The works that are the subject of the EIA Study include the construction and operation phases of the Project. The key components of the Project include the following:
i. Dredging of a series of seabed pits within the proposed East of Sha Chau Facility Boundary (Part 1, Figure 2.4b);
ii. Backfilling each dredged pit with contaminated mud that has been classified as requiring Type 2 disposal in accordance with ETWBTC 34/2002; and,
iii. Capping each backfilled pit with uncontaminated mud and/or public fill effectively isolating the contaminated mud from the surrounding marine environment.
Despite the proven acceptability and close proximity of the existing facility, the purpose of the assessment was to thoroughly evaluate the East of Sha Chau Facility in terms of acceptability of predicted impacts to water quality from dredging, backfilling and capping of the pits and also concurrent activities.
This Section describes an engineering design for the proposed East of Sha Chau Facility, which is based on maximising disposal capacity, ensuring continuity in use of the site, and ensuring that environmental impacts are environmentally acceptable and no greater than those associated with existing CMP operations. The information presented in this section is taken from the preliminary design and will be refined at the detailed engineering design stage.
The Project involves the sequential disposal of contaminated mud into a series of dredged pits, provisionally titled Pits A, B, C and D. The sequential construction and operation of the pits has been used to develop scenarios for sediment transport modelling, assess marine traffic issues and identify key environmental issues for water quality, ecology, fisheries, human health, noise and heritage assessments.
Previous purpose-dredged contaminated mud disposal facilities at the East of Sha Chau area have had backfill levels to 3m below original seabed. Recently, however, the very large disused sand borrow pits now used as CMPIV have an allowed backfill level up to 6m. This greater depth was deemed necessary because of the greater surface area of material that would be subject to exposure to the effects of storm waves.
In contrast to the above, the preliminary design of the East of Sha Chau Facility consists of smaller, shallower pits that would minimise exposure of contaminated mud thus reducing the potential for dispersion outside of the pit boundary. As a result of these design features, the same backfill level design of 3m below original seabed level that has been employed in the design of the East Sha Chau Facility.
Caps at previous CMPs in the East Sha Chau Area have consisted of 3 m layer of uncontaminated material placed by controlled bottom dumping from barges. Additional clean mud has been added later to compensate for long-term consolidation of the contaminated mud. Such practice has been employed in the design of the East of Sha Chau Facility. The rationale for the design of the cap design () () is to keep the contaminated material beyond the reach of bioturbation and to protect it against storm erosion.
The potential for damage and breaching of the cap due to anchorage has been considered, but the shallow water of the East of Sha Chau Facility restricts the size of vessel which can anchor in the area which, in turn, restricts the size of anchor and the penetration depth.
Once the EIA Report has been formally approved by Government, CEDD will obtain an Environmental Permit (EP) for construction of the Project. Once the EP has been obtained the first pit is expected to be dredged during 2008 in order to be ready to receive contaminated mud in early 2009. According to arisings estimates the fourth pit at the East of Sha Chau Facility will be backfilled and capped during the first half of 2015. It should be noted that should the rate at which contaminated mud arises change (either increasing or decreasing) then the fourth pit maybe capped earlier or later than 2015. The tentative construction programme is presented in Figure 1.1a. It should be noted that the timeline presents predicted timeframes for each works component.
Figure 1.1a Indicative Works Sequencing at the East of Sha Chau Facility
A requirement in the Study Brief is to examine the cumulative effects of other projects concurrent with construction and operations at the East of Sha Chau Facility. Projects that have been identified as occurring potentially at the same time are detailed below:
· Disposal at North Brothers
· Reclamations along North Lantau Coastline
- Potential New Town Extension at Tung Chung East and Tung Chung West
- Lantau Logistics Park
- Potential Theme Park
- Reclamations at Yam O
· Permanent Aviation Fuel Facility (PAFF)
· Highway Projects
- Tuen Mun to Chek Lap Kok link
- North Lantau Highway Connection to the Hong Kong - Zhuhai - Macao Bridge
· Sewage Discharges
- Siu Ho Wan Sewage Treatment Work (STW)
- Pillar Point Sewage Treatment Work (STW)
The significance of the above Projects to the proposed East of Sha Chau Facility is discussed in more detail in the Water Quality Impact Assessment (Part 3, Section 2).