Science Centres: Oceans

Revealing the forces that shape beaches

A time-averaged picture of Tairua Beach. The two white bands indicate the positions of the shoreline (left) and sandbar (right). Hourly Cam-Era images from eight New Zealand beaches can be accessed from the Cam-era website.

Understanding what causes some parts of beaches to erode and others to build up is important for coastal dwellers and developers. An international research team led by NIWA’s Dr Giovanni Coco has been investigating how sandbars influence beach shape.

The scientists analysed digital video footage of Tairua and Pauanui beaches, on the Coromandel Peninsula. The footage was taken at hourly intervals over several years as part of NIWA’s Cam-Era project. The positions of the shoreline and sandbar were automatically determined from variations in colour and brightness, then compared over time using sophisticated statistical techniques.

The results of this FRST-funded study show that the shape of the shoreline usually mirrors the shape of the sandbar, although any undulations in the bar are generally larger than undulations in the beach.

‘This coupling probably arises from the sandbar setting up a particular pattern of water circulation, which then reinforces itself by shaping the sandbar and shoreline. In other words, a little wiggle reinforces itself and gets bigger and bigger’, says Dr Coco.