In this section the ‘construction’ of the mitigation measures is described in more detail.
1. Relocation of coral communities
The environmental study identified a significant colony of healthy Elkhorn coral that could be affected by the dredging works. Prior to those works, a specialist marine diving company was employed to examine the condition of the Elkhorn and advise on possible methods for relocating the colony to a suitable site. It was considered feasible to move the best specimens to a location outside the harbour basin.
- 300 healthy Elkhorn corals identified for relocation;
- Relocation site found to the north of the harbour basin;
- Transport under water using air bags;
- Set in new location using grout;
- Completed in 1 month.
2. Water quality and current monitoring
Extensive numerical modelling was carried out to assess the impact of the proposed harbour on the currents and the water quality of the broader area. The modelling showed that the currents would not adversely be affected and that the water quality in the lagoon would not deteriorate as a consequence of the works. However the sediment modelling showed that to avoid any return of sediment from the pipe onto the coral reefs, the pipe outlet would have to be located at the 40 m depth contour to the south of the project site.
Since the local economy is highly attached to the existence of the coral reef, project and authorities raised great public concern on the possible adverse impact of the works. Therefore, an extensive water quality monitoring was established including continuous turbidity monitoring stations and current monitoring stations, plus vessel based water quality monitoring (turbidity and suspended solids).
An environmental study had been performed well in advance of any construction works to establish the baseline water conditions in the area surrounding the project location. Based on this study and after discussions between governmental representatives, financiers, the owner and the contractor, a monitoring protocol was developed that addressed the concerns for the protection of surrounding coral reefs.
Up to a distance of 300 m from any construction point the following restrictions apply:
- Early warning level: the 60-minute depth average turbidity level at the monitoring station should not exceed the 95% of the baseline values plus 20 NTU.
- Action level: the depth average suspended solid level should not exceed 95% of the baseline values plus 17 mg/l.
- Target level: the depth average suspended solid level should not exceed 95% of the baseline values plus 20 mg/l.
3. Construction of two settlement basins and placement of a silt screen
The dredged material was to be discharged into an onshore and offshore reclamation area. Approximately 85% of the dredged material was to be pumped onshore. The remaining volume was to be pumped along the inside of the breakwater. Excess water was to be treated such that suspended solid levels released back into the sea were kept to a minimum.
At the onshore reclamation two settlement basins were created. To minimise the spread of suspended solids outside the harbour basin the following measures were put in place:
- Commencement of this reclamation was only permitted to start if a sufficient lead of breakwater core was available.
- A silt screen was to be put in place 100 m downstream of the discharge point. The silt screen was to force settlement of suspended solids close to the reclamation area.
The area of settlement of suspended solids would always be inside the harbour basin. The extremely low currents in the harbour basin would not create sufficient shear strength to allow re-suspension transporting the settled suspended solids outside the harbour basin. The area would be cleaned up by the cutter suction dredger (during the final clean-up phase) or would be covered via dry filling.
4. Discharge of reclamation water below the 40 m depth contour
Due to the volume of excess water being pumped onshore, despite passing through two settlement basins discharge of reclamation water could have an adverse impact on the water quality. Coral surveys had demonstrated that coral communities below the 25 m depth contour were virtually non-existent. As an extra safe guard it was decided to discharge the excess water below the 40 m depth contour via a pipeline.
The above mentioned mitigation measures and water quality monitoring were part of the total contract package. Compliance with these measures, installation, operation and maintenance of the water quality monitoring as well as compliance with the water quality limits were a contractual obligation to the contractor.