Sedimentation

Sedimentation of estuaries and seabed habitats is a natural process, caused by slips in native forest.

However, our activities on land and in the sea have accelerated erosion rates and led to dramatically increased rates of sediment entering our waterways.

Excessive sedimentation is can smother habitats and thereby change ecological composition by killing and displacing marine invertebrates, shellfish, and algae.

Forestry, farming, road building and washouts, seabed dredging and river management all contribute excessive sediment levels to the coastal environments, especially to the estuaries in the inner Pelorus Sound/Te Hoiere and Kenepuru Sound.

Dating of seabed sediment layers shows sedimentation rates have increased about 10 times over historical rates.

The removal of wetlands and the destruction by dredging of extensive natural mussel beds, which filtered and removed sediment in the Pelorus Sound, has compounded the ecological effects.

These effects include clogging fish and shellfish gills, and reduced light to large and small algae affecting photosynthesis and the flow of energy through the food web.

Council is undertaking research into the causes and consequences of sedimentation from different land-uses.

The research includes:

• Seabed coring to identify the sources and rates of sedimentation.
• Review of the science around forestry effects on the Sounds.
• Inventory of slips on Sounds roads which entered coastal waters.
• Capturing historic aerial photo surveys to identify land use change.

The results of these initiatives will inform Council's policy formulation and the development of codes of industry best practices to mitigate and minimise sediment.

The information gathered will also assist in restoration of estuary and seabed habitats.

To find out more email Council's Coastal Scientist