Sustainable sand extraction: sand mapping first step in 3-year study 

Hume, T. M.; Smith, Quentin H. T. 

2000 

 

 

Article in journal 

More than one billion cubic metres of sand lie at the entrance of the Kaipara Harbour. Swept up the west coast by large ocean waves, some of the sand enters the harbour where it forms shoals. Because of its particular grade, the sand is dredged and then barged to Helensville where it is used to manufacture ready-mix concrete and concrete blocks. When resource consents for sand extraction were re-negotiated in 1997, the Minister of Conservation specified that a study be undertaken to obtain a better understanding of sand movements in the harbour. Shoreline erosion and extraction rates were identified as key issues for the study. A joint venture was formed between the two extraction companies, Winstone Aggregates Ltd and Mt Rex Shipping, plus the Auckland Regional Council and the Northland Regional Council to fund the study. NIWA, in association with the University of Auckland, was appointed to undertake the study. Terry Hume, a coastal research scientist with NIWA in Hamilton, is leading the research, which is by far the largest of its type undertaken in New Zealand. "The Kaipara Harbour has a very big sand system," says Terry. "There are two consents for 150,000 cubic metres of sand to be extracted a year and, although this is a relatively small amount compared to the massive volumes we believe are in the system, this is such an important resource it is vital that we understand its dynamics and behaviour." Measuring an underwater sand resource requires both experience and technology. NIWA has undertaken research into the Pakiri–Mangawhai sand resource on Northland’s east coast, and the movement of sand in the Manukau Harbour and at Piha Beach on Auckland’s west coast. NIWA has an armoury of highly sophisticated equipment for mapping sand reserves and measuring sand movements under the waves. The swift currents and ocean waves that roll in through the entrance of the Kaipara Harbour create an undulating seafloor. Sand forms in banks, which are separated by channels or pathways along which the sand is transported as it moves about the harbour. A sonar unit is towed behind a research vessel and takes pictures of the seafloor out to 100m either side of the vessel. These images show the pathways where sand is channeled and areas of rock and shell where strong currents strip the bed clean of sand. Cameras are lowered into the sea to take photos of the seabed to confirm the sonar pictures. Seabed samples are also dredged up for analysis. "Our immediate goal is to map where the sand is," says Terry. "We measure the speed at which it is moving and determine how much sand is in the system. We also want to know where it is coming from and where it is going to in the harbour." The study could take up to three years to complete and the results will likely determine the sustainability of the Kaipara Harbour sand resource for many years to come. NIWA contact: Terry Hume, Hamilton, [email protected] ARC contact: Quentin Smith, [email protected] 

 

sand mining; Biogeophysical 

http://www.niwa.cri.nz/pubs/an/archive/aniwaniwa14_sand.html [Web page no longer active]