2 Description of the Project

2.1 Introduction

2.1.1 The proposed site of the Project is located in Mui Wo, Southern Lantau. Most of the watercourses in Mui Wo remain untouched at their upper reaches and are partially channelised at their downstream ends near River Silver. Pak Ngan Heung River, Luk Tei Tong River and Tai Tei Tong River merge at Silver River before passing Mui Wo township to Silver Mine Bay (see Figure 1.1). The Luk Tei Tong (2) River is a tributary of Luk Tei Tong River and flows through an existing floodplain to the north of the Luk Tei Tong Village. Ling Tsui Tau, a small village adjacent to Pak Ngan Heung River, is a flooding black spot. Due to inadequate drainage system, floodwater from Butterfly Hill cannot be effectively discharged into Pak Ngan Heung River.

2.1.2 As these existing rivers do not meet flood protection standards, drainage improvement works are recommended accordingly.

2.2 Need of the Project

2.2.1 Lantau Island is well known for its spectacular countryside and natural landscape, the study areas of this project are planned for conservation and recreation purposes under the South West New Territories Development Strategy Review. It also plays an important role in sustaining the future development of Hong Kong

2.2.2 In particular, Mui Wo and South Lantau Coast having historical interests and recreational value, are considered as one of the tourist attractions in Lantau Island. Taking cognizance of the high conservation and recreation value of these areas, drainage improvements will have to be implemented to address the existing flooding problems, otherwise tourism that has been considered as one of the economic mainstay industry will have to be compromised.

2.2.3 Southern Lantau is served by its existing drainage systems. Most of these drainage systems remain natural at their upper reach and are channelized at the downstream ends. Village expansions around the rural townships in Mui Wo and South Lantau Coast have resulted in higher runoff. Some of the existing drainage systems that have been designed with lower flood protection standards were no longer adequate to provide the flood protection standards required for developed area.

2.2.4 According to the DMP Study, most parts of these four rivers have flood protection level less than 2-year. The maximum flood protection levels for the rivers are 10-year. The rivers will not be able to contain flood flow within its banks during heavy rainstorms. The existing drainage conditions are summarized in Table 2.1.

Table 2.1 Summary of Existing Drainage Conditions in Mui Wo

Locations	Drainage Conditions
Pak Ngan Heung River	(a) In some parts of the river, the flood protection level is less than 1-in-2 years and the river will not be able to contain flood flow within its banks during heavy rainstorms. (b) The previous flood records have revealed that floodwater overtopped an existing bridge near Mui Wo Rural Committee Road and flooded houses immediately downstream of the river. (c) For the upstream area of Pak Ngan Heung River, the flooding problems are caused by very fast super-critical flow around bends. The super-critical flow has a great inertia and prefers to flow straight ahead rather than around bends and hence causing overtopping. In addition, cross-waves and bow-waves are created. A box culvert has been proposed to convey the flow directly to downstream without passing through sharp bend during heavy rainstorm. (d) Around the main confluence at downstream of Pak Ngan Heung River, the problem is mainly due to low bank levels. The area is at risk of flooding caused by both high sea water level and extreme rainfall.
Ling Tsui Tau	(a) Ling Tsui Tau is a flooding black spot. (b) Due to the inadequate drainage system, surface runoff from Butterfly Hill cannot be effectively discharge to Pak Ngan Heung River during heavy rainstorm.
Luk Tei Tong (2) River	(a) The Luk Tei Tong (2) River is a tributary of Luk Tei Tong River. It flows through an existing floodplain in the north of Luk Tei Tong Village. (b) Most parts of the Luk Tei Tong (2) River has a flood protection level of less than 1-in-2 years and it is far too small (1m – 1.5m wide) for conveying flow during extreme storm events. This is the main cause of widespread flooding in northern edge of Luk Tei Tong Village which cuts off the footpaths and isolates houses in floodplain.
Luk Tei Tong River	(a) Luk Tei Tong River is the continuation of a newly engineered channel that crosses the Mui Wo Marsh. The upstream section of the river was constructed by the Home Affairs Department under the Rural Planning and Improvement Strategy (RPIS) Project from in 1998. (b) The Luk Tei Tong River stretches from the confluence with the Luk Tei Tong (2) River to the confluence with the Silver River. (c) Most parts of downstream of Luk Tei Tong River remain natural and have flood protection of less than 2 years.
Tai Tei Tong River	(a) The downstream of Tai Tei Tong River has a flood protection level of less than 1-in-10 year. (b) During heavy rainstorm, floodwater will overflow from existing rivers and inundate adjacent houses.

2.2.5 During the rainstorm event on 5 November 1993, flooding occurred in Mui Wo and caused risk to the public life. In this case, FSD came to Mui Wo to rescue the public from the flooding hazard. For the Typhoon Sam event in August 1999, floodings occurred at various locations in Mui Wo. The areas included Tsoi Yuen Tsuen, San Lung Wai Tsuen, Lung Mei Tsuen, Kau Tau Tsuen, Chung Hau, Ling Tsui Tau, Pak Kung Bridge, Luk Tei Tong Village and also the Rural Committee Road.

2.2.6 During the recent rainstorm event (which was only Amber Rainstorm Signal) on 29 August 2004, serious flooding also occurred at various locations of Mui Wo. Flooding photos for a similar storm event in May 1993 are shown in Appendix 2.1.

2.2.7 During the consultation with respective Villagers’ Representatives and Mui Wo Rural Committee, they stated that the flooding frequently occurred during heavy rainstorm and the flooding situation was particularly severe in Luk Tei Tong. However, they did not report all the flooding incidents to the relevant government departments for record. During the consultation process, they repeatedly urged the government to carry out the drainage improvement works as soon as possible.

2.2.8 According to the drainage models, most of the study area in Mui Wo would be subject to a 1m – 1.4m flooding depth during the 200-year rainstorm event. In addition, the flooding situation is particularly severe in Luk Tei Tong as most of the section of existing Luk Tei Tong (2) River and Luk Tei Tong River have a drainage capacity less than 2-year protection level.

2.2.9 It is anticipated that the subsequent development in the rural part of the study areas would result in a higher surface runoff and the flood protection level provided by the existing drainage system would be further downgraded. Drainage improvements in the Mui Wo are therefore needed to overcome existing flooding problems and also to sustain future developments. To meeting the current DSD protection standard, 1-in-50 flood protection level will be provided to the Mui Wo after the completion of the drainage improvement works.

2.2.10 If the Project does not proceed, flooding risks to the community in Mui Wo would continue and future development would be compromised. There are no simple, small-scale works that can be implemented to reduce these risks. The only effective solution to provide adequate flood relief is to implement the drainage improvement works identified in this Project

2.2.11 Pursuant to the 2004 Policy Address, the Lantau Development Task Force led by the Financial Secretary was set up in February 2004 to provide a high-level policy steer on the economic and infrastructure development in Lantau. As a start, the Task Force has drawn up a Concept Plan for Lantau. The Concept Plan aims to provide an overall planning framework to ensure a balanced and coordinated approach for the proposed developments while meeting the nature conservation needs. According to the Concept Plan, Mui Wo was recommended to promote sustainable recreational uses, compatible countryside recreation in and around the country parts.

2.3 Continuous Public Involvement

2.3.1 Public consultation have been conducted since early planning stage with District Councils (DC), Rural Committee, village representatives, local residents, green groups and interested parties. The scope and development of the Project have subsequently been reviewed, revised and fine-tuned to meet the interests of concerned parties as far as practicable. After the completion preparation of the EIA Report in end March 2004, the process of continuous public involvement (CPI) commenced continued in May 2004. The meetings conducted are summarized in Table 2.2.

Table 2.2 Summary of Key Continuous Public Involvement

Date	Subject
21 May 04	Public Consultation with Village Representatives in Mui Wo
21 May 04	Public Consultation with Green Lantau Association
4 June 04	Follow up discussions with DSD to review the proposed works at Tai Tei Tong
29 June 04	Follow up discussions with DSD to review the proposed works at Tai Tei Tong
15 July 04	Public Consultation with Village Representatives in Mui Wo
15 July 04	Public Consultation with Green Lantau Association
29 July 04	Joint site inspection with green groups including Green Power and World Wild Fund to Mui Wo
3 Aug 04	Presentation of the proposed works to green groups
25 Aug 04	Joint site visit with Green Lantau Association – Proposed Luk Tei Tong (2) River
25 Aug 04	Mui Wo Rural Committee Meeting – Proposed Works in Mui Wo
18 Oct 04	Consultation with Islands District Council
26 Nov 04	Consultation with Village Representatives and Mui Wo Rural Committee

2.3.2 Key comments and recommendations from the public are summarized in Table 2.3 below:

Table 2.3 Summary of Key Comments/Recommendations from the Public

Concerned Parties	Key Comments/ Recommendations	Responses
Villagers, Village Representatives and Rural Committee	(a) The project was supported. (b) To revise the alignment of the proposed river in order to avoid resuming building lots and houses. (c) The flooding situation at Tai Tei Tong was not severe and agreed to reduce the scope of works at Tai Tei Tong.	(a) The alignment of Luk Tei Tong River was revised to avoid resuming existing building lots and houses. (b) The scope of works at Tai Tei Tong River was reduced from full-scale training to only improvement of 3 bottlenecks.
Green groups	(a) Existing riverbed should be untouched as far as possible. (b) More ecological friendly design should be included. (c) To avoid drawing down the water table at Luk Tei Tong Marsh	(a) The masonry and grasscrete lining was revised to gabion lining with planting pits. (b) Natural substrates, pools and riffles will be provided at the riverbed. (c) The existing river will be untouched and will not be deepened as far as possible. (d) Shallow but wide section is proposed for the Luk Tei Tong (2) River. The proposed river channel is only 0.8m deep to avoid drawing down the ground water table of Luk Tei Tong Marsh. In addition, 200mm high embankment will be constructed at both sides of the river banks to avoid surface runoff flowing to the proposed river. (e) Gabion blocks will be provided at the outlets to enhance the ecological environmental wherever the space is adequate.

2.3.3 During the course of consultation with green groups, the pros and cons of alternative drainage options, as well as the project need, were presented. The green groups provided a number of suggestions to improve the design and most of these suggestions to improve the design in Mui Wo have been incorporated in the design.

2.3.4 The draft Final EIA Report, including the recommendations from the public and green groups, was prepared and circulated to the green groups for comment. Green groups had no major comments on the draft Final EIA Report. For Luk Tei Tong (2) River, the Conservancy Association advised that the landscape design should be dre-use the existing planting as far as possible.

2.4 Alternative Considerations (Drainage Options and Alignments)

2.4.1 The following drainage improvement options were considered for relieving flooding in Pak Ngan Heung, Luk Tei Tong and Tai Tei Tong.

Option A Maintenance of existing river – Carry out more frequent maintenance works of the three watercourses to improve the drainage capacity of the existing rivers. The works mainly include desilting activities, removal of overgrown vegetation, debris and obstructions within the river channel.

Option B By-pass channel or box culvert – Construct by-pass channel or box culvert to divert part of the flow from the existing river to downstream.

Option C Interception and Storage Pond – Intercept the flow and provide surface storage detention pond to hold surface runoff from the upstream during extreme storm events in order to reduce the flow downstream. At some location, floodwalls will also be required along the existing rivers to eliminate the overspill of water as the riverbank levels are lower than the extreme high tide level.

Option D Floodwall and Pumping System – Construct floodwall along the riverbanks to prevent floodwater overflowing from the existing rivers and provide pumping station to pump surface runoff to downstream.

Option E River Training of Existing Drainage System – To widen and/or deepen the existing rivers in order to increase the drainage capacity of the drainage system.

Option F Construction of Earth Bunds at both sides of Luk Tei Tong (2) River – For the drainage improvement works for Luk Tei Tong (2) River, EPD and AFCD had advised to consider constructing earth bunds at both sides of the Luk Tei Tong (2) River in order to contain the floodwater from overflowing to adjacent area.

2.4.2 The considerations, environmental benefits and dis-benefits for the various drainage options are reviewed and shown in Table 2.4.

2.4.3 After the consideration of all options detailed in Table 2.4, the preferred option is to widen the existing rivers and construct a drainage channel with gabion lining and natural bed to increase the drainage capacity while retaining the river ecology and improving the aesthetic, visual and environmental aspect of the river widening works. This option is also the most direct and effective way to discharge storm flow.

2.4.4 As the rivers will be widened mainly along the existing streams, there will be significant reduction of land resumption as compared to other options. River widening is a more reliable and low cost option to improve the drainage capacities of rivers compared to the flood storage option. Hence, river training works are recommended to rivers. However, the Pak Ngan Heung River downstream and Luk Tei Tong River will not be deepened.

2.4.5 For Tai Tei Tong River, river widening option is considered the most appropriate drainage option. During the public consultation, it is noted that the flooding situation can be tolerated by the locals. To minimize the possible impacts to the existing rivers, only 3 bottlenecks are proposed to be widened and reinstated by gabion blocks. The proposal was accepted by locals and green groups. The full-scale training of Tai Tei Tong River will depend on the future population and development pace in Mui Wo.

Table 2.4 Considerations of Various Drainage Options

Drainage Options	Environmental Benefit	Environmental Dis-benefit	Recommendation	Reason & Considerations
Maintenance of existing river	(a) The area to be affected by the proposed drainage improvement works will be the least. (b) The existing condition of rivers will be improved. For example, the contaminated soil and waste settled in the river bed will be removed. (c) The disturbance to the ecological system and water quality in existing rivers will be of small scale.	No major environmental dis-benefits are anticipated.	Not recommended.	(a) This option can only marginally improve the capacity of the existing rivers. (b) The flood protection level provided is still very low due to the small size of existing rivers. Severe flooding in the areas will continue.
By-pass channel or box culvert	(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.	Extensive land resumption is required for the construction of by-pass channel or box culvert. The public will be seriously disturbed.	By-pass box culvert Pak Ngan Heung River. By-pass channel Luk Tei Tong (2) River	(a) The construction of by-pass channel or box culvert will require extensive land resumption and will impose adverse impact to other areas further away from the flooding spots. (b) Along Luk Tei Tong (2) River and Pak Ngan Heung River, most of the areas are abandoned agricultural lands and marsh. There are sufficient available space for the construction of the by-pass box culvert and channel. The diversion scheme is considered feasible and acceptable. Locals and green groups also accepted the proposal. (c) The impact to existing marsh at Luk Tei Tong would be minimum by adopted ecological friendly design details. The existing river will be untouched.
Interception and Flood Storage	(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.	(a) Extensive land resumption is required for construction of storage pond or interception channel. (b) Environmental impacts to other areas will result. The upstream area, which is mainly composed of dense vegetation area, agricultural land, residential area and nurseries, will be seriously affected.	Not recommended	(a) This option is also not a cost-effective solution given the large volume of runoff needed to be stored which results in resumption of large piece of land for the construction of a large stormwater storage pond. The plan area of the pond can be reduced by half if the depth of the pond is increased to 6m. However, a pumping station is then required for pumping the water away after the storms. The required land resumption is still very substantial and the objection from public cannot still be avoided. (b) This option is therefore considered not acceptable.
Floodwall and Pumping Station	(a) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.	(a) Floodwall has limited scope for improving the aesthetic. The visual impact is of concerns, in particular, if the floodwall is high.	Not recommended	(a) Floodwall can be constructed along the rivers to contain the floodwater during extreme rainstorm events. However, the capacities of the concerned rivers are very low and a high floodwall of minimum 2m is needed. (b) The floodwall protection does not provide a long-term solution because it contains floodwater at a higher water level and causes flooding of the local drainage systems behind the floodwalls. Flooding in adjacent areas still persists. (c) To eliminate the severe flooding outside the floodwalls, the floodwalls are required to be constructed further away from the existing rivers such that an additional flood plain flow area is allowed for conveyance of flow during extreme rainstorms. The formation of flood plain flow area requires substantial land resumption. (d) Consideration has been given to pumping the surface runoff in the rivers to the downstream in order to reduce the water level and hence height of the floodwalls. However, the peak discharges are very substantial and temporal. It is technically impracticable to pump the peak surface runoff away without the use of detention pond. (e) Therefore, this option is not further considered.
Deepening and Widening of Existing Rivers	(a) The existing condition of the rivers will be improved by the proposed engineering works by applying appropriate ecological friendly design. For example, Gabion lining with planting pits can enhance the ecological condition and provide shelter for the river habitats; the contaminated soil and waste settled in the riverbed will be removed, eroded river bank will be re-constructed. (b) As the improvement works will be carried out along the existing rivers, it can avoid major resumption of land and environmental impacts in other areas.	The existing rivers and ecological system will be disturbed.	River widening ü Luk Tei Tong River Improve bottlenecks ü Tai Tei Tong River	(a) The prime reason for flooding is that the existing rivers do not have adequate drainage capacity. The most effective solution is to widen and deepen the existing rivers. (b) As the river widening works will be carried out mainly along the existing rivers, there will be significant reduction of land resumption as compared to other options. (c) River widening is more reliable and low cost option to improve the drainage of rivers compared to the other options. (d) The channel design should provide an ecological and aesthetical acceptable solution, taking into account hydraulic performance and operation requirements of the channel. (e) Appropriate mitigation measures are required to be implemented such that the environmental impact is kept to acceptable limit. (f) This option is recommended and the proposed channels are lined with gabion block, and natural bed to retain the river ecology, and to improve the aesthetic, visual and environmental aspect of the river widening works. (g) For the Pak Ngan Heung River, river widening would affect the existing shrine and Fung Shui Tree. This area is culturally sensitive and any change to the shrine and Fung Shui tree at the existing bridge will be objected by the villager. Besides, one of the reasons for flooding is the sharp bend of the river near the existing bridge. Widening and deepening cannot remove the sharp bend and the problem. Therefore, this option is not recommended to Pak Ngan Heung River.
Construction of Earth Bunds at both sides of Luk Tei Tong (2) River	(a) No excavation was required for the affected area except the area covered by the foundation of the earth bunds. (b) The existing river would be untouched.	(a) Permanent loss of marsh area. (b) Visual impact due to the proposed earth bunds would be substantiated.	Not recommended	(a) The earth bunds would occupy substantial area of the marsh area and cause permanent loss of marsh area and associated ecology. (b) The earth bunds would obstruct surface runoff flowing into the existing river and aggravate the flooding situation. (c) Visual impact is of concern.

Pak Ngan Heung River

2.4.6 The option of resolving flooding by upgrading the watercourse in Pak Ngan Heung River has been considered. The existing Pak Ngan Heung Bridge would require to be re-constructed for widening the river. However, objections to changes to the Pak Ngan Heung Bridge due to the presence of an old tree and other Fung Shui issues were received during the discussion with the local villagers during the DMP Study.

2.4.7 The feasible option is to construct a by-pass culvert at the upstream of a sharp bend meander such that flood flow can be diverted directly to the downstream by-passing the problem bridge and sharp bend. A low flow diversion dam and low flow pipes are also provided at the upstream of the meander in order to maintain base flow to sustain the natural habitat of the meander.

Ling Tsui Tau U-Channel

2.4.8 The flooding at Ling Tsui Tau is mainly due to inadequate drainage system. Runoff from Butterfly Hill cannot be effectively discharged to Pak Ngan Heung River during heavy rainstorm.

2.4.9 Due to the small quantity of the flow at Ling Tsui Tau and the area are well disturbed urban area, flood storage, pumping station, floodwall are not recommended as it is not cost effective in preventing flooding.

2.4.10 Construction of U-channel along the downhill slope to intercept runoff from Butterfly Hill and convey to Pak Ngan Heung River is considered the most cost-effective and direct solution. In addition, the land resumption area and impact to the local residents are the least.

Tai Tei Tong River

2.4.11 Several options to increase the flood protection level of the Tai Tei Tong River have been considered. The option of diversion of floodwater to adjacent watercourses is not feasible as this would deteriorate the flooding situation of adjacent watercourse that already has its own flooding problems. Flood containment scheme to construct levees/floodwalls at both banks of river will require extensive private land resumption and clearance of adjacent houses along the river to provide the flood plain flow area and this option is therefore not considered further. The most appropriate option is to widen and deepen the Tai Tei Tong River to alleviate flooding. This option is the most direct and effective way to discharge floodwater.

2.4.12 During the consultation, the Villagers’ Representative advised that the flooding situation at Tai Tei Tong River could still be tolerated by locals. As agreed with the locals, the scope of works at Tai Tei Tong River is reduced from full-scale training to the improvement of 3 bottlenecks.

2.4.13 The proposal was presented to the Mui Wo Rural Committee and green groups. They had no comments on the captioned proposal.

Luk Tei Tong (2) River and Luk Tei Tong River

2.4.14 The existing Luk Tei Tong (2) River is extremely small and it lies adjacent to the Luk Tei Tong Village. Diversion of floodwater from Luk Tei Tong (2) River to Tai Tei Tong River is technically possible. However, this would require a diversion of a significant portion of floodwater to the Tai Tei Tong River that has its own flooding problems and environmental constraints. The diversion of floodwater will need an increase in hydraulic capacity by further widening of Tai Tei Tong River but this is restricted by the existing houses located at the river downstream.

2.4.15 The watercourse in Luk Tei Tong (2) is an ecological sensitive receiver. During the DMP Study, different options were explored and discussed among AFCD, EPD and DSD, and consultations with The Green Lantau Group were carried out.

2.4.16 AFCD and EPD advised to consider the proposal of construction of earth bunds at both sides of Luk Tei Tong (2) River (i.e. Option F) in order to avoid excavation in the existing marsh area. It is noted that the foundation and slope of the proposed 1m high earth bunds would occupy substantial area of marsh area and cause permanent loss of marsh area and associated ecology. Even though compensatory plantings could be provided on the proposed earth bunds, the ecological environment would become different compared with the original condition. Furthermore, the proposed earth bunds would obstruct the surface runoff flowing into the existing river and aggravate the residual flooding situation. The visual impact due to the 1m high earth bunds was also of concern. As such, Option F is not preferable.

2.4.17 The recommendation in DMP Study includes an environmental friendly channel with natural bed and grass concrete slopes. The invert of the channel should be kept similar to the existing level of the river to minimize draw-down of water table. The option was considered acceptable with further improvements. Improvements to this design option would include:

· The Luk Tei Tong (2) channel bed would be reinforced with geo-textile matting with silty mud. The matting would be overlain with turf taken from adjacent areas of Luk Tei Tong Marsh that would be directly affected by the proposed works. This area would be maintained as compensatory marshland habitat.

· A shallow but wide section is proposed for the Luk Tei Tong (2) River in order to avoid drawing down the ground water table in adjacent marsh.

· The banks of the whole proposed Channel would be lined with rock-filled gabion.

· A minimum of 200mm high embankment will be constructed at most of the section of the channel to avoid surface runoff from existing marsh flowing to the proposed by-pass channel. In order to match with the downstream Luk Tei Tong Rriver, the height of the embankment would be gradually increased to about 1500mm at only the non-sensitive areas.

2.4.18 Under the original proposal, the proposed alignment of the Luk Tei Tong (2) River followed the existing river. During the consultation with the public, local villagers advised that the proposed alignment of the Luk Tei Tong (2) would affect an approved licensed house. In addition, green groups also advised that the existing river should be untouched as far as possible. To avoid affecting the licensed house and the existing river, a Luk Tei Tong By-pass Channel is proposed which passes through the existing marsh area and abandoned agricultural land. During heavy rainstorm, runoff from upstream will enter the by-pass channel and then be discharged to Luk Tei Tong River. Under normal condition, the flow to the existing streamcourse will be maintained.

2.5 Preliminary Design of Drainage Improvement Works

Design Criteria

2.5.1 The design methodology and assumptions for the preliminary design of the drainage improvement works are as follows:

(a) Design in accordance with DSD Stormwater Drainage Manual.

(b) The proposed channels are classified as “Main Rural Drainage Channel” with a design return period 1 in 50 years and should contain floodwater under a design return period 1 in 200 years.

(c) Examples of environmentally friendly drainage channel designs arising from environmental impact assessment.

(d) Examples of environmental friendly drainage channels in other countries as recommended by the green groups.

(e) Recommendations from continuous public involvement.

Channel Alignment

2.5.2 The considerations in determining the alternative alignment are listed below:

(a) In design of the alignment of the proposed rivers in Mui Wo, the horizontal alignment has been designed to follow the existing river in order to minimize the total area of the lands resumed for the drainage channels. In other words, the total area of existing farmland to be used for channel can be minimized.

(b) At Pak Ngan Heung, a diversion box culvert is proposed due to the existing Fung Shui Tree and Shrine as well as the presence of sharp bend. The alignment of the box culvert shall provide a smooth connection of the river at upstream to downstream and avoid resumption of agricultural land and dwelling as far as possible.

(c) For Luk Tei Tong By-pass Channel, the alignment of the proposed river shall avoid affecting the building lots as far as possible, as well as the existing streamcourses.

(d) Along the rivers, there are several buildings and dense vegetation areas. The alignment should avoid encroaching the dense vegetation areas and the buildings.

(e) In some locations, there are several buildings at both banks. This prevents the widening of the channel and only a rectangular drainage channel remains feasible and is proposed.

2.5.3 As for determining the vertical alignment, the invert level at the upstream end of the channel is designed to be slightly lower than that of the existing watercourse at immediately upstream of the channel. At the downstream end, the channel invert level is designed to match with the existing riverbed level. In both cases, the purpose is to allow the flow in the upstream watercourses discharging smoothly into the receiving channels.

Channel Shape and Lining

2.5.4 In determining the details of the drainage channels, the followings have been considered:

(a) Open Concrete channel

(b) Trapezoidal channel with natural bed and grass concrete/masonry side slopes

(c) Rectangular drainage channel

(d) Box Culvert

(e) Gabion Channel with natural substrates

2.5.5 Concrete lined channel is the most effective channel for the conveyance of stormwater. It is also the easiest for maintenance. However, it will impose adverse ecological, visual and aesthetic impacts to the existing environment. Therefore, this type of drainage channel is not recommended.

2.5.6 For the proposed trapezoidal channels, natural bedding and gabion linings will be provided. To make the proposed rivers more environmentally friendly, the channel base is lined with rip-rap (i.e. rock base) and natural substrates to produce a suitable environment similar to the existing rivers for some living species such as small fishes. The existing riverbed will be untouched as far as possible. The excavated material will be re-used as the natural substrates in the riverbed.

2.5.7 Rectangular open channels have limited scope for improving the aesthetics when compared with trapezoidal channels. Therefore, rectangular open channels shall be avoided as far as possible. However, it requires less land for construction and will be used when adequate space is not available. Rectangular open channels will only be proposed at section of the rivers which the site is bounded by existing small houses

2.5.8 To improve the ecological environment at the rectangular channel, gabion blocks will be provided at both sides of river bed (up to +2.5mPD) wherever there are adequate spaces.

2.5.9 The Luk Tei Tong By-pass Channel flows through a large expanse of abandoned wet agricultural land (Luk Tei Tong Marsh), which is of potential high ecological value. Widening of the channel would result in direct impact to the marsh. Hence, gabion wall and silty mud bedding with small plants are proposed for the Luk Tei Tong (2) River in order to minimize the potential impact to existing marsh.

Dry Weather Flow Channel

2.5.10 No dry weather flow channel is proposed. The base of the channel will be reconstructed to width similar to existing river and the base is maintained with water at all time to replicate the existing river.

Operation and Maintenance Requirement

2.5.11 To minimize the impact to the areas, no maintenance access will be provided for the proposed rivers in Mui Wo. Only an access points to the channel bed will be provided at the downstream end of the Pak Ngan Heung River for desilting purpose. Maintenance vehicles can access Tai Tei Tong River, Luk Tei Tong By-pass Channel and Luk Tei Tong River via the access point.

Collection of Local Runoff

2.5.12 At locations where embankments are required along the proposed channel, the surface runoff from the adjacent areas behind the embankments will be intercepted by toe channels and discharged into the proposed channel through embankment drains.

2.5.13 For local drainage pipe inlets, if they are located in areas where the ground levels are lower than the design flood levels, flap valves will be provided to prevent backflow of water from the proposed channel.

Reprovision of Accesses

2.5.14 Some of the existing pedestrian or vehicular accesses, or crossings on the existing rivers are affected by the proposed channel upgrading works. These accesses or crossings will be demolished and reprovided under the channel upgrading works.

2.5.15 During public consultation, the local residents in Mui Wo reported that the flooding situation at Tai Tei Tong was acceptable and not severe. Only 3 bottlenecks are now proposed to be widened, as such, the scope of works at Tai Tei Tong was reviewed as per agreed with the locals and Green Lantau Association.

2.5.16 The major items of works proposed are summarized in Table 2.5 below.

Table 2.5 Summary of Project Elements

Works Programme	Mid 2006 to Mid 2009
Major Elements in Pak Ngan Heung River and Ling Tsui Tau U-Channel
(a) Open Trapezoidal Channel	Length = 80m	Depth = 2.5m – 3.5m		Bed width = 8m
	Gabion walls with rip-rap bases and natural substrates.
(b) Rectangular Channel	Length = 100m	Depth = 3m – 3.5m
	Rip-rap bedding and concrete vertical walls of rip-rap bedding
(c) Diversion Box Culvert	Length = 180m	3 cells 3m(W) x 2m(H)
(d) U-Channel (Ling Tsui Tau)	Length = 200m	750mm U-channel
Major Elements in Luk Tei Tong By-pass Channel
(a) Rectangular Shape	Length = 350m	Depth = 0.8m	Bed width = 14m – 18m
	A minimum of 200mm high embankment will be constructed at most of the section of the channel to avoid surface runoff from existing marsh flowing to the proposed by-pass channel. In order to match with the downstream Luk Tei Tong river, the height of the embankment would be gradually increased to about 1500mm at only the non-sensitive areas.AverageMinimum 200mm high earth embankment to avoid surface runoff from existing marsh overflowing to the proposed channel.
(b) Reprovision of crossings	2 numbers (1 vehicular crossing and 1 pedestrian crossing)
Major Elements in Luk Tei Tong River
(a) Open Trapezoidal Channel	Length = 240250m	Depth = 3m	Bed Width = 3m
	Gabion walls with rip-rap bases and natural substrates.
Major Elements in Tai Tei Tong
(a) River Widening	3 bottlenecks only
	Existing river bed will be untouched and the bank will be reinstated by gabion blocks.

2.5.17 The general layout plans for the improvement works at Pak Ngan Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk Tei Tong By-pass Channel are shown in Figures 2.1 to 2.2. Cross sections of the general layout plans are shown in Figure 2.3.

2.5.18 It is noted that natural lowland and estuarine river are potentially valuable ecological habitats. The design of the drainage improvement works should target to maintain the potential ecological value of the existing rivers, and natural features of the rivers should be retained wherever possible.

2.5.19 The design of channel improvement works should strike a balance between hydraulic and environmental aspects. By adopting appropriate channel design, potential environmental impacts from channelisation of a river such as increased velocity, leading to higher sediment loads and poorer water quality, and loss of natural habitats and landscape features would be avoided and minimized.

2.5.20 The following environmental-friendly measures have been incorporated as basic design principles:

(i) Use natural bed along the watercourses.

(a) Riverbeds should not be lined with concrete. The natural stream bed and substrate should be retained or recreated.

(b) Riverbanks should not be lined with concrete. Soft engineering materials such as rock-filled gabion should be utilised as far as possible.

(ii) Retain and/or recreate natural river channels and water levels.

(a) Existing river should be untouched as far as possible and no deepening works is proposed.

(b) Consideration is given to the preservation of meanders and water level management to preserve river habitats. For example, diversion channels are proposed in the Pak Ngan Heung River and Luk Tei Tong (2) River. Only during peak flow events would over-flow from the river be conveyed to the downstream section by passing through the proposed channel/box culvert. Flow conditions would be maintained in the meander to preserve existing ecological condition.

(c) Natural features of the rivers (such as pool/riffle sequences and meanders) should be retained or recreated wherever possible.

(iii) Maintain water table to minimize the impact to existing riparian life.

(a) Consideration is given to maintain water table in the rivers. Minimizing the excavation to the bottom level of the rivers can maintain water tables. For example, shallow and wide section of Luk Tei Tong By-pass Channel River is proposed instead of deepening of the river to increase its hydraulic capacity.

(iv) Provide planting of native species along channels

(a) Tree preservation and planting near open channel should be considered on a case-by-case basis.

(b) Compensatory planting should be provided to mitigate for the loss of riparian vegetation.

2.6 Landscape Design

2.6.1 Landscape design aims at preservation of natural features as far as practicable, compensation of loss of existing trees, reinstatement of disturbed areas, enhancement of landscape value of the concerned rivers, and in general, create landscape and visual settings which are compatible with the surrounding natural environment so as to minimize potential adverse visual impact and enhance the environment as far as possible. Landscape design measures to address landscape and visual issues during the construction and operation stages are presented below, and shown in the Preliminary Landscape Plans (Figures 2.4 to 2.6).

2.6.2 Landscape and visual issues of the proposed works:

a) Loss of vegetation due to channel re-alignment and excavation works of box culvert

b) Exposed channel bank and bed after channel re-alignment

c) Exposed wall surfaces created by construction of retaining walls and wing walls

d) Creation of flat areas due to backfill of existing channel

2.6.3 In order to address landscape and aesthetic requirements, proposed landscape design to mitigate landscape and visual impacts are summarized below:

a) Reinstatement planting by hydroseeding grass seed mix should be provided for the peripheral disturbed areas during construction. Transplantation of existing trees should be located in this area. Compensatory planting, using the existing species as far as possible, should also be provided for any loss of vegetation.

b) Amenity planting that generally includes trees, shrubs and ground cover for roadside planting and woodland planting should be provided along re-provisioned footpaths and slopes. The proposed species are generally wet-soil tolerated or adaptable to the site situation, and compatible with the existing vegetation. Backfilled areas should be planted to enhance the landscape value. For area above covered box culvert where soil depth is 500 mm, groundcover and low-growing herbaceous planting is proposed.

c) Natural Colonisation for Gabion will take place for the trapezoidal channel banks to minimize the visual impact, except the channel sections where tidal influence happens at most of the time that masonry lining would be proposed.

d) Grasscrete linings with hydroseeding for low flow diversion dam and ramp. Hydroseeding mix shall include herbaceous or graminaceous species that are adaptable to normal or wet soil condition when it is not flooded, or flood tolerant and suitable for margin of water bodies, or even able to submerge into water.

e) Channel bed planting at particular location is proposed so as to mimic the existing site-specific natural setting. Details are provided under Chapter 7 (Ecological Impact) of the EIA report.

f) Exposed wall surfaces should be finished with masonry to match with existing wall surfaces in adjacent areas

2.6.4 Planting design in the detailed design stage should consider the existing species as recorded in Section 7 of the EIA report. For woodland planting species and roadside planting species, reference should be made to Section 7.4 of the EIA report. A preliminary species list proposed for grasscrete and amenity planting is presented in Table 2.6. Photomontages to illustrate the effects of the proposed landscape design are provided. (Figures 2.7 to 2.9)

Table 2.6 Proposed Planting Species

Grasscrete	Amenity Planting (Roadside & Woodland)
Bacopa monnieri Centella asiatica Colocasia esculenta Cyperus malaccensis Eleocharis dulcis Eleocharis equisetina Eleocharis spiralis Equisetum debile Eragrastis atrovirens Fimbristylis acuminata Fimbristylis camplanata Hydrocotyle sibthoripioides Juncus effuses Leersia hexandra Lepironia articulata Ludwigia ascendens Phragmites australis Polygonum hydropiper Sagittaria guayanensis	Acacia confusa (Tree) Acacia mangium (Tree) Albizia lebbek (Tree) Bischofia trifoliate (Tree) Casuarina equisetifolia (Tree) Celtis sinensis (Tree) Cinnamomum camphora (Tree) Eriobotrya japonica (Tree) Ficus hispida (Tree) Ficus microcarpa (Tree) Ficus superba (Tree) Glyptostrobus pensilis (Tree) Litsea glutinosa (Tree) Macaranga tanarius (Tree) Melaleuca leucadendron (Tree) Sapium sebiferum (Tree) Sterculia lanceolata (Tree) Syzygium jambos (Tree) Terminalia catappa (Tree) Thespesia populnea (Tree) Tristania conferta (Tree) Caesalpinia crista (Salt-tolerant Vine) Clerodendrum inerme (Salt-tolerant Shrub) Crinum asiaticum (Salt-tolerant Herb) Vitex rotundifolia (Salt-tolerant Shrub)
	Alocasia odorata (Shrub) Calliandra haematocephala (Shrub) Cestrum noctumum (Shrub) Ficus microcarpa ‘Golden leaf’ (Shrub) Gordonia axillaries (Shrub) Lantana spp. (Shrub) Philodendron spp. (Shrub) Rhaphiolepis indica (Shrub) Syngonium spp. (Shrub) Scindapsus aureus (Climber) Wikstroemia indica (Shrub) Centella asiatica (Herbaceous) Chrysopogon aciculatis (Herbaceous) Lepironia articulata (Ground cover) Iris tectorum (Herbaceous) Miscanthus floridulus (Herbaceous) Nephrolepis exaltata (Ground cover) Rhapis excelsa (Herbaceous)

2.6.5 Approval in principle from government departments to manage and maintain the proposed landscape works should be in accordance with WBTC No. 14/2002 and ETWBTC 2/2004, while construction details could be agreed at a later stage. A detail tree survey was conducted in a separate report “Tree Survey and Landscape Proposal” to assess if any roadside tree will be affected in according with the ETWBTC 2/2004 and WBTC 14/2002 on Tree Preservation.

2.7 Construction Methods and Engineering Requirements

2.7.1 For the construction of river channels, earth bunds or wall will be placed along the river channel to separate the construction site and existing river. Excavation works will be carried out at the either side of the river banks. Gabion blocks will be applied to the river banks to form the permanent lining. Sand, rubble and rock will be placed on the bottom to form the channel bed. After half of the channel is completed, the river will be diverted to the newly completed channel. The remaining part of the channel will be constructed similarly.

2.7.2 The proposed construction method ensures that the existing river can be maintained at any time such that the impact to the river ecology is kept to minimum. Construction in wet conditions along the whole river sections is not recommended as the construction activities will adverse by affect the whole section of the river. Besides, the siltation caused by excavation in existing river is more serious and adversely affects the water quality. Temporary culverting the rivers to form a dry conditions for construction is also not suggested as the river will be temporarily destroyed during the construction period.

2.7.3 Due to limited space available, some of the vertical wall inside the channel will be constructed by mini-bored piles. Mini-bored pile walls are recommended as it will not cause excessive noise and vibration during construction. Percussion and driven pile for wall construction is not recommended.

2.7.4 Construction of the Pak Ngan Heung diversion box culvert will be carried out by conventional methods that generally involve site clearance, excavation, installation of temporary supports to the excavated face, erection of formwork, reinforcement fixing and concreting followed by backfilling and compaction.

2.7.5 Vehicular and pedestrian crossings will be constructed in Luk Tei Tong By-pass Channel. Construction of these crossings will be carried out by conventional methods that generally involve site clearance, excavation, installation of temporary supports to excavated face, erection of formwork and reinforcement fixing, concreting followed by backfilling and compaction.

Special Considerations

2.7.6 The main construction activities for the river training include: excavation and disposal of wet spoil from existing channel bottom. Excavation and earth moving will be carried out using a tracked excavator or backhoe. The side slopes will be laid with gabion. Rubble and rock will be placed at the channel bed.

2.7.7 The works activities involved in stream water may adversely affect the local surface water system during construction. Excavation of bottom sediments may increase the suspended solid levels in the river water and release of soluble nutrients into the water.

2.7.8 Prevention of water quality deterioration during construction involves two basic elements:

(i) executing the works in the dry behind temporary bunds, thus minimising the rivers from potential impact by high levels of suspended solid generated from the disturbed soil surface; and

(ii) following site practice outlined in ProPECC PN 1/94 Construction Site Drainage.

2.7.9 The potential for erosion and sedimentation can also be limited by the following means:

(i) where possible, excavation work should be scheduled for the dry season to minimise the silty water running downstream; and

(ii) earthworks should be surrounded by temporary flood protection bunds.

2.7.10 This project is a typical river training project involving the deepening and widening of existing river. The construction activities mainly include earthwork, concreting works and foundation works. The construction methods for the proposed works and consideration of alternative methods are summarized in Table 2.7.

Table 2.7 Summary of Construction Methods and Alternative Consideration

Major Works	Locations	Major Activities	Reasons & Consideration of alternative method
River Training	River widening and deepening ü Luk Tei Tong River River widening ü Luk Tei Tong By-pass Channel ü Sections of Pak Ngan Heung River	Earthworks include excavation by mechanical plant; disposal of excavated soil by dump trucks; and compacting by mechanical plant.	It is a common construction method. The impact to the environment is local and temporary. It can be mitigated by appropriate measures. It is recommended to maintain the existing river on one side and to carry out the widening works on the other side. This can ensure the river is maintained at any time to minimise the impact. The construction works will be carried out at dry condition to minimize the adverse effect on the rivers. Working in wet condition inside the rivers or temporary culverting the rivers are not recommended.
Retaining Walls, Box Culvert and Floodwall	Retaining walls Bottlenecks, inlet, outlet of rivers, as well as back side of gabion channel Box Culverts ü Pak Ngan Heung Diversion Box Culvert ü Crossings at Luk Tei Tong By-pass Channel	Earthworks include excavation by mechanical plant; disposal of excavated soil by dump trucks; and compaction by mechanical plant.	It is a common construction method. The impact to the environment can be mitigated by appropriate measures.
		Concreting works include formwork erection, reinforcement fixing and in-situ concreting. Piling works include installation of steel casings, reinforcement fixing and concreting.	The walls will be cast in-situ and it is a common construction technique. Precast unit for the retaining walls and box culvert is not preferable. As the size of retaining wall and box culvert is large, it requires a large plant to handle and install the precast units. Due to limited space, it is not feasible to transport the precast units to site for installation. Bored piles are used for the construction of retaining wall where space is limited. Percussion and driven piles are not recommended due to noise generation during construction.

2.8 Operation of the Projects

2.8.1 During the operation of the proposed channel, routine desilting will be required. In general, desilting activities would take place during the dry season.

2.8.2 Desilting will be carried out by excavators. The excavators will be lowered into the channels through the ramps at the downstream of Pak Ngan Heung River. The excavators will be used within the channels to move and transport sediments or debris towards the ramps for removal. The removed sediment and debris will then be loaded into covered lorries and transported to proper disposal sites.

2.9 Works Programme

2.9.1 The drainage improvement works at Pak Ngan Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk Tei Tong (2) River is currently scheduled from mid 2006 to mid 2009. A preliminary construction programme is presented in Appendix 2.2.

2.9.2 The preliminary construction programme was determined according to the following constraints and factors:

(a) High flooding risk for the outlets - the improvement works for the outlets should be carried out in the dry season only.

(b) Impact to the existing marcsh at Luk Tei Tong - as agreed with the green group, the construction works for the proposed Luk Tei Tong By-pass Channel should be carried out in the dry season. As such, the potential impact to the existing marsh area at Luk Tei Tong could be minimized.

(c) Cumulative impacts – in order to minimize the potential impacts to the existing sensitive receivers and public, the cumulative impacts for the concurrent projects, as well as those the proposed works under this project have been duly considered. The Contractor should be encouraged to programme the construction activities under the project in such a manner that ongoing activities would should be separated physically as far as possible from one another and other construction sites.

2.10 Interaction with Other Projects

2.10.1 Major Concurrent projects with likely interaction with this Project are identified and summarized in Table 2.8. Figure 2.10 also shows the locations of these concurrent projects. Potential cumulative impacts identified will be addressed in the relevant sections of this EIA Report. The construction programme of the concurrent projects is based on the available information at the time of the submission of this Report. It should be noted that the implementation of individual projects would be subject to the on-going review by relevant project proponents.

Table 2.8 Concurrent Projects

Concurrently Projects in the Vicinity	Programme
Outlying Sewerage Stage 2 – Mui Wo Village Sewerage (Package 1A)	Late 05 to Mid 07
Construction of Emergency Vehicular Access and Sewerage Works at Luk Tei Tong, Mui Wo	Late 04 to Late 06