2.
DESCRIPTION OF PROJECT
2.1
Justifications and Benefits of the Project
2.1.1
In
Hong Kong, life and property are from time to time under threat of flooding due
to heavy rainfall. A key objective for stormwater management and flood control
is to provide a reasonable level of protection against flooding and hazardous
water flow, thereby reducing to an acceptable level the potential risk of loss
of life and property damage.
2.1.2
In an urbanised area such as Northern Hong Kong,
stormwater management is focused on the physical stormwater drainage systems.
Stormwater drainage systems are designed to collect and convey rainfall-runoff
for safe discharge into a receiving watercourse or the sea. Traditionally
stormwater drainage systems are constructed underneath existing public roads. However,
in the project area space within the public roads is very limited and is
congested with utilities.
2.1.3
The
Drainage Services Department (DSD) commissioned the Northern Hong Kong Island
Drainage Master Plan (DMP) Study in 1996 to assess the existing drainage
systems. The Study area covers about 30 km2, comprising 16 districts
from Kennedy Town in the west to Siu Sai Wan in the east, with a residential
population in excess of half a million people. Many of the existing drainage
systems in the lower catchment are situated in major residential and commercial
districts, warranting a high standard of flood protection to mitigate against
possible major flood damage costs and consequential disruption.
2.2
Consideration of Flood Alleviation Options
2.2.1
A number
of alternative options were considered in the DMP for providing the recommended
flood protection standard to the study area. The investigated options which
have been identified based on the need to control runoff and flooding within
the urbanised environment of the study area are:-
· Reduce flows entering the drainage system
(runoff control)
· Attenuate stormwater flow (retention)
· Increase conveyance capacity of existing
drainage systems
· Flow diversion
· Overland flow control
· Mechanical pumping
· Flood proofing
2.2.2
Runoff
control is used to reduce the peak runoff generated from a rainfall event
through increased vegetation cover, storage in surface depressions, infiltration
into the soil, etc. These methods are most effective at controlling the peak
runoff generated during smaller rainstorm events. Within Hong Kong Island, the
availability of land for implementing runoff control measures is extremely
limited.
2.2.3
Attenuation
refers to the temporary storage of water such that the peak outflow is less
than the peak inflow. Methods of retention storage include constructed basins
(impounding reservoirs, cavern storage), roof top storage, dry and wet ponds,
etc. The volumes of water
associated with extreme rainfall events are very large and accordingly, the
volume of storage required for effective flood control would be large. Within
the Study Area, there are no areas suitable for large storage basins to be
constructed.
2.2.4
Conveyance
capacity improvement refers to increasing the size of the existing system so as
to enable the design flows to safely pass. This is the most common method for
increasing the capacity of urban drainage systems. However, increasing the size
of the existing drainage pipes would necessitate considerable disruption to
traffic flows and lives as existing drainage systems are dug up and replaced.
In addition, there are a number of other utilities and underground obstructions
which could make construction in these urban areas very expensive and
disruptive. This option can therefore be considered for limited areas but not
as a solution for the entire Study Area.
2.2.5
Flow
diversion involves interception of the flows and channelling them through a new
route to the sea. For urban areas
like Northern Hong Kong Island, flow diversion would require the new route to
be constructed within an underground tunnel to minimise the impact of
construction. This is the preferred
option and is the focus of the present EIA.
2.2.6
Overland
flow control is essentially controlled flooding. For this option, flooding
would be allowed to occur but the flood waters would be channelled at surface
level along specially designed and designated flood routes. Currently there is
a lack of space within Hong Kong to construct such flood routes and the
suddenness of flooding means that public safety could not be guaranteed.
2.2.7
Mechanical
pumping could be used for increasing the rate of discharge to the sea. This is
effective for areas where flooding is due to limited discharge rates rather
than the capacity of the existing system.
However, there is lack of space within Hong Kong from Causeway Bay via
Central to Western to build the pumping station and the associated pipework.
2.3
Project
2.3.1
Assessment
of the possible flood alleviation options for Northern Hong Kong Island during
the DMP Study identified that diversion of the flood flows through a tunnel is
the preferred option as it effectively deals with the large volumes of water
whilst minimising the impact on property, people and the environment.
2.3.2
The
feasibility of constructing the Project has been confirmed through extensive
investigations and study. The preferred option of a stormwater drainage tunnel
to divert flows was evaluated and confirmed to be the preferred option based on
technical, economical and environmental impacts. An environmental review of the preferred
scheme has confirmed that there are no insurmountable environmental impacts
associated with the construction and operation of the scheme.
2.3.3
Following
confirmation of the drainage tunnel option as the preferred scheme, a number of
alternative alignments were identified and evaluated. The selection of the
preferred alignment has considered issues such as technical requirements, land
issues, project cost, construction programme, environmental impact and
landscape impact. The key points relating to environmental issues are reported
in this chapter.
2.3.4
The Project
will intercept and divert storm flows from significant watercourses that drain
into the commercial and residential areas of Northern Hong Kong Island, and
convey the intercepted flow via a tunnel system to the sea. The general alignment of the main tunnel
starts from Tai Hang in the east of the project area and discharges to the western
portal outlet that will be located on the western coast of Hong Kong Island adjacent
to the Lamma Channel. Two locations
for the western portal are considered in the present Chapter with the preferred
site being located near Cyberport. The
locations of the Project including the tunnel alignment and the intake shafts
are shown on Figures 2.1, 2.1(a) to 2.1(c).
2.3.5
The
horizontal alignment will pass beneath the fringe of the urban areas in the
mid-levels of North-Western Hong Kong Island. The tunnel alignment passes below the
Tai Tam, Aberdeen, Pok Fu Lam and Lung Fu Shan Country Parks, but the portals
and intake shafts are outside of these Parks. From the eastern portal on Tai Hang
Road, adjacent to the Haw Par Mansion, the tunnel is generally aligned in a
south-western direction and passes beneath the western edge of the Tai Tam
Country Park. The tunnel passes
underneath the Braemer Hill Gas Tunnel, WSD's Tai Tam Conduit and
2.3.6
Since
a portion of the proposed tunnel route passes below the boundaries of Pok Fu
Lam, Aberdeen, Lung Fu Shan and Tai Tam Country Parks, the project is
classified under item Q1 in Schedule 2 of the EIAO as a Designated Project. The location of Country Park areas along
the tunnel alignment are shown on Figures 2.4, 2.4A & 2.4B.
2.3.7
Seven horizontal
alignment options have been considered as part of the preliminary design of the
project and these are discussed in more detail in Chapter 2.6.
2.3.8
The
vertical alignment of the main tunnel is restricted by the elevations of the
proposed eastern and western portals of the tunnel, and the elevation of the
Aberdeen Tunnel over which the tunnel must past. The elevation of the eastern portal is
relatively high within the project area (about 43mPD) while the elevation of
the western portal is near sea level.
In order for the tunnel to pass over the Aberdeen Tunnel the main tunnel
will have an initial gradient of 1 in 349 which steepens after the Aberdeen
Tunnel to 1 in 204. Figure 2.2
shows the horizontal and vertical alignments of the preferred tunnel system.
2.3.9
The
diameter of the tunnel is designed to convey the intercepted flood flows for a
200-year storm event occurring across the complete catchment. The internal diameter of the initial
section of the main tunnel, before the Aberdeen Tunnel, is 6.25m, while the
remaining tunnel has an internal diameter of 7.25m.
2.3.10
Thirty
five intake locations have been identified within the project area that are
suitable for intercepting flood flows and thereby benefiting the lower urban
areas by reducing the flood risk. To
maximise the objectives of flood alleviation in the lower catchment all intakes
are located as low as possible within their respective catchments in order to
intercept as much flow as possible.
The choice of sites to locate individual intakes is limited by the
following constraints: for ideal flood alleviation in the lower catchment the
intake must be as low as possible within the catchment; the site must be owned
by the government; the site must have sufficient area for the intake structure;
the site must have vehicle access for construction and maintenance access; and,
in order to connect the intake to the main tunnel there must be a clear
alignment for the adit that does not pass beneath privately owned land. Figures 2.3-1 to 2.3-35 show the
locations and boundaries of each intake site.
2.3.11
The most
upstream intake structure, which intercepts flow within the stream adjacent to
the Tiger Balm Gardens and is the largest intercepted flow, is combined with the
eastern portal. This intake
structure is unique compared to the other intake structures as it consists of a
weir that impounds the existing watercourse and a diversion channel for the
flood water to flow into the start of the main tunnel (Figure 2.14). The general design of the intake is to
initially dissipate the energy within the highly turbulent and fast flowing
stream flow within an energy stilling basin before discharging the flow into a
short tunnel that links the natural watercourse with the main tunnel. The stilling basin will also trap large
stones and boulders mobilised by flood flows and enhance the sedimentation of
fine particles. A trash and
security screen will prevent unwanted public access to the tunnel and trap most
debris.
2.3.12
Investigation
to identify other possible location for the Eastern Portal has been carried out
but the site that is near to Haw Par Mansion is considered the most ideal
location with due consideration of the following factors:
-
The
stream by the side of Haw Par Mansion collects a significant amount of flow
from the catchment area above Haw Par Mansion. The catchment area is about 19% in area of
the total catchments to be intercepted by the proposed drainage tunnel. Without intercepting the flow from this
stream, it will significantly reduce the flood protection provided by the
tunnel scheme to the lower catchment area;
-
In
order to maximize the volume of water interception, the Eastern Portal has to
be located as near to the downstream end of the catchment as possible. The selected site is the nearest site on
the outskirt of the urban area;
-
Relocation
of the portal site further upstream means reduction in the catchment area to be
intercepted. In addition, it will inevitably involve a substantial site
formation works resulting in a greater loss of natural habitats;
-
Relocation
of the portal site further downstream means to build the portal in the urban
area. No ideal sites that is large
enough for the portal is identified; and
-
The
Eastern Portal is a gateway to the tunnel for daily operation and maintenance
purpose. The current site for eastern portal is in close proximity to Tai Hang
Road, providing an easy access to the portal.
2.3.13
The remaining
intakes are all located on steep watercourses across the project area. Because the watercourses are extremely
steep there is no opportunity to construct sediment and debris traps upstream
of the intake locations. Therefore
through necessity these intakes have an innovated design that was specifically
developed to ensure large stones, boulders and debris are prevented from
entering the intake while maximising the intakes hydraulic efficiency and
conveyance of sediments. Their
design is shown in Figure 2.17.
These intakes are a combination of two hydraulic structures.
i.
Firstly,
the flow will enter the intake structure through a bottom rack intake which
comprises of a screen with bars aligned with the watercourse placed on its
invert. The flow ‘falls’ through
the screen into the structure beneath.
The proposed spacing between bars is sufficiently wide enough to ensure effective
flow diversion while maximising the prevention of large stones, boulders,
larger wood, and debris from
entering the tunnel system. The bar
sizes are also large enough to support material resting on the bars and also will
allow for some leaves and litter to be trapped. Lateral beams below the bars will
facilitate structural support and aid trapping of leaves and litter. It is intended that all reject material (large
stones, boulders, debris, leaves and litter) will accumulate at the downstream
end of the screen ready for removal by maintenance staff. The screen will effectively be self
cleansing and should not block during flood events.
ii.
Below
the screen the flow passes into a vortex inlet before flowing down the vertical
drop shaft. The vortex inlet forces
the flow entering the drop shaft into a helical flow pattern which reduces the
vertical acceleration of the flow while increasing the energy dissipation
within the drop shaft, and provides a stable central air core that allows
entrained air within the flow to escape back up the drop shaft.
iii.
All
the untrapped sediment, small stones and leaves will be conveyed through the
screen down to the tunnel system.
In light of this, a sand trap will be provided at the bottom of each
drop shaft to trap the coarser particles and small stones (Figure 7.9). The trap will be most effective for low
flow condition. It should be noted
that there is unlikely to have litter present within the flow approaching the
intake because the catchments are mostly natural catchments or country
parks.
iv.
Any
vegetation and leaves that pass through the intake will be flushed through the
tunnel system by the storm flow and will discharge into the sea similar to what
occurs for all currently operating stormwater intakes and drainage
systems. Some of this material will
float on the surface of the sea but because of flow conditions within the
tunnel system and site constraints at the tunnel outlet (the Western Portal)
there are no formal facilities proposed to further trap leaves or other
floating material. Maintenance,
including the removal of leaves and other potential floating debris, will be
carried out on the water courses at the intakes and within the tunnel system
itself before the onset of every wet season to minimise the amount of leaves
and floating debris that may discharge to the sea. Leaves and other debris floating within
the discharge plume would be collected should it be necessary following large
storm events. Based on the balance
of maximising the effectiveness of the tunnel system preventing flooding in the
lower catchments of Northern Hong Kong Island and the occasional discharge of
leaves during extreme storm events into the sea, the latter is considered a
reasonable residual impact.
2.3.14
All
intakes will include a low flow bypass to allow pre-determined baseflows to bypass
the intake structure and discharge into the existing drainage system (Figure 2.18).
2.3.15
The
intakes are connected to the main tunnel via vertical drop shafts and adits. The depths of the drop shafts range from
15 metres up to about 180 metres.
Chambers are placed at the bottom of the drop shafts to dissipate energy
and remove entrained air within the flow.
The flow then passes through a system of adits which are smaller tunnels
with an internal diameter of 2.3 metres, before joining the main tunnel. The gradient of all adits is 1 in 200.
2.3.16
The outlet
structure at the western portal will dissipate the energy of the diverted flow
within the tunnel before discharging into the Lamma Channel using an energy
stilling basin (Figures 2.19 & 2.20).
The energy stilling basin will reduce the velocity of the discharging
flow thereby helping to protect the existing sea floor of the Lamma
Channel. To increased protection an
armour layer of riprap will also be placed on the sea floor in the vicinity of
the western portal.
2.3.17
Two
sites that are investigated as being suitable for the location of the western
portal are constrained by existing land use, slopes and the coastline to build
a sediment trap at the western portal.
However the reduction in the flow velocity within the stilling basin
could enhance the sedimentation of the fine particles, reducing the amount of
fine particles being flushing through the tunnel system into the sea. Nevertheless,
for EIA assessment, the possible adverse impacts on water quality without any sedimentation
at the discharge outlet are considered and assessed, and mitigation measures are
proposed wherever necessary.
2.3.18
Both
the eastern and western portals provide vehicle access to the tunnel system and
all drop shafts are designed with sufficient internal diameters to allow entry
for maintenance staff using suitable equipment.
2.3.19
Limited
improvements to urban drainage system have been recommended in flood prone
catchments not serviced by the Project.
These urban drainage improvements are not subject to the EIA process.
2.4
Consideration of Construction Techniques
Construction of Tunnels and Adits
Tunnel Boring
2.4.1
Tunnel
boring machines (TBM) are commonly used for the excavation of longer
tunnels. The advantages of
excavation by TBM are relatively high production rates compared with
alternative methods for rock excavation, a controlled excavation profile, and low
vibration and noise generation.
2.4.2
Given
the length of the main tunnel, the unit cost for excavation using a TBM
compares favourably with other methods of excavation. TBM construction is therefore proposed for
the main tunnel excavation. A
double-shielded machine is proposed that will minimise water ingress, although
progress rates for an un-shielded TBM are generally slightly higher. This could avoid seepage of groundwater
and thus drawdown of watertable.
2.4.3
TBM
excavation consists of an approximately 180m long machine including back-up
units. Excavated material is
carried by conveyor over the back-up units from where it can be carried out of
the tunnel by either conveyors or rail cars. Operation of the TBM requires
electricity to be delivered to the unit via an 11kv cable within the tunnel.
Ventilation and lubrication of the cutting face is necessary to prevent the
build up of dust arising from excavation.
Drill & Blasting
2.4.4
Drill
and blasting is commonly used for the excavation of hard rock tunnels. Blasting is a relatively cheap method of
construction. In addition, relative to TBM excavation, blasting does not
require the lengthy period up-front for procurement nor the large area for
assembly. Excavation by blasting is not a continuous operation and consequently
has lower production rates than TBM excavation. The progress rates for blasting
are affected by the time to drill the holes, charge the explosive, remove the
material and install temporary supports. In addition, the progress is affected
by the delivery of the explosives and the need to retain blast induced
vibrations within the threshold limits. Generally, blasting results in greater
vibration than TBM excavation, but for less duration.
2.4.5
Blasting
is seen as the most suitable method of excavation for the adits due to the
relative cost of alternative methods of construction. Blasting
is not seen as a suitable method of excavation for the shafts due to the noise
generated. Given the proximity of the intake shaft locations to sensitive
receivers, blasting would need to be severely restricted to remain within the
noise restrictions. Construction of shafts by drill and blast would also
necessitate the shafts to be constructed larger than the 2.3m diameter required
to enable spoil to be removed. This would consequently lead to slower progress
and would also cause difficulties at many intake shaft locations, where the
working area is very restricted.
Chemical Blasting
2.4.6
An
alternative to the use of conventional explosives is to carry out drill and
blast operation using chemical explosives.
These include penetrating cone fracture and propellant systems. These
methods are subject to the same restrictions and regulations as conventional
blasting. In theory, chemical blasting should result in lower vibrations
although this depends on finding experienced operators.
2.4.7
Similar
to conventional blasting, the use of chemical blasting requires a number of
discrete operations (drill holes, place explosive, blast, muck out and place
supports). However, the cost of the chemical explosives is considerably greater
than that for conventional explosives. In addition, the blast cycle takes
considerably longer than conventional blasting, which limits the number of
blasts achievable in a day. For the above reasons, these methods are normally
only used in specialised applications such as blasting very close to existing
structures.
2.4.8
The
use of chemical blasting for the construction of the tunnels and adits is seen
as unlikely given that conventional blasting is a more economical method of
construction. In addition, chemical blasting offers no advantages in terms of
programme as its use is subject to obtaining blasting permits and addressing
the same issues of storage, delivery and handling as conventional blasting.
Mechanical and Non-Explosive Systems
2.4.9
Mechanical
and non-explosive systems have the advantage that they are not subject to the
approvals process of blasting nor the need for special precautions. There are a number of systems available,
although these are generally more expensive than blasting and many of these
systems have not been fully proven.
2.4.10
Examples
of non-explosive systems for rock excavation include radial-axial splitters,
controlled foam injection, water injection, and plasma blasting (electrical).
Many of these systems produce lower vibrations than conventional blasting.
These systems are however more expensive and production rates are lower than
for blasting.
2.4.11
It is
likely that some of these systems are employed for construction of adit junctions,
surface work and some of the smaller adits. Unlike blasting, using these
systems work can be carried out at the same time in other parts of the tunnel.
Construction
of Drop Shaft and Intakes
Raise Boring
2.4.12
Raise
boring method (RBM) is a technique which can be used for vertical construction
of a shaft in very competent rock. Excavation
is carried out by a reamer which is pulled back towards the rig. As such, this technique can only be used
for construction of straight shafts or adits where access is available to both
ends. The system does not have any provision for temporary support of the
excavation and as such is only suitable for areas where the rock is self
supporting.
2.4.13
Raise
boring method does offer the advantage of continuous production with good
advance rates and can be operated without requiring a large working area at
ground level. This is a particular advantage for the construction of the
shafts, where the working area is often tight. Shafts built using this
technique could only be constructed once the main tunnel and adits have been
completed.
Reverse Circulation Drilling
2.4.14
This
drilling technique is used for pile construction from the surface but it is
equally appropriate for shaft construction. It consists of a drilled hole with
compressed water or drilling mud used for flushing out the cuttings. Excavation
can either be carried out using a rotary coring bit or a percussive hammer.
Reverse circulation drilling (RCD) is the fastest of the surface excavation
techniques, although it is more expensive.
2.4.15
Relative
to other surface construction techniques, RCD rigs require less working space.
However, the process generates slurry which needs to be dewatered prior to
removal from site. The dewatering
plant requires additional land take and as such, this method may not be
suitable for some of the more restricted sites.
2.4.16
RCD
construction has the advantage that this method is well suited to a wide range
of geological conditions. As such, this method of construction is well suited
for those shafts which cannot be constructed using the raised boring machine.
Hand Excavation
2.4.17
Hand
excavation of shafts is particularly suited to excavations within softer
material and shallow excavations. Hand excavation generally requires much
smaller working areas at the surface than alternative excavation methods
although progress rates are considerably lower. Temporary support and
ventilation must be carefully designed to ensure the safety of workers during
construction.
Conclusion
2.4.18
By
comparing the pros and cons of the various construction methods as mentioned
above, Tunnel Boring Machine, Raise Boring method and Drill & blasting
method are the most practical construction method for the drainage tunnel,
intake shafts and adits respectively.
The assessment results, recommendations and conclusions have been
addressed in this EIA report based on the proposed construction
techniques/methods.
2.5
Consideration of Tunnel Alignment Options
and Intake Locations
2.5.1
Seven
possible tunnel alignments were investigated before selecting the Project. These seven alignment options are summarised
in Table 2.1 and shown in Figures 2.5-2.12. In all cases the intakes are positioned on
existing main drainage paths that intersect with the tunnel alignment. At these locations the flows are intercepted
and directed to the tunnel. Photos
taken at the proposed intake locations are shown in Figure 2.13.
2.5.2
While
the engineering team has identified a set of seven different alignment options,
in environmental terms the potential for impact is simplified into three impact
zones:
·
Eastern
Portal located adjacent to the Haw Par Mansion (this is common to all tunnel
alignment options);
·
The
tunnel alignment between the Eastern Portal and the Western Portal of which
there are 7 options considered; and
·
The Western
Portal location. There are two alignment
options considered, the first immediately north of Cyberport and the second is
located on the very western edge of Hong Kong Island adjacent to the Sulphur
Channel.
2.5.3
Where
there are specific differences between alignment options, they are discussed in
terms of environmental indicator areas e.g. air quality (dust), noise, ecology,
cultural heritage, visual and landscape and water quality.
2.5.4
There
are three main selection criterions that have been used to identify the
locations of the proposed intakes.
The first is that the sites must be located adjacent to the watercourses
to allow for flow to be intercepted and diverted into the Project; the second
is that vehicle access to the intakes must be possible for construction and
maintenance reasons; and thirdly their location should be low within the
catchment to maximise the amount of flow that is diverted thereby maximising
the effectiveness of the project.
At each intake site the layout of the site is dependent on the
surrounding environment including access arrangements, existing nearby
structures, ground topography, position of existing trees, and existing
habitat.
Table 2.1: Alignment Selection
|
Option |
Alignment |
Tunnel Alignment Description |
Western Portal Location |
Abbreviated Option Name / Figure Number |
|
|
|
|
Eastern Portal to Lung Fu Shan |
Lung Fu Shan to Western Portal |
|
|
|
A |
1 |
passes under urban areas in Jardine's
Lookout and Mid Levels |
passes below urban areas, below Mt Butler to the
Sulphur Channel |
Sulphur Channel |
A1 / Figure
2.6 |
|
3 |
a sinuous alignment below Pok Fu Lam to
Cyberport |
Cyberport |
A2 / Figure
2.7 |
||
|
B |
9 |
a sinuous alignment south of Option A and
outside the footprint of the urban area |
passes below urban areas, below Mt Butler to the
Sulphur Channel |
Sulphur Channel |
B1 / Figure
2.8 |
|
11 |
B2 /
Figure 2.9 |
||||
|
C |
21 |
a straighter alignment than Option B and
south of Option A outside the footprint of the urban area |
C1 /
Figure 2.10 |
||
|
23 |
a sinuous alignment below Pok Fu Lam to
Cyberport |
Cyberport |
C2 / Figure
2.11 |
||
|
29 |
a straighter alignment below Pok Fu Lam to
Cyberport |
C3 /
Figure 2.12 |
|||
2.6
Consideration of Environmental Issues
2.6.1
The
key environmental issues for assessment were divided into two sub-sections:
construction and operation. These are described in the following paragraphs and
were used for environmental evaluation of the seven alignment options.
2.6.2
Potential
impacts during the construction phase:
·
Dust
impacts on Sensitive Receivers (SR) at the tunnel portals;
·
Noise
impacts on Noise Sensitive Receivers (NSRs);
·
Water
quality impacts from polluted construction water migrating offsite;
·
Excavated
spoil handling, including identification of material reuse;
·
Permanent
or temporary ecological impacts affecting directly or indirectly the stream habitats
at the portals and intake sites; and
·
Impacts
on sites of cultural heritage.
Potential impacts during the operation phase:
·
Marine
water quality impact at the tunnel outfall during storm water discharge; and
·
Visual
and landscaping impacts at the Western and Eastern portals.
2.7
Eastern Portal
2.7.1
The Eastern
Portal, which is common to all tunnel alignment options, will be established adjacent
to an existing watercourse at the southern end of a car park immediately east
of the Haw Par Mansion and south side of the Tai Hang Road. During construction, activities in the
works area will include:
·
initial
excavation/breaking to establish the portal;
·
excavation
and construction of the stream diversion structure;
·
assembly
and operation of the tunnel boring machine;
·
handling
of the spoil;
·
supply
of materials for construction of the stream diversion structure and tunnel; and
·
establishment
and dis-establishment of site offices.
2.7.2
Standard
working procedures will be required to ensure adverse impacts are
minimised. It is expected that construction
activities will generate dust, although these can be mitigated through simple
site based measures.
2.7.3
Construction
and operation of the Eastern Portal will have limited ecological impact. For
the construction phase, the impact will mostly be associated with minor
construction activity within the existing streambed.
2.7.4
Although
the Eastern Portal is located adjacent to Haw Par Mansion, construction and
operation should have no significant impact on the cultural heritage of the
building. During the operation
phase the only foreseeable impact is visual which can be mitigated with
suitable landscaping and architectural measures.
2.8
Tunnel Alignment
2.8.1
The
tunnel boring machines will create a tunnel running from the Eastern Portal to
the Western Portal located on the west side of HK Island. There are three routes identified. Alignment
Options A1 and A3 lie beneath the urban area and are considered to have a
higher potential for creating impact from structural borne noise.
2.8.2
Spoil
from the excavation of the tunnel will be removed via the Eastern and Western
Portals. These will be the principal working areas for access and egress to the
tunnel construction areas. Although there are limited differences in respect of
spoil quantities removed, the selected alignment Option C29 will generate the
least quantities of Construction and Demolition material (C&D) material.
2.9
Western Portal
2.9.1
The
Portal will be an engineered structure specifically designed to discharge
tunnel flow into the sea while maximizing dissipation of the flow’s energy. The
emerging tunnel will have a diameter of 7.25 metres (internal diameter), it
will pass into a rectangular profile channel that will widen to about 18 metres
over a distance of about 80 metres.
Thereafter the flow will pass into an energy dissipating structure that
discharges directly into the sea.
The layout of the proposed structure is shown in Figure 2.3-35. Most of the structure will be below sea
level and it is proposed that all of the structure will be below final ground
level to make the site more visually attractive. The Portal will also be the site where a
TBM will start construction of the tunnel alignment working from the western portal.
In both cases, no reclamation is required to position the stilling basin.
2.9.2
There
was two tunnel discharge locations considered depending on the tunnel
alignment. The first is located
immediately north of Cyberport and for the second is located immediately west
of Mount Davis, within the Sulphur Channel.
Cyberport Portal Site
2.9.3
The
portal will be constructed on an existing reclaimed area of land beneath the
northern access road to Cyberport. The portal will be close to SR's at
Cyberport and its tunnel alignment passes beneath the Queen Mary Hospital. Potential impacts of this alignment include
the potential impact on residential property and the Queen Mary Hospital from
Structure Borne Noise. During the
operation phase concerns may include the portal’s visual impact, the visual
impact from silty discharge, possible smothering of marine fauna and possible impact
on Fish Culture Zones (FCZ).
2.9.4
The
closest sensitive receivers to the Cyberport site are those that are located
high on the headland of Point Breeze about 150 metres to north of the site. The portal may have visual impact on
those SR’s located high enough, however the structure will be shielded by the
Cyberport access road and will be covered by reinstated ground leaving only the
air vent, access road and outlet at the shoreline visible. SRs in Cyberport will be shielded from
direct line of sight by the existing Sewerage Treatment Works whereas SRs
located on the water, to the west of the site, will have uninterrupted views of
the portal structure.
2.9.5
During
construction dust could be generated from construction activities. Simple site precautions will be required
to ensure that adverse impacts are minimized.
2.9.6
The
impact of structure borne noise during construction of the tunnel on the Queen
Mary Hospital will need to be assessed to determine acceptable levels of impact.
At the portal sensitive receivers are remote from the site and severe impact is
not anticipated if normal precautions are adopted.
2.9.7
The
sea floor immediately adjacent to the portal site is already altered due to the
construction of the reclamation. A
temporary pier will be constructed on the sea floor and then replaced with an
area of permanent armour rock (refer to Figures 2.15 and 2.16). Because the sea floor is already in an
altered state the impact of the proposed pier and armour is unlikely to
adversely affect the existing environment.
However, possible smothering of benthic organisms due to additional silt
discharge will need to be addressed.
Likewise, the effect on benthic organisms on the seabed and the Fish
Culture Zones (although they are remote being 4.5km from the site) on the east
side of Lamma Island will need to be addressed.
2.9.8
There
are seawater intakes north of the site for Queen Mary Hospital (300 metres)
Sandy Bay (1200 metres) and proposed intake at Telegraph Bay
(500 metres). The discharge
plume into the sea will need to be modelled to determine if the suspended
solids within the plume and the freshwater both interfere with the intakes. Similarly, any silt plume migration
would have an impact over a large area and would be visible to elevated SRs on
the west side of HK Island including Cyberport.
2.9.9
No
sites of cultural heritage have been identified in relation to either the
proposed tunnel alignment or the western portal site.
Mount Davis Portal Site
2.9.10
Alignments
A1, B1, B2 and C1 pass beneath Mount Davis to a portal structure on rocky shore
immediately south of the Sulphur Channel. The alignment and portal are remote
from residential developments on Mount Davis Road. The entrance into Victoria
Harbour through the Sulphur Channel is considered to be one of the
"gateways" to Hong Kong. The presence of a large outer structure on
the promontory would be highly visible to observers on boats entering and
leaving Victoria Harbour. There is potential for silt to enter Victoria Harbour
creating visual impact and impact on sea water intakes in addition to loss of
rocky shore habitat and possible smothering of benthic organisms.
2.9.11
There
are no land based SRs in the immediate area. The closest are isolated developments on
Victoria Road however these are screened by intermediate topography. Receivers on the water to the west will
have uninterrupted views of the portal and any spillway structure. Benthic organisms on the seabed could be
affected by silty discharges, though currents are swift and advection and dispersion
is likely to be high in this area. There
are seawater intakes within Victoria Harbour serving Kennedy Town.
2.9.12
During
construction, dust could be generated from general portal construction.
However, the site is remote and simple site precautions should ensure
acceptable levels of impact.
2.9.13
The
discharge plume into the sea will need to be modelled to determine if the
suspended solids within the plume and the freshwater both interfere with
domestic intakes. Likewise the
model will show the risk of smothering of benthic organisms due to additional
silt discharge. Any silt plume
migration would have an adverse impact over a large area and could be visible
within Victoria Harbour.
2.9.14
No
sites of cultural heritage have been identified in relation to either the
proposed tunnel alignment or the western portal site.
2.10
Intake Structures
2.10.1
On the
alignment between the Eastern Portal and the Western Portal 35 intake
structures will be constructed on existing stormwater flow paths to collect
surface water. An intake diversion structure and drop shaft will be constructed
to take water down to the level of the tunnel and a horizontal adit will take
the flow from the base of the shaft into the tunnel.
2.10.2
The
intake structures will require the construction of a concrete flow interception
structure with safety and operational covers, and secure access points. The structures will be more visually
prominent than the existing flow channel. The intake sites can be close to
existing residential properties that are sensitive to noise, air quality and
visual impact. While the structures themselves may occupy an existing built
site, some may encroach upon vegetated slopes and temporary work areas
typically extend into vegetated slopes.
2.10.3
The
vertical shafts will require the excavation, removal of spoil material and the
fixing of pre-cast concrete liners. Most of the shafts will be constructed from
the bottom up, using a Raised Boring Machine. If the shaft is remote from the
main tunnel a connecting adit will be constructed underground.
2.10.4
It is
anticipated that the adits will be constructed using drill and blast methods.
As the adits will not be directly connected to the surface and will be
constructed at depth (in general, 60m – 200m below the surface), the noise associated
with this construction method will be very limited.
2.10.5
The intakes
for Alignment Option A are either sited directly over, or close to, the tunnel
alignment. For these intakes some noise impact is expected.
2.10.6
For Alignment
Options B and C the intake locations are further away from the urban area. The expected noise impacts during
construction activities are therefore lower than that expected for alignment
options.
2.10.7
Spoil
from the construction of the adits will be removed via the tunnel to the two
portals. Similarly, shafts constructed using the Raise Bore method will have
spoil removed via the tunnel to the portals. It is anticipated that this
construction method will be used for the majority of shaft excavation although
alternative methods will be required for those few shafts where the ground
conditions are unsuitable for Raised Bore construction techniques. In these
cases, spoil from the shafts would be removed at the surface at each intake site.
2.10.8
During
the construction of the intakes all flow within the watercourses will bypass
the construction sites and continue flowing in to the existing downstream
drainage system. Therefore the
existing drainage capacity in the downstream lower catchments will not be
affected or get worsen by the construction of the Project. When the tunnel system is operating the
potential flooding in the lower catchments will be substantially reduced.
2.11
Conclusion
on Option Selection
2.11.1
For
the Eastern Portal near Haw Par Mansion which is common to all alignment options
and there are no clear environmental differences. On the alignment itself, the
TBM will have minimal environmental impact but as Alignment Option A
passes beneath the urban area there is a higher possibility of structure borne
noise and for this reason it is considered to be less environmentally attractive
than Alignment Options B and C.
2.11.2
The locations
of the proposed intake structures is restricted by watercourse alignments, land
availability and construction and maintenance access so provision of the intake
structures will need to address visual impacts and potential to adversely
affect cultural and heritage sites, noise, air (dust), water quality and solid
waste management during construction phase.
2.11.3
The
Western Portal may impact on seabed ecology due to the local introduction of
silt and fresh water, potential impacts on seawater intakes and the visual
impact at the portal structure itself.
However the impacts are broadly similar for both tunnel alignments and
at both western portal sites. The portal at Cyberport will be closer to SRs and
there is a higher potential for impact, though mitigation is available. The
Mount Davis site is undeveloped and it could be argued that this increases
severity of the visual impact and silt could enter Victoria Harbour. For this
reason the Mount Davis portal is considered to be marginally less
environmentally attractive.
2.11.4
From
an environmental perspective there are only minor differences between alignment
options but in engineering terms there were more definitive differences and Alignment
Option C3 was selected as the Project. The main advantage of this
alignment option is to avoid encroachment upon private land lots and with greater
separation between the tunnel centre line and the private lots (wherever
possible). This has the added environmental benefit of minimizing the potential
construction noise and fugitive dust impacts to the residential / GIC units on
private lots. The adopted portal site will prevent damage to rocky shore habitats
under the current Project design since it is proposed to be constructed on
existing reclaimed land.
2.12
Hong Kong West Drainage Tunnel
2.12.1
Following
an extensive engineering, environmental and economic review of possible intake
shaft locations, outfall locations and tunnel alignments, the following option
was chosen:
·
A
continuous main drainage tunnel – the first section of 4.5 kilometres with an
internal diameter of 6.25 metres from Tai Hang Road to Aberdeen Tunnel, and
from the Aberdeen Tunnel to Cyberport the second section of tunnel of about 6.0
kilometres in length with an internal diameter of 7.25 meters;
·
A
system of Adits with total length of about 7.5 kilometres and internal diameter
of 2.3 meters that connect the intakes with the main drainage tunnel;
·
Thirty
five intakes that will intercept existing flows and divert them via 30
dropshafts to the drainage tunnel. The intakes will include the in-stream flow
diversion structure (including screen for preventing debris and large stones
from entering the tunnel system), vortex inlet to facilitate stable flow within
the drop shaft, the drop shafts, a low flow bypass channels and maintenance
platforms. The drop shafts vary in
height with the shortest being 8 metres and the longest being approximately 180
metres;
·
An
outlet to the sea at Cyberport that includes an energy stilling basin to ensure
low velocity outflows into the Lamma Channel;
·
Two
tunnel portals at the east and west end of the tunnel alignment. The eastern portal in combined with the
largest intake structure and the western portal is combined with the outlet
structure. Both portals provide for
vehicle accesses for maintenance purposes.
2.13
Design and Construction Programme
2.13.1
The
design and construction phase is expected to commence in end 2005. Tentative
Construction Programme for the proposed drainage tunnel is May 2007 to Nov 2011. The maximum tentative construction
programme for the deepest intake shaft is about than 12 months.
2.14
Concurrent Projects and Potential
Cumulative Impacts
2.14.1
There
are no scheduled concurrent public works in the vicinity of the proposed tunnel
portals, intakes or tunnel alignment. No cumulative construction impacts are
likely to arise from this Project.
2.14.2
Potential
cumulative impacts in terms of water quality during the operation phase with
the concurrent operation of the Cyberport Sewage Treatment Works in the short
term are provided in Section 7.