Sand mining along the Mangawhai-Pakiri coastline (New Zealand)
Background information
Sand is an essential mineral resource in a modern economy
Located 50km North of Auckland, it is convenient for the nations largest and most economically active region
Auckland population is 1.5 million (1/3 of total) and produces 35% of GDP
Growing business, finance and tech industry
Coastal tourism is growing (2.3 million foreign visitors in 2015)
Offshore sand mining and the sediment budget
Nearshore sand dredging along the 20km coastline between Mangawhai and Pakiri has operated for over 70 years
Between 1994 and 2004, 165,000 cubic metres per year was extracted
Current rates of attraction are 75,000 cubic metres per year until 2020
Mining at Mangawhai ended in 2005 but continues at Pakiri
Large amounts of sand used to replenish Auckland's beaches
Sand is a non-renewable resource along this coastline and was deposited 9000 years ago
Few sizeable rivers in the area and most sand was derived from offshore
Coastal sediment budget is essentially a closed system so outputs by sand mining is not replaced by inputs from waves or rivers
Extraction rates at Pakiri exceed inputs by a factor of 5
Mining will deplete the total sand supply, stored in dunes, beaches and sea bed
Movements of sand between major stores has diminished as a result of this
Impacts on coastal landforms
Beaches starved of sediment have become wider and flatter and are less effective at absorbing waves
Higher energy waves erode beaches and landforms like dunes and spits are now vulnerable
Foredune ridges are undercut by wave action, developing steep, seaward facing scarps
Loss of vegetation cover makes them susceptible to wind erosion
Storms in 1978 caused a 28m breach at the bas of Mangawhai spit, along with a second breach, led tidal currents to be altered. Leading to the sedimentation of Mangawhai's harbour
Shallower water in the harbour threatened the waterfront community with flooding
Dredging of the harbour and groyne construction on the spit helped to restore some equilibrium
Increased rates in the future (Auckland regional council)
Coastal retreat by the end of the century is estimated at 35m
Width of coastal zone susceptible to erosion varies from 48m to 111m
This estimate is higher than all 123 other beaches in Auckland's regions