Mangawhai-Pakiri CS

• Where is the Mangawhai-Pakiri coastline?

  New Zealand
• What is sand used for?

  1. Construction
  2. Concrete making
  3. Glass manufacturing
  4. Beach replenishment
• Why is the sand in the nearshore zone at Mangawhai-Pakiri useful?

  1. High quality
  2. Suitable for the construction industry
  3. Just 50km north of Auckland it is convenient for NZ's largest and economically most dynamic metropolitan region
• What is the population of the Auckland region?

  Over 1.5m (accounts for 2/3 NZ's total pop and 35% of country's GDP)
• How is the region growing rapidly?

  Apart from business, finance and high-tech industry, tourism centred on Auckland's outstanding coastal amenities is booming (2015 saw 2.3m foreign visitors)
• How long has sand dredging operated for and how much has been extracted?

  1. Operated for over 70 years
  2. 1994-2004 165,000 m3/year was extracted
• What are current rates of extraction

  Ended (2005) at Mangawhai but has continued at Pakiri Beach

  Current rates of extraction = 75,000 m3/year until 2020
• What is the sand extracted used for?

  1. Replenishing Auckland's tourist beaches
  2. Construction industry (many holiday home and hotels being built in area)
• How has sand been deposited on the coastline?

  1. During Holocene (past 9,000 years) = non-renewable
  2. Few sizeable rivers in area
  3. Most thought to have been derived from offshore
• What type of system is the coastline essentially and why significant?

  1. Essentially a closed system
  2. Outputs of sand not replaced by inputs from rivers, and waves from offshore
  3. Extraction rates at Pakiri Beach exceed input rates by a factor of 5
  4. Effect = deplete total sand supply stored in dunes, beaches and on sea bed (up to 2km offshore) = movements of sand between major stores have diminished
• How is the depletion of sand impacting on coastal landforms?

  1. Beaches (starved of sediment) = wider and flatter = less effective in absorbing waves
  2. Higher energy waves erode beaches and landforms (i.e. dunes and spits) become vulnerable
  3. Foredune ridges undercut by wave action, developing steep, seaward-facing scarps
  4. Loss of vegetation cover = susceptible to wind erosion
• What happened in the 1978 storms?

  1. 28-metre breach at the base of the Mangawhai spit- this (and second breach) altered tidal currents = sedimentation of Mangawhai’s harbour
  2. Shallower water in the harbour threatened Mangawhai’s waterfront community with flooding

  ALTHOUGH subsequent dredging of the harbour and groyne construction on the spit has helped restore some equilibrium
• What complicates the fact that coastal retreat is attributed partly to sand extraction?

  Climate change and rising sea level
• What is long-term retreat by the end of the century estimated at?

  1. 35m
  2. Width of coastal zone susceptible to erosion varies (48-111m) but this estimate is higher than any of Auckland region's other 123 beaches