2. consideration of alternatives
2.2 Alignment Option Review and Selection
2.1 Introduction
2.1.1.1 Several options and alternatives have been considered in the development, refinement and selection of the scheme of the TM-CLKL to be taken forward for environmental assessment and preliminary design. This section of the report provides the details of the alignment options considered and the constraints and considerations assessed in adopting the recommended scheme.
2.1.1.2 In addition, as part of this process, the various construction methodology options available have been reviewed in order to determine the most effective means of building the Project. The review has taken into account engineering feasibility, site conditions, programme constraints and environmental considerations. Details of the construction method alternatives are also provided below.
2.2 Alignment Option Review and Selection
2.2.1 Background
2.2.1.1 In 2005, Highways Department (HyD) commissioned an engineering feasibility study (FS), namely Tuen Mun Chek Lap Kok Link and Tuen Mun Western Bypass – Feasibility Study (Agreement No. CE 28/2005 (HY)), to evaluate the technical feasibility and impacts of the Project. The FS recommended that the TM-CLKL shall be a dual 2-lane road with a total length of about 9 km with about 4 km long submarine tunnel and 5 km long elevated structure.
2.2.1.2 However, the subsequent site selection study for the HKBCF, completed in 2007, proposed to integrate the TM-CLKL southern landfall reclamation with the HKBCF reclamation to the northeast of Chek Lap Kok. This arrangement was deemed to provide a cost-effective connection between the two Projects and was adopted for the TM-CLKL alignment study.
2.2.2 Development of Previous Alignment Options for TM-CLKL
2.2.2.1
In the former FS, the FS Consultants developed and
assessed a number of alignment options for the TM-CLKL. The FS
Consultants initially considered five alignment options for the main crossing,
as shown in Figure
2.1, with three connection options at the northern end at Tuen Mun and
five connection options at the southern end at
2.2.2.2
Both the above mentioned short-listed options included
an artificial island at Tai Mo To. Option CLKL-2 comprised an immersed tube
(IMT) tunnel between the artificial island and an area of proposed reclamation
adjacent to the east side of the Tuen Mun River Trade Terminal, whereas Option
CLKL-5 comprised an IMT tunnel between the artificial island and an area of
proposed reclamation protruding from
2.2.2.3
Option CLKL-2 would have required the diversion of
various submarine utilities, including CLP 132kV power cables, HGC and NWT
telecommunication cables and a China Gas proposed gas main. On the other hand,
Option CLKL-5 would have affected
2.2.2.4
For the section of the alignment to the south of the
artificial island, the alignment of TM-CLKL depended on the alignment of the
HZMB North Lantau Highway Connection (NLHC). In respect of the NLHC sea
viaduct option, the TM-CLKL would head southwest and connect with NLHC off the
southeast coast of
2.2.3
Development
of HZMB
2.2.3.1
In 2007, HyD commissioned a FS, namely
2.2.3.2 The recommendation to locate the HKBCF off the northeast coast of Chek Lap Kok, also, affected the layout of the NLHC, in so far as the NLHC land tunnel option became obsolete and, at that time, the NLHC sea viaduct option was redirected to land at the HKBCF (and was also renamed the Hong Kong Link Road (HKLR)). The most up to date HKLR alignment comprises a combination of marine and land viaduct, tunnel and at-grade road. The HKBCF was linked to NLH by the southern section of TM-CLKL, as shown in Figure 2.3.
2.2.3.3 The development and assessment of the previous alignment options for TM-CLKL, under Agreement No. CE 28/2005 (HY), did not take account of the proposal to locate the HKBCF off the northeast coast of Chek Lap Kok, nor did it take account of the proposal to integrate the artificial island for TM-CLKL with the proposed reclamation for the HKBCF. As such, it was necessary for this Assignment to develop and consider new alignment options which took full account of the HKBCF.
2.2.3.4 Ideas regarding the previous connection options were incorporated into the new alignment options. Furthermore, the previous preferred alignment Option CLKL-2 which was used to apply for the EIA Study Brief and included in EIA Study Brief ESB 175/2007 issued in December 2007 was also included in the option evaluation process under this Assignment in order to reinforce the previous recommendation under Agreement No. CE 7/2007 (CE) of integrating the artificial island for TM-CLKL with the proposed reclamation for the HKBCF.
2.2.4 Initial Alignment Options
2.2.4.1 The Initial Alignment Options are described below, and comprise two Northern Connection Options at Tuen Mun, two Southern Connection Options at Lantau and three Main Crossing Options. The options for the north, south and main crossing are shown on Figures 2.4, 2.5 to 2.5a, and 2.6 respectively.
2.2.4.2 There are quite a number of key constraints that needed to be taken into account in developing and assessing the options and these include:
·
Geological Conditions Across
· Construction Practicality;
· Marine Borrow Pits and Mud Disposal Pits;
· Contaminated Mud Pit at South of the Brothers;
· Requirements of Safe Navigation of Vessels;
·
Marine Restricted Zones around the
· Tuen Mun Immigration and Sham Shui Kok Anchorages;
· Existing Submarine Utilities;
·
Designated Area of
·
Connection for Hong Kong – Zhuhai –
· Runways & Associated Airport Height Restriction (AHR);
· Sky Pier and the East Fire Rescue Station;
· Aeronautical lights (ie. Aviation beacons);
· Meteorological Equipment;
· Premises at Tuen Mun Area 40;
· River Trade Golf at Tuen Mun Area 46;
· Fireboat Station and the associated berth at Tuen Mun River Trade Terminal;
·
· Future Tung Chung East and West Developments;
·
· Operations of Helicopters;
· Airport Development and Facilities;
·
Developments at
· Marine Accesses;
· Existing and Proposed Drainage / Sewerage Networks;
· Crematorium, Columbarium, Funeral Services Centre in Tuen Mun Area 46; and
· Existing graves near Tuen Mun Area 46.
Northern Connection Options at Tuen Mun
2.2.4.3 As discussed above, Option CLKL-2 was recommended as the preferred alignment option in the former FS. Therefore, the development of the Northern Connection Options all focussed on optimising the connections between TM-CLKL, TMWB and the local road network and minimising reclamation. Two options, Options N1 and N2 have been evaluated in detail and are shown in Figure 2.4 and described below.
Northern Connection Option N1
2.2.4.4 Option N1 has a permanent reclamation adjacent to Tuen Mun River Trade Terminal to form the northern landfall for the submarine tunnel. From the submarine tunnel, this option heads north eastward, passes over the land of sawmills (2 nos.) and an open space car park under short-term tenancy (STT) on viaduct and crosses over the re-aligned Lung Mun Road. Before connecting to the TMWB, there is a combined toll plaza northeast of the River Trade Golf where a toll plaza has been proposed to allow tolling for travelling solely on TM-CLKL or TMWB or both.
Northern Connection Option N2
2.2.4.5
Option N2 avoids the sawmills in Tuen Mun Area 40 and
the proposed Crematorium, Columbarium, Funeral Services Centre in Tuen Mun Area
46. Permanent reclamation is required east of the sawmills to form the northern
landfall for the submarine tunnel. From the tunnel portal, this option
heads northward east of Butterfly Beach Laundry and crosses over
Southern
Connection Options at
2.2.4.6
The development of the Southern Connection Options has
mainly focussed on the connections between TM-CLKL, HKBCF and NLH. Two
southern connections, a viaduct scheme, Option S1 (as shown in Figure
2.5), and a tunnel scheme, Option S1a (as shown in Figure
2.5a) have been assessed. Both options require the realignment of
NLH and
Southern Connection Option S1
2.2.4.7 Option S1 is based on the preferred alignment option recommended in the former FS, but takes into account of the current recommended scheme for the HKBCF. From the HKBCF, this option heads south eastward on elevated structure across the Tung Chung Navigation Channel and the waters west of the proposed contaminated mud pits at south of the Brothers, before crossing the Airport Express Railway and connecting with NLH between the Future Tung Chung East Development and the proposed Lantau Logistics Park. This option also provides connection with the HKIA via the connection roads at the HKBCF. This option may also connect with the future Lantau Road P1 at Tai Ho Wan.
Southern Connection Option S1a
2.2.4.8 Option S1a is a tunnel option which requires permanent reclamation at the south-east side of the proposed HKBCF to form a landfall for the submarine tunnel. From the submarine tunnel, this option, similar to Option S1, heads south eastward towards another landfall located at west of the proposed contaminated mud pits at south of the Brothers, before crossing the Airport Express Railway and connecting with NLH between the Future Tung Chung East Development and the proposed Lantau Logistics Park.
2.2.4.9 Similar to Option S1, this option also provides connection with the HKIA via the connection roads of the HKBCF and future Lantau Road P1.
Main Crossing Options
2.2.4.10 The three Main Crossing Options presented below utilise the space between Tai Mo To, the HKBCF and Chek Lap Kok. They also take account of the latest developments and modifications to the layout of the HKBCF. The three options all comprise a submarine tunnel assumed for the EIA to be constructed by tunnel boring machine (TBM) method as opposed to an immersed tube (IMT) tunnel. The engineering and environmental advantages of the TBM solution compared to an IMT tunnel is described in Section 2.3.1 below. An overview of the Main Crossing Options is shown in Figure 2.6.
Main Crossing Option M1
2.2.4.11 Option M1 is based on the recommended alignment option recommended in
the former FS, but takes account of the current recommended scheme for the
HKBCF. From the HKBCF, this option heads north eastward across the waters
between the HKBCF and Tai Mo To on elevated structure, before passing onto an
area of reclamation adjacent to Tai Mo To that forms the Southern Landfall for
the main submarine tunnel crossing. From this landfall, the submarine tunnel
heads north westward crossing the proposed sand borrow area and passing under
2.2.4.12 Option M1 can connect with any of the Northern Connection Options and
any of the Southern Connection Options described above. Therefore, the
toll
Main Crossing Option M3
2.2.4.13 Option M3 follows the recommended scheme of the HKBCF FS, except that the toll plaza is located in Tuen Mun. From the HKBCF, this option heads northward in submarine tunnel under the waters west of Tai Mo To, before crossing the proposed sand borrow area and passing under Urmston Road. The Southern Landfall comprises an area of reclamation combined with the reclamation for the HKBCF alongside the eastern edge of the HKBCF.
2.2.4.14 Similar to Option M1, Option M3 can connect with any of the Northern Connection Options and any of the Southern Connection Options described above.
Main Crossing Option M4
2.2.4.15 Option M4 follows the option recommended in the HKBCF FS, except the
toll plaza is located in Tuen Mun. The HKBCF FS recommended that such option
should be considered in the subsequent investigation assignment, hence Option
M4 is included in this evaluation. From the HKBCF, this option heads
northward in submarine tunnel under the waters northeast of Chek Lap Kok,
before passing under
2.2.5 Detailed Evaluation Methodology
2.2.5.1 In order to fully evaluate the Initial Alignment Options for TM-CLKL described above with a view to identifying the Recommended Scheme to be taken forward to the subsequent preliminary design and impact assessments, etc, a comparative assessment was undertaken. The comparison comprised a preliminary desk-top assessment and was undertaken based upon the following criteria sets, which were also divided into sub-sets as required:
(i) Transport and Operation Planning;
(ii) Marine Impact;
(iii) Engineering;
(iv) Environmental Impacts;
(v) Planning and Lands Issues; and
(vi) Public Perception.
2.2.5.2 The environmental impacts criteria were further divided as per Table 2.1 below:
Table 2.1 Detailed Environmental Evaluation Criteria
|
Environmental Parameter |
Rating Principle |
|
Air quality and noise impact |
Options with less potential noise and air quality impacts during both the construction and operation phases will be rated higher. |
|
Water quality |
Options causing less impact on water quality and hydrodynamics during the construction and operation phases will be rated higher. |
|
Ecology and fisheries |
Options causing less effect on the Chinese White Dolphin and its habitat and less potential impact to fisheries and other marine and terrestrial ecology resources during the construction and operation phases will be rated higher. |
|
Waste management |
Options which generate smaller amounts of disposal materials and maximise the use of public fill for construction/reclamation works will be rated higher. |
|
Landscape and visual |
Options which result in less landscape and visual impacts during the construction and operation stages will be rated higher. |
2.2.5.3 Each of the main criteria groups were weighed to reflect their relative importance and each option assessed, compared and ranked from these different perspectives. The robustness of the detailed evaluation exercise and selection of the Recommended Scheme was subjected to sensitivity testing.
2.2.5.4 The environmental assessment of the initial alignment options are presented below.
2.2.6 Environmental Assessment of the Initial Alignment Options
Northern Connection Options
Air Quality and Noise Impacts
2.2.6.1
Two of the northern options land in the industrial
area between the River Trade Terminal and
2.2.6.2 The key air sensitive receivers in the area are clustered at the west end of the industrial area north of the River Trade Terminal (RTT). These sensitive receivers are relevant to all the northern alignment options and have the potential to be subject to construction impacts and vehicular emissions during the operational phase.
2.2.6.3
Option N1 would comprise a large reclamation adjacent
to the RTT and then an elevated viaduct section towards the toll plaza and
2.2.6.4
Option N2 is similar to Option N1 in that it would
comprise a reclamation area for the tunnel portal and a viaduct section heads
northwards and to the east of Butterfly Beach Laundry, crossing over
Water Quality
2.2.6.5 In respect of water quality impacts, the key issue for the northern landing options is the need for and size of any reclamation. Both options comprise relatively similar levels of reclamation, with Option N1 comprising 16.5ha and Option N2 comprising 19.1ha.
2.2.6.6
Option N1 presents a reclamation which has been
reduced in size when compared with Option N2 as the toll plaza is proposed to
be located on land and not on the reclaimed land. The alignment would
also be shifted westwards closer to the RTT reclamation so as to minimise the
land reclamation needed. Reducing the size of the reclamation is
preferable from a construction impact perspective (reduced dredging and
quantities of waste sediment generated) and also reduces direct loss of
ecological habitat. Operationally, the elongated reclamation would
protrude slightly outside of the 10m depth contour but somewhat in the wake of
the RTT. As such, while significant effects on tidal flows and large
scale water quality are unlikely, localised acceleration in the flows
at the end of the reclamation could occur. The reclamation, however, could
also affect wave action on
2.2.6.7
Option N2 would have a larger reclamation overall due
to the location of the toll plaza in this area. This larger size means that,
irrespective of the TBM tunnel construction, greater dredging and seabed
disturbance would occur for Option N2 as compared to Option N1. In terms
of flows, as with Option N1, there could be some acceleration in
the flows at the end of the reclamation, although large scale water
quality impacts are unlikely to be affected. Unlike Option N1, however, Option
N2 would create a large embayed area between the reclamation and the RTT and
floating debris and local water flushing could be a problem. The
reclamation could also affect wave action on
Ecology and Fisheries
2.2.6.8
In terms of the marine environment, several important
ecology sensitive receivers are present within the broad study area. These
include sites of conservation importance (e.g., the Sha Chau and
2.2.6.9 The terrestrial environment of the Tuen Mun landing area is mostly disturbed urbanised area (heavy / special industrial use) or contains habitats of relatively low ecological values (e.g., plantation woodland and fire-disturbed grassland). These habitats are unlikely to support significant species of ecological concern. The main area of ecological value comprises the Siu Lang Shui SSSI butterflies overwintering site but this is some distance (~1.7km) northwest of the landing area.
2.2.6.10 Both Options N1 and N2 would require permanent reclamation works for the
tunnel portal and, in respect of Option N2, the toll plaza as well. These
marine works have the potential to cause marine ecological impacts during both
the construction and operational stages. The land works for both options
would be located in already disturbed areas dominated by disturbed areas,
currently occupied by the River Trade Golf, and of low ecological value.
The marine works for Option N2 would be larger than for Option N1 increasing
the potential for impacts and also resulting in more direct seabed loss.
In addition, the reclamation for Option N2 would be located close to
Waste Management
2.2.6.11 As mentioned above, all the northern options land just east of the RTT in a largely developed area with industrial land use. Based upon the land use in this area, there is potential for contaminated land issues which could affect Options N1 and N2.
2.2.6.12 Both Options N1 and N2 all pass through the potentially contaminated land associated with the works areas adjacent to the coast on viaduct and while quantities would be relatively small and likely to restricted to surface deposits, this material would need to be managed carefully to avoid secondary impacts. Both options would require marine sediment removal as part of the works to form the reclamations. The approximate amount of material to be dredged would be 1.46Mm3 and 1.71Mm3 (bulk volume with 30% bulk factor) for Options N1 and N2 respectively. While the quantity of sediment is higher for Option N2 given the larger reclamation works, this option has less land excavation works than Option N1 as the toll plaza is located in the reclaimed land. On balance, both options were deemed to be similar and based upon the amount of marine dredged material generated by each and the potential for interface with contaminated land, both were ranked “Poor” for waste management.
Landscape and Visual
2.2.6.13 Under Option N1, the sources of landscape and visual impacts will
include the reclamation works (16.5ha) at seafront near RTT and the toll plaza
at the site of River Trade Golf. The proposed works will cause permanent loss
of 16.5ha seawater body near Pillar Point and the impact is considered as
substantial. The proposed works will cause permanent loss of the temporary
River Trade Golf within the planned Crematorium, Columbarium, Funeral Services
Centre and open space. However, as the River Trade Golf is under STT, its
sensitivity is considered as medium. Therefore, the impact on River Trade Golf
is moderate. Besides, proposed works will slightly affect nearby urbanized
vegetation and planting. The proposed works will moderately affect an inshore water
landscape near Pillar Point; and would slightly affect Pillar Point
miscellaneous urban fringe landscape and Pillar Point industrial urban
landscape. The proposed works will be visible to small populations of workers
near RTT (closest viewing distance: close to 100m), high populations of
recreational users at
2.2.6.14 In terms of Option N2, the sources of landscape and visual impacts would
include reclamation works (19.1ha) and the tunnel portal for the northern
landing point connecting to the proposed TMWB at the seafront near the RTT. The
proposed works would cause permanent loss of 19.1ha seawater body near Pillar
Point and the impact is considered as substantial. The proposed works
would moderately affect landscape resources of nearby urbanized vegetation and
planting. The proposed works would moderately affect and inshore water
landscape near Pillar Point; and would slightly affect Pillar Point
miscellaneous urban fringe landscape and Pillar Point industrial urban
landscape. The proposed works would also be visible to small populations of
workers near RTT (closest viewing distance: close to 200m), high populations of
recreational users at
2.2.6.15 Based on the above, the Northern Connection Options were scored for the Environmental Impacts criteria set as summarised in Table 2.2 below.
Table 2.2 Northern Connection Options Scores – Environmental Impacts
|
Sub-Criterion |
Option |
|
|
N1 |
N2 |
|
|
Air quality and noise impact |
60 |
80 |
|
Water quality |
60 |
40 |
|
Ecology and fishery |
60 |
40 |
|
Waste management |
40 |
40 |
|
Landscape and visual |
40 |
40 |
2.2.6.16 Environmentally, Option N1 resulted in the higher score of the two options. Option N1 is also the more favourable option after taking into account other parameters including traffic performance and impact, operation and maintenance issues, construction practicability and safety and cost and programme.
Southern Connection Options
Air Quality and Noise Impact
2.2.6.17 There are two possible options on the south, Options S1 and S1a, the
former comprising a viaduct landing at Tai Ho Wan and the latter a tunnel
option that makes landfalls adjacent to Tai Ho Wan and the south-east of
proposed HKBCF together a viaduct landing at Tai Ho Wan. In terms of
existing sensitive receivers, both areas are generally unpopulated but with
some village houses present at
2.2.6.18 The potential sources of noise and air impacts for Option S1 would be from the open section of the southern viaduct and would be an issue for the villagers at Pak Mong and the future sensitive receivers in the Future Tung Chung East Development and the potential Lantau Logistics Park, although the number of sensitive receivers is likely to be small and the additional impacts given the background of the NLH are not expected to be significant. Construction phase impacts on the villagers at Pak Mong, some 500m away, are also not expected to be significant given the baseline environment. As such, Option S1 was ranked “Good”.
2.2.6.19 In terms of Option S1a, the alignment is largely in tunnel, the only potentially affected area is at Siu Ho Wan which is close to the portal and where the tunnel ventilation building will be located and also the connecting slip roads. The concentrated pollutant emissions from the tunnel ventilation could have the effect of contributing to the overall air pollution in the area but this should be balanced against the fact that there are no air and noise sensitive receivers present in the tunnel portal area. Another potential sources of noise and air impacts for Option S1a would be from the open section of the southern viaduct and would also be an issue for the villagers at Pak Mong and the future sensitive receivers in the Future Tung Chung East Development and the potential Lantau Logistics Park, although the number of sensitive receivers is likely to be small and the additional impacts given the background of the NLH are not expected to be significant. Construction phase impacts on the villagers at Pak Mong, some 500m away, are also not expected to be significant given the baseline environment. Therefore, Option S1a was also ranked “Good”.
Water Quality
2.2.6.20 Option S1 presents the scheme recommended by the Feasibility Study and
consists of a viaduct section between the TM-CLKL tunnel portal/HKBCF and the
connection with the
2.2.6.21 For Option S1a, part of the viaduct of Option S1 is replaced with the
IMT tunnel which is about 1.4km long. Operationally, as the tunnel
requires reclamations (totally about 14.0ha) at either end of the alignment for
the tunnel portals, this option will effectively form a barrier across the Tung
Chung Bay, with only a small channel left open in between the 2
reclamations. It would be predicted, therefore, to have significant
effects on the hydrology of the area and the flushing of the
Ecology and Fisheries
2.2.6.22 Ecologically, the area contains many ecologically sensitive receivers.
Tai Ho Wan represents a sensitive ecological resource, comprising the Tai
Ho Stream SSSI, mangroves and seagrass stands, numerous fish species and the
brown fish owl has been noted in this area. Intertidally, mangroves,
seagrass and horseshoe crabs are located in
2.2.6.23 The land connection of Option S1 occurs at Tai Ho Wan, with elevated
slip roads directly connecting with the
2.2.6.24 Option S1a would require an IMT tunnel which would need dredging and
permanent reclamations at either end of the alignment and as such, this option
would have greater disturbance to the marine environment and the potential for
temporary impacts at least to Tai Ho Wan (via the flushing culvert), Tung Chung
and the horseshoe crab habitats. Notwithstanding the permanent habitat losses,
while the habitats temporarily disturbed would be expected to regenerate in
time, this could take up about 3-5 years. Terrestrially, Option S1a would
be the same as Option S1 with the slip roads connecting directly with the
Waste Management
2.2.6.25 Waste management issues for Option S1 would be relatively minor and limited to the dredging for the viaduct piers and the excavation in the transport corridors to accommodate the new slip roads. The amount of marine sediment to be removed would be in the region of 0.03Mm3 (bulk volume with 30% bulk factor). The small amounts of the sediment would not be expected to be contaminated and to require disposal in confined disposal mud pits. As such, Option S1 was ranked “Good”.
2.2.6.26 Option S1a would generate substantially more material, largely in the form of dredged sediment for the IMT tunnel and reclamations. The sediment generated would be a quantity of 0.6Mm3 (bulk volume with 30% bulk factor). Based upon this, Option S1a was ranked “Poor”.
Landscape and Visual
2.2.6.27 In terms of Option S1, the sources of landscape and visual impacts would
include the sea viaduct in open seascape from the south portal on the proposed
HKBCF to the Southern Connection and the viaduct. These would substantially
affect the seawater body north of Tai Ho Wan, moderately affect the roadside
planting along
2.2.6.28 For Option S1a, the alignment would largely be in tunnel and, as such,
the sources of landscape and visual impacts would be restricted to the portals
at the HKBCF and the Southern Connection and also a short section of viaducts
connection to NLH. These would substantially affect the seawater body
north of Tai Ho Wan, moderately affect the roadside planting along NLH and
slightly affect the vegetation at Tai Ho and Pak Mong. The affected landscape
character areas would include inshore water landscapes near Tai Ho and the
2.2.6.29 Environmentally, Option S1 was preferred as it would potentially cause less impact on the marine environment and generate less waste. Option S1 was, also, the more favourable when taking into account all other parameters, being ranked significantly better for transport and operation planning, planning and land issues and engineering.
Main Crossing Options
Air Quality and Noise Impact
2.2.6.30 All three main crossing alignments would comprise long tunnels for the majority of their lengths and are only in the open when they connect to the HKBCF and to the north portal. In addition, there are no air or noise sensitive receivers in the vicinity of the alignments. As such, air quality and noise impacts are not a key issue for these main crossings, given that the dominant factor will be the HKBCF itself. However, all options would require a ventilation building at each end of the tunnel, which would contribute to the air pollution in the broad study area. On balance, all options were considered to be equal and were ranked “Good”.
Water Quality
2.2.6.31 The Main Crossing Option M1 is similar to the alignment recommended by
the previous FS Study and included in the EIA Study Brief and would comprises a
small reclamation of about 13.7ha at Tai Mo To for the tunnel portal. The
alignment would then connect to a further reclamation area of about 14.6ha
connected to the HKBCF via a small stretch of viaduct which would cross an area
of stronger flows. As the tunnel would be constructed by TBM, no dredging
or disturbance to the seabed is required and, as will be the case for the other
two options also, therefore, will not cause any water quality impacts during
either the construction or operational stages. In addition, given that
the Option M1 reclamation would be located in relatively shallow water adjacent
to the Brothers, no significant impacts on local tidal flows or the flows in
2.2.6.32 In total, Option M1 would require 28.3ha of reclamation to accommodate the tunnel portals and connections roads, in addition to an extra stretch of viaduct. While no effects on the overall flows in the area would be expected from Option M1 works, given that the HKBCF will be dominant, it would cause more seabed loss and disturbance than the other options and as such, was ranked “Poor”.
2.2.6.33 The second Main Crossing Option M3 would have the reclamation connected to the HKBCF on the eastern side and, therefore, would avoid the need for a separate tunnel landing reclamation. The area of reclamation needed for Option M3 would be 19.1ha. Given its size, the HKBCF reclamation would likely affect the tidal flows in the study area and along the North Lantau coastline and could possibly increase flows through the Airport sea channel leading to increased erosion which could affect the seagrasses and mangroves in Tung Chung Bay. However, the contribution of the main combined TM-CLKL reclamation for Option M3 would be unlikely to increase the magnitude of any HKBCF impacts and could present a net decrease in construction impacts than if the reclamations were constructed and operated separately as with Option M1. However, in the design assessed, in order to accommodate an alignment gradient of 5% as it exits from the tunnel to the portal, an elongated nib of reclamation would be required to the north. As this nib extends to the 10m contour, it would have a possibly significant impact on flows between Chek Lap Kok and Tai Mo To and possibly cause a redistribution of the larger scale flows.
2.2.6.34 Based upon this, while the area of reclamation as compared to Option M1 would be smaller and as such construction phase impacts would be less, given the potential operational implications of the nib, Option M3 was also ranked “Poor”.
2.2.6.35 Main Crossing Option M4 would be similar to Option M3 but assumes that the connection with the HKBCF would be made on the west instead of the east. The size of the reclamation needed and the need for a nib would be comparable to Option M3 and as such the predicted construction and operational impacts would also be expected to be about the same. Based on this, Option M4 was ranked the same as Option M3 at “Poor”.
Ecology and Fisheries
2.2.6.36 The location of the main crossing options contains important ecological sensitive receivers including habitats and species of conservation importance. The Chinese White Dolphin (CWD) is the single most important species in the study area and the one that has the potential to be impacted, both by the TM-CLKL and cumulatively. The proposed alignments of the main crossing options would be within the prime habitat for the local population of CWD. The potential disturbance and habitat degradation/destruction could represent a significant threat to the long-term sustainability of this species and options which minimise potential impacts on this species were favoured.
2.2.6.37 The area is also a fishing ground and protection of other species including horseshoe crabs, which are occasionally found close the Brothers, and corals of which some have been noted in the area, were also an issue when comparing the options. As such, while all options require reclamation of about the same size, the method of construction will have a notable effect on the potential impacts to the marine environment and options which can reduce the amount of dredging required will be preferable.
2.2.6.38 Besides the 14.6ha reclamation attached to the HKBCF, Option M1 would also have an extra area of reclamation for the tunnel portal at Tai Mo To which would increase the direct habitat loss by 13.7ha for marine ecology and fisheries. In addition, the area around the Brothers has increased in importance for the CWD in recent years and is currently highly utilised by this key species. As mentioned above the area around The Brothers is also habitat for horseshoe crabs and occasional corals. As such, construction impacts and direct loss of habitat in this area would be of concern and this option was ranked “Very Poor” accordingly.
2.2.6.39 Main Crossing Options M3 and M4 would be very similar. Both reclamation connect to the HKBCF and would be away from the CWD habitat at The Brothers. While there would be direct habitat loss and construction impacts, the effects on corals and benthos would be similar for both the options and less than for Option M1. As such, both Options M3 and M4 were ranked “Fair” from an ecology perspective.
Waste Management
2.2.6.40 All three main crossings would comprise a long tunnel formed by TBM. The lengths of the tunnels would vary slightly. Due to the different lengths, the amount of material generated by each would be about 1.09Mm3 for Option M1, 1.48Mm3 for Option M3 and 1.39Mm3 for Option M4 in bulk volume with 30% bulk factor.
2.2.6.41 For the reuse of excavated alluvium, it is necessary to use a slurry treatment plant (STP) to separate the slurry and the alluvium generated from the TBM tunnelling and then the alluvium could be used as public fill. On the basis of the similar amount of waste that would be generated, Options M1, M3 and M4 were all classed as “Fair”.
Landscape and Visual
2.2.6.42 In terms of Main Crossing Option M1, the sources of landscape and visual impacts would include reclamation works at Tai Mo To and the sea viaduct to/ from the airport and HKBCF. The proposed works would cause a permanent change to the island landscape of Tai Mo To and permanent loss of the inshore water landscape near Tai Mo To which is of high sensitivity. Therefore, landscape impacts would be considered to be substantial. The proposed works would be mainly visible to high populations of visitors and workers at the HKIA (closest viewing distance: 2.8km), high populations of residents at Tung Chung New Town (closest viewing distance: 4km), high populations of residents at Tuen Mun (closest viewing distance: 4.4km), the proposed Lantau Logistics Park (closest viewing distance: close to 2.8km), possible LLP Extension or other compatible uses (closest viewing distance: close to 2.7km) and the Future Tung Chung East Development (closest viewing distance: close to 3.5km). Mitigation measures that would likely be required would include minimised construction area, the use of appropriate (visually unobtrusive and non-reflective) building materials and colours in built structures and screen planting. With the implementation of mitigation measures, the view would still be changed from an open seaview to partial blockage by the marine viaduct. Therefore, the visual impact would be considered as moderate to substantial. Based on the substantial landscape and moderate to substantial visual impacts and the permanent change of Tai Mo To island landscape, Option M1 was ranked “Very Poor”.
2.2.6.43 For Option M3, the sources of landscape and visual impacts would include reclamation works at HKBCF. The proposed works would cause permanent loss of the seawater body near HKBCF which is of high sensitivity. Therefore, it was considered that the landscape impact would be substantial. The proposed works would be mainly visible to high populations of visitor and workers of the HKIA (closest viewing distance: 2km), high populations of residents at Tung Chung New Town (closest viewing distance: 4km), high populations of residents at Tuen Mun (closest viewing distance: 6km), the proposed Lantau Logistics Park (closest viewing distance: close to 2km), possible LLP Extension or other compatible uses (closest viewing distance: close to 1.9km) and the Future Tung Chung East Development (closest viewing distance: close to 2km). Mitigation measures that would likely be required would include minimised construction area, the use of appropriate (visually unobtrusive and non-reflective) building materials and colours in built structures and screen planting. With the implementation of mitigation measures, the visual impact would be considered as insubstantial to slight. Based on the substantial landscape and insubstantial to slight visual impacts, Option M3 was ranked “Fair”.
2.2.6.44 The sources of landscape and visual impacts for Main Cross Option M4 would include reclamation works at HKBCF. The proposed works would cause permanent loss of the seawater body between the HKBCF and the HKIA which is of high sensitivity. Therefore, it is considered that the landscape impacts would be substantial. The proposed works would be mainly visible to high populations of visitor and workers of the HKIA (closest viewing distance: 20m), high populations of residents at the Tung Chung New Town (closest viewing distance: 2km), high populations of residents at Tuen Mun (closest viewing distance: 6km), possible LLP Extension or other compatible uses (closest viewing distance: close to 2.3km) and the Future Tung Chung East Development (closest viewing distance: close to 2.4km). Mitigation measures that would likely be required would include minimised construction area, the use of appropriate (visually unobtrusive and non-reflective) building materials and colours in built structures and screen planting. With the implementation of mitigation measures, the view would still be changed from open seaview to partial blockage by the marine viaduct. Therefore, the visual impact is considered as moderate to substantial. Based on the substantial landscape and moderate to substantial visual impacts, Option M4 was ranked “Poor”.
2.2.6.45 Based on the above, the Main Crossing Options were scored for the Environmental Impacts criteria set as summarised in Table 2.4 below.
Table 2.4 Main Crossing Options Scores – Environmental Impacts
|
Sub-Criterion |
Option |
||
|
M1 |
M3 |
M4 |
|
|
Air quality and noise impact |
80 |
80 |
80 |
|
Water quality |
40 |
40 |
40 |
|
Ecology and fishery |
20 |
60 |
60 |
|
Waste management |
60 |
60 |
60 |
|
Landscape and visual |
20 |
60 |
40 |
2.2.6.46 Options M3 and M4 were ranked better than Option M1 environmentally, largely because of the need for the second reclamation at Tai Mo To for Option M1 which increased construction and operational marine impacts. The overall weighted scores for all parameters favour Main Crossing Option M3 which was preferable on the basis of engineering programme, transport connectivity and traffic performance and impact.
2.2.7 Alignment Selection
2.2.7.1 Following the detailed evaluation of the alignment options based on all the criteria detailed in Section 2.2.5 above, the highest rankings are given to Northern Connection Option N1, Southern Connection Option S1 and Main Crossing Option M3 in the base assessment. This overall conclusion did not conflict with the results of the environmental assessment, the environmentally preferred alignment being a combination of Option N1, Option S1 and Option M3.
2.2.7.2 In order to test the robustness of the detailed evaluation, sensitivity tests were undertaken based upon one relevant criteria set having its overall weighting boosted by 50% (multiplier 1.5) or by 100% (multiplier 2.0). In both cases, the highest scores remained for Option N1, Option S1 and Option M3 and were, therefore, consistent with the overall conclusion.
2.2.7.3 Thus, according to the outcome of the option assessment and the associated sensitivity tests, combined alignment N1, S1 and M3 was selected as the recommended option for further assessment under the Assignment.
2.3 Construction Alternatives
2.3.1 Tunnel Construction
2.3.1.1 The key options for constructing the main tunnel connecting the northern landfall at Tuen Mun and the southern landfall at the HKBCF are either by tunnel boring machine (TBM) or immersed tube (IMT) tunnel.
TBM
2.3.1.2 Based on the internal traffic envelope and ventilation requirements, the twin bored TBM tunnels is envisaged to have an external diameter of approximate 14.0m. The tunnels will be constructed in parallel configuration with a typical separation of about one times the diameter of the tunnel or 14.0m.
2.3.1.3 The twin bored tunnel of external diameter of approximate 14.0m with central separation of about 14.0m will be constructed by Tunnel Boring Machine (TBM) launched from Tuen Mun side and retrieved from reclamation beside the HKBCF. Cross passages connecting the twin tunnels will be provided at regular intervals. Generally ground freezing method will be employed for the cross passage construction.
2.3.1.4 The TBM moves forward as it excavates the tunnel by extending the pushing jacks at the back. When the advancement of the machine reaches distance of the length of a ring, the excavation stops and the pushing jacks are retrieved, a concrete circular ring in form of a numbers of segments were then put together at the tail of the shield. The pushing arms are once again extended in full contact with the concrete ring just erected and excavation is resumed. The cycle of excavation and ring erection is repeated as the TBM is advanced to form the lining of the tunnel.
2.3.1.5
The TBM tunnel would require the extension of the
northern and southern landfall reclamations in order to provide the necessary
tunnel cover as the tunnel ascends to the portals. In order to provide
safe tunnel construction by TBM, soil cover of about 14.0m will be provided to
the tunnel at the ends of the reclamations. This results in the
reclamations requiring protruding nibs at both ends. In the north, while
significant effects on tidal flows and large scale water quality are unlikely,
localised acceleration in the flows at the end of the
reclamations could occur. The reclamation, however, could also affect
wave action on
2.3.1.6
The major advantages of employing TBM for the
construction of submarine tunnel across
· TBM scheme involve limited dredging operation;
·
No need for diversion of 132kV cable and
telecommunication cables connecting Tuen Mun and
·
No impacts on existing navigation channel
· Minimum ecology impact of Chinese White Dolphin; and
· Construction of TBM tunnel does not depend on the weather and can be operated with day and night shifts
2.3.1.7 The major concern of TBM is the possible risk to human health and safety of exposure to hyperbaric environment on rare occasions. Maintenance of the TBM cutterhead is required from time to time particularly for changing worn or damaged cutter discs. Cutterhead tools can be changed within the spokes of the cutterhead under atmospheric conditions. Notwithstanding, the possibility exists during emergency situation for entering the excavation chamber under hyperbaric condition if such operations cannot be carried out within the cutterhead spokes.
IMT
2.3.1.8 The proposed immersed tube tunnel (IMT) is in the form reinforced concrete rectangular box structure of dimensions 9.6m high x 33.2m wide and is designed to accommodate 2 x 2 lanes.
2.3.1.9 An immersed tube tunnel consists of numbers of prefabricated tunnel elements are first fabricated in the casting basin. After the tunnel elements are constructed, they are floated to the site, installed one by one, and connected to one another under water. There will be around 32 nos. tunnel elements and each element is about 120m long. An immersed tunnel is generally installed in the trench that has been dredged previously in the bottom of the sea. The space between the trench bottom and the invert of the tunnel was filled with sand foundation. As construction proceeds, the tunnel is backfilled. The completed tunnel is covered with a protective layer over the roof.
2.3.1.10 The major advantages of employing immersed tube for the construction of
submarine tunnel across
· Shallow soil cover (about 2-3m) making the gradients of road more gentle and have a better connection to the approach tunnels on both ends; and
· The extents of reclamations are smaller as compared with TBM tunnel option; and
· Cross passages can be more easily provided within the internal tunnel structure
2.3.1.11 The major concerns of IMT construction for TM-CLKL include environmental and ecological impact due to extensive dredging of seabed, location and availability of land required for casting basin, risk due to deep sea diving operation under high currents and low visibility conditions, impact on marine traffic of existing navigation channels especially Urmston Road during towing and sinking operations and diversion of 132kV cable and telecommunication cables connecting Tuen Mun and Chek Lap Kok Airport Island.
Selected Method
2.3.1.12 The TBM tunnel will have significant environmental advantages over the IMT tunnel scheme. For the IMT tunnel construction, dredging for the tunnel trench would be required, which would be about 140-210m wide and about 17-30m deep (from seabed) and this would be required for the full submarine tunnel alignment. The total amount of marine sediment to be dredged for the IMT tunnel construction is approximately 11.5Mm3 bulk volume (with 30% bulk factor). During the construction phase, the need for dredging would disturb the seabed resulting in temporary losses of marine habitat and would, also, release sediment into the water column increasing the risk of impacts to water quality and marine life. As the alignment for the tunnel passes though fisheries, Chinese White Dolphins, coral and other marine life habitats, avoiding such impacts through the use of TBM is a notable environmental advantage.
2.3.1.13 Based upon the notable environmental advantages during the construction phase and the engineering justification, the TBM method is preferred and selected.
2.3.2 Reclamation Methods
Seawalls
2.3.2.1 The project involves extensive reclamation, with 16.5ha required for the northern tunnel landfall and 19.1ha required for southern landfall to the cope line (or 21.1ha and 25.4ha, respectively, for the footprint area to the bottom of the seawall where it intersects the seabed). Three types of seawall are proposed for these areas as follows:
· vertical blockwork seawall, where berthing of vessels is required;
· vertical wave absorption seawall:
· sloping seawalls elsewhere, to provide better wave dissipation.
2.3.2.2 There are several ways to construct the seawall foundation but each have different engineering and environmental implications. The key construction methods are listed below and further details on each provided in the following sections:
· stone column method;
· deep cement mixing;
· sand compaction piles; and
· fully dredged.
Stone Column Method
2.3.2.3 This method can be applied when the increases to the overall bearing capacity and strength of the treated layer are needed. The stone columns are installed with a vibratory probe and the void created as the probe is removed is filled with rock. The formed columns allow transfer of the load from the fill layer to firmer stratum. The extraction of the probe has the possibility of causing minor sediment plumes due to the adherence of fine clay material to the outside. However, it is expected that the presence of a sand capping layer, covering the area as a pre-treatment to provide sufficient ground stability for the construction equipment, would filter most of this material and, as such, the sediment plumes should be minimal.
2.3.2.4 Notwithstanding, this technique is not deemed suitable for treatment of soft marine deposits as are present in the study area. The technique when applied in marine site conditions is slow and, therefore, expensive and furthermore has difficulty in providing an adequate level of quality that can be checked with transparent methods. In addition, the volume of stones placed in each column is not able to be accurately measured and the stone consumption over depth in each column is unable to be checked. Hence, there is a great risk of leaving poorly treated zones which may lead to excessive settlements or instability for the reclamations and the structures on them.
Deep Cement Mixing
2.3.2.5 The deep cement mixing method requires that cement is used to mix with the marine deposits to improve the shear strength of the bearing stratum and to reduce the consolidation settlement of the bearing stratum under load.
2.3.2.6 Cement slurry is mixed with the sediment by a rotating paddle inside a metal casing. When the mixing system reaches the seabed and is no longer contained, the injected cement has the potential to leak into the environment. Leakage of cement slurry in the last section of the column formation can be as much as 5% of the application, and could affect water quality in terms of suspended solids and local changes to pH, potentially leading to an increase of ammonia toxicity to sensitive species such as juvenile fish. The waters in the study area are often nitrogen enriched and the water quality objective (WQO) of 0.021 mg/l for un-ionised ammoniacal nitrogen level could be exceeded by the leaked cement even though any impact would likely be localised to the leakage area.
2.3.2.7 On the other hand, the reliability of this method can be variable as the strength of the soil-cement pile will be influenced by factors such as sediment type, consistency of soil/cement mixing and the curing environment. Other constraints include the fact that it is difficult to control the quality of the works and establish the diameter of the pile formed which would require lengthy field trials and, also, the technique would be very costly compared to other methods. As such, notwithstanding that the overall impacts on the water quality are likely to be small and localised, this method is not proposed.
Sand Compaction Pile
2.3.2.8 Sand Compaction Pile (SCP) refers to the construction of a column of dense sand through the full thickness of the sediment and broadly follows the following method:
· Possibly following the laying of a sand blanket, a steel tube, typically varying in diameter from 0.4 to 0.8 m, is pushed into the sediment to the required depth. Insertion of the tube can be assisted by vibration at the top of the tube and air/water injection at the base of the tube. The tube is blocked during insertion and there is no boring or removal of spoil as the tube is inserted;
· When the steel tube has reached the required level it is withdrawn a short distance and at the same time sand is forced out of the base of the tube by compressed air;
· The level of the sand in the tube is monitored to ensure that the tube always contains sand and that the sediment around the tube is not allowed to collapse below the tube;
· The tube is then pushed back into the sand and vibrated back on top of the sand that has been deposited in the ground below the tube in order to increase the diameter of the sand column by pushing it out against the sediment and increasing the density of the sand at the same time; and
· When the desired diameter has been achieved (determined from the known volume of sand placed in the column) more sand is added to the tube and the process of sand placement and compaction is repeated. The process is continued until the sand compaction pile has reached the desired level.
2.3.2.9 The composite pile formed would increase strength and result in a decrease in consolidation settlement and lateral displacement. This method, also, requires the shortest period for sediment stabilisation and improvement of strength of the ground conditions and, as such, is used in the stabilisation of foundation soil supporting structures offshore.
2.3.2.10 Though the dredged volume when using SCP will be decreased by 70% when compared with the fully dredged method, the use of SCP will increase the construction cost by three times and may cause an overall delay of the programme by 30 months.
2.3.2.11 Furthermore, in view of the fast track of the construction works and the
lack of local experience on the construction of SCP in
2.3.2.12 Setting aside the time and cost factors, SCP is technically not applicable for use in the southern landfall reclamation where excessive upheaval of the existing seabed induced by the SCP installation could adversely affect the use of the seawall for berthing both during construction and operationally. The existing seabed at the southern landfall is approximately -4mPD to -11mPD, and the thickness of the marine deposits is about 14m. Using this technique, the seabed would heave by about 4m after installation of the SCPs, and such upheaval would adversely affect the operation of barges both during construction and in future if the heaved materials are not dredged away. Doing so, however, will inevitably increase the quantity of material for disposal and potential water quality impacts.
2.3.2.13 In view of the prolongation to the construction programme, increase in construction cost, inability of a total avoidance of dredging marine deposits and other factors mentioned above, the use of SCP is not recommended to form the seawall foundation at either landfall.
Fully Dredged for Seawall Foundation
2.3.2.14 Complete removal of marine deposits by dredging and backfill with
sand/rock fill for seawall foundation is a commonly used method in
2.3.2.15 The fully dredged method has the following advantages from an engineering perspective:
· avoids long term settlement and deformation of the seawall; and
· is the most rapid technique where programme constraints are present.
2.3.2.16 During dredging works fine material will be displaced and may be carried downstream of the works area. The extent of the suspended sediment plume will depend on the rate of release, the particle size of the dredged material and its characteristic settling velocity, the prevailing currents and hydrodynamic conditions. It is proposed that grab dredgers be used for this activity, and this will result in instantaneous sediment plumes, formed throughout the water column as the grab is pulled to the surface.
2.3.2.17 Based on a comparison of the methods above, the fully dredged option for the formation of the seawall foundations has the potential to release the most suspended solids into the water column and would be the least environmentally favourable option.
Selected Method
2.3.2.18 Based upon the above, from an engineering perspective, both the Stone Column Method and the Deep Cement Mixing have been rejected as unsuitable for use in the Project. Also, as the use of the SCP technique will result in an overall delay of the programme by 30 months compared with the fully dredged method, does not give the same level of ground strength and would result in sediment heave at the southern landfall, this method is not proposed to form the foundations for the landfall reclamation seawalls. The comparison between the various construction methods for seawall foundation are summarized in Table 2.5 below:
Table 2.5 Summary of Comparison between various construction methods for seawall foundation
|
|
Stone Column Method |
Deep Cement Mixing |
Sand Compaction Piles |
Fully Dredged |
|
Total Cost |
High |
Very High |
High |
Low |
|
Construction Period |
Slow |
Slow |
Slow |
Fast |
|
Dredging |
No |
No |
Partially Dredged |
Fully Dredged |
|
Remarks |
As mentioned in section 2.3.2.4, there are some engineering difficulties encountered for installation of stone column into the soft marine deposits. Hence the method is not feasible |
It
is very expensive when compared with other method. Also, there is no local
track record in |
It
is expensive, long construction period and dredging is unavoidable due to
sediment heave. Also, there is no local track record in |
Low cost and short construction period when compared with others. Feasible solution for seawall foundation. |
2.3.2.19 The fully dredged method, being the only remaining feasible solution, is, therefore, selected.
Land Formation
2.3.2.20 The main purposes of the reclamations for the TM-CLKL are to provide land for the launching and receiving shaft for the TBM as well as providing minimum soil cover of 14.0m to facilitate the safe operation of the TBM. The reclamations will, also, contain the portal and tunnel approach roads.
2.3.2.21 Reclamation can be formed using either fully dredged or non-dredged methods.
Non-dredged Method
2.3.2.22 For the most common non-dredged method, the marine deposits are left in place and band drains installed to accelerate primary consolidation so that the reclamation can be constructed more rapidly. If the highly compressed marine and alluvial deposits are not dredged, these layers would consolidate under the weight of the fill material and this would result in settlement of the reclaimed land. The consolidation process is slow and in order to speed this up, band drains can be installed to drain the pore water. The band drains are installed through the full depth of the compressible layer and cut off at the sand drainage layer at the top. Surcharge in the form of soil embankment is then placed on the land after the band drains have been installed.
2.3.2.23 While a cost-effective process, the time required for consolidation of the marine deposits can present severe constraints on the construction programme.
2.3.2.24 Environmentally, this process has notable advantages of avoiding the construction phase impacts associated with dredging and the associated potential water quality, marine ecological and waste management implications that results. The extraction of the band drain device has the potential to cause minor sediment plumes due to the possible adherence of fine clay material to the outside, but the sand drainage layer would be expected to filter most of the material and, as such, the sediment plumes should be minimal.
2.3.2.25 Other non-dredged methods include those which may be applicable to seawall construction such as stone column method, sand compaction pile (SCP) and deep cement mixing (DCM), the details of which are described in the foregoing sub-sections. However, these methods are either not deemed suitable for the treatment of the marine deposits in the study area, or the reliability of which can be variable as discussed above. Moreover, the use of SCP is more applicable to the strengthening of seawall foundation rather than general reclamation.
Fully Dredged Method
2.3.2.26 For the fully dredged method, the marine deposits will be removed by dredging, and the post reclamation settlements can be reduced significantly. It is suitable for the reclamation requiring short construction period and provides good foundation for the structure above.
2.3.2.27 As with the fully dredged seawalls, any dredging works for the land formation would also result in the loss of suspended solids into the water body and, as the works would be extensive and undertaken over a notable time period, unmitigated impacts could be significant. However, depending on the progress of the seawall, a partially completed seawall or leading edge of seawall could act as a barrier to reduce the losses of dredged sediment. Notwithstanding, the potential environmental impacts will be more significant than those from the non-dredged method and, as such, would not be environmentally preferred.
Selected Method
2.3.2.28 Given the notable environmental advantages of the non-dredged method of land formation, this has been proposed for the majority parts of both the northern and southern landfalls. The use of band drains and surcharging are preferred over the other alternative non-dredged methods which have been dismissed on engineering grounds.
2.3.2.29 However, at the southern portion of the northern landfall and the northern portion of the southern landfall, deep excavation will be required for constructing the TBM shafts and the deeper section of the cut-and-cover tunnel. This would inevitably have removed the marine deposits located above the proposed structures and, therefore, the fully dredged method is proposed in conjunction with the seawall construction.
2.3.3 Piling Methods
2.3.3.1 Piling activities will be required for the viaduct piers of the southern section located within the sea channel between the southern reclamation and north Lautau shoreline.
2.3.3.2
The type of piling method to be used will have a
bearing on the potential impacts to cetaceans, specifically the Chinese White
Dolphin (CWDs), that resides the study area. Cetaceans use two functional
classes of sound, these are echolocation and communication. Echolocation is
used for orientation, navigation, prey detection and learning about the
surrounding environment, whilst communication is used for intra-species
signalling (
Percussive Piling
2.3.3.3 Percussive piling creates loud, impulsive sounds that have the potential to physically injure or disturb dolphins. Most of the energy from pile driving occurs below 20 kHz, with a peak at about 200 to 1,500 Hz (1/3 octave spectra) (Nedwell and Howell 2004). The lower acoustic frequencies of pile driving can be transmitted for as far as about 40 km in water deeper than several meters (David 2006), so the impacts can in some cases be quite long-range.
2.3.3.4 Because most pile driving results in considerable energy into the single kHz digits, it has the potential to be a serious issue for dolphins. This is the range where small cetaceans, such as humpback dolphins, produce most of their communication sounds, and are most acoustically-sensitive, although they also may produce sounds to frequencies as high as 200 kHz and beyond (Goold and Jefferson 2004). Loud sounds are, also, known to cause (short-term) behavioural reactions such as increased swimming speed and also interfere with communication and induce physiologic impairment in cetaceans (Richardson, 1995; Jefferson, 2000a; Wursig et al., 2000). Certain research in relation to marine piling for wind farms (Madsen et al. (2006)) suggests that pile driving is the noisiest and most disturbing part of the process and that it could cause behavioural disturbance and even hearing impairment. Other research (David (2006)) speculates that pile driving could mask strong bottlenose dolphin vocalizations at 9 kHz within 10-15 km and weak vocalizations up to 40 km. However, this was not based upon direct behavioural or physiological information.
2.3.3.5
However, there is not much detailed information on
reactions of marine mammals to pile driving. Harbour seals (Phoca
vitulina) may vacate areas of pile driving (for example, Rodkin and Reyff
2004), but in the Arctic, Blackwell et al. (2004) found that a related species,
the ringed seal (Phoca hispida) appeared to ignore the pile-driving work
that was being conducted from a nearby artificial island. The latter
case, however, may represent an example of habituation, as the
2.3.3.6 Intensive monitoring of CWD behaviour during percussive pile driving for construction of the aviation fuel receiving facility (AFRF) at Sha Chau in 1996, did, however, show that dolphin density was lower for the period of piling (and other activities) but recovered once construction of the AFRF has been completed (Jefferson 2000; Wrsig et al. 2000). There were, also, probable impacts on dolphins swimming speed, patterns and dive times (Jefferson 2000).
2.3.3.7 Although percussive piling does produce high-intensity noise impacts capable of inducing physical injury, the piling duration is usually shorter than would be required for bored piling.
2.3.3.8 Notwithstanding, the use of percussive piling may result in at least temporary abandonment of habitat (see Jefferson 2000), forcing animals to spend time in lower-quality habitats which in turn may result in lowered fitness, reduced reproductive output, or reduced survival.
Bored Piling
2.3.3.9 Noise created by bored piling methods tends to be a less intensive continuous noise, rather than the pulsed high power sounds emitted through percussive piling. Bored piling usually creates a steady sound that is less disruptive to dolphins than the pulsed or burst sounds associated with activity such as percussive piling (Wursig et al., 2000). Dolphins are known to habituate to low-level sounds such as those produced through bored piling (Greene and Moore, 1995).
2.3.3.10 However, bored piling work generally takes a longer time period to drive the piles, thereby lengthening the overall time period of impact and likely resulting in chronic lower level impacts as opposed to the an acute high-energy noise impact of percussive piling. The programming for the southern viaduct piling is approximately 14 months.
2.3.3.11 Almost no work has been done on its effects on dolphins or porpoises, but it is reasonable to assume that it may also cause behavioural disturbance, although it is generally accepted that the impacts would be much less severe than for percussive piling.
2.3.3.12 Notwithstanding the longer construction period, based upon the potential for less severe impacts to this key species and, together with the engineering preferences above, bored piling techniques have been selected for the viaduct works.
Selected Method
2.3.3.13 Due to the heavy loads imposed by the relatively long spans of the viaduct, the viaduct piers will be required to sit on piles founded on rock. In order to minimise the number of piles located in the sea channel, large diameter piles of diameter between 1.5m and 2.5m will be required. Piles of this size will be too large to be driven and will need to be constructed by bored methods. Therefore, percussive piling will not be used for construction of the piles supporting the viaduct.
2.3.4 Sand Filling Methods
2.3.4.1 Sand filling is required for the land formation at the southern landfall, and the sand can be deposited using either a pelican barge or a bottom dumping hopper barge. Land formation at the northern landfall uses public fill.
2.3.4.2
Since pelican barges will used for the transportation
of marine sand imported from
2.3.4.3 It should, also, be noted that the pelican barge would not “rainbow” in the same way as a trailer. A pelican barge has a conveyor belt extending from the central part of the barge to the front edge and during the unloading process, the material is transferred from the body to the front edge and then merely drops down. The conveyor belt can, therefore, at the most cast the material out for a very small distance only. This process may be better described as “dropping” and does not compare to the fluidised and pumped sand and water mixture that is commonly described as “rainbowing”. As such, the potential environmental impacts of the use of pelican barges in terms of sediment losses would be similar to that of the bottom dumping hopper barge.