For at least 150 years New Zealand has been pushing its harbours, towns and effluent out beyond the shifting shoreline. Wharves and harbour breakwaters reach seaward, often causing damage to the coastal seabed. When structures are built on the water’s edge, the sand that naturally moves along a coastline can become trapped. This can lead to coastal erosion in some areas and a build-up of silt or sand in others.
In many places along New Zealand’s shores, shallow seabeds have been reclaimed to supplement scarce flat land. As its name suggests, Wellington’s downtown street, Lambton Quay, used to be the waterfront. The engineering technology involved in reclamation is relatively simple. Rocks and rubble are poured into the shallow water until they rise above it. This fill is compacted and allowed to settle.
A bit of Bristol
The beach at Oriental Bay, Wellington, has only a small natural supply of sand. A sandy beach had been engineered there long before it was re-nourished with South Island sand in the early 2000s. On 9 October 1944 the Evening Post reported that 10,000 tons of clean, Bristol Channel sand, originally used as ships’ ballast in vessels from England, found a new use and was tipped over the sea wall.
As standards of hygiene improved around 1900, reticulated sewerage systems discharged waste, often through short seabed pipelines, into the ocean. Except in a few places where ocean currents carried the effluent away, this replaced one health hazard with another. Swimming and collecting shellfish had to be banned in areas close to many of these outfalls. More recent offshore pipelines have been kilometres long, and carry only treated effluent to areas where coastal currents will disperse it away from populated coastlines.
Pipelines also carry other products, mainly oil and gas. One off Waikato carries ironsands from the beach out to Japanese ships moored 3 kilometres offshore.
There has been some belated respect in recent times for the fragility of the seabed and shoreline. This has been largely due to the Resource Management Act 1991, which placed tight restrictions on coastal engineering works and structures built along the shore and on the sea floor.