what is Holocene sand?
this is the more recent (last 10,000yrs) deposits of sand from the sea level rising in this time
what is Pleistocene sand?
Pleistocene Sand is from 100,000 - 11,000 yrs ago
this is NOT being replenished
glacial period, glacial deposits
iron rich sand so not good for construction (iron rusts)
how do the beaches location make it attractive for sand mining?
the sand is high quality, “clean” and is therefore suitable for the construction industry.
also 90-100km (close!) to Auckland
convenient for NZ’s largest and most economically dynamic metropolitan region
Auckland = 1.5 mill pop = 1/3 of NZ’s overall pop and 35% of country’s GDP
region is growing rapidly (business, finance and high tech industry as well as tourism (2.3mill in 2015)
give an overview of past and current sand mining
PAST:
- between 1994 and 2004, 165,000 m3/yr
- mining ended in 2005 at Mangawhai
- nearshore dredging on the 20km coastline has operated for over 70 yrs
CURRENT:
- current rates are 75,000 m3/yr until 2020
- large proportion used to replenish tourist beaches (worthwhile economically)
where was the sediment originally sourced from?
input of sand from rivers only 8000 m3/yr, less than 1/10th of sand mined today
= sand is a non-renewable resource and this is an unsustainable process as sand isn’t replaced
sand originally deposited during the Holocene (past 10,000 yrs) - sand is a nonrenewable resource!
coastal sediment budget essentially a closed system
what is the ratio of inputs to outputs at Pakiri beach?
outputs of sand through nearshore mining are not replaced by inputs from rivers/waves offshore
extraction rates at pakiri beach exceed inputs by 5 (5:1)
what impacts is sand mining having on the coastal landforms?
beaches are starved of sediment = narrower and flatter, and are less effective at absorbing waves, so higher energy waves erode beaches and dunes, and spits become vulnerable
fore dune ridges are undercut by wave action developing steep, seaward facing scarps. loss of vegetation cover = susceptible to wind erosion
1978 spit breach
what happened in 1978?
storms caused a 28m breach at the base of the Mangawhai spit.
this and a 2nd breach altered tidal currents, which led to the sedimentation of Mangawhai’s harbour
subsequent dredging of the harbour and groyne construction has helped restore equilibrium
what statistics for coastal retreat in the future are there?
increased rates of coastal erosion are likely in future w/ declining natural protection from extreme storm events
coastal retreat is also already evident, and LT retreat is estimated at 35 metres by the end of the century
this is higher than any of the Auckland region’s other 123 beaches
where do waves act here?
waves only act/break in the shallowest 10 metres of the beach
sediment here is moved between the beach and the shallower water (8-10m deep, no more)
this is part of the dynamic equilibrium of this coastal system
what are humans doing to modify this landscape?
dredging for holocene sand (no iron) used for construction in Auckland to support the growing population (35% GDP, 1/3 population)
planting pine trees behind the dunes = wind forced up and over them which means aeolian processes can’t replenish the dunes = starved of sediment and will deflate
what impacts on the coastal system has this had on the landforms and coastal system?
beach becoming narrower and flat (profile change)
can’t absorb wave energy therefore dunes are more subject to erosion
higher, more energetic waves are now able to erode beaches even further (positive feedback)
spit breached in 1978 over 28m (has been restored now)
beach profile change = sand wetter for longer, can’t be picked up by aeolian processes to replenish dunes
the loss of vegetation on the foredunes also makes them susceptible to wind erosion, whilst the planting of pines on the backshore has deflected wind higher and thus impedes replenishment.
shallower water threatens the waterfront community with flooding
