In dredging the Harbour channel and disposing of material offshore, Port Otago is responsible for measuring the turbidity of the receiving waters. Turbidity is the concentration of suspended particles, or more simply, the cloudiness of water.
An overview of the current status of all aspects of monitoring can be found on the following page: Monitoring Summary - Dashboard
Dredging involves picking up sediment from the Harbour floor and relocating it to other locations. This is carried out by Port Otago for maintaining the Harbour channel and deepening it as part of the Next Generation project. Stirring up sediment like this leads to a higher concentration of particles floating in the water, and therefore higher turbidity or cloudier water.
If the turbidity of water becomes too high the particles in the water can reduce the sunlight filtering through to the plants and animals. The biology in the Harbour naturally has a degree of tolerance due to the nature of their habitat. However, if high turbidity is sustained then it can have a negative impact.
Port Otago has 7 fixed turbidity monitoring sites within the Harbour which continually measure the concentration of particles. The recorded data is sent to Port Otago and if the turbidity exceeds particular values then prescribed actions are undertaken. There are several thresholds of turbidity, with each threshold having an associated action plan which is outlined in the Environmental Management Plan. The actions range from notifying the Otago Regional Council within 24 hours of the event, to stopping dredging.
There are 3 moored buoys offshore in the vicinity of the A0 disposal site that measure the receiving coastal waters. If the measured turbidity exceeds the trigger level or turbidity limit, actions outlined in the consent and Environmental Management Plan need to be taken to reduce the turbidity levels.
The compliance limits (of B minus A) within the consents and outlined also in the EMP are: