2.1.1.1
Ngong Ping is located on a
plateau area to the west of Fung Wong Shan (
2.1.1.2 At present, there is no comprehensive drainage planning for the area. A heavy rainstorm on 7 June 2008 caused severe flooding at the Po Lin Monastery and adjacent areas and further storms pose a serious flood risk to the local communities including the Po Lin Monastery and nearby villagers. The livelihood of about 1600 local residents and the normal operation of Po Lin Monastery were affected, in addition to the Po Lin Monastery being closed to the public for about a week after the incident. As a result, the Ngong Ping tourism activities were seriously impeded. In late 2008, Drainage Service Department (DSD) commissioned the “Drainage Study for Ngong Ping” (“the 2008 Drainage Study”) to identify and quantify the flood hazard and formulate cost effective drainage improvement plans. The 2008 Drainage Study has set out a comprehensive strategy and programme to safeguard the concerned area from flood risk.
2.1.1.3 The 2008 Drainage Study identified that the flood protection level of the existing watercourse to the north of the Po Lin Monastery and Ngong Ping 360 Terminal represents a less than 1 in 10 year design return period and needed to be improved to a flood protection level of 1 in 50 year design return period. The 2008 Drainage Study, also, recommended a series of first aid and short-term flood improvement measures to alleviate flood risks in the local areas. These measures have been implemented starting from 2009 and have subsequently been completed. The long-term measures recommended to improve the overall drainage capacity of the area form the basis of this Project. The existing drainage system and the long term measures to provide the necessary improvements are discussed below.
2.2.1.1 The catchment of Ngong Ping area covers part of Nei Lak Shan, Lantau Peak, Po Lin Monastery, Ngong Ping Terminal and local village area, etc with total area of about 1.4km2 (Appendix A1).
2.2.1.2 The existing stormwater drainage system comprises seven portions, (see Figure 2.1). Brief descriptions of these seven portions are given below:
· Portion A - Watercourse near water storage tank at the North of Po Lin Monastery;
· Portion B - Box culvert underneath Po Lin Monastery;
· Portion C - The 2.85m (W) x 1.65m (H) box culvert located at the south of Po Lin Monastery;
· Portion D - The 1650mm diameter twin-pipe;
·
Portion E - The
· Portion F - Gabion channel near Ngong Ping 360 Terminal; and
· Portion G - The natural stream of Ngong Ping Stream at the downstream of Portion F gabion channel.
2.2.1.3 The runoff from the sub-catchment of Nei Lak Shan is collected by the box culvert underneath the Po Lin Monastery (Portion B) and the runoff from the sub-catchment of the Lantau Peak is collected by the 2850mm (W) x 1650mm (H) box culvert located to the south of the Po Lin Monastery (Portion C). These collected runoffs are discharged into the 1650mm diameter twin-pipe (Portion D). The runoff is then conveyed westward, through the existing watercourse near the local village area (Portion E) and passing through the embanked gabion channel (Portion F) and natural stream (Portion G) near Ngong Ping 360 Terminal before being discharged downhill along the downstream section of Ngong Ping Stream.
2.2.2
Flooding Incidents and Flooding Risk Analysis
2.2.2.1 Flooding incidents have been previously recorded in a few locations within the Study Area (see Figure 2.2). The 2008 Drainage Study concluded that flooding in the area was mainly caused by:
(i) the backwater effect of the bottleneck section of the existing drainage system (Portion E);
(ii) the blockage of entrance of the box culvert underneath Po Lin Monastery (Portion B); and
(iii) the blockage of flow path for the 1650mm diameter twin pipe (Portion D);
2.2.2.2 The 2008 Drainage Study included a detailed hydraulic modelling assessment of the existing drainage system and the findings are summarised in Table 2.1 and described below.
Table 2.1 Predicted Maximum Flood Depth of Existing Drainage System
|
Portion |
Max. flood depth (m) for Design Return
Period (Yr) |
|
50 |
|
|
Portion
A - Watercourse near Water Storage Tank to the North of the Po Lin Monastery |
0.15 |
|
Portion
B - Box Culvert underneath |
- |
|
Portion
C - Box Culvert Located to the South of the |
- |
|
Portion D - The 1650mm Diameter Twin-pipe |
- |
|
Portion E- The Stream between the 1650mm
Diameter Twin-pipe and the Gabion Channel next to the Ngong Ping 360 Terminal
|
0.81 |
|
Portion
F - Gabion Channel near Ngong Ping 360 Terminal |
- |
|
Portion
G- The Natural Stream downstream of the Gabion Channel |
0.22 |
2.2.2.3 From the hydraulic results of the 50 years return period, the following areas were identified to have high flood risk:
· Due to the marginal flow capacity of upland u-channel of water storage tank, floodwater will overflow from upland u-channel through the overland flow path (existing footway) and then downward to the northern inlet of existing box culvert of Po Lin Monastery;
· The box culverts underneath Po Lin Monastery generally have a flood protection level of 50 years, the flooding is mainly due to the lack of desilting, maintenance and blockage at inlets by utilities intrusion; and
·
Local flooding will occur at
three locations including the bottleneck of stream near
2.2.3
Interim Measures to Improve the Flood Protection Standard
2.2.3.1 In order to provide immediate relief to the flooding problem in some local areas, a series of interim (first aid and short-term) drainage improvement measures were identified in the 2008 Drainage Study and these are shown in Appendices A2 and A3.
2.2.3.2 These first aid and interim measures were implemented starting from 2009 and have subsequently been completed. These measures do not form part of the current Project.
2.3.1.1
The flood protection standards
in
2.3.1.2
As indicated in Table 2.1, based on the 50 years
design return period, flooding between 0.15 to 0.81m was predicted at several
locations including the northern side of the Po Lin Monastery (Portion A),
stream near
2.3.1.3 Hence, the Project needs to be implemented to protect the Po Lin Monastery, the Ngong Ping 360 Terminal, the local villagers and the local habitats.
2.4.1.1
A
fundamental project alternative is the option not to implement the recommended
long terms drainage improvement measures in the study area, which in
environmental terms is referred to as the “Do-nothing” option.
2.4.1.2 If the proposed Project is not implemented, the potential for serious flooding of the Study Area will continue. Flooding not only affects the social, tourist and business activities of the area, but, also, deteriorates the visual appeal of the area and prolonged flooding can damage historical buildings. Post-flooding clean up / restoration, also, translates into additional waste generation and off-site disposal issues. Therefore, flooding can, also, lead to re-current undesirable environmental outcomes (see Table 2.3).
2.4.1.3 Based upon the above, the “Do-nothing” option is not preferred and not considered to be the environmentally preferred solution to the flooding issue and is not further discussed in this report.
2.5.1.1 The primary purpose of the proposed drainage improvement works is to reduce the flooding risk of the area and bring the local flood protection to the current standard of 50 years, specifically at Portions A, E and G, which are particularly prone to high flooding risk (see Table 2.1). Therefore, a viable scheme must be able to meet this design drainage hydraulic performance requirement.
2.5.1.2 Two main drainage improvement alternatives (Options A and B) have been considered for the Project. These two alternative schemes are indicated in Figure 2.3 and key features summarised in Table 2.2 below.
Table 2.2 Key Features of the Alignment Design Options
|
Water Course |
Option A |
Option B |
|
Upstream Section |
An underground DN1500 drain pipe (interception
drain) connecting Portions A and E. About 500m long. |
An underground DN1500 drain pipe (interception
drain) connecting Portions A and E. About 440m long. |
|
Midstream Section |
Widening and realignment of stream section in
Portion E. The length of the alignment is about 140m and the top width of the
channel would be about 8.65m to align with the existing gabion channel. |
A loop system in the form of underground box
culvert. About 223m long. |
|
Downstream Section |
Widening of natural stream section at Portion G. The
length of the alignment is about 240m and the top width of the channel would
be at least 8.65m to align with the existing gabion channel. |
An underground DN1800 drain pipe (flood relief
drain) connecting Portion F and bypass Portion G. About 198m long. |
2.5.1.3 As indicated in Table 2.2 and Figure 2.3, the upstream sections of both schemes have the same alignment and pipeline dimensions. The main differences between the two options are in the middle and downstream sections. Option A directly tackles the identified bottlenecks by widening and realigning the existing stream courses, while Option B address the same issue by provision of an alternate floodway by-passing the bottlenecks. A preliminary appraisal of the potential environmental benefits and dis-benefits are outlined in the sections below and summarised in Table 2.3.
2.5.2.1 Based on the hydraulic performance of the existing drainage system as described in Section 2.2 above, the existing natural stream between the 1,650mm diameter twin-pipe (Portion D) and the gabion channel (Portion F) was found to be a key bottleneck of the existing drainage system. In order to mitigate the flood risk, long-term drainage improvements could be achieved by widening and realignment of the section of natural stream.
2.5.2.2 In addition, there is a need to address the flooding risk at Portion A. However, an alternative floodway, provided as an upgrade (widening) of the existing box culvert (Portion B) underneath the Po Lin Monastery has been dismissed as it would require works entirely within private land lots which could significantly affect the normal operation of Po Lin Monastery. In addition, such widening of the existing box culvert would be expected to result in significant noise, dust and visual impacts to the Po Lin Monastery for the duration of the construction works and would not be recommended. Therefore, a long underground interception drain commencing at a location near the water storage tank located to the northeast of the Po Lin Monastery and passing underneath the existing footpath/access to connect Portion A with Portion E, was proposed. Similarly, to eliminate the bottleneck downstream of the gabion channel (Portion G) and, hence, reduce flooding risk at Portion F, the natural stream in Portion G would be proposed to be widened. The Option A scheme provides traditional engineering migration measures to address the existing drainage bottlenecks.
2.5.2.3 However, further study indicated that this option would not be feasible due to the following constraints:
· The proposed river widening works at Portion E would fall within a number of private lots and land resumption would be required. Any land resumption could significantly delay the project implementation;
· The existing stream section at Portion E is constrained by adjacent structures and clearance of the site would have been required and the provision of an alternative crossing would be required, increasing the complexity of the project; and
· The natural stream section at Portion G is partly within the Lantau North Country Park and was purposely preserved under the Tung Chung Cable Car Project although stream bed rock has been trimmed to improve its hydraulic performance.
2.5.2.4 Replacement of streams in Portions E and G by engineered drainage channels is, also, not ecological favourable and could result in impacts on the ecology of the existing water course, including:
· Permanent loss of natural streams/rivers habitats, and also natural sediments and other substrates important for maintaining species biodiversity;
· Loss of aquatic species biodiversity, including a decline in macro-invertebrate, fish and other aquatic species; and
· Loss of bank-side terrestrial and aquatic vegetation, which provide shelter, shade and a food source for aquatic species.
2.5.2.5 In terms of water quality, during the construction stage, significant water quality impacts would be anticipated as the existing stream course will be affected if Option A is implemented. In addition, as the existing steam course is the only drainage path, a temporary alternative drain would have to be provided, thus, leading to additional off-site impacts. Alternatively, while it may be possible to just work on one half cross section of the stream while leaving another half as the drainage path, controlling the site runoff from the working half would be difficult and the sediment laden water from the working half section could easily affect the downstream section. Since about half of the drainage capacity would be temporary unavailable, the area would be more prone to flooding during the construction and this is not acceptable. This arrangement would, also, prolong the construction period as only half of the site would be available at any one time, leading to a longer potential construction phase environmental impacts to the sensitive receivers.
2.5.2.6 In respect of landscape and visual impacts, Option A would represent a permanent change of the natural stream landscape to an artificial watercourse. While the use of gabion linings and, also, some landscaping works could reduce magnitude of any landscape and visual impacts, this is, also, not preferable as avoidance of impacts is the first priority. In addition, given that the recent objectives in the designs of drainage works has been to restore lined channels to their natural state as far as possible (for example, the recent revitalisation of Kai Tai Nullah to Kai Tak River), Option A would not be favourable from the landscape and visual perspective. While without the implementation of the Project, a flooding incidence could temporary deteriorate the visual appeal of the Study Area, the conversion of a natural stream landscape to a constructed water course would represent a permanent deterioration of the landscape quality which is, also, not favourable.
2.5.3.1 In order to alleviate the flood risk arising from the bottleneck of drainage system (i.e. the stream in Portion E) and to provide relief from flood risks from the existing drainage system but without widening of the stream in Portion E, the alternative proposal was to provide a loop system in the form of a combined box culvert of approximately 174 m long (size 3m(W) x 2.5m(H)) and a box culvert of approximately 49 m long (size 2.5m(W) x 2.5m(H)) is proposed. In order to minimise impacts to the terrestrial ecology, the alignment would mostly follow the existing road as far as possible. Similarly, to protect the natural stream section at Portion G, in the Option B scheme, an underground flood relief drain is proposed. As Portion G comprises mostly vegetated habitats, a trenchless construction method will be used to construct this section of the drainage system to minimise surface works and impacts and only a small bank-side area would be disturbed for construction of Outfall B.
2.5.3.2 While the Option B overall alignment scheme is slightly longer than Option A due the loop at Portion E, it offers several environmental advantages over Option A during both the construction and operation phases. Both the loop system and flood relief drain would avoid directly affecting the existing stream ecology in Portion E and minimises the works in the natural stream section at Portion G to only a small bank-side area for construction of an outfall (Outfall B). Therefore, direct ecological impacts to the natural stream and associated riparian ecology would be almost completely eliminated during the construction stage.
2.5.3.3 In respect of water quality, as the existing watercourse would remain untouched overall, with the exception of only five small areas for the inlets and outfalls, the potential construction phase water quality impacts would be minimised while still achieving the overall design objectives. Nonetheless, good site management would still be required to ensure the potential water quality impacts would be further minimised.
2.5.3.4 In terms of construction phase air quality and noise impacts, while the slightly longer alignment under Option B may mean there could be more sensitive receivers (mainly residential) along the alignment and exposed to construction disturbances at Portion E, the minimum separation between the alignment and sensitive receivers increases under this scheme. Under Option A, the few rural houses to the west of the Monkey’s Tale Theatre will be immediately adjacent to the alignment and there would be no space for the implementation of any necessary mitigation measures. However, under Option B, there is sufficient space between the alignment and the sensitive receivers to allow mitigation and noise attenuation measures to be applied during the works.
2.5.3.5 Since Option B avoids the natural stream and associated riparian habitats, the operational phase ecological impact (permanent loss of aquatic habitat) is, also, substantially reduced. While this is at the expense of more terrestrial habitats being directly affected (mostly temporary only), the adoption of an underground drainage design would reduce the permanent impact (habitat loss) to a few manholes only and these manholes are mostly in developed areas rather than natural habitats. Given that streams in the area are hydro-dynamically linked to the Ngong Ping Stream in the Ngong Ping SSSI which is an important habitat for the endemic Romer’s Tree Frog as well as other fauna and the extent of terrestrial habitat to be lost compared to the overall habitat available in the Study Area is very low, preserving the aquatic habitats is considered more essential than the terrestrial habitats, if none can be avoided. Similarly, with Option B, the natural stream landscape will be preserved during the operation phase and, hence, would, also more preferable from the landscape and visual point of view.
2.5.4
Environmental Option Comparison
2.5.4.1 As described above, the Option B drainage scheme is considered to have less environmental dis-benefits compared to Option A during both the construction and operation phases and, as such, would be the preferred and recommended option for the Project. A summary comparison of the two options is presented in Table 2.3 below.
Table 2.3 Potential Environmental Benefits and Dis-benefits of Alternative Drainage Design Options
|
Environmental Issues |
Design Options |
|||
|
Construction Phase |
Operation
Phase |
|||
|
Option A |
Option B |
Option A |
Option
B |
|
|
Noise |
X |
ü |
- |
- |
|
Air Quality |
- |
- |
- |
- |
|
Water Quality |
X |
ü |
- |
- |
|
Ecology |
X |
ü |
X |
ü |
|
Waste |
- |
- |
- |
- |
|
Landscape and Visual |
X |
ü |
X |
ü |
|
Cultural Heritage |
- |
- |
- |
- |
Note: “ü” = Environmentally preferred option; “X” =
Environmentally not preferred option; “-” = No environmental preference
2.5.5
Selected Drainage Improvement Scheme Design
2.5.5.1 The whole proposed drainage improvement scheme under Option B, including the interception drain, loop system and flood relief drain, provides a flood protection of 50 years or above and, therefore, meets the required design objectives and is, also, considered to present the best environmental option overall and is the selected drainage improvement scheme design.
2.5.5.2 The scope of the selected drainage improvement works under the Project is summarised in Table 2.4 below and the overall layout illustrated in Figure 1.1. A schematic general layout of the works, including the works boundary, alignment chainage and locations of works areas is illustrated in Figures 2.9a-2.9g.
Table 2.4 Key Components of the Proposed Works
|
Location |
Works |
Approx. Length |
Dimensions |
Proposed Construction Method |
|
Northern side of the Po Lin
Monastery (Upstream Section) |
New underground drainage
pipe (Interception Drain) |
440m |
DN1500
drain pipe |
Cut-&-Cover
excavation 250m) and trenchless method
(190m) |
|
Northwest of the Po Lin
Monastery near (Midstream Section) |
New underground box culvert
(Loop System) |
223m |
49m 2.5m x 2.5m box culvert + 174m 3mx2.5m box culvert |
Cut-&-Cover excavation |
|
Northern side of the Ngong
Ping 360 Terminal and Columbarium (Downstream Section |
New underground box drainage
pipe (Flood Relief Drain) |
198m |
DN1800
drain pipe |
Trenchless method |
2.5.6.1 Since the selection of the preferred Alignment (Option B), the alignment and design has continued to be modified in order to reduce potential environmental impacts and to improve the scheme where possible. The modifications have concerned the pipeline alignment between the Ngong Ping 360 Terminal and the Columbarium area (Works Section 6, see Figures 2.9a-2.9g).
2.5.6.2 Under the preferred selected alignment (Option B), the pipeline would start near the Ngong Ping 360 Terminal and the alignment was proposed to run north-west west, across the existing road in order to avoid the burial ground and graves (including a historical grave NP27, see Section 8) in the burial ground. The pipeline alignment would then proceed to north-west toward the outfall at the steep cliff area near the Columbarium. Under this option, the outfall would be constructed at the cliff area and would require some rock cutting/drilling. The location of the outfall close to the steep cliff area would, also, increase the difficulties of the construction works as well as future maintenance. A certain degree of landscape impacts could be expected. The location of this scheme, developed under the “2008 Drainage Study” during the planning stage, is shown in Figure 2.4 and denoted Sub-option B1.
2.5.6.3 However, Sub-option B2 was developed during the detailed design stage to avoid and minimise some environmental impacts. As shown in Figure 2.4, Sub-option B2 only slightly varies from that of Sub-option B1, with both alignments avoiding the burial ground and the historical grave. However, the Sub-option B2 pipeline alignment would run parallel to the existing road, instead of crossing under the road as with Sub-option B1, to link to an outfall located at the existing shallow pool area near the Columbarium. Under Sub-option B2, the outfall is shifted southward by about 13m such that it would not be located at the cliff and, therefore, would be less visually prominent and, hence, more desirable. In addition, the shift in the alignment allows a patch of protected orchids species in the cliff area (see Section 6) to be avoided and retained.
2.5.6.4 Subsequently, Sub-option B3 was developed. This option retained the location of the outfall at the existing shallow pool area near the Columbarium. However, the location of a proposed manhole between the Ngong Ping Cable Car Aerial Ropeway and the Columbarium has been relocated to north of the road. The relocation means that the jacking pit during construction and associated working area, would avoid an area rich in flora species of conservation interest located in the original location of the manhole and jacking pit for Sub-Options B1 and B2. As such, Sub-option B3 combines the benefits of Sub-option B2 but presents an improvement avoiding key species of conservation interest which would otherwise have been affected during both the construction and operational phase.
2.5.6.5 Overall, Sub-option B3 is preferred as it avoids affecting protected species and has reduced landscape and visual impacts. It, also, represents a better balance of various construction and operational aspects, including environmental performance, site condition, engineering and technical requirements, as well as cost-effectiveness.
2.6.1 Construction Methodology Options
2.6.1.1 In general, open trapezoidal channels provide the most economical cross-section for the conveyance of stormwater, both in terms of construction and maintenance costs . However, to suit the site conditions and to minimise the associated environmental impacts, underground box culvert and pipelines have been adopted for construction instead. It is, also, by the use of underground facilities that the Option B scheme design offers environmental advantages over Option A, as discussed above.
2.6.1.2 Since the geometry and size of the drainage system is determined by the hydraulic performance required for the Project, the choice of construction methods must be able to meet the design requirements. Rectangular box culvert is commonly constructed by cut-and-cover method. On the other hand, the commonly available construction techniques for the circular pipe include traditional cut-and-cover (C&C) method and trenchless techniques such as pipe jacking method.
2.6.1.3 The proposed construction methodologies of C&C and trench-less method and their suitability to this project are discussed below.
Cut and Cover Method
2.6.1.4
The cut-and-cover (C&C)
method of construction involves excavating a trench, erection of lateral
support, then constructing a structure (or placing a pre-formed structure)
within the trench and then backfilling the structure with soil. Underground drainage
box culverts and pipelines are usually constructed using this open
cut-and-cover methods which is generally suitable to relatively softer ground. It is a proven and commonly used method of
excavation and construction in
2.6.1.5 However, in terms of environmental impacts, this construction method induces a higher level of potential impacts as any natural habitats along the alignment will be directly impacted. The near-by human sensitive receivers would, also, be affected by the construction noise, as well as potential dust nuisance. The temporary exposed open trench would, also, be visually intrusive during the construction phase. For alignment sections that are to be constructed underneath an existing access (path or road), temporary traffic (vehicular and/or pedestrian) diversion would typically be required.
2.6.1.6 For this Project, the C&C method is a suitable method for constructing the 2.5m x 2.5m box culvert at Works Section 4 (see Figures 2.9a-2.9e for location of the Works Sections), the 3.0m x 2.5m box culvert at Works Section 5 and, also, the connecting junction between the box culverts and the upstream interception drains (at Chainage B4+49). Due to land availability issues, the interception drain at Works Section 1 is planned to be located along the existing access road and, therefore, a straight alignment is not possible but instead it has to keep turning every 20-30m along the access road alignment. For the same reason, a straight alignment for the interception drain at Works Section 3 is, also, not possible although the alignment variations are less often in this portion. Because of need for frequent changes in the alignment direction, the C&C method is preferred construction method meeting the design requirement.
Trenchless Method
2.6.1.7 A trenchless method installs a pipe underground without the need of open trench excavation from the surface, although entry and exit pits would be required. One such trenchless technique is pipe-jacking where jacking pits of typically 4m (W) x 8m (L) x 5-10m (D) and receiving pits of 4m (W) x 5m (L) x 2-8m (D) are constructed.
2.6.1.8 Pipe jack tunnelling uses a specialist jacking system, which consists of a tunnelling shield in front (i.e., the jacking head / cutting shoe), a string of drain pipe segments and a hydraulic drive. This system is placed inside jacking pit, except for a control room and sedimentation tank which are placed at the ground level. In short, the shield is jacked forward incrementally by the hydraulic jack at the jacking pit. As the shield advances, jacking pipes are inserted behind the shield one by one and the whole string is jacked forward. Once a pipe segment is completely driven into the soil, the hydraulic drive is with-drawn, a new pipe segment added and the jacking process repeats until the pipeline reach the receiving pit where the tunnelling shield can be retrieved. The excavated materials are then transported to the surface through a slurry discharging pipe (for slurry shield), or a screw conveyor to the jacking pit by a trolley system, and then lifted up to the ground surface. In order to reduce the friction during the jacking process, often a small amount of the lubricant, typically bentonite slurry, is required.
2.6.1.9
In terms of environmental
performances, the trenchless method offers several advantages over the
traditional C&C method. With the trenchless method, construction of an open
trench along the alignment is not necessary and the directly affected areas are
limited to the construction pits only. Thus, direct impacts associated with
natural ecological habitats, dust, air quality, water quality, landscape and
visual and cultural heritage are limited to the development and operation of
the pits during the construction stage. In addition, the amount of powered
mechanical equipment required is, also, reduced in the trenchless technique as
excavation and backfilling works are substantially reduced. Also, the jacking system would be located at
the bottom of the pit, about 6m below ground level in the present case, and,
thus, the noise of the operation would be effectively shielded from the
sensitive receivers.
2.6.1.10 The trenchless method, however, is not without environmental disadvantages. Firstly, the pit area will be occupied throughout the construction period and the duration of the occupancy of the excavated area will be longer than the C&C method as a C&C trench segment can be backfilled once the segment of works have been completed. This means that any sensitive receivers nearby the construction pits would be subject to a longer period of potential disturbance/impacts. Secondly, unlike the C&C method where a trench can be temporary covered, the construction pits cannot be covered during the process as it is the point of entry for all the pipe segments and point of exit for all excavated material, with the erection of a lifting gantry. Therefore, if the technique is chosen, the construction pits should be located to avoid any existing access roads or paths and be placed as far away as possible from any sensitive receivers. Finally, the bentonite slurry lubricant can be polluting to the water environment if it is released but can be controlled with good site management.
2.6.1.11 As discussed above, and detailed in Table 2.6 below, the trenchless pipe jacking method, overall, has some environmentally benefits over the C&C method. However, with due consideration to the site constraints including land use status, engineering requirements of the construction method and the drainage alignment, the pipe jacking method is only feasible for Works Sections 2 and 6 (see Figures 2.9a-2.9g). The selection rationale of construction method for each Works Section is detailed in Table 2.5 below.
Table 2.5 Preferred Construction Methods
|
Drainage
Requirement |
Works
Section (1) |
Construction
Method |
Selection
|
|
Circular
DN1500 drain pipe (interception drain) |
1
|
C&C |
Engineering constraints associated
with catering for alignment changes. |
|
Circular
DN1500 drain pipe (interception drain) |
2 |
Pipe jacking |
Environmental preference
associated with minimising the directly affected area and associated
environmental impacts e.g. air, noise and visual. |
|
Circular
DN1500 drain pipe (interception drain) |
3 |
C&C |
Engineering constraints
associated with catering for alignment changes. |
|
Square 2.5m x 2.5m
box culvert (loop system) |
4 |
C&C |
Engineering constraints
associated with catering for alignment changes and also the traditional method
for construction of box culvert. |
|
Rectangular 3.0m x
2.5m box culvert (loop system) |
5 |
C&C |
Engineering constraints
associated with catering for alignment changes and also traditional method
for construction of box culvert. |
|
Circular DN1800 drain
pipe (flood relief drain) |
6 |
Pipe jacking |
Environmental preference
associated with minimising the directly affected area and associated
environmental impacts e.g. air, noise and visual. |
Notes:
1. Please refer to Figures
2.9a-
2.9g for the location of Works Section.
2.6.2
Environmental Comparison of Construction Methodology Options
2.6.2.1 A comparison of the environmental benefits and dis-benefits of the C&C method and trenchless pipe jacking construction methods is summarised in Table 2.6 below.
Table 2.6 Environmental Benefits and Dis-benefits of Drain Pipe Construction Methods
|
Construction Method |
Benefits |
Dis-benefits |
|
Cut-and-cover Method |
Relatively quicker technique
for short sections of trench. Possibility of re-using
excavated material or surpluses fill from other projects. Catering for alignment
changes. |
Works directly affect habitats
along the complete length of the alignment. All sensitive receivers along
the alignment have the potential to be affected. More construction plant will
be involved and this would generate relatively more noise and dust impacts. Larger amount of material
handing due to excavation and backfilling. More potential for
construction run off due to open excavation. |
|
Trenchless Method |
Surface works limited to the construction pits.
Hence, reduced direct impacts on habitats and vegetation. All works underground so minimising the disturbance
to sensitive receivers along the alignment. Less use of PMEs and also less spoil to be disposed
of compared with C&C method. The underground works will not be visible to the
public and hence reduced visual impacts.
|
Sensitive receivers near by the construction pits
will be subject to a longer period of environmental disturbance. A wider pit is required compared to C&C method
and sufficient area may not be available at congested site. Requires treatment of surplus bentonite before
disposal. |
2.6.2.2 Based upon the environmental preference of the trenchless method, as detailed in Table 2.6, it has been proposed for areas where engineering constraints do not restrict its use. It is noted that the pipe-jacking method would be expected to involve the maximum size required for any construction pits for a trenchless construction method and as such, pipe-jacking and its associated pits have been used as the basis for the environmental assessment as a worst case.
2.6.3.1 The original construction programme anticipated during the early stage of the Project by the Engineer is shown in Figure 2.8a. Based on the programme, essentially two concurrent work fronts were proposed, one for the cut and cover (C&C) works of the drain and box culvert and another one for the trenchless drain construction. After the establishment of a site office at Works Section 2, the C&C pipe laying for the interception drain would begin from Works Section 1 and trenchless works at Works Section 2 for the interception drain will commence at the same time by constructing the jacking pit JP1 at the site office (SO) area. The C&C works and pipe jacking works for the interception drain were proposed to be completed in about 15 months. Afterward, the pipe jacking system will be transferred to Works Section 6 for the pipe jacking works of the flood relief drain while the C&C works would begin the box culvert construction works at Works Sections 4 and 5 for the Loop System. The whole construction works was proposed to be completed in 30 months.
2.6.3.2 However, after review of the original construction programme and the nature of drainage system, it has been recommended that an alternative construction programme be adopted as indicated in Figure 2.8b. The main difference between the two programmes is that the trenchless works at Works Section 6 for flood relief drain will be constructed first instead of the interception drain at Works Section 2 (see the dash line box in Figure 2.8a). The advantages of the recommended construction programme are outlined below:
· There is no change to the overall completion timeframe;
· The flood relief drain at the downstream can be completed and be in operation within 18 months and provide advanced flood protection to the downstream area about 12 months earlier than the original programme where necessary. It is noted that the early completion of the interception drain at the upstream will not provide any similar advantages as the interception drain cannot operate until the loop system is completed; and
· The works in Works Sections 1 and 2, which are located in close proximity, will not be carried out concurrently and, hence, there would be a general reduction in the associated disturbance to the sensitive receivers (see Sections 3 and 4 of the report) to the north of Po Lin Monastery for 13 months (see Figure 2.8a). While the use of the trenchless method in Works Section 2 reduces any environmental disturbance, the avoidance of simultaneous works in this area can further help to minimise any environmental disturbance.
2.6.3.3 Given the above discussed operational and environmental advantages, the alternative construction programme, as indicated in Figure 2.8b, is, therefore, the preferred construction programme and forms the basis of the detailed impact assessment in the subsequent chapters of the EIA Report.
2.6.3.4 In order to further limit the extent and duration of the localised impacts, the cut and cover construction works will proceed section by section. Each active constructing cut and cover section will be about 40m long which should only take 2 months to complete subject to the site condition.
2.7.1.1 As described in Section 2.5.4, Option B is the preferred drainage scheme which can provide adequate flood protection to the Study Area. The scheme design and construction method has been refined where possible to minimise water quality, ecology, cultural heritage and landscape and visual impacts, especially for the natural stream and riparian habitats. The general layout of the preferred alignment is shown in Figure 2.5. The preferred alignment consists of three main systems:
·
An upstream Interception Drain
- an approximately 450m long underground drain pipe (size 1500mm diameter) and
associated inlets. The interception
drain starts at a location near the water storage tank located at northeast of
Po Lin Monastery and is aligned underneath the existing footpath/access/adjacent
vegetation land. The drain ends at the
north of
·
A Loop System - an approximately
179m long (size 3m(W) x 2.5m(H)) box culvert, an approximate 45 m long (size
2.5m(W) x 2.5m(H) box culvert and associated inlet and outfall. The loop system
starts at the
·
A downstream Flood Relief Drain
- an approximately 198m long underground drain pipe (size 1800mm diameter) and
associated inlet and outfall. The flood relief
drain starts at the exiting gabion channel to the north of
2.7.1.2 The full alignment is shown in Figures 2.6a-2.6d while the general arrangement of the intakes and outfalls are shown in Figures 2.7a-2.7e. A geological map of the site, longitudinal profile and typical cross sections of the proposed drainage system are shown in Appendix A4.
2.7.2.1 Based on the linear nature of the drainage alignment and, also, the type of construction works involved, the construction works has been divided into six main Works Sections as summarised in Table 2.7 and shown in Figures 2.9a-2.9g.
Table 2.7 Works Sections and Construction Works
|
Drainage
System |
Works
Section (Chainage) |
Location |
Works
Description |
|
- |
2 (A1+60 – A2+10) |
Northwest of PLM,
around between 法嚴精舍 and 覺淨 |
· Site clearance and construction of site
office (SO). · The area will also be used for stocking
of materials. · The size of the SO is about 1350 m2. · Site hoarding/chain fence will be
established around the perimeter. |
|
Interception Drain |
1 (A0+00 – A1+20) |
Northeast of PLM,
around between the water storage tank and 慧海淨舍 |
· C&C construction of DN 1500mm drain
pipe (120m long), manholes and Intake A. · About 120m excavation: 2.5m wide, depth
range between 4-6m. · A temporary stockpiling area (SA1) of about
110 m2 at the eastern side of the PLM water storage tank. |
|
Interception Drain |
2 (A1+20 –A3+00) |
Northwest of PLM,
around between 法嚴精舍 and 常樂林 |
· Construction of DN 1500mm drain pipe
(180m long) by trenchless method, associated pits and manholes. ·
Jacking pit (JP1) at A1+70, about 10m deep. · Receiving pit
(RP1) at A1+20, about 8m deep. · Receiving pit
(RP2) at A3+00, about 8m deep. · Temporary works areas (WA1 of about 130m2,
and WA2 of about 110 m2) will be established around the two
receiving pits. |
|
Interception Drain |
3 (A3+00 – A4+40) |
Northwest of
PLM, about between常樂林and north end of |
· C&C construction of DN 1500mm drain
pipe (150m long) and manholes. · About 150m excavation: 2.5m wide, depth
range between 4-6m. |
|
|
4 (B0+00 – B0+45) |
North end of |
· C&C construction of 2.5m (W) x 2.5m
(H) box culvert (45m long) and Intake B. · About 45m excavation: 5.5m wide, depth
range between 4-6m. · A temporary stockpiling area (SA2) of
about 880 m2 along the alignment. · Intake B at existing open channel. |
|
|
5 (B0+45 –B2+22) |
West of PLM, around
between north end of |
· C&C construction of 3.0m (W) x 2.5m
(H) box culvert (182m long) and Outfall A. · About 182m excavation: 6.0m wide, depth
range between 4-7m. · A temporary stockpiling area (SA3) of
about 2440m2 along the western half of the alignment. · Outfall A at existing gabion channel. · A temporary haul road may be required to
allow crossing of the existing open channel. |
|
Flood Relief Drain |
6 (C0+00 – C1+96) |
North of the Ngong
Ping 360 Terminal, around between north of |
· Construction of DN 1800mm drain pipe (198m)
by trenchless method, associated
pits, manholes, Intake C and Outfall B. ·
Jacking pit JP2 at C1+10, about 5m deep. · Receiving pit
(RP3) at C0+00, about 5m deep. · Receiving pit
(RP4) at C1+96, about 2m deep. · Temporary works areas (WA3 of about 180m2,
and WA4 of about 100 m2) will be established around the two
receiving pits. · A temporary stockpiling area (SA4) of
about 300m2 at JP2. · Intake C at existing gabion channel. · Outfall B at existing natural stream. |
Notes:
1. JP: Jacking Pit; PLM:
2. Please refer to Figures
2.9a-2.9g
for the location of Works Sections and working areas.
2.7.3 Construction Programme and Sequence
2.7.3.1 The recommended construction programme is shown in Figure 2.8b. It is anticipated that the construction works will begin in the second quarter of 2014 and last for about 30 months. The major anticipated works activities are summarised in Table 2.8 below.
Table 2.8 Summarised Construction Programme (Tentative)
|
Main ID |
Major Task (1) (2) |
Period |
|
1 |
Site preparation and
site office (SO) establishment |
Month: 1 - 2. |
|
2 |
Interception Drain
(C&C at Works Sections 1 and 3) |
Month: 3 - 15. |
|
3 |
|
Month: 16 - 30. |
|
4 |
Flood Relief Drain
(Pipe jack at Works Section 6) |
Month: 3 – 16. |
|
5 |
Interception Drain
(Pipe jack at Works Section 2) |
Month: 16 - 28. |
|
6 |
Demolition of site
office, site reinstatement and landscaping work |
Month: 29 - 30. |
Notes:
1.
Please see Figure
2.8b
for the detailed tentative construction programme.
2.
Please refer to Figures
2.9a-2.9g
for the location of Works Section.
2.7.3.2 The indicative construction sequence is illustrated in Appendix A5. During the first stage of the construction, the site will be cleared and the site office (SO) established. There will be two concurrent work fronts for the main works, one for the cut and cover (C&C) works of the drain and box culvert and another one for the trenchless drain constructions.
2.7.3.3
The first work front commences
with the C&C pipe laying in Works Section 1 at the Po Lin Monastery water
storage site and will proceed westwards. Once the C&C pipe laying at Works
Section 1 is completed, Intake A will be constructed and C&C pipe laying at
Works Section 3 will commence from the north of
2.7.3.4 At the same time as the commencement of C&C pipe laying in Works Section 1, trenchless works in Works Section 6 will, also, begin. The jacking pit JP2 and the receiving pit RP3 will be constructed first and the pipe jacking of DN1800mm pipe will proceed initially eastwards to RP3. When the eastward jacking works reach Chainage C0+00, the jacking head will be taken out from RP3 and jacking works at JP2 will commence again and work westward to the receiving pit RP4. The RP4 will be constructed when the jacking works approach Chainage C1+96. The soil from the jacking works will be removed at JP2 and also new concrete pipe sections will be inserted at JP2. When the pipe jacking works are completed, a manhole will be constructed at JP2. Intake C will be constructed at RP3 while RP4 will be for Outfall B.
2.7.3.5 The jacking system will then be transferred to Works Section 2 for trenchless works at the site office (SO) area and modified for the DN1500mm pipe. The working cycle will be similar to Works Section 6, but from jacking pit JP1 towards receiving pit RP1 and then RP2. The soil from the jacking works will be removed at JP1 and also new concrete pipe sections will be inserted at JP1. When the pipe jacking works is completed, manholes will be constructed at JP1, RP1 and RP2.
2.7.4
Construction Plant Inventory
2.7.4.1 During the first stage of the construction, the site will be cleared and the site office (SO) established. Minimum amount of construction plant will be used and the powered mechanical equipment to be deployed and their numbers are summarised in Table 2.9
Table 2.9 Tentative Construction Plant Inventory for Site Clearance and Construction of Site Office
|
Activity |
Powered Mechanical
Equipment (PME) |
No. of PME (1) |
|
Activity 1 - Site Clearance |
Excavator, wheeled |
1 |
|
Saw, chain, hand-held |
1 |
|
|
Dump truck |
1 |
|
|
Activity 2 – Construction of
Site Office |
Concrete lorry mixer |
1 |
|
Excavator, wheeled |
1 |
|
|
Generator (silenced) |
1 |
|
|
Lorry, with crane,
<38tonne |
1 |
Notes:
1. Only one number of each
listed equipment will be available for any works during Activities 1 and 2.
2.7.4.2 During the C&C process for pipe laying, a 2.5m wide trench will first be excavated, typically with an excavator and breaker. The trench will then have to be stabilised with a lateral support system (e.g., sheet-pile wall) to prevent it from collapse. Pre-cast reinforced concrete pipe is then laid with the aid of crane and winch. After the pipe laying, the trench is backfilled to the original level and the pavement reinstated. If a box culvert is to be constructed, a larger trench of 5.5m (for 2.5m x 2.5m box culvert) to 6.0m (for 3.0m x 2.5m box culvert) wide will be excavated. Instead of the laying of the pre-cast concrete pipe, formwork and steel fixings will be erected for in-situ casting of concrete to form the box-culvert. The trench will then be backfilled and the works area similarly reinstated. The powered mechanical equipment to be deployed during these activities and the numbers required are summarised in Table 2.10.
Table 2.10 Tentative Construction Plant Inventory for Cut-and-cover Construction Works
|
Works |
PME |
Activity 3 – C&C: Drain
pipe (1) |
Activity 4 – C&C: Box-culvert (1) |
|
Excavation and earth lateral
support system |
Piling, vibrating hammer |
1 |
1 |
|
Dump truck |
1 |
1 |
|
|
Excavator |
1 |
1 |
|
|
Generator (silenced) |
1 |
1 |
|
|
Lorry, with crane,
<38tonne |
1 |
1 |
|
|
Water pump (electric) |
1 |
1 |
|
|
Laying of pipe |
Crane, mobile (diesel) |
1 |
- |
|
Generator (silenced) |
1 |
- |
|
|
Lorry, with crane, <38tonne |
1 |
- |
|
|
Water pump (electric) |
1 |
- |
|
|
Winch (electric) |
1 |
- |
|
|
In-situ casting of Box-culvert (erection of formwork, steel fixing and
concreting) |
Bar bender and cutter (electric) |
- |
1 |
|
Concrete lorry mixer |
- |
1 |
|
|
Crane, mobile (diesel) |
- |
1 |
|
|
Saw, circular, wood |
- |
1 |
|
|
Poker, vibratory, hand-held (electric) |
- |
1 |
|
|
Water pump (electric) |
- |
1 |
|
|
Backfilling the trench |
Compactor, vibratory |
1 |
1 |
|
Dump truck |
1 |
1 |
|
|
Excavator, wheeled |
1 |
1 |
|
|
Reinstatement of original pavement (concrete or bitumen) |
Concrete lorry mixer |
1 |
1 |
|
Generator (silenced) |
1 |
1 |
|
|
Poker, vibratory, hand-held (electric) |
1 |
1 |
Notes:
1. Only one number of each listed equipment
will be available for any works during Activities 3 and 4.
2.7.4.3 For the trenchless construction of drain pipes, for example using the pipe jacking method, a specialist jacking system will be applied. The jacking system will consist of a tunnel shield in front (i.e., the jacking head / cutting shoe), the concrete / steel pipe segments and a hydraulic drive. The general sequence of works normally consists of the following steps:
(a) Precondition Survey: A full scale precondition survey is conducted to record the existing condition of the adjacent structures and to assess the feasibility of the proposed pit location, temporary traffic arrangement (if need) and pipe jacking operation including reconfirmation of the alignment and levels of pipe jacking and the propositions of pits.
(b) Construct Jacking Pit / Receiving Pit: A shaft or pit is constructed as the jacking pit. For this project, the jacking and receiving pits would have dimensions of about 4m (W) x 5-8m (L) x 2-10m (D) and range in size of between 18m2 to 32m2.
(c) Ground Treatment: In case of serious ground water or unstable soil is encountered, grouting will applied to improve the ground condition. This will also reduce the possibility of accidental release of bentonite slurry through fractures (see Section 5 for further details).
(d) Pipe Jacking by Tunnel Boring Machine:
· The jacking shield is placed on pipe jacking track adjusted to achieve the desired line and grade.
· The shield is jacked until there is sufficient space for the first jacking pipe to be installed. The jacking machine is stopped and retracted for insertion of the jacking pipe. After insertion of a pipe segment, the jacking process is repeated until it reaches the receiving pit.
· Small amounts of lubricant, typically bentonite slurry, are added from the cutting face as required to reduce friction during the jacking.
· As the shield advances, excavated spoil is removed from the cutting face using mucking skips which are typically rail-mounted and winched to and from the face by a continuous rope system. Alternatively, there may be a conveyor-belt which loads into a hoisting system at the shaft bottom or by using slurry discharge system.
2.7.4.4 The receiving pit is constructed at the location where the drain pipe ends. For this project, to allow for alignment turning, two receiving pits are required for each pipe jacking section.
2.7.4.5 The jacking system and concrete / steel pipe segments are lowered into the jacking pit with a crane and winch while the excavated soil is removed from the pit using a hoisting winch. The powered mechanical equipment to be deployed for the pipe jacking works is summarised in Table 2.11.
Table 2.11 Tentative Construction Plant Inventory for Pipe Jacking Construction Works
|
Activity / Works |
Powered Mechanical Equipment (PME) |
No. of PME ( 1) |
|
Activity 5 – Laying of Pipe by Pipe-jacking Method (2) |
||
|
Construction of
jacking pits |
Breaker,
excavator mounted (hydraulic) |
1 |
|
Excavator |
1 |
|
|
Water pump
(electric) |
1 |
|
|
Generator
(silenced) |
1 |
|
|
Air blower
(electric) |
1 |
|
|
Lorry, with
crane, <38tonne |
1 |
|
|
Pipe jacking |
Crane, mobile
(diesel) |
1 |
|
Winch (electric) |
1 |
|
|
Grout Pump |
1 |
|
|
Grout mixer |
1 |
|
|
Lorry,
<38tonne |
1 |
|
|
Pipe jacking
machine |
1 |
|
|
Tunnel boring
machine3 |
|
|
|
Construction of
receiving pits |
Breaker,
excavator mounted (hydraulic) |
1 |
|
Excavator |
1 |
|
|
Water pump
(electric) |
1 |
|
|
Generator
(silenced) |
1 |
|
|
Air blower
(electric) |
1 |
|
|
Lorry,
<38tonne |
1 |
|
|
Backfilling the
pits |
Compactor,
vibratory |
1 |
|
Dump truck |
1 |
|
|
Excavator,
wheeled |
1 |
|
Notes:
1. Only one number of each listed equipment
will be available for any works during Activity 5.
2. For the construction of RP4 and Outfall B at WA4 (task ID 5), a
mini excavator (CNP 082) with SWL in 94 dB(A) will be used instead of the
normal excavator (CNP 081) due to lack of space near the works area and access
constraints.
3.
Pipe jacking system, grout pump and tunnel boring
machine would be located at the bottom of the pit which is about 6m below
ground level. The noise of the machines
would be effectively shielded from the sensitive receivers and thus would not
be considered as a potential noise source in the construction noise assessment.
2.7.4.6
The last process of the
drainage works comprises the construction of the associated inlets, manholes
and outlets. The C&C construction of drain pipes and box-culvert includes
the intermediate manholes construction as necessary. There will be no manholes
for the jacked pipe section except at the jacking and receiving pits. When all
works are completed and operative, the site office will be demolished and the
area reinstated with associated landscaping works. The powered mechanical equipment to be
deployed for construction of manholes, inlets and outfalls structures and also
demolition of the site office are summarised in Table
2.12.
Table 2.12 Tentative Construction Plant Inventory for Manholes, Inlets and Outfalls Construction Works and Site Office Demolition
|
Construction Activity |
Powered Mechanical Equipment (PME) |
No. of PME ( 1) |
|
|
Activity 6 – Construction of
Manholes (2), Inlet and Outfall Structures (3) |
|||
|
Excavation |
Breaker, excavator mounted (hydraulic) |
1 |
|
|
Dump truck |
1 |
||
|
Excavator |
1 |
||
|
Water pump (electric) |
1 |
||
|
Generator (silenced) |
1 |
||
|
Lorry, <38tonne |
1 |
||
|
Erection of formwork, steel fixing and concreting |
Bar bender and cutter (electric) |
1 |
|
|
Concrete lorry mixer |
1 |
||
|
Crane, mobile (diesel) |
1 |
||
|
Saw, circular, wood |
1 |
||
|
Poker, vibratory, hand-held (electric) |
1 |
||
|
Water pump (electric) |
1 |
||
|
Backfilling |
Compactor, vibratory |
1 |
|
|
Dump truck |
1 |
||
|
Excavator, wheeled |
1 |
||
|
Activity 7 – Demolition of
Site Office |
|||
|
Demolition of Site Office, Reinstatement (including landscaping works) |
Breaker, excavator mounted (hydraulic) |
1 |
|
|
Dump truck |
1 |
||
|
Lorry, with crane, <38tonne |
1 |
||
Notes:
1. Only one number of each listed equipment
will be available for any works of Activity 6 and 7.
2. For manholes at the jacking pits only.
3. For the construction of RP4 and Outfall B
at WA4 (task ID 6), a mini excavator (CNP 082) with SWL in 94 dB(A) will be
used instead of the normal excavator (CNP 081) due to lack of space near the
works area and access constraints.
2.7.5.1 The tentative works boundary indicated in Figures 2.9a-2.9g and it covers the entire alignment as well as the working space for the project. Nine major working areas can be identified for use during the construction period of the Project and these will be used for locating site office (SO), stockpiling (SA) of excavation material and storage of materials and for working space (WA), etc. These works areas are adjacent the drainage alignment and their locations are shown in Figures 2.9a-2.9g and described in Table 2.13 below. It should be noted that for C&C construction method, the works will be spread along the alignment within the works boundary. For the pipe jacking method, the works will be limited to those identified works areas and the larger works boundary as indicated in Figures 2.9a-2.9g is defined for the purpose of site management with engineering constraints associated with catering alignment changes due to unforeseeable underground pipe-jacking process and conditions; or any emergency situation resulting from the breakdown of pipe jacking machine.
Table 2.13 Details of Proposed Works Areas
|
Works
Area ID |
Location
(Chainage) |
Current
Status |
Proposed
Use |
|
SA1 |
East of the PLM
water storage tank (A0+00) |
Vegetated developed
area |
Works area for
Intake A and also for temporary storage of construction material for Works
Section 1. |
|
WA1 |
Northwest of
PLM, near 慧海淨舍 (A1+05) |
Open developed area |
Working space
for receiving pit RP1. |
|
SO |
Northwest of PLM, around between 法嚴精舍 and 覺淨 (A1+50– A2+00) |
Open ground fringed
with vegetation |
Site office and
for storage of construction material. The jacking pit JP1 is located in this
area. |
|
WA2 |
Northwest of PLM, near 常樂林 (A2+90) |
Vegetated
developed area |
Working space
for receiving pit RP2. |
|
SA2 |
North end of (B0+00 – A4+25) |
Vegetated
developed area and disturbed plantation woodland |
Works area for
box culvert at Works Section 4. Also for temporary storage of construction materials. |
|
SA3 |
West of PLM, around between 眾善蓮苑 and Walking with Buddha (與佛同行) (B1+20 – B2+22) |
Vegetated
developed area and secondary woodland |
Works area for
box culvert and for storage of materials. |
|
WA3 |
North of the (C0+00) |
Gabion channel |
Working space
for receiving pit RP3 and Intake C. |
|
SA4 |
North of the
Ngong Ping 360 Terminal and within the (C0+35 – C0+75) |
Shrubland and
plantation |
For storage of construction
materials and also for jacking pit JP2. |
|
WA4 |
North of the PLM
Columbarium (C1+96) |
Shrubland |
Working space
for receiving pit RP4 and Outfall B. |
Notes:
1. JP: Jacking
Pit; PLM:
2. Please refer
to Figures 2.9a-2.9g
for the location of Works Sections and Works Areas.
2.7.6
Operation of Drainage System
2.7.6.1 The proposed drainage system (Interception Drain, Loop System and Flood Relief Drain) will be operated by gravity and no pumping stations or similar active systems are required. There will be a weir wall of 500mm high at each intake and outfall, and the stream water will not enter the proposed drainage system under the regular flow regime. Hence, the normal flow of the existing stream will not be affected and the proposed drainage system will serve as a by-pass system during period of heavy rain to prevent flooding.
2.7.6.2 The proposed drainage system is essentially a flood way by-pass system and, as such, significant sedimentation is not expected and there shall be no flow inside the drain under typical flow regimes. Hence, maintenance desilting works are not anticipated. As also indicated in Appendix A4, there will be a sand trap at each intake and any maintenance desilting shall take place at the intakes. Should desilting be found necessary, it will be carried out by mechanical means and hydraulic jets will not be used to avoid the sediment laden waste water from being discharged into the downstream water course.
2.7.7
Interface with Other Projects
2.7.7.1 There are no known proposed or planned development projects in the Ngong Ping area during the construction stage of the proposed drainage works. Hence, consideration of cumulative impacts from concurrent projects is not necessary.
2.7.7.2
The gabion channel at Portion F
was constructed under the Tung Chung Cable Car Project - Diversion of the
· Clause 2.1: To avoid any adverse environmental impacts to the adjacent environmentally sensitive areas, no desilting or maintenance works shall be carried out any time between April and September inclusive, unless otherwise approved by the Director in writing.
· Clause 2.2: No temporary access or haul road shall be formed for carrying out desilting or any other maintenance works.
·
Clause 2.3: No material of any kind shall be dumped in any
environmentally sensitive areas including the
2.7.7.3 Outfall A and Intake C of the proposed Project are located at the gabion channel. Outfall A is at about chainage 50 of the gabion channel while Intake C is at about chainage 180 (Figure 2.10a). These portions of the diverted stream are completely gabion lined (both the bottom and bank). The design width of the channel is 8.65m with a bottom width of 3.65m. The central section of the gabion channel is lined with rip rap or has a natural rock bottom (between chainage 80 and 165) (Figure 2.10b) and will not be affected by the proposed Project even though part of it is marked as being within the works boundary of the proposed Project (Figure 2.10a).
2.7.7.4 Since Outfall A and Intake C are within the gabion channel, the construction and operation (maintenance desilting works, if any) of both will need to comply with the Specific Conditions EP-192/2004 unless otherwise approved by the DEP in writing. Based on the tentative construction programme (Figure 2.8b), specific exemption is, also, not anticipated.