Coast casestudy: Mangawhai-Pakiri

Question 1

Where is Mangawhai-Pakiri?

Answer 1

• A coastline on New Zealand’s Northern Peninsula.

Question 2

Why is sand mining important in Mangawhai-Pakiri to Auckland?

Answer 2

• Sand mining
• Sand is an essential material resource in New Zealand’s modern economy.
• It has a wide range of uses from construction, concrete, glass manufacture and beach replenishment.
• High quality sand from this area is used in Auckland, convenient for an economically dynamic metropolitan region. Auckland has a population of over 1.5 million and accounts for 1/3 of the total population and 35% of GDP.
• Auckland is growing rapidly, requiring construction. In addition, Auckland’s coast attracts over 3 million foreign visitors - requiring beach maintenance.

Question 3

What is the impact of sandmining in Mangawhai-Pakiri on processes and flows of material and energy?

Answer 3

• Offshore sandmining impacts the sediment budget.
• Sand is considered a non-renewable resource on the Mangawhai-Pakiri coastline as most material in the system is considered to have offshore sources.
• The coastal sediment budget is essentially a closed system, so outputs of sand from mining IS NOT replaced by fluvial inputs and waves from offshore.
• Extraction rates have exceeded inputs by a factor of five!
• Mining therefore depletes the total sand supply stored in dunes, beaches and seabed.
• As a result, movement of sand between the major stores have diminished.

Question 4

Give a background on Mangawhai-Pakiri’s sand mining industry

Answer 4

• Nearshore sand dredging occurring on the 20km coastline has occurred for over 70 years.
• Sand mining ceased at Mangawhai in 2005, yet continued in Pakiri.
• Current rates of extraction are 75,000m³/year, measured up to 2020.
• A large proportion of sand is used for tourist reasons: replenishing Auckland’s beaches. Also, it is used in construction of holiday homes and hotels.

Question 5

What is the effect of sand mining in Mangawhai-Pakiri on coastal landforms?

Answer 5

• The unsustainable depletion of sand is having an impact on landforms and landscapes.
• Beaches are starved of sediment so they become wider and flatter, and become less effective in absorbing and dissipating wave energy.
• Depositional landforms such as dunes and spits become vulnerable.
• Foredune ridges are undercut by wave action, developing steep, seaward-facing scarps.
• This is combined with a loss of stabilizing vegetation, so they are susceptible to wind energy.

Question 6

What are the observable consequences of sand-mining on landscapes overall in Mangawhai-Pakiri?

Answer 6

• In 1978, storms caused a 28m breach at the base of the Mangawhai spit
• This, and a second breach, altered tidal currents which led to sedimentation of Mangawhai’s harbour.
• This shallower water in the harbour threatens Mangawhai’s waterfront community with flooding.

Question 7

Studies conducted by the Auckland Regional Council suggest increased rates of coastal erosion are likely in the future, true or false.
Explain

Answer 7

• True
  There is declining natural protection from extreme storm events.
  So, coastal retreat from mining is complicated by climate change and rising sea level.
• Long term retreat by the end of the century is estimated to be 35m with the width of the coastal zone susceptible to erosion being from 48-111m.

Question 8

What is the only strategy being used in Mangawhai-Pakiri to counteract the negative effects?

Answer 8

• Dredging of the harbour and groyne construction on the spit has helped to restore some equilibrium
• This is in response to sedimentation of Mangawhai’s harbour from altered tidal currents.