Benthic Mitigation & Monitoring

Construction Mitigation
The key concern on water quality impacts during the construction phase of the project is sediment
dispersion from dredging, jetting and water pumping operations.  Sediment release rates for
sediment dispersion modeling were estimated based on the selected working rate for dredging,
jetting speed of the jetting machine, and pumping rate for seawater removal from suction
caissons.  The predicted results showed no adverse water quality impacts.  Mitigation measures to
minimise water quality impacts arising from the operations with potential sediment release are
recommended as follows:

  • Working rate for dredging in Junk Bay should not exceed 6,300 m3/day for two dredgers.
  • Jetting speed should not exceed 75 m/hr in the section, which starts at the dredging end
    point on the seaward side of the transmission power cable in Junk Bay and extends
    towards the offshore direction of the transmission power cable.
  • Jetting speed should not exceed 150 m/hr for jetting operation carried out in the remaining
    sections of the transmission power cable and the array cable at the wind farm.  
  • Pumping rate for seawater removal from suction caissons during foundation installation
    should not exceed 1,200 m3/hr per foundation or 300 m3/hr per pump.  
  • Closed grab dredgers should be used for sediment dredging in Junk Bay.  The mechanical
    grabs should be properly maintained to minimise spillage of sediment.
  • Silt curtains should be provided surrounding the dredging point to minimise dispersion of
    sediment plumes.  Arrangement of silt curtain is presented HERE.
  • Barges for disposal of dredged marine sediment:
  • Bottom of the barges should be fitted with tight seals to prevent leakage of sediment
    during transport.
  • Filling of dredged marine sediment should only be up to a level that sediment would
    not spill over during transport to the disposal site.
  • Adequate freeboard should be provided to avoid washing the sediment overboard by
    wave action.
  • Dredging operation should be carefully controlled to avoid splashing sediment into
    the sea when transferring the dredged sediment to the barge.  
  • Excess material from decks and exposed fitting of barge should be cleaned before
    the barge is towed to the disposal site.
  • The decks of the barges and other marine vessels should be kept clean and tidy,
    and are free pollutants, i.e. oil and grease.
  • Good site management practices should be implemented to avoid water pollution at all
    times during the construction phase.


Construction Monitoring
Water quality and coral monitoring will be included as part of the Environmental Monitoring and
Audit programme to ensure that water pollution during the construction phase of the project is
minimal.  Details of the water quality monitoring and audit requirements are included in the EM&A
Manual.


Operational Mitigation and Monitoring
There would be no unacceptable hydrodynamic and water quality impacts during the operational
phase.  Water quality monitoring and audit is considered not necessary.
Epifauna community at a South China
Sea Oil Rig
Source: Asiatic Marine