Deposition

Farewell Spit is a spectacular coastal feature that extends for 25 kilometres into the sea at the northern tip of the South Island. This image shows clearly the power of the ocean currents that have washed sediment north and eastwards from the West Coast. Image: GNS Science, Lloyd Homer.

Farewell Spit is a spectacular coastal feature that extends for 25 kilometres into the sea at the northern tip of the South Island. This image shows clearly the power of the ocean currents that have washed sediment north and eastwards from the West Coast. Image: GNS Science, Lloyd Homer.

There are three main processes responsible for the formation of depositional features, such as bars

Rivers supply huge amounts of sediment to add to the pebbles and sand that have been eroded by wave action along the shoreline. After heavy rainfall, plumes of brown muddy water can often be seen extending from the mouths of our rivers out into the sea.

Longshore drift is the sideways transport of beach sediment along the coast due to waves striking the shore at an angle. It is largely dependent on the prevailing wind direction relative to the orientation of the shoreline. Spits are coastal features that are created by longshore drift. Our most famous example is Farewell Spit in Gold Bay, which is one of the longest in the world. The sand that makes up the spit has been eroded by rivers from the Southern Alps,and then washed up the west coast into Golden Bay by strong currents and prevailing westerly winds.

Wind: Dunes are drifts of windblown beach sand. In New Zealand there are some quite extensive areas of dunes around our coasts. As time passes they are stabilised by highly adapted plants that have long roots and can withstand the harsh salty winds as well as the alternation of wet and dry weather.

Extensive dunes next to the entrance of Hokianga Harbour in Northland. Image: GNS Science, Lloyd Homer.

Extensive dunes next to the entrance of Hokianga Harbour in Northland. Image: GNS Science, Lloyd Homer.