(a) Construction Runoff
(I) At all parts of all works areas and construction sites, and throughout the full duration of the construction contract(s), the following shall be implemented:
(i) sediment tanks of sufficient capacity to handle the predicted water flows shall be provided for settling out waste water prior to its disposal. The sediment tanks shall be constructed from pre-formed individual cells of approximately 6 to 8 m3 capacity. The system capacity shall be flexible and able to handle multiple inputs from a variety of sources including influent that is pumped;
(ii) all drainage facilities and erosion and sediment control structures shall be regularly inspected and maintained to ensure proper and efficient operation at all times and particularly following rainstorms. Deposited silt and grit shall be removed regularly and disposed of by spreading evenly over stable, vegetated areas;
(iii) open stockpiles of construction materials (for example, aggregates, sand and fill material) of more than 50 m3 shall be covered with tarpaulin or similar fabric during rainstorms. Measures shall be taken to prevent the washing away of construction materials, soil, silt or debris into any drainage system; and
(iv) all temporary and permanent drainage pipes and culverts provided to facilitate runoff discharge shall be designed for the controlled release of storm flows. All sediment traps shall be regularly cleaned and maintained. The temporary diverted drainage shall be reinstated to the original condition when the construction works have finished or the temporary diversion is no longer required.
(II) At all parts of the works areas and construction sites that may potentially experience spillages of oil or fuel associated with construction works, and throughout the full duration of the construction contract(s), oil interceptors shall be provided in the drainage system downstream of any oil/fuel pollution sources associated with construction. The oil interceptors shall be regularly emptied to prevent the release of oil and grease into the storm water drainage system after accidental spillages. The interceptor shall have a bypass to prevent flushing during periods of heavy rain.
(III) At all parts of all works areas and construction sites, and throughout the full duration of the construction contract(s), existing drainage arrangements shall not be adversely affected during construction works, and any flow from the construction site must be diverted to settling traps/ponds before being discharged into the public drainage system.
(b) Dredging and Marine Disposal of Dredged Sediment
During dredging works associated with the construction of foundations for viaducts over the Shing Mun River, and Siu Lek Yuen and Tai Shui Hang Nullahs, the following mitigation measures shall be implemented to minimise the potential water quality impact during dredging:
(i) contaminated sediments shall be dredged using grabs of no more than 8 m3;
(ii) barges shall be fitted with tight fitting seals to their bottom openings to prevent leakage of material;
(iii) barge loading shall be accurate to avoid splashing dredged material into the surrounding water; and
(iv) construction works shall cause no visible foam, oil, grease, scum, litter or other objectionable matter to be present in the water within the site or dumping grounds;
(c) General Construction Activities
At all parts of all works areas and construction sites, and throughout the full duration of the construction contract(s), debris and rubbish on site shall be handled and disposed of to avoid entering the water column and causing water quality impacts. Temporary on-site storage of excavated materials from station and depot construction works shall be covered with tarpaulin or similar fabric during rainstorms. Any washout of construction or excavated materials should be diverted to the drainage system via sediment traps. Stockpiling of the excavated material can be minimised by scheduling the construction programme in a way that one section of the alignment can be constructed and completed before the excavation works of the next section commence.