Rivers: the land-coast connection

Rivers: the land-coast connection

Gravel extraction, Waimakariri River, Canterbury. (Photo: above, Graham Fenwick, NIWA)

(Photo: above, Alistair McKerchar, NIWA)

NIWA scientists are developing models to predict the impact of changes in river sediment loads on coastal erosion. Rivers are a key connection between the land and the sea, carrying sand, gravel, and cobbles down river, and depositing them at the coast. This material is then redistributed and groundup by waves and currents. Coastlines slowly advance where the river sediment supply is plentiful relative to the capacity of waves to transport the sediment and/or abrade it away. Conversely, coastlines may retreat where the river sediment supply is insufficient to balance the losses at the coast.

When humans intervene – for instance by extracting sand and gravel from river beds or by building dams for hydroelectricity or irrigation – this can cause changes in the amount of sediment arriving at the coast. For example, a naturally stable coastline may begin to erode, or erosion may accelerate on a naturally eroding coastline. Both cases can cause problems for people managing the coastal zone.

Scientist Murray Hicks says that there is still a lot which is not understood. ‘Every catchment and its adjacent coast is unique, and impacts depend on local conditions. There may also be a long time lag before the effects of upstream catchment works are seen at the coast’, says Murray.

Research subject: Coasts