CASE STUDY - Mangawari Pakiri (human use of a coastal landscape)

Question 1

How is the area economically significant?

Answer 1

• Sand is an essential mineral resource in a modern economy, has a wide range of uses including construction, concrete making, glass manufacture and beach replenishment
• A high-quality sand resource occurs in the nearshore zone at Mangawhai-Pakiri on the east coast of New Zealand’s Northland Peninsula - this sand is high quality and suitable for the construction industry
• Located just 50km north of Auckland, it is convenient for New Zealand’s largest and economically most dynamic metropolitan region
• Population of over 1.5 million - Auckland region accounts for 1/3 of New Zealand’s total population and 35% of the country’s GDP
• Apart from business, finance and high-tech industry, tourism centred on Auckland’s outstanding coastal amenities is booming, 2015 saw a record 2.3 million foreign visitors

Question 2

Detail the process of offshore sand mining occuring

Answer 2

• Nearshore sand dredging on the 20km long coastline between Mangawhai and Pakiri has operated for over 70 years - between 1994 and 2004, 165,000m^3/ year were extracted
• Although mining ended at Mangawhai in 2005, it has continued at Pakiri Beach - current rates of extraction are 75,000m^3/yr, a large proportion of this sand is used for replenishing Auckland’s tourist beaches

Question 3

How has this impacted the sediment cell balance?

Answer 3

• Deposited during the Holocene (past 9000 years), sand is a non-renewable resource on the Mangawhai-Pakiri coastline
• There are few sizeable rivers in the area and most sand is thought to have been derived from offshore
• The coastal sediment budget is essentially a closed system, thus outputs of sand through nearshore mining are not replaced by inputs from rivers and waves from offshore - extraction rates at Pakiri exceed inputs by a factor of five
• The effect of mining is therefore to deplete the total sand supply, stored in dunes, beaches and on the sea bed (up to 2km offshore), as a result, movements of sand between the major stores have diminished

Question 4

How has this impacted coastal landforms?

Answer 4

• Given closed nature of the sediment cell at Mangawhai-Pakiri, current rates of sand extraction are unsustainable, depletion of sand is already having an impact on landforms and landscapes
• Beaches starved of sediment have become smaller and flatter and are less effective in absorbing waves, higher energy waves thus erode beaches, and landforms such as dunes and spits become vulnerable
• Foredune ridges are undercut by wave action, developing steep, seaward-facing scarps, loss of vegetation cover makes them susceptible to wind erosion
• 1978, storms caused a 28m breach at the age of Mangawhai spit, this and a second breach altered tidal currents, which led to the sedimentation of Mangawhai’s harbour, shallower water in the harbour also threatened Mangawhai’s waterfront community with flooding, subsequent dredging of the harbour and groyne construction on the spit has helped restore some equilibrium

Question 5

How has this impacted rates of coastal erosion?

Answer 5

• Studies of the Mangawhai-Pakiri coastline by the Auckland Regional Council suggest increased rates of coastal erosion are likely in future with declining natural protection from extreme storm events - coastal retreat is already evident and is attributed party to sand extraction, though this is complicated by climate change and rising sea levels
• Long-term retreat by the end of the century is estimated at 35m and the width of the coastal zone susceptible to erosion varies from 48 to 111m, significantly, this estimate is higher than any of the Auckland region’s other 123 beaches