2.1 The proposed landslide preventive works will be constructed to protect the existing residential developments at the toe of the project site. The location of the project is shown in Figure 1.1. As the proposed works at Po Shan Area will fall within the boundary of the existing Pok Fu Lam Country Park and Special Area, the Project is classified under Category Q.1 of Part 1 in Schedule 2 of the Environmental Impact Assessment Ordinance (EIAO) as a Designated Project. Location of Pok Fu Lam Country Park is illustrated in Figure 1.1. Figure 1.2 shows the general layout of the Project.
2.2
The study area can be
broadly defined as encompassing the mostly undeveloped hillsides above the
residential development at
2.3 The Agreement was awarded by CEDD to Maunsell Geotechnical Services Ltd. (MGSL) as the engineering consultant in April 2005 for design and construction of the underground drainage adits.
2.4
Following a shallow
landslide on the hillside in June 2005, the CEDD carried out a natural terrain
landslide risk assessment. The assessment concluded that natural terrain risk
mitigation works is necessary on the hillside above
2.5 The scope of works includes the installation about 700 numbers of soil nails and about 60 numbers of raking drains on the natural terrain within the concerned area as shown in Figure 1.4. The length of the soil nails is about 20m with a spacing of 2m horizontally and 3m vertically; the length of raking drains is about 10m with a spacing of 5m horizontally and 15m vertically.
2.6 Rock slope stabilisation works will be provided for the rock outcrop / boulders at the upper portion of the natural terrain. Measures such as scaling, installation of rock bolts / dowels, construction of concrete buttress and provision of wire mesh protection will be provided where necessary.
2.7 No tree felling will be proposed under this Project.
2.8 The Project will be carried out concurrently with the construction works of drainage adit within the Po Shan area as shown in Figure 1.1
2.9 The purpose of this EIA Study is to provide information on the nature and extent of environmental impacts arising from the construction and operation of the proposed landslide preventive works, and to contribute to decisions on the overall acceptability of the Project, after the implementation of proposed mitigation measures.
2.10 The EIA study has been prepared in accordance with the guidelines provided in Annex 11 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM) for the report contents and Annexes 12 to 19 for the impact assessments of various environmental issues. The general approaches and methodologies adopted for this EIA study are described below.
Existing Environment Condition
2.11 The characteristics of the existing environment were reviewed for identification and prediction of environmental impacts. Baseline surveys were conducted to determine the existing environmental conditions on the Project site and environs likely to be affected by the Project. The baseline conditions of the key issues as identified in the EIA Study Brief including ecology, landscape & visual, noise, water quality and air quality are described in the assessment.
Impacts Prediction
2.12 The EIA Study was undertaken in accordance with the guidelines on assessment methodologies given in Annexes 12 to 19 of the EIAO-TM.
2.13 Quantitative approach was adopted for assessing the construction noise impacts. The predictions were conducted based on well-proven, internationally accepted methods.
2.14 The applied methodologies for the Project had previously been adopted in other EIA studies. They have been generally accepted for use in predicting environmental impacts and for comparison of assessment results with the EIAO-TM requirements. Limitations are however envisaged of these methodologies. The accuracy of the prediction result will be affected by uncertainty in input data such as sound power levels of construction plants to be used on-site. The most realistic data have been used in the prediction in order to provide a result with as small as possible the quantitative uncertainty.
Mitigation Measures
2.15 Mitigation measures are proposed to alleviate the potential impacts predicted. The proposed mitigation measures were adopted in other environmental assessment study with similar construction works. Environmental Monitoring and Audit programme is recommended to ensure the recommended mitigation measures could effectively mitigate the impacts to comply with the corresponding criteria.
Considerations of Different Technology and Design Options
2.16 The concerned area had been studied under Agreement No. CE 29/2003(GE) for assessing the risk to facilitates downslope generated by shallow geological processes. Based on the findings by the Quantitative Risk Assessment (QRA), the proposed mitigation works scheme had been optimized as the only possible method to adequately reduce the natural terrain landslide risk, which includes a combination of soil nails and raking drains; otherwise, massive upgrading works such as retaining wall and backfilling may have to be applied that would cause excessive nuisance to the environment.
2.17 The QRA carried out under the captioned study had identified four groups of landslide hazards (Figure 2.1) that have been significant in the geomorphological development of the site as below.
1. Retrogressive / retreating failures of the residual colluvium on the eastern and western spurs of the study area within the locally steeper portions of slope.
2. Debris avalanche and debris flow type failures sourcing from the upper rock dominated part of the study area, between the cliffs.
3. Rock slide (wedge and planar) and rock fall failures on the rock cliffs in the mid to upper portions of the catchment.
4. Deep-seated failures may have occurred in the past within the residual colluvium, and possibly within the buried saprolite below the colluvium.
2.18 In order to improve the stability of the Po Shan area against shallow slope failures (i.e. the aforementioned hazard group No. 1), the proposed mitigation scheme by means of localised soil nails was proposed under this Study.
Considerations of Alternative Construction Methods and Sequence of Works
2.19 The detailed design of the proposed works was accepted by GEO Checker in March in 2007 under this Agreement No. CE 28/2004(GE). Under this specific design, all soil nails and raking drains have been well located so that no tree felling will be involved. However, exact locations for the proposed soil nails and raking drains can still be slightly adjusted on site (if necessary) to minimize nuisance to those existing trees on the captioned area.
2.20 As recommended under the QRA, it is concurred that provision of soil nails and raking drains would cause the least impact to the environment as comparing with retaining wall construction, filling and slope cutting. If other slope upgrading measures have to be applied instead of the current proposed scheme by means of soil nails and raking drains, bulk excavation is inevitably required and tree felling or transplant has to be carried out.
2.21 Regarding the rock slope stabilisation works, measures such as scaling, installation of rock bolts / dowels, construction of concrete buttress and provision of wire mesh protection installations on locally unstable rock outcrops would be proposed. The scope for such works is considerably small in extent and shall not have any environmental impact.
Selection of the Preferred Scenario
2.22 The soil nailing scheme for the slope stabilization works involves no slope cutting or filling that may cause unnecessary tree felling. Also, it shall induce significantly lower ground disturbance when comparing with the construction of massive retaining structure. Hence, it is considered to be more environmental friendly.
2.23 Following the review of available mitigation measures, the natural terrain hazard risk mitigation at the study area was therefore designed with a combination of soil nails, raking drains and some localised rock slope stabilisation works. After applying the proposed measures, the risk generated by shallow geological process would be reduced to acceptable levels.
Implementation and Planning of the Proposed Project
2.24 The proposed works are scheduled to commence in November 2007 with duration of 10 months. The preliminary construction programme for the Project is shown as shown in Table 2.1 and presented in Appendix 2.1.
Table 2.1 Preliminary Construction Programme
|
Activity |
Time period |
|
Site clearance and set up |
November 2007 to mid November 2007 |
|
Soil nail installation |
Mid November 2007 to mid July 2008 |
|
Soil nail head construction |
December 2007 to July 2008 |
|
Rock slope stabilization works |
Mid November 2007 to July 2008 |
|
Raking drain installation |
March 2008 to July 2008 |
|
Landscaping works |
August 2008 to Early September 2008 |
|
Site reinstatement works |
August 2008 to Early September 2008 |
2.25 There are interactions with another designated project “Agreement No. CE 28/2004 (GE) Landslide Preventive Works at Po Shan, Mid-levels – Design and Construction” (under Environmental Permit EP-235/2005/A) on-going during the same period within the captioned area which will also be carried out by the Project Proponent.
2.26 The project "Agreement No. CE13/2005(WS), Improvement to Hong Kong Central Mid Level and High Level Areas Water Supply – Remaining Works" proposed by Water Supplies Department, will be commenced in end of 2007. With reference to the proposed development at Hatton Road Service Reservoirs, it is noted that such works will be carried out at location away from the works areas of the Project. No interaction between the two projects is expected.
Scenarios with or without the Project
2.27 A qualitative risk assessment was performed to identify the optimal scheme with consideration of environmental disturbance during installation and visual appearance following installation. Based on the findings, the proposed scheme involving localised soil nailing works and minor rock slope / boulder treatment works has been optimised to adequately reduce the natural terrain landslide risk to below the current risk acceptability criteria.
2.28 Without having the proposed works implemented, the facilities situated downslope of the Po Shan Catchment are considered vulnerable to future natural terrain landslides. Based on the rational assessment of the likely debris flow paths, lateral spread of debris deposition, and the debris volume sourcing from natural terrain landslides initiating with the catchment, the vulnerable facilities are defined laterally as the frontage of Po Shan Mansions, the nearby amenity plot to the east and the drainage channel to the west.
Scope of Key Environmental Issues
2.29 The identified key environmental issues regarding the construction of the proposed Project are listed as below:
· potential losses or damage to flora, fauna and natural habitats and the potential ecological impacts from the Project during its construction and operation stages;
· potential landscape and visual impacts to nearby sensitive receivers from the Project during its construction and operation stages;
· potential construction noise impacts to nearby sensitive receivers from the Project;
· potential construction water quality and dust impacts to nearby sensitive receivers, as well as waste management implications, from the Project; and
· potential cumulative impacts to nearby sensitive receivers for the above key issues, taking into account the existing construction works mentioned in paragraph 2.13.
2.30 The assessments of these key environmental issues are to be discussed in the following chapters of this Report.
2.31 It is identified that the proposed Project has no impacts at all on environmental aspects of cultural heritage and agriculture & fisheries. Hence these impacts are not assessed in this EIA Study.
Key Assessment Assumptions and Limitations of Methodologies
2.32 The key assessment assumptions, limitation of assessment methodologies and all relevant prior agreements with the EPD on individual environmental media assessment components are summarised in Appendix 10.2.