To manage environmental impacts from dredging and reclamation works, Boskalis is implementing various mitigating measures in compliance with EIA requirements and in addition to these requirements. All of the mitigating measures are focused on reducing the spreading of sediments in the water as much as possible. To make sure these measures are helping, environmental monitoring is carried out daily to ensure water quality meets the limits set in the EIA.
Environmental Mitigation
Environmental Management – In Compliance with EIA Requirements
Reclamation - Perimeter bund construction
Construction of a perimeter bund (a barrier to contain the sediment) on the southern boundary of Gulhifalhu is the main mitigating measure to minimise plumes of fine sediments, created by the Project reclamation works, from entering s into the waters outside Gulhifalhu lagoon.
To reduce sediments spreading from the bund construction itself, several mitigating measures are used during the construction of the bund:
An optimized bund construction methodology will be used. Sand from the previously reclaimed land will be used to construct the bund. The bund will be constructed in small sections and covered with a sheet of geotextile, to keep the sand in place. Sand will be placed to form the bund when the current pushes the sediment towards the lagoon, and the section will be covered with geotextile when the current would otherwise take the sediment southwards, away from the lagoon.
Reclamation - Silt Screen
In addition to the perimeter bund, silt screens will be used, where practical, to further limit sediment plumes entering into the waters outside Gulhifalhu lagoon. A silt screen is planned to be used at the northern end of the construction works inside the lagoon, to minimize sediment plumes from exiting the Gulhifalhu lagoon through the entrance channel.
It is also planned to use silt screens at the bund construction site, during the construction of the perimeter bund. These silt screens will help to minimize sediment plumes reaching Hans Hass Place and Villingili.
It can be difficult to use silt screens effectively in situations where there is a strong current. In order to ensure that the silt screens used are as effective as possible, various test installations of the silt screens will be placed at the start of the construction works to help decide on the final silt screen layout.
Sand Borrow Area - Dredging Restrictions
In addition to the perimeter bund and silt screens installed in the area of the construction activities, an additional measure to help limit impacts from sediment plumes is the implementation of a “Green Valve” on the dredging vessel. A Green Valve helps reduce the amount of air bubbles in dredgers overflow (excess water mixed with some sediment), this in turn helps the sediment in the overflow to sink down, reducing the area over which the sediment plume spreads.
Controlled Overflow Zones are designated within the area of the dredging activities in case the Green Valve fails or needs to be turned off. In these situations overflow from the dredger will be avoided or minimized.
In addition to the Green Valve and Controlled Overflow Zones, a ‘No Dredging Zone’ has been established where dredging activities are not allowed. In other areas there are time restrictions on the dredging activities meaning that no dredging can take place during certain times in these locations. The details of the No Dredging Zone and the time period restrictions are provided below.
1) Dredging restrictions in the Primary Borrow Area
– Parts of the Primary Borrow Area are marked as a Controlled Overflow zone
2) Dredging restrictions in the Alternative Borrow Area A
– The whole of Alternative Borrow Area A is marked as a Controlled Overflow zone.
3) Dredging restrictions in Alternative Borrow Area B
– No dredging will be undertaken at the eastern arm of the Alternative Borrow Area B, the No Dredging Zone.
– Dredging will be avoided between 15 December and 15 January.
– The remaining half of the borrow area is a “Controlled Overflow” zone for the dredger.