2.1.1 The project is entitled “Improvement of Fresh Water Supply to Cheung Chau” and is referred hereafter to as “the Project”.
2.1.2 Cheung Chau, with a population of around 23,000, is currently supplied with treated water from Silver Mine Bay Water Treatment Works on Lantau via two submarine water mains, 10” (about 250 mm) and 500 mm in diameter, across Adamasta Channel. Treated water is normally provided by the 500 mm diameter main, with the 10” main serving as emergency backup.
2.1.3 Laid in 1963, the existing 10” submarine water main providing partial emergency back up fresh water supply to Cheung Chau is approaching the end of its design life of 50 years. Repairs would be uneconomical and take a long time given the difficulty of the task and the condition of the main. To improve the reliability of water supply to Cheung Chau, it is necessary to strengthen the emergency back up by replacing the existing 10” submarine water main with a new 500 mm diameter submarine water main.
The Project is located in the southeastern coast of Chi Ma Wan
Peninsula at South Lantau, in the northwestern coast of
2.2.2 The Project is to construct and operate a new submarine water main across Adamasta Channel from Lantau to Cheung Chau to replace the existing 10” submarine water main, which is serving as emergency back up, to improve the reliability of water supply to Cheung Chau. The Project will comprise the following:
(i) Laying of a submarine water main of approximately 1,400 m in length and 500 mm in diameter across Adamasta Channel;
Construction of landfall and associated works within
(iii) Construction of landfall and associated works near Tai Kwai Wan, Cheung Chau.
General Description of the Project Area
Cheung Chau Landfall
2.2.4 Cheung Chau is situated south-east of Lantau and according to the 2001 Census has a population of 23,000. Cheung Chau is the most densely populated outlying island. It is dumbbell-shaped with vegetated knolls in the north and south and a narrow strip of flat land in the central part which is bounded by a typhoon shelter to the west and Tung Wan to the east. The central lowland contains the development core where most of the existing village areas, commercial uses and major community facilities are concentrated.
2.2.5 The northern part of the island is hilly and largely undeveloped. A private low-rise residential development, a rural public housing estate and infrastructure facilities serving the whole island (such as refuse transfer station, sewage treatment works, abattoir and fresh water pumping station) are the key features at the north-western promontory.
2.2.6 Regular licensed passenger ferry service is the major means of public transportation for Cheung Chau. Ferry services to Central, Mui Wo, Chi Ma Wan and Peng Chau are provided. Cheung Chau enjoys a car-free environment with only a limited number of village vehicles (Plate 5), fire engines, police vehicles and ambulance vehicles serving local needs.
The proposed landfall on Cheung Chau
would be constructed at a low headland in the southern end of Tai Kwai Wan (Plate 2), which is a sandy
bay situated at the northwestern coast of
2.2.8 The proposed landfall and works area on Cheung Chau fall on areas zoned “Government, Institution or Community” (G/IC) on the approved Cheung Chau Outline Zoning Plan (OZP) No. S/I-CC/4.
After making landfall, the proposed
land-based main will be laid along
The proposed landfall site on Lantau
would be located at Ha So Pai, in the southeastern corner of Chi Ma Wan
Peninsula, Lantau (Figure 2.3).
It is a remote coastal bay at the southern boundary of
2.2.11 After making landfall, the proposed land-based main will be connected to the nearby existing exposed main on Lantau (Plate 7).
2.2.12 The site is not accessible by vehicles, but can be reached by hiking the Lantau Trail via Chi Ma Wan or using commercially-hired boats.
The Adamasta Channel
The proposed submarine main would be
laid from Cheung Chau to Ha So Pai of Lantau across the Adamasta Channel. The
Adamasta Channel, with water depth ranging from 5 – 10 m, is a narrow passage
between Chi Ma Wan Peninsula of Lantau
2.2.14 Beside the two existing 500 mm and 10” submarine fresh water mains, twelve submarine cables consisting of three 33kV and nine 132kV electricity supply circuits were laid across the Adamasta Channel.
The Adamasta Channel is the main
traffic route for high speed ferry services between Hong Kong and
2.3.1 Currently, potable water supply in Cheung Chau is provided by the Silver Mine Bay Water Treatment Works in Lantau. After treatment, treated fresh water gravitates from Silver Mine Bay Service Reservoir on Lantau to Cheung Chau Fresh Water Pumping Station via two land mains and two submarine mains. Water is then pumped from Cheung Chau Fresh Water Pumping Station to three existing service reservoirs on Cheung Chau where it gravitates via distribution mains to consumers on the island.
2.3.2 The land mains in Lantau were laid in the fifties and are currently being rehabilitated/replaced under WSD’s Contract No. 2/WSD/07 – “Replacement and Rehabilitation of Water Mains, Stage 2 – Mains on Lantau Island East”, which is scheduled for completion in 2010. The two submarine mains across Adamasta Channel, 10” and 500 mm in diameter, were laid in 1963 and 1986 respectively. Treated water is normally provided by the 500 mm diameter main, with the 10” main serving as emergency back-up so as to maintain water supply to Cheung Chau.
2.3.3 With the 10” submarine main in service for over 46 years and approaching the end of its service life, repairs would be uneconomical and take a long time given the difficulty of the task and the condition of the main. Moreover, the supply through this 10” submarine main alone can only partially meet the demand of Cheung Chau for treated water. To improve the reliability of water supply to Cheung Chau, it is necessary to strengthen the emergency back-up by replacing the existing 10” submarine water main with a new 500 mm diameter submarine water main.
Need for the Project
2.3.4 The need for the Project has evolved from the requirement to provide a secure water supply to Cheung Chau. Specifically, it has been determined that the existing water main serving as emergency backup is approaching its design life of 50 years. There will be increasing risk of having one water main under maintenance while another main has to be taken out of service without warning.
2.3.5 Being the only water supply to Cheung Chau, it is necessary to lay a new submarine water main from Lantau to Cheung Chau to maintain the reliability of water supply to Cheung Chau.
2.3.6 The maximum mean daily demand for Cheung Chau is about 9,500 m3/day in 2013 and beyond. The 10” submarine main alone (carrying capacity 7,600 m3/day), is not sufficient to meet the projected fresh water and flushing water demand. For replacing this 10” submarine main, it is assessed that the proposed main, running at roughly the same alignment, should be 500 mm in diameter so that it alone can cope with the projected water demand for Cheung Chau.
Purpose and Objective of the Project
2.3.7 The purpose of the Project is to construct and operate a new submarine water main across Adamasta Channel from Lantau to Cheung Chau to replace the existing submarine water main, which is serving as emergency back up, to improve the reliability of water supply to Cheung Chau.
Scenarios With and Without Project
2.3.8 The existing 10” submarine water main in Cheung Chau was laid almost 50 years ago and is approaching the end of its serviceable life. Consequences of not proceeding with the Project will cause serious water supply interruption, disruption of everyday life and businesses in Cheung Chau leading to adverse inconvenience to the public and tourists in the event the current water main is taken out of service unexpectedly.
2.3.9 The proposed Project will provide a secure and reliable water supply to Cheung Chau.
Justification and Benefits of the Project
2.3.10 This Project is in line with the WSD’s objective to plan and develop reliable and efficient treated water supply and distribution systems for the territory.
2.3.11 The estimated population expected to be directly benefited by the Project will be the residents of Cheung Chau of about 23,000 as well as the tourists visiting Cheung Chau.
2.4 Consideration of Alternative Options of Water Supply, Water Main Alignments, Landfall Locations and Construction Methods
2.4.1 Various alternatives of improving the fresh water supply to Cheung Chau, water main alignments, landfall locations as well as the construction methods have been investigated to identify feasible options for further detailed evaluation.
Alternatives for Improvement of Fresh Water Supply to Cheung Chau
2.4.2 A number of alternatives for improving the adequacy and reliability of water supply to Cheung Chau have been identified. These included:
· laying new water mains from Lantau to Cheung Chau;
· constructing a desalination plant at Cheung Chau; and
· rehabilitating the existing water main.
2.4.3 Other alternatives such as new water mains directly from Lamma Island, Hong Kong Island or Kowloon to Cheung Chau are considered to be not practical given the expected large distance separation, the associated adverse environmental impacts and the high construction costs, therefore are not considered further.
Laying New Water Mains from Lantau to Cheung Chau
2.4.4 Currently, potable water supply in Cheung Chau is provided by the Silver Mine Bay Water Treatment Works (WTW) in Lantau. Treated fresh water gravitates from Silver Mine Bay Service Reservoir to Cheung Chau via two land mains and two submarine mains (near Ha So Pai at Chi Ma Wan peninsula). Currently the land mains are being rehabilitated/replaced under WSD’s Contract No. 2/WSD/07 – “Replacement and Rehabilitation of Water Mains, Stage 2 – Mains on Lantau Island East”, for completion in 2010.
2.4.5 Possible options for laying water mains from Lantau to Cheung Chau include new mains from the Chi Ma Wan peninsula coast and new mains directly from Silver Mine Bay Water Treatment Works.
2.4.6 Both the current 500 mm and 10” water mains cross the Adamasta Channel from Ha So Pai at the southern part of Chi Ma Wan peninsula to Tai Kwai Wan in Cheung Chau. The Adamasta Channel is about 1.4 km wide and feasible options include laying a new 500 mm diameter submarine fresh water main or two smaller submarine fresh water mains in stages using conventional open trench or tunneling methods.
2.4.7 Silver Mine Bay WTW is over 6 km from Cheung Chau. Due to this large distance, either tunnel boring or submarine laying will be necessary. Potential constraints on tunneling works near WTW include risk and hazard from chlorine facilities at the WTW, spoil management and high construction cost. Tunnel boring is also unsuitable for small diameter pipeline, thus will not be considered further. The other feasible option is to lay a new 10 km long pipeline directly connecting Silver Mine Bay WTW to Cheung Chau. The main constraints are environmental, cost and marine traffic issues.
Desalination Plant in Cheung Chau
2.4.8 An alternative to constructing new pipeline to Cheung Chau would be to construct a reverse osmosis desalination plant on Cheung Chau. The reverse osmosis (RO) process uses pressure to force pure water through semi-permeable membranes that inhibit the passage of salts.
2.4.9 The most important limitations to the use of RO desalination plant are the cost, high energy consumption, ultimate disposal of waste streams and land availability. Based on the estimated water supply demand of 9,500 m3/day for Cheung Chau, a permanent site of approximately 0.5 ha would be required. As shown in Figure 2.6, there are very little possible sites of suitable size in Cheung Chau for the desalination plant.
Rehabilitating the Existing 10” Water Main
2.4.10 The existing 10” (~250 mm) submarine water main could be rehabilitated by using slip lining, swagelining or cure in place pipe which entails inserting a new pipe inside the old pipe and pushing and/or pulling it into place to extend the serviceable life of the pipe.
2.4.11 Such option can preclude the need of laying new submarine main. The completed main will, however, have a significantly smaller internal diameter (about 150 mm) than the original pipe (about 250 mm). This main alone would not be adequate to provide the necessary water supply to Cheung Chau.
2.4.12 Moreover, there will be high operational risk in the event the 500 mm main is taken out of service unexpectedly while the 10” main (which serve as emergency backup) is being rehabilitated leading unacceptable water supply interruption to Cheung Chau.
Alternatives for Water Main Alignment
2.4.13 The length of the main from Chi Ma Wan to Cheung Chau (about 1,400 m) is within the bounds of possibility for HDD. As shown in Figure 2.6, the alignment for Cheung Chau is constrained by natural coastlines (most of which have been zoned as CPA), Tai Kwai Wan Archaeological Site and existing submarine pipelines. The factors governing the water main alignments between Chi Ma Wan and Cheung Chau are presented in Figures 2 & 3 of Appendix 2.
2.4.14 It is recommended that the water main alignments should follow the existing alignment from Ha So Pai at Chi Ma Wan peninsula to Tai Kwai Wan at Cheung Chau as it is generally shorter and can be more easily connected to the existing water main. For Cheung Chau, this alignment is close to the Cheung Chau Fresh Water Pumping Station and can be benefited by shorter length of land-main. There are also suitable landfall locations (evaluated in the next section below) for this alignment.
The alignment of the water main at Lantau is constrained by
the nearby natural coastline and
Alternatives for Landfall Location
As shown in Figure 2.6, the southern part of Chi Ma Wan
peninsula is located within the
2.4.17 As for Cheung Chau (Figure 2.6), the northern portion is natural shoreline zoned as Coastal Protection Area (CPA) while the central portion is all built-up area. There are sandy shores near Tai Kwai Wan which has also been zoned as CPA. Part of the sandy shore is also within the Tai Kwai Wan Archaeological Site. The only feasible landfall location of suitable size for works site is the headland at Tai Kwai Wan which is zoned as G/IC and has been reserved for waterworks. The site is currently used as temporary works compound. As the site has already been formed and highly disturbed, the associated ecological impact is considered to be negligible. The site is also close to the Cheung Chau Fresh Water Pumping Station, thus a shorter land-based main for connection to the existing main.
2.4.18 For HDD works, a sufficiently sized flat area is required to accommodate the drill rig and supporting equipment. In addition, the entry point location also requires good access as well as stable ground conditions to support heavy equipment. The headland at Tai Kwai Wan in Cheung Chau is the only suitable landfall site.
2.4.19 Another important consideration for the launching site is the availability of water supply for the drilling fluid and flushing of the water main. The site at Cheung Chau is located near water supply and has available flat area for the necessary storage and treatment.
Alternative Construction Methods and Sequences of Works
2.4.20 The common methods for constructing submarine pipelines include the conventional trenching methods and horizontal directional drilling method. Tunnel boring is generally for large diameter pipeline (1m or above) and is therefore considered not suitable for this case.
Conventional Trenching Methods
2.4.21 Conventional trenching involve forming a trench in the sea bed, installing the pipe and subsequent backfilling. There are a variety of techniques to form a trench and number of ways of installing the pipe (see Figure 4 of Appendix 2).
2.4.22 Trenching may be done by dredging or jetting and the pipe can be installed by lay barge, bottom pull, or float and lower methods. After the pipe has been placed in position, the trench can be backfilled by bottom dump, tremie, or other methods. An alternative is to simply lay the pipe on the sea bed with or without additional protection.
Grab or Suction Dredger. Dredging involves
the removal of marine sediment from the seabed to form a trench, into which the
pipeline is laid. Dredging by grab
dredger is commonly used in
2.4.24 Jetting (Trenching). This uses water jets to break-up, remove or liquefy the soil from under a marine pipeline allowing the pipeline to settle to an elevation below the seabed. Jetting is a post-trenching technique, which means the pipeline is laid on the seabed before jetting is carried out. The marine sediment in front of the machine is fluidised to allow the pipeline to settle into the sediment. The sediment to be fluidized may be suspended in the water column and thus have an impact on the water quality.
2.4.25 Jetting (Injection). The injection process is an improved post-trenching jetting method. The jetting machine uses two swords to cut into the sediment and then fluidises the marine sediment in front of the machine and between the two swords. The pipeline is positioned between the two swords and hence settles down into the fluidised sediment. Depending on the strength of the sediment it may require several passes of the machine to lower the pipeline to the designated elevation. The technique limits the pipeline protection layer within the width of the machine. Submarine pipelines requiring a wider protection layer against damage from dragged anchors may require extra trench dredging.
2.4.26 Lay Barge Pipe Installation. Sections of the pipe are welded together on board a barge and the continuous length of welded pipe is allowed to descend from the barge into the trench. The barge moves slowly along the pipe route as new sections of pipe are welded together. The lay barge requires a considerable distance ahead and behind to allow the pipeline to descend into the trench. This can cause considerable problems in areas of busy marine traffic and the method is probably not suitable for short crossings.
2.4.27 Bottom Pull Method of Pipe Installation. Pipes are assembled into a complete string or strings in which the first string is pulled into the trench by a winch on the opposite side (or on a barge) and subsequent strings are jointed sequentially to the trailing end by welding as the pipeline is advanced along its alignment. This method typically applies to steel pipes, which are installed empty and then filled with water. The pipes have a concrete weight coat to make them just sink when empty and this fine balance of weight and buoyancy helps keep pulling forces low. Where there is not enough space, the pipes can be joined and pulled pipe by pipe or, if required, space can be formed by temporary reclamation or from a temporary jetty or a moored flat top barge with launching ramp and welding facilities. The pipe will be towed along the sea bed from the launching site to the receiving site.
2.4.28 Float and Lower. The pipe string is welded together and towed floating to the installation location. It is then lowered into position onto the sea bed by filling it or by filling attached buoyancy tanks.
2.4.29 Conventional trenching methods are not preferable as it has the potential to cause deterioration in water quality with subsequent secondary impacts on marine ecology and in particular to fisheries. This is because the dredging activities will disturb the seabed and the water column, dispersing the sediment which will spread in to the wider water body increasing suspended solid (SS) concentrations and depleting the dissolved oxygen (DO). The sediment may be contaminated and this will impact on nearby sensitive receivers such as fish and other marine organisms. During subsequent deposition, the sediment may smother plants and animals living on the sea bed.
Horizontal Directional Drilling Method
Directional Drilling (
2.4.31 Construction of any submarine pipe line will have environmental impacts but compared with conventional trenched methods, the environmental impacts associated with HDD are likely to be significantly less as the pipes are installed at considerable depths below the seabed. Impacts will typically be limited to the entry and exit portals and along the alignment of any pipe strings during construction.
2.4.32 A particular concern with HDD is the potential for inadvertent loss of drilling fluid either through break out through the overlying soils or at either end of the drill path. However, such impact can be mitigated by proper drilling fluid management and provision of adequate drilling fluid recovery, containment and treatment systems.
2.4.33 A detailed comparison of HDD and conventional laying method is presented in Figure 5 of Appendix 2.
Alternative Options Considered
2.4.34 From the above discussed alternatives, the following options are evaluated further in Table 2.1.
Option 1 – construct a new 500 mm diameter submarine fresh water main from Chi Ma Wan peninsula to Cheung Chau using HDD method.
Option 2 – construct two smaller diameter (about 300 mm) submarine fresh water mains in stages from Chi Ma Wan to Cheung Chau.
Option 3 – construct a submarine fresh water main directly from Silver Mine Bay Water Treatment Works to Cheung Chau using conventional trenching method.
Option 4 – construct a reverse osmosis desalination plant in Cheung Chau.
Option 5 – rehabilitate the existing 10” water main to prolong its serviceable life.
2.4.35 The environmental benefits and dis-benefits of each option have been evaluated in particular to the potential impacts to water quality, marine ecology and fisheries.
2.4.36 Other factors or constraints such as construction, long term operational and maintenance, marine traffic, cost issues have also been considered in the evaluation.
Summary of the Evaluation of Alternative Options for the Improvement of Fresh Water Supply to Cheung Chau
Construct a 500mm diameter submarine fresh water main from Chi Ma Wan peninsula to Cheung Chau using HDD method
§ No dredging of seabed is required for this option, thus no direct impact to water quality, fisheries resources (including fishing activity) and marine sensitive receivers is expected. There is also no need to dispose the excavated sediment.
§ As the work is generally land-based drilling works, there will be no direct ecological impact on marine and intertidal habitats.
§ By adopting forward reaming and
pipe pushing from Cheung Chau, the size of the exit pit at Lantau and the
associated works can be significant reduced, thereby minimizing the impacts
§ The alignment will not encroached into Tai Kwai Wan Archaeological Site, thus be no direct impact to archaeological resources is expected.
§ The site at Cheung Chau is already partly formed for use as works compound of low ecological value, therefore has little ecological impact. Given the adequate size, the CPA zone can be avoided and trees can be retained by adjusting the site boundary.
§ Potential water quality and ecological impacts from inadvertent spillage / loss of drilling fluid to the marine environment.
§ Potential construction noise impact at the launching site in Cheung Chau.
§ A small area of rocky shore at Lantau will be affected during construction of the exit pit.
§ Marine geophysical and ground investigation data indicate the subsurface geological profile of the water main alignment to be suitable for HDD.
§ Adequate site area at Cheung Chau is available as the staging area of the works and can facilitate set-up of storage / treatment tanks.
§ Impact on marine traffic and fishing activities in Adamasta Channel during construction can be largely avoided.
§ HDD crossings offer maximum depth of cover and can be installed deep beneath the rock head profile at little or no extra cost, thereby affording maximum protection and minimizing maintenance costs. They are inherently better protected from wave action, sea bed movement, trawling, and anchor damage than pipes installed on the sea bed or trenches.
§ The construction cost is in the range of $150M - $170M.
§ The construction time using HDD is generally shorter than for traditional dredging methods.
§ Similar HDD projects have been
As HDD is essentially land-based drilling works, the associated impact to water quality, fisheries and marine sensitive receivers can be substantially avoided. Other environmental impacts can be mitigated to acceptable level. This option is therefore recommended.
Construct two smaller diameter submarine fresh water mains in stages from Chi Ma Wan peninsula to Cheung Chau as demand requires. (The first pipe could be installed by HDD or submarine laying. The second pipe could be provided in the same way, or by rehabilitating the existing 10” main.)
§ Similar to Option 1.
§ Less environmental impact as smaller HDD plant and work site is needed.
§ If both pipes are installed during the same period of construction, environmental issues would be similar to those for single HDD pipe option.
§ Similar to Option 1.
§ Potential environmental impacts to mobilize the drilling rig and associated construction plant twice if the pipes are installed in stages.
§ Potential environmental impact
during construction including to
§ The installation of the second pipe would benefit from a thorough knowledge of the geophysical and environmental conditions that would have gained during design and construction of the first one.
§ One more submarine pipe needs to be maintained.
§ The construction cost for two smaller pipes is in the range of $160M - $180M.
§ Smaller HDD plant and drill holes will be required, hence smaller works site is needed.
§ Potential risk of on-line rehabilitation of long length of 10” submarine pipeline.
This option would require mobilizing works twice, and would not offer environmental or engineering benefits. As such, this option is considered not practicable and is therefore not recommended.
Construction a submarine fresh water main directly from Silver Mine Bay Water Treatment Works to Cheung Chau using conventional trenching method
§ No works required in
§ Dredging of seabed will lead to adverse impacts on water quality, fisheries and sensitive marine habitats.
§ Require marine disposal of excavated sediment.
§ The length (10 km) is beyond the reach of HDD installation and construction will have to be by one of the conventional laying methods.
§ Impact on marine traffic and fishing activities in Adamasta Channel during construction.
§ Rock armour for protection of the submarine main could affects fish trawling activity.
§ Substantially longer construction time is required as dredging rate may have to be restricted to minimize environmental impact.
§ The construction cost is estimated to be about $450M.
§ Could provide alternative routing
also for the land mains on
Given the expected adverse water quality and marine ecological impacts from excavation of marine sediment and substantially higher construction cost, this option is therefore not recommended.
Construct a reverse osmosis desalination plant in Cheung Chau
§ No works required in
§ No excavation of seabed is required.
§ Potential water quality impact from construction of intake and discharge pipes.
§ The desalination processes are energy-intensive, with high energy consumption.
§ Potential water quality impact from discharge of concentrated brine back to the sea.
§ Potential noise impact from operation of the plant.
§ Site formation will be necessary to ensure the site at Cheung Chau is suitable to build the plant.
§ In addition to the plant, it will be necessary to construct both an intake pipe and a brine concentration discharge pipe out into the sea.
§ The plant will require continual operations and maintenance staff.
§ The plant will incur high capital cost and high recurrent operational cost.
§ The construction cost including intake and outfall pipes is estimated to be about $105M.
As this option requires continuous energy consumption and has long term operational and maintenance requirements, this option is considered not practicable and is therefore not recommended.
Rehabilitate the existing 10” water main to prolong its serviceable life
§ No works required in
§ No excavation of seabed is required.
§ As marine works is not required, no adverse environmental impact is envisaged.
§ Rehabilitation of the existing 10” pipe would only be possible by slip lining using a PE lining with an outside diameter of 200mm and an internal diameter of 150mm.
§ The internal diameter of the rehabilitated pipe will be substantially smaller and hence will not be adequate to provide the required water demand.
§ The construction cost is estimated to be about $20M.
§ There will be no additional pipe that needs to be maintained.
§ As the existing 10” pipe is serving as emergency backup, there will be unacceptable operational risk that the existing main being taken out of service while the emergency backup main is being rehabilitated leading to water supply disruption to Cheung Chau.
§ Potential risk of on-line rehabilitation of long length of 10” submarine pipeline.
§ As this option will not be able to provide adequate water supply to Cheung Chau, it is therefore not recommended.
2.4.37 Current evaluation indicates that Option 1 – a single 500mm diameter polyethylene (PE) pipe within a steel casing from Chi Ma Wan peninsula to Cheung Chau installed by Horizontal Directional Drilling (HDD) as the recommended option for the alignment across Adamasta Channel. After making landfall, the land-based water main (500mm diameter ductile iron (DI) pipe) will be laid underground using conventional open cut method.
2.4.38 The technology and process of HDD is briefly described below.
Horizontal Directional Drilling (HDD)
2.4.39 Horizontal Directional Drilling (HDD) is a form of trenchless technology. It is a technique for installing pipes or utility lines below ground using a surface-mounted drill rig that launches and places a drill string at a shallow angle to the surface and has tracking and steering capabilities. The equipment and procedures are intended to minimize surface damage, restoration requirements, and disruption of vehicular or maritime traffic with little or no interruption of other existing lines or services*.
* ASTM Standard F1962-05 Standard Guide for Use of Maxi-horizontal Directional Drilling for Placement of Polyethylene Pipe or Conduit under Obstacles, Including River Crossings.
Advantages of HDD
2.4.40 The main advantages of HDD are:
· The technology offers maximum depth of cover under the obstacle, thereby, affording maximum protection and minimizing maintenance cost.
· Traffic is not interrupted, as most of the work is confined to both works area only.
· HDD has a predictable and short construction schedule.
· Needs little or no excavation and hence little or no restoration is required.
· In many cases, HDD is less expensive than other methods.
· Has least environmental impacts.
· Reduces public inconvenience from road / navigation fairway closure.
2.4.41 The main disadvantages or limitations of HDD are:
· The feasibility of HDD is dictated by the length of the bore hole to be drilled, the diameter of the pipe string, as well as the subsurface soil / bedrock conditions.
· It requires a suitably sized working area to accommodate drilling rig, pipes and other auxiliary equipment.
· Constraint by site geometry and topography.
· Potential water quality impacts from spillage or escape (frac-out) of drilling fluids.
· Requires specialized equipment and contractor in undertaken the works.
2.4.42 The method comprises a three stage process wherein first stage drills a pilot hole on the designed path and the second stage enlarges the hole by passing a larger cutting tool known as reamer. The third stage places the product or casing pipe in the enlarged hole. The elements of a HDD installation are:
· A rig, which provides the physical means – thrust and torque, to open the hole and pull in the product.
· A transmitter / receiver system for tracking the location of the bore.
· The down-hole equipment – drill pipe, drill bits, and reamers, which converts the physical properties of the rig to open the hole and pull in the product.
· The drilling fluid, which serves to stabilize the hole, cool the down-hole equipment, and remove the cuttings from the hole.
· The drilling fluid delivery, recovery and containment systems, made up of tanks, mixing systems, pumps and, when recycling fluids, a system of screens, filters, shakers, cones, etc. to separate cuttings from the fluid so that it can be reused.
2.4.43 The typical arrangement and procedures for undertaking a HDD project are described below.
* HDD Consortium (2001) Horizontal Directional Drilling, Good Practices Guidelines.
2.4.44 The drill rig is off-loaded and positioned over the bore centerline a sufficient distance behind the entry point to allow for the carriage height and entry angle, such that the drill bit will enter the ground at the correct location and angle. Depending on the rig size and entry angle, the distance may be 1 to 6 m. The entry angle is usually between 8 and 16 degrees. To make the drill string entry easier, a small pit is usually excavated over the entry point. The pit facilitates entry of the bit at the proper angle and helps contain the drilling fluid. Another pit is usually excavated at the exit point to facilitate containment of drilling fluids once the pilot bore reaches the exit point.
2.4.45 Drilling fluid is composed of a carrier fluid (water) and drilling fluid additives (bentonite and/or polymers). Bentonite is a naturally occurring clay mineral (montmorillinite) that forms a mud when mixed with water.
2.4.46 The functions of drilling fluids used in HDD are:
· Transporting drilling cuttings to the surface by suspending and carrying them in the fluid stream flowing in the annulus between the bore wall and the drill pipe/product.
· Cleaning off build-up on drill bits or reamer cutters by directing fluid streams at the cutters.
· Cooling the downhole tools and electronic equipment.
· Lubricating to reduce the friction between the drill pipe/product and the bore wall.
· Stabilizing the bore and exerting a positive hydrostatic pressure against the bore wall.
2.4.47 Mixing systems are used to mix the proper drilling fluid additives with water to create a drilling fluid mixture suitable for the local geological conditions.
2.4.48 Holding (frac) tanks for pre-mixed volumes of drilling fluid are used mainly during large diameter and mud motor applications. They are also used to temporarily store drilling fluids prior to disposal.
2.4.49 Cleaning systems are used for removal of cuttings and recycling of drilling fluids.
2.4.50 The recycling process begins with a mud pump delivering drilling fluid under high pressure from the holding tank to the drill head nozzle. Drill cuttings are then transported with the drilling fluid into a mud tank for temporary storage. A submersible pump delivers the drilling fluid and slurry with cuttings to a shaker. The shaker helps to separate large cuttings for washing, storage and subsequent removal. Lighter density liquid is then transferred to desander/desilter where finer solids are separated for storage and removal. After removal of cuttings, the drilling fluid is pumped to the holding tank ready for conditioning and reuse. Wastewater generated during the recycling process, from the cleaning systems along with wastewater from wheel washing facility and site runoff will be treated (e.g. sedimentation) and will be reuse for washing within the site or re-circulated back to the bore hole. No discharge of wastewater during the drilling fluid treatment and recycling process is expected.
2.4.51 Prior to drilling, the drilling fluids should be mixed and sufficient quantities should be available to complete the pilot bore, pre-reaming, and reaming passes.
2.4.52 The pilot bore is drilled along the planned alignment from entry to exit. Another pit is usually excavated at the exit point to facilitate containment of drilling fluids and entry of the pipe during pullback operations.
2.4.53 The type of drill bit used will vary depending on ground conditions and contractor preferences.
2.4.54 The drill head is tracked by monitoring an electromagnetic signal transmitted from the transmitter mounted in the drill head to the receiver at the surface via a wireline or wireless non-walkover system. The drill locator determines drill head location and calculated depth, drill head inclination, or tool face angle, and orientation of the slanted face. The drill locator provides the tracking information to the driller or provides instructions to the drill rig operator for steering.
Reaming / Hole Enlargement
2.4.55 If the pilot bore is to be enlarged, a reamer hole opener is attached to the drill pipe and the drill pipe is pressurized to ensure the jets are open. The reamer is then rotated and pulled (or pushed in some cases) back through the pilot bore to enlarge the bore in one or more reaming passes. The number of reaming passes depends on the diameter of the product compared to the diameter of the pilot bore, ground conditions and contractor preferences.
2.4.56 Reaming can be accomplished by pulling or pushing the reaming tool through the hole. The final bore diameter must be larger than the product diameter to reduce frictional pullback loads and to facilitate flow of drilling fluids around the product. As a rule of thumb, the final bore diameter should be 1.5 times the diameter of the product.
Pipe Layout, Fabrication and Testing
2.4.57 The product is usually prepared for installation while the bore is being reamed.
2.4.58 During pullback, a pulling head is attached to the product. A swivel is installed between the reamer and the pulling head to prevent rotation of the product.
2.4.59 The new product is finally connected to existing pipes.
Demobilization, Site Cleanup and Restoration
2.4.60 After the product is installed and satisfactorily tested, the entry and exit pits are cleaned of drilling fluids and cuttings and backfilled with native soil or selected backfill. Bentonite slurry is generally reconditioned and reused wherever practicable. In the case of this Project, the drilling fluid (bentonite) will be reuse in the borehole throughout the drilling process. Spent bentonite will be dewatered and transported offsite to public fill reception facilities for final disposal after completion of drilling works.
2.4.61 The recommended works area requirement for large scale HDD works is typically about 45 m x 75 m.
Preferred HDD Works for this Project
2.4.62 Marine geophysical survey and ground investigation survey were conducted to study the existing geological data and to assess the subsurface conditions and characteristics likely to be encountered during the drilling process. As shown in Figure 2.7, the underground conditions of the drill path are considered to be geotechnically feasible for HDD. HDD technique, consisting of entirely land-based drilling works, will be used to lay the submarine water main across Adamasta Channel within bedrock layer. No dredging or marine works is required.
The length and diameter of the proposed main is
also within the bounds of possibility for HDD. The bore diameter of 900 mm is
recommended as the final bore size. To avoid / minimize adverse environmental
i. The pilot hole will be drilled from Cheung Chau side to within about 50 m of exit.
ii. The bore will then be enlarged in stages to the required diameter using forward reaming.
iii. All drilling fluid recycling equipment will be on Cheung Chau.
iv. The reamed hole will then be totally cleaned by flushing with water until the returns are totally clear.
v. The last section will be drilled using water only instead of drilling fluid so that any fluid exiting from the Lantau side of the bore will be just water.
vi. The pipe will be welded up in sections as it is pushed in from Cheung Chau side or pulled from the Lantau side using a winch, or both.
2.4.64 A schematic layout of the proposed launching site at Cheung Chau is shown in Figure 2.4. The work site has been adjusted to retain a number of trees. The site is about 2,340 m2 and can be separated into clean and dirty working areas. The clean area will be used to accommodate the site offices and storage of water pipes. The dirty area will be the main area for the drilling works consisting of the drilling rig, entry pit, drill pipes storage, holding tanks, drilling fluid recovery, recycling and containment systems, stockpiling area and wastewater treatment facilities. Concrete bund will be constructed around the entire dirty area to contain and divert polluted site runoff to wastewater treatment tank for treatment. Concrete bund will also be provided around the entry pit to provide additional tier of containment to prevent spillage of drilling fluid.
2.4.65 A schematic layout of the reception site at Lantau is shown in Figure 2.5. A small exit pit of 2 m x 3 m will be excavated during the final boring process with water as the drilling fluid. Concrete bund will be provided around the exit pit to prevent spillage of drilling fluid. A temporary working platform (8 m x 4 m) using steel decking or wooden scaffolding will need be to be constructed to provide a suitable working area in retrieving the water main and connection. Such temporary working platform can be easily erected and dismantled without the need for site formation hence minimizing impact to the surrounding rocky shore habitat.
2.4.66 It is estimated that approximately 980 m3 of drilling fluid (bentonite) will be required for the Project. Most of the drilling fluid will be below ground level within the bore, while portion of the drilling fluid will be stored in the holding tank and mud tank above ground in Cheung Chau.
Sterilization of Water Main Prior to Commissioning
2.4.67 All new fresh water mains must be cleaned and sterilized before being put into operation. Typically, water mains are sterilized by chlorination. The purpose of chlorination is to disinfect the water main, resulting in an absence of coliforms as confirmed by laboratory analysis, before they are placed in service. Chlorine solutions are normally used as the sterilizing agent.
2.4.68 The cleaning and sterilization procedures of fresh water mains according to the General Specification for Civil Engineering Works Volume 2, 2006 Edition (Section 22.73) and WSD Departmental Instruction No. 805: Mainlaying - Cleaning and Sterilization of Fresh Water Mains are listed below. Notwithstanding the procedures below, the Contractor should also adhere to the Project contract document with regards to sterilization of the water main.
1. All extraneous materials should be removed before the fresh water pipeline is laid and connected.
2. The pipeline should be filled slowly with water and tested to the required pressure.
3. After the pipeline has been successfully pressure tested, it should be cleaned internally and flushed through with potable water. For long length of small diameter mains which are inaccessible for cleaning (size below DN600), swabbing should be carried out to remove dirt and materials inadvertently left in the pipeline.
4. The pipeline should then be completed filled with water that has been dosed with a homogeneous solution of sterilising chemicals (e.g. chloride of lime) such that the final concentration of free chlorine in the water is at least 30 ppm. The water should be left in the pipeline for at least 24 hours.
5. After the 24-hour period, the pipeline should be drained down and the sterilizing water should be flushed out using potable water until the remaining chlorine level in the pipeline is less than 1 ppm. Bacteriological and chemical samples should be taken and submitted to the Waterworks Chemist for analysis in accordance with the contract requirements.
6. To minimize or avoid any possibility of contamination, sterilization of the pipeline should take place not more than 7 days before putting it into operation.
2.4.69 Approximately 590 m3 of water will be required to clean and flush the completed water main. Any solids flushed out will be settled out through the sedimentation tank. Approximately 295 m3 of chlorinated water will be used to sterilize the water main.
2.4.70 If high chlorinated water reaches an aquatic system, it has the potential to kill fish and other aquatic organism because of the chlorine concentration. One mode of action is likely through damaging the gills, thus preventing the fish from breathing. Therefore, it is necessary to dechlorinate the water in order to make it safe for discharge to the environment.
2.4.71 Currently, sodium bisulfite, sodium sulfite and sodium thiosulfate are most frequently used for dechlorination. Sodium thiosulfate is preferred since it is less hazardous and consumes less oxygen than sodium bisulfite and sodium sulfite. The Contractor is required to submit for the Engineer approval, details of the processes and chemicals including dosage to be used in the chlorination and dechlorination of the sterilizing water.
2.4.72 The sterilization water must be treated to the relevant discharge requirement stipulated in “Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters” (TM-DSS) before discharging. With the implementation of appropriate treatment such as dechlorination to control water discharge from sterilization of water main, adverse impact on water quality is not expected. Monitoring of the total residual chlorine concentration at the discharge point is recommended.
The remote coastal bay of the proposed landfall location in
Lantau is located within the
Access to Cheung Chau will rely on marine transportation.
Construction equipment and materials could be offloaded along the
2.5.1 The design phase of the Project is scheduled to commence in late 2010 to late 2011. The implementation programme for the proposed works is anticipated to commence in early 2012 for completion in early 2014 for approximately 24 months. Detailed construction programme is not available at this current stage.
2.6.1 According to information provided by various works department, only the project below is likely to have potential cumulative impact with this Project.
Agreement No. CE 31/2007 (DS) – Outlying
2.6.2 This DSD project consists of construction of village sewers in Tai Kwai Wan San Tsuen and other unsewered villages in Cheung Chau. The tentative programme is 2013 – 2017. A short section of about 30 m of the sewer alignment is located where the land-based water main will be laid and connected to the existing land main. Close liaison will be undertaken with the project proponent of the interfacing project to avoid concurrent works as far as possible. Alternatively, feasibility of entrusting the works or sharing a common excavated trench during laying of the water main and sewer will be explored to minimize the nuisance to the public from repeated excavation. By avoiding concurrent works and given the short section of the interfacing works and the small scale works involved, adverse cumulative environmental impact is not expected.
2.6.3 Notwithstanding, continuous liaison will be conducted with all works department and utilities companies to ensure adequate phasing with each concurrent projects is taken into consideration, if interfacing does eventually occurs.
2.7.1 The completed water main is only to convey treated water from Lantau to Cheung Chau. No maintenance of the water main is necessary during the operational phase.
2.8.1 The process of Continuous Public Involvement (CPI) through public consultation have been undertaken during the EIA study.
2.8.2 The stakeholders included relevant Green Groups, Rural Committees, village representatives, Island District Council and local fishermen / trade association have been or are being consulted.
2.8.3 The preliminary design was reviewed and revised according to the views and comments received. The main comments and recommendations are summarized in Table 2.2 below.
Summary of Public Consultation
Main Comments / Recommendations
Responses / Outcomes
· Conservancy Association (CA)
· Friends of the Earth (FoE)
· Green Lantau Association (GLA)
· Green Power (GP)
· Kadoorie Farm and Botanic Garden (KFBG)
· World Wide Fund for Nature
Consultation paper (9 January 2009)
Meeting (8 May 2009) attended by KFBG and WWF
· Express concern on site practices and site runoff
· Recommend using existing boulder to shield the view of the exposed water main at Lantau to minimize visual impact
· Noting the works are mostly on disturbed land, they expressed no particular concern on the ecological impact.
· Propose two tiers of containment to contain and divert polluted site runoff away from the sea
· Propose tanks of adequate number and sufficient capacity to contain and treat site runoff
· Propose using boulders and rocks to shield view of the exposed water main
Island District Council member (Ms. K.C. Lee, MH) and local mariculture/fishermen association
Meeting (28 August 2009)
· Fishermen association recalled unpleasant experience from previous dredging / reclamation works and expressed concern about polluted marine water impacting the nearby fish culture zone
· Suggest having monitoring team to check the water quality and to ensure the mitigation measures are implemented properly.
· To have close liaison with the fishermen
· No adverse objections in principle to the Project
· The Project will adopt HDD, no dredging or marine works will be required.
· Propose two tiers of containment to contain and divert polluted site runoff away from the sea
· Propose tanks of adequate number and sufficient capacity to contain and treat site runoff before discharge
· Implement water quality monitoring and audit programme to ensure no adverse impact to marine waters during construction
Island District Council (Tourism, Agriculture, Fisheries & Environmental Hygiene Committee)
Meeting (21 September 2009)
· Some member raised question about potential noise during construction.
· All members supported the Project
· Adequate measures including use of quiet plant, erecting temporary noise barrier and good site practices have been recommended in the EIA. Noise monitoring is proposed to ensure no adverse noise impact will arise during construction.
2.8.4 Overall, no adverse comments were received from Green Groups, mariculture/fishermen association and District Council.
2.9.1 According to Item 3.4.7 of the EIA study brief, “The Applicant shall include in the EIA report the design proposals, covering appearance, façade treatment, colour scheme and texture of materials used, etc., of the landfalls and the landscaping design measures at the landfalls locations. Annotated illustrative materials such as computer-generated photomontages, oblique aerial photographs, photographs, plans, elevation and section drawings shall be used as appropriate to illustrate the visual/aesthetic effects of the landfalls.”
2.9.2 Based on the preliminary design, the landfall at Cheung Chau will be an underground chamber, approximately 2 m x 3 m, to accommodate an isolation valve and an air releasing valve. The proposed submarine main will be connected to the chamber at the landfall 2 to 3 metres below ground level. The underground chamber will be made of reinforced concrete. The chamber will be covered with precast concrete slab flushed with the ground level and ductile iron covers for the openings. The remaining parts of the landfall will be reinstated to its original condition after construction. It should be noted that the landfall location is currently zoned as G/IC and reserved for several waterworks sites.
2.9.3 After making landfall at Cheung Chau, the proposed water main will be laid along Cheung Kwai Road and connect to the existing main near Cheung Chau Pumping Station. The proposed main (500 mm diameter ductile iron water pipe) will be entirely underground. The works area will be reinstated to its original condition and ground profile after construction (i.e. concrete footpath).
2.9.4 The existing view of the landfall location and land main at Cheung Chau and the photomontage showing the completed water main is shown in Figure 2.9.
2.9.5 The landfall at Lantau will be an underground chamber, approximately 2m x 3m, to accommodate an isolation valve and air release valve. The proposed submarine main will be connected to the chamber at the landfall 2 to 3 metres below ground level. The underground chamber will be made of reinforced concrete. The slab will be flushed with ground level as far as possible, with precast concrete slab covers and ductile iron covers for the openings.
2.9.6 After making landfall at Lantau, the proposed water main will connect to the existing exposed land main currently being rehabilitated. The proposed main will be 500 mm diameter ductile iron pipe.
2.9.7 To minimize potential visual impact of the chamber and exposed main, natural materials such as boulders / rocks sourced from nearby area will be used to shield and to blend in with the surrounding coastal environment. The locations where the boulders / rocks will be collected and placed as well as the method statement should be proposed by the Contractor for verification by the Environmental Team Leader and approval by the Engineer to ensure no adverse ecological or visual impacts will arise from the removal and placement.
2.9.8 In general, the boulders / rocks to be removed should preferably be located above the high water mark to avoid impact on intertidal organisms. No vegetation should be disturbed during collection. The size should be in the range of 200mm – 400mm for safe and easy handling withoutThe existing view of the landfall location and land main at Lantau and the photomontage showing the completed water main is shown in Figure 2.10.