10               conclusion

10.1          Introduction

10.1.1     This EIA Report has provided an assessment of the potential environmental impacts associated with the construction and operation phases of the drainage improvement works at Pak Ngau Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk Tei Tong By-pass Channel under the Project, “Drainage Improvement in Southern Lantau”. 

10.1.2     Specific mitigation measure requirements for the Project, as well as an environmental monitoring and auditing programme, have been developed during the assessment of the proposed drainage improvement works.  The Implementation Schedule of the recommendations are presented in Section 11.  The principal findings of this Report are summarized below.

 

10.2          Air Quality Impact

10.2.1     Impacts arising from the construction of the Project would primarily relate to dust nuisance from excavation works, material handling and gaseous emissions from the construction plant.  With the implementation of mitigation measures as stipulated in the Air Pollution Control (Construction Dust) Regulation, any construction dust nuisance associated with the construction of the proposed Project would be minimised.

 

10.3          Noise Impact

10.3.1     Noise arising from the construction activities of the proposed Project would have potential impacts on NSRs located in the vicinity of the proposed work areas.  Unmitigated construction noise levels at representative NSRs were predicted to exceed the EIAO-TM daytime construction noise criteria.

10.3.2     Sufficient noise control measures would be required to mitigate the potential construction noise impacts, such as adopting quiet construction plant, using movable/temporary noise barriers, and implementing good site practices.

10.3.3     With the recommended control measures in place, the mitigated noise levels at most of the NSRs would comply with the EIAO-TM noise criteria.  A NSR located in very close proximity to the works site of the retaining wall at Pak Ngau Heung River outlet would still be exposed to noise levels in exceedance of the EIAO-TM noise criterion, despite all practicable direct noise mitigation measures exhaustively employed.  However, the residual impacts are minimised and it is expected that the residual impacts would be short-term and localised.

 

10.4          Water Quality Impact

10.4.1     The key issue in terms of water quality would be related to excavation works during the construction phase of the proposed drainage improvement works.  To minimize potential impacts on water quality during the channel construction, special working method controls and pollution control measures have been recommended.  In particular, excavation works at each river channel would be carried out in dry condition by use of containment measures within the channel.  The recommended water quality mitigation measures should be enforced by the provision of specifications of pollution control measures into works contracts.

10.4.2     Other potential sources of water quality impact associated with the proposed drainage improvement works would comprise construction site runoff and drainage; debris, refuse and liquid spillages from general construction activities; and sewage effluents from the construction workforce.  Minimization of water quality deterioration could be achieved through implementing adequate mitigation measures, such as control measures on the runoff and drainage from the works areas to minimize construction run-off.  Proper site management and good housekeeping practices would also be required to ensure that construction wastes and materials would not enter the adjacent waterbodies or storm water drains.  Sewage effluent arising from the construction workforce would also require appropriate treatment through provision of portable toilets. 

10.4.3     As such, with the implementation of these recommended mitigation measures, the construction works for the proposed drainage improvements to the river channels in Southern Lantau would not be anticipated to result in unacceptable impacts on water quality.  Site inspections should be undertaken routinely to inspect the construction activities and works areas in order to ensure the recommended mitigation measures are properly implemented.  In addition, given that Pak Ngan Heung River, Tai Tei Tong River and Luk Tei Tong River discharge to the three-river confluence at the head of the Silver River, which ultimately discharges to Silver Mine Bay, monitoring of downstream water quality within the rivers was recommended during the construction phase. 

 

10.5          Waste Management Implications

10.5.1     The main waste arising during the construction phase would be excavated material from the river improvements works at Pak Ngan Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk Tei Tong Bypass Channel. The excavated material would comprise soil, gravel and small rocks.  It was recommended that the excavated material with suitable characteristics be reused on-site, as far as practicable, such as for fill material for backfilling of temporary drainage channels or construction of embankments.  Surplus excavated material would require disposal to a public filling area. 

10.5.2     Other wastes generated by the construction activities are likely to include general refuse from the workforce, chemical wastes from plant and vehicle maintenance, and C&D material from the demolition of existing structures and site clearance.  Provided that these identified waste arisings are to be handled, transported and disposed of using the recommended methods and good site practices are to be strictly followed, adverse environmental impacts would not be expected during the construction works.  The recommended mitigation measures should form the basis of the Waste Management Plan to be developed by the Contractor.

 

10.6          Ecological Impact

10.6.1     A literature review and ecological surveys covering the wet and dry seasons were undertaken to establish the ecological profile of the assessment area.

10.6.2     Habitats considered of moderate/moderate-high ecological value included Pak Ngan Heung River, Tai Tei Tong, Luk Tei Tong River and Luk Tei Tong (2) River, secondary woodland on the hillsides surrounding Mui Wo Valley and Butterfly Hill, and Luk Tei Tong Marsh. Other habitats recorded such as active agricultural land and villages were considered of lower ecological value.

10.6.3     Several faunal species of conservation interest were recorded in the assessment area, comprising two fishes, one amphibian, and 16 birds. Of particular interest were the fish species Flagtail (Kuhlia marginata) and Predaceous Chub (Parazacco spilurus). Avifauna of conservation interest included White-shouldered Starling (Sturnus sinensis), which have been recorded breeding adjacent to the proposed LTT works area near the confluence, and various species of avifauna recorded feeding at the lower reaches of the three rivers, and to a lesser extent, Luk Tei Tong Marsh. Previous records of the rare and locally protected Romer’s Tree Frog (Philautus romeri) have also been made from the Mui Wo Area. Although not recorded during recent surveys, it is possible this species still occurs in the Assessment Area.

10.6.4     The key ecological impact resulting from the Project would be the direct impacts to 175m of PNH River, 160m of TTT River, and 225m of LTT River. The Project would also directly impact approximately 0.13ha of moderate-high value secondary woodland, and 0.8ha of Luk Tei Tong Marsh. Although the Marsh as a whole was considered of moderate-high ecological value, areas directly affected by the proposed works were observed to be relatively dry, dominated by weedy species, and not to be of particular conservation interest.

10.6.5     Measures to avoid, minimise and compensate for identified ecological impacts have been recommended. These included measures to control sedimentation, the design of drainage channels (such as the use of natural lining for channel beds, rock-filled gabion to line the channel banks, and compensatory planting along the channel banks), design of LTT (2) River Bypass Channel to include compensatory marsh habitat, avoidance of works close to White-shouldered Starlings (Sternus sinensis) during the main breeding season of the Starling, and translocation of fish and amphibian species of conservation interest

10.6.6     With the implementation of the recommended mitigation measures, residual impacts from the proposed drainage improvement works are expected to be ecologically acceptable.

 

10.7          Cultural Heritage

10.7.1     Based on cultural heritage field surveys undertaken, key potential archaeological impacts would be limited to small areas of minor excavation works associated with the proposed 0.75m U-channel at Ling Tsui Tau Tsuen within the Chung Hau Archaeological Site. Further investigation and a watching brief are recommended.

10.7.2     There may be potential vibration impact to the existing retaining wall and associated structures of the Yuen Compound during the construction of rubble seawall and retaining wall at the Luk Tei Tong River, requiring careful construction and vibration monitoring.

 

10.8          Overall Conclusion

10.8.1     The findings of this EIA have provided information on the nature and extent of environmental impacts arising from the construction and operation of the Designated Project elements of the Project.  The EIA has, where appropriate, identified mitigation measures to ensure compliance with environmental legislation and standards.

10.8.2     Overall, the EIA Report has predicted that the Project would be environmentally acceptable with the implementation of the proposed mitigation measures for construction and operation phases.  An environmental monitoring and audit programme has been recommended to ensure the effectiveness of recommended mitigation measures.  Table 10.1 summarises the key environmental outcomes arising from the EIA study and environmental benefits of the environmental protection measures recommended.

Table 10.1     Summary of Key Environmental Outcomes/ Benefits

 

Issue

Environmental Outcomes/Benefits

 

The Project

After completion:

·       relieve the risk of flooding

·       minimise economic loss

·       minimise disruption to everyday life especially during the wet season in Mui Wo.

·       sustain the residential developments

 

Benefited areas:

·       Tai Tei Tong, Luk Tei Tong, Mui Wo Town, Chung Hau, Ling Tsui Tau, Pak Ngan Heung.

 

Potential consequences without the Project:

·       Persistent flooding along the Pak Ngan Heung River, Luk Tei Tong River, Luk Tei Tong (2) River, Ling Tsui Tau and Mui Wo Town; 

·       Hinder future development in Mui Wo, Southern Lantau.

·       Damage to property and nuisance to the public.

 

Drainage Options, Alignment and Construction Methods

The factors such as social, ecological and other environmental aspects have been considered in light of different drainage options, alignment and construction method. The following outcomes are found:

(a)   The proposed alignments of the river channels follow the existing ones as far as possible. The affected area and hence land resumption and house clearance are minimum.

(b)   Diversion box culvert and channel are recommended in Pak Ngan Heung and Luk Tei Tong (2) River respectively to avoid affecting the existing rivers.

(c)   The proposed works provide a direct and effective way to discharge stormwater and prevent flooding.

(d)   The potential environmental impacts would be local and limited to the construction stage. The impacts could be mitigated by appropriate measures and choice of suitable construction methods.

(e)   The proposed drainage improvement works provide a longer-term solution for flooding.

 

The following ecological-friendly design would be incorporated into the channel designs in order to mitigate the environmental impact.

 

For Pak Ngan Heung, Tai Tei Tong and Luk Tei Tong Rivers:

·       The existing river will not be deepened and the existing riverbed will be untouched as far as practicable.

·       Channel beds would be constructed using natural materials such as small cobbles and boulders. Where possible, construction would utilise suitably sized cobbles/boulders found in the existing rivers. This recreated habitat would provide a suitable environment for many organisms dependent on the existing watercourses.

 

·       Channel banks would be largely trapezoidal, and lined with gabion blocks.

 

·       The depth of the channels would be varied to re-create the pool-riffle sequence found in natural streams.

 

·       Larger pools in the stream and river currently maintained for crop irrigation would be re-instated.

 

For Luk Tei Tong (2) River:

·       A bypass drainage is proposed to convey floodwater to Luk Tei Tong River during heavy rainstorm.

·       The Luk Tei Tong By-pass Channel bed would be reinforced with geo-textile matting. The matting would be overlain with turf taken from adjacent areas of Luk Tei Tong Marsh that would be directly affected by the proposed works. This area would be maintained as compensatory marshland habitat.

·       The banks of the whole proposed Channel would be lined with rock-filled gabion.

 

Construction Programme

The preliminary construction programme was designed to minimize the potential environmental impact by taking account of the following factors:

(a)   High flooding risk for the outlets - the improvement works for the outlets should be carried out in the dry season only.

 

(b)   Impact to the existing marsh at Luk Tei Tong - as agreed with the green group, the construction works for the proposed Luk Tei Tong By-pass Channel should be carried out in the dry season in order to minimize the potential impact on the existing marsh area at Luk Tei Tong.

 

(c)   Cumulative impacts – in order to minimize the potential impacts to the existing sensitive receivers and public, the cumulative impacts for the concurrent projects, as well as those proposed works under this project have been duly considered. The Contractor should be encouraged to programme the construction activities under the project in such a manner that ongoing activities would be separated physically as far as possible from one another and other construction sites.

 

Construction Air Quality Impact

·       Potential air quality impacts: dust nuisance and gaseous emissions from the construction plant and vehicles

 

·       Mitigation measures: implement the Air Pollution Control (Construction Dust) Regulation and good site practices.

 

·       No adverse air quality impact would be anticipated.

 

Construction Noise Impact

·       Unmitigated cumulative construction noise levels: 62 dB(A) to 100 dB(A).

 

·       Recommended mitigation measures: adopting quiet construction plant; using movable/temporary noise barriers; and implementing good site practices.

 

·       Under mitigated scenario, all the NSRs would be protected to comply with the daytime construction noise standard as set out in the EIAO-TM, except the NSR in close proximity to the works sites of retaining wall at Pak Ngau Heung River outlet.  However, it is expected that the residual impacts would be short-term and localized.

 

·       An EM&A programme would be implemented to monitor construction noise impact.

 

 

Water Quality Impact

·       Key potential source of water quality impact: excavation works during construction phase.

 

·       Minimisation of potential impacts: working method controls recommended e.g. excavation works carried out in dry condition and containment measures used within the channel.

 

·       Other potential sources of impacts: construction site runoff and drainage; debris, refuse and liquid spillages from general construction activities; and sewage effluents from the construction workforce. 

 

·       Minimisation of water quality deterioration: implementation of adequate mitigation measures, proper site management and good housekeeping practices; and appropriate treatment of sewage effluents.

 

·       Site inspections would be conducted during the construction phase.

 

·       No unacceptable impacts on water quality would be anticipated. 

 

Waste Management Implications

·       Main waste: excavated materials from the three rivers (approx. 45,200 m3).

 

·       Excavated material with suitable characteristics should be reused on-site as far as practicable (approx. 6,400 m3). Surplus material would require disposal to a public filling area.

 

·       Other wastes generated by construction activities: general refuse from the workforce, chemical wastes from plant and vehicle maintenance, and C&D material from the demolition of existing structures and site clearance.

 

·       With implementation of the recommended mitigation measures for the handling, transportation and disposal of the identified waste, no residual waste management impacts would be expected.

Ecology

·       River channels designed to provide suitable habitats for existing aquatic communities, and to largely compensate for impacts to marsh habitats. A specific EM&A programme would be implemented to monitor the relative effectiveness of river and marsh habitat recreation.

 

·       Potential impacts to a breeding colony of the locally rare White-shouldered Starling would be largely avoided through scheduling of works close to the colony outside of the breeding season. A specific EM&A programme would be implemented to ensure impacts to the colony are avoided and minimised.

 

·       Impacts to woodland habitats and trees compensated through landscaping measures.

 

·       Fish and amphibian species of conservation interest potentially occurring within works areas would be temporarily translocated from works areas to suitable nearby habitats or holding aquariums to minimise impacts.

 

·       A fish ladder constructed in Pak Ngan Heung River would allow migration of fish species in this river, thus improving the value of the river to fish communities.

Impact of Cultural Heritage

·       Further investigation and a watching brief are recommended for the excavation works associated with the proposed U-channel at Ling Tsui Tau Tsuen within the Chung Hau Archaeological Site.

 

·       Careful construction and vibration monitoring are recommended to minimize the potential vibration impact to the existing retaining wall and associated structures of the Yuen Compound during the construction of rubble seawall and retaining wall at the Luk Tei Tong River.