4.B - Sand mining along the Mangawhai-Pakiri coast

Question 1

how was sandmining along the mangawhai-pakiri coastline unintentional?

Answer 1

• intentional action
• UNINTENTIONAL impact on coastline

Question 2

why is sand in demand for industry?

Answer 2

• 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)

Question 3

what is the sand type in Pakiri?

Answer 3

• Holocene, deposited at the end of the last Ice Age
• clean, doesnt contain iron
• non-renewable resource

Question 4

what is Holocene sand?

Answer 4

• this is the more recent (last 10,000yrs) deposits of sand from the sea level rising in this time

Question 5

what is Pleistocene sand?

Answer 5

• 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)

Question 6

how do the beaches location make it attractive for sand mining?

Answer 6

• 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)

Question 7

give an overview of past and current sand mining

Answer 7

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)

Question 8

where was the sediment originally sourced from? (Mangawhai - Pakiri)

Answer 8

• 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

Question 9

what is the ratio of inputs to outputs at Pakiri beach?

Answer 9

• 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)

Question 10

what impacts is sand mining having on the coastal landforms?

Answer 10

• 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

Question 11

what happened in 1978?

Answer 11

• 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

Question 12

what statistics for coastal retreat in the future are there?

Answer 12

• 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 2100
• this is higher than any of the Auckland region’s other 123 beaches

Question 13

what is holocene sand?

Answer 13

• this is the more recent (last 10,000yrs) deposits of sand from the sea level rising in this time

Question 14

what is pleistocene sand?

Answer 14

• 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 will rust)

Question 15

where do waves act here?

Answer 15

• 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

Question 16

what are humans doing to modify this landscape?

Answer 16

• 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

Question 17

what impacts on the coastal system has this had on the landforms and coastal system?

Answer 17

• 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

Question 18

what are the potential impacts for the coastal landscape in the future?

Answer 18

• dredging and groyne construction on Mangawhai spit has helped restore equilibrium so hopefully spit is restored
• however elsewhere natural protection is decreased from extreme storm events
• this has been complicated by rising sea level and climate change
• the width of the coastal zone susceptible to erosion varies from 48-111m higher than any other of Auckland’s 123 beaches