2 Description of the Project

2.1 Project Details and Design Considerations

2.1.1 The Project is located in Sai Kung, which is an area bounded by the high ground of Ma On Shan, Buffalo Hill and Kowloon Peak. Surface runoff drains across the area into Port Shelter to the East via several watercourses. The Project involves drainage improvement works at three of the watercourses, namely Sai Kung River, Pak Kong River and Ho Chung Channel.

2.1.2 The total length of Sai Kung River is approximately 1.3km from upstream grassland flowing through Sai Kung town centre before entering into the Inner Port Shelter (Sai Kung Hoi). The currently proposed Sha Ha Diversion Box Culvert will connect the Sai Kung River to the downstream box culvert constructed by Civil Engineering and Development Department (CEDD). The details of the concerned project section located at the mid section of the Sai Kung River, are summarised in Table 2.1.

2.1.3 The Pak Kong River runs initially alongside with the Hiram’s Highway and finally entering a road crossing under the Hiram’s Highway and discharging to Hebe Haven. The total length of Pak Kong River is approximately 1.32km. The concerned section is 600m long which is located at the downstream end of the river. The details of the concerned project section are summarised in Table 2.1. During public consultation, the local residents in Pak Kong objected the improvement works because of the resumption of village houses along both sides of the river bank. The local residents requested during the public consultation to minimise the extent of works as far as possible and they accepted a higher flooding risk instead of resumption of village houses. As compromised with the local residents, this project would only re-construct two bottlenecks in Pak Kong River and stabilise an existing channel slope adjacent to Hiram’s Highway. Details of public consultation is given in Section 2.3 – Continuous Public Involvement.

2.1.4 The Ho Chung Channel runs alongside with Ho Chung Road before passing under the Hiram’s Highway and discharging to Hebe Haven in the south. The total length is approximately 1.6km. The concerned project section is 650m long located near the outlet area. The details of the concerned section are shown in Table 2.1.

Table 2.1 Details of the Concerned Project Sections

Location	Bed width (m)		Bank Width (m)		Depth (m)
Location	Existing	Future	Existing	Future	Existing	Future
Sai Kung River	2 - 3	5	10 - 12	16 - 24	1 - 2.5	3 - 3.5
Pak Kong River (box culverts)	7 - 13	16	12 - 13	16	3	3
Pak Kong River (Channel Widening)	4 - 12	7 - 18	5 - 18	13 - 18	3	3
Pak Kong River (Slope stabilization)	-	-	-	-	6	6
Ho Chung Channel	7 - 28	17 - 34	15 - 30	17 - 42	3 - 5	3.8 - 5

2.1.5 Natural lowland and estuarine river habitats are relatively rare in Hong Kong, and are known to provide habitats for a diversity of aquatic organisms. The design of the drainage improvement works should maintain the potential ecological value of the existing rivers, and natural features of the rivers should be retained wherever possible. The following ecological-friendly design has been incorporated as basic principles:

(a) Riverbeds should not be lined with concrete: the natural stream bed and substrate should be retained.

(b) Riverbanks should not be lined with concrete: soft engineering materials should be utilised as far as possible.

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

(d) Compensatory planting should be provided to mitigate the loss of natural riparian vegetation.

2.1.6 The channel design has been consulted with the green groups for whom have no adverse comments. The details of the proposed works are summarised below and the typical sections are shown in Figure 1.6.

(A)	Sai Kung River and Sha Ha Diversion Culvert
(a)	The concerned existing river section would be widened and deepened.
(b)	Gabion lining with planting pits is proposed along the channel. In addition, natural substrates and rip-rap bedding would be provided at the channel bed.
(c)	Pools and riffles would be constructed along the riverbed.

(B)	Ho Chung Channel
(a)	Gabion walls with planting pits would be provided along the concerned river channel. At the channel bed, natural substrates and rip-rap bedding would also be provided.
(b)	Since there are many village houses adjacent to the concerned river section, the structural stability of these houses is the major concern. As such, a retaining wall would be provided behind the gabion walls. To allow movement of water between the existing ground and river channel, drain pipes would be installed at the retaining wall.
(c)	At the mid section of the concerned channel, a bored pile wall would be constructed due to the limited space available adjacent to existing village houses. Masonry lining and climbers are proposed at the wall surface.
(d)	At the downstream area, no deepening works would be carried out. Moreover, the riverbed adjacent to the existing Ho Chung Road would remain untouched as far as possible. During the construction stage, an earth bund or concrete blocks would be constructed along the river in order to avoid the construction activities affecting the existing riverbed.

(C)	Pak Kong River
(a)	One footbridge and one vehicular crossing would be reconstructed to 3-cell box culverts.
(b)	A retaining wall would be constructed to stabilise the existing river bank adjacent to the Hiram’s Highway.
(c)	The existing riverbed would remain untouched as far as possible.

2.1.7 The major items of works are summarised in Table 2.2 below.

Table 2.2 Summary of Project Elements

Works Programme	Dec 2005 to April 2008
Major Elements in Sai Kung River and Sha Ha Diversion Culvert
(a) Open Trapezoidal Channel	Length = 150m		Depth = 3m – 3.5m
	Bank width = 16m – 24m		Bed width = 5m
	Gabion walls with rip-rap bases and natural substrates
(b) Diversion Box Culverts	Length = 280m	3 cells 3m(W) x 3m(H)
(c) Low Flow Pipes	Length = 12m	3 x 600mm diameter

Major Elements in Pak Kong River
(a) Channel Widening	Length = 100m		Depth = 3m (approx.)
	Bank width = 13m – 18m		Bed width = 7m – 18m
	To be reinstated by gabion blocks and concrete retaining wall
(b) Reprovision of crossings	2 numbers (1 vehicular crossing and 1 pedestrian crossing)

Major Elements in Ho Chung Channel
(a) Open Trapezoidal Channel	Length = 300m			Depth = 3.8m – 5m
	Bank width = 17m – 42m			Bed Width = 17m – 34m
	Gabion walls with rip-rap bases and natural substrates
(b) Rectangular Channel	Length = 350m	Depth = 5m			Width = 17m – 25m
(c) Reprovision of crossings	4 numbers (1 vehicular crossing and 3 pedestrian crossings)
(d) Floodwall	Length = 300m		Height = 1m

2.1.8 Comparison of environmental benefits, alternative drainage options, considerations and constraints for the proposed drainage improvement works are presented in the following Sections.

2.2 The Need for the Project

2.2.1 Over the last 10 years, rapid development in Sai Kung has resulted in a higher surface runoff and the existing drainage systems are found no longer adequate to provide the current flood protection standard required for the developed areas. The deficiencies in the existing drainage systems in Sai Kung Catchment are revealed by flooding records.

2.2.2 Sai Kung River, Pak Kong River and Ho Chung Channel are the watercourses associated with flooding problems in Sai Kung town centre, Sha Kok Mei, Pak Kong and Ho Chung respectively. These watercourses remain in natural with irregular cross-sections along most of their lengths. Although engineering modifications have been locally carried out in some parts of the watercourses, flooding still re-occurs due to inadequate flow capacities of these rivers and is exacerbated by the development in Sai Kung.

2.2.3 According to flooding records, there are flooding problems in the Sai Kung River, Pak Kong River and Ho Chung Channel. Existing drainage conditions in these areas are summarised as follows:

(i) Sai Kung River

Sai Kung town centre and Sha Kok Mei are located at the downstream of the Sai Kung River. Hiram’s Highway Bridge, Sai Kung Nullah and Sai Kung Township Culvert in the downstream section of the Sai Kung River can only provide a 1-in-10-year flood protection level to the adjacent commercial areas in Sai Kung Township. The inadequate capacity of the Sai Kung township culvert and Hiram’s Highway Bridge would cause flooding in storm events greater than 1-in-10-year event. Floodwater would spill out of the nullah and flood the roads and areas in the town centre.

After completion of the drainage improvement works, some of the floodwater from Sai Kung River will be diverted to Sha Ha Diversion Box Culvert. The flooding conditions at Sai Kung Town and Sha Kok Mei will be greatly alleviated accordingly. A 1-in-50-year level of protection will be achieved.

The flow to downstream of Sai Kung River will be maintained at all times via the diversion pipes. During heavy rainstorms, the floodwater will overflow into the Sha Ha Diversion Box Culvert without causing flooding to downstream area of Sai Kung River and Sai Kung Town.

(ii) Pak Kong River

The existing nurseries and houses along the downstream of Pak Kong Rivers are located at the north east of Tai Chung Hau. The downstream section of the river and culvert has a protection level of less than 1 in 5 years. Nurseries and houses at the downstream section of the Pak Kong River are occasionally flooded mainly due to low banks, inadequate channel size and constriction caused by existing crossings.

DSD had conducted several consultations and presented the current flooding hazards of Pak Kong River to the local residents. DSD proposed the improvement of the existing river to the current flood protection standard of 1-in-50-year. However, the widening of the river would resume some of the houses in the proximity. The residents objected the full-scale training works and would accept the flooding risks as a trade-off. They also proposed DSD to remove two existing bottlenecks at Pak Kong River and stabilise an existing riverbank. As such, the scale of works as stated in the Project Brief was reduced as the people to be protected compromised to a higher flooding risk in oppose to the resumption of houses.

(iii) Ho Chung Channel

Ho Chung is a flooding black spot identified by DSD. The immediate downstream of the confluence near the Ho Chung Lowland Pumping Station and Ho Chung Village is a flooding problem area. Flooding occurs due to the following reasons:

· Flooding occurs as a result of the two very fast tributaries of Ho Chung Channel joining at a very sharp angle at the confluence. Re-profiling the junction is required to improve the drainage condition.

· Flooding onto Ho Chung Road adjacent to Ho Chung Village is exacerbated by the high level and long length of weirs alongside the Ho Chung Lowland Pumping Station. Modifications to the weirs are required to reduce the flooding at the Ho Chung Village.

· Ho Chung Channel has a flood protection level of less than 1 in 5 years at the section alongside with the Ho Chung Road mainly due to the inadequate size. River widening and bank raising along the right bank of the river is therefore required. After completion of drainage improvement works at Ho Chung Channel, the flooding situation at Luk Mei Tsuen, Ho Chung Valley, Shui Hau, Ho Chung Village will be alleviated. A 1-in-50-year level of protection will be achieved.

2.2.4 According to the records, numerous flooding complaints were received as summarised in Table 2.3. The flooding incidents affected the electricity and water supplies, caused landslides and slope failure, and blocked existing roads.

Table 2.3 List of Flooding Complaints Recorded

Ho Chung	Date	Location/Details
	09/06/1998	Ho Chung Road (Pak Sha Wan)
	28/08/1998	13 Ho Chung Road
	05/08/1999	13 Ho Chung Road
	22/08/1999	Ho Chung Road
	22/08/1999	17 Ho Chung Road
	25/08/1999	Hiram’s Highway (Ho Chung Road No. 23)
	25/08/1999	Hiram’s Highway (Ho Chung Road turning area)
	14/04/2000	Ho Chung Tsuen
	23/04/2000	Hiram’s Highway (Nam Pin Wai to Ho Chung)
	09/06/2001	Ho Chung Road
	09/06/2001	Ho Chung Tsuen
	27/06/2001	Ho Chung Road
	27/06/2001	Ho Chung Tsuen
	19/05/2002	Hiram’s Highway (Ho Chung Road near pm5866)
	20/05/2002	Ho Chung Road
	10/08/2002	Ho Chung Road

Sai Kung Town	Date	Location/Details
	02/07/1997	Po Tung Road
	19/07/1997	28 Fuk Man Street
	22/08/1997	Po Tung Road
	09/06/1998	Po Tung Road near Lok Yuk Kindergarden
	08/06/1999	120 Man Nin Street
	05/08/1999	120 Man Nin Street
	25/08/1999	1795 Ground Floor, Po Tung Road
	15/09/2000	Sai Kung Plaza
	01/09/2001	94A, Sai Kung Road
	19/06/2002	182 & 6 Po Tung Road

Pak Kong	Date	Location/Details
	09/06/1998	Tai Chung Hau Road
	09/06/1998	Tai Chung Hau River
	10/06/1998	8B, Fung On Village (Landslide, failure slope adjacent to a Crown Land License squatter)
	11/06/1998	49, Tai Chung Hau Village (Landslide)
	14/04/1999	Tai Chung Hau Village

2.2.5 Hydraulic performance analysis confirms that the capacities of the existing drainage systems are inadequate and need to be upgraded. It is anticipated that future urbanisation will result in higher surface runoff in these areas. Flooding will persist and will become more serious.

2.2.6 Public consultation with local residents, village representatives, Rural Committee Members, the District Council and green groups had been conducted since October 2003 and was completed in June 2004. The layout and details of the proposed works have been revised in response to the comments from relevant parties.

2.2.7 To summarise, drainage improvements are recommended to alleviate the flooding problems in the above areas and to meet the current flood protection standard. This Project includes the proposed river training works in Sai Kung River, Pak Kong River and Ho Chung Channel. The communities will benefit from the proposed drainage works, for which will reduce the threat of disastrous floods and minimises economic loss and disruption to daily life, especially during the wet season.

2.3 Continuous Public Involvement (CPI)

2.3.1 The process of continuous public involvement commenced in October 2003 and the meetings conducted were summarised in Table 2.4.

Table 2.4 Summary of Continuous Public Involvement

Date	Subject
20 Oct 03	Public Consultation Meeting with VRs (Sha Ha, Sha Kok Mei, Pak Kong, Tai Chung Hau and Ho Chung)
3 Nov 03	Public Consultation Meeting with Ho Chung VR and Villagers
2 Dec 03	Public Consultation with Tai Chung Hau VR and Villagers
8 Dec 03	Public Consultation with Sai Kung Rural Committee
17 Feb 04	Public Consultation with Tai Chung Hau VR and Villagers
24 Feb 04	Public Consultation with Sai Kung District Council
30 Apr 04	Public Consultation with Green Groups (CA, GP, WWF, AFCD, DSD/PM and MCAL)
11 Jun 04	Public Consultation with Green Groups (GP, WWF, DSD/PM, AFCD, EPD and MCAL)
16 Jun 04	Public Consultation with the Conservancy Association

2.3.2 During the course of CPI, the channel details were continuously reviewed according to the comments and recommendations from the public. Major comments and recommendations from the public are summarised below:

Table 2.5 Summary of Comments/Recommendations from the Public

Concerned Parties	Major Comments/Recommendations	Responses/Outcomes
Tai Chung Hau VR and villagers	The proposed works at Pak Kong should be reduced from full-scale training to widening of 2 existing bottlenecks.	The local residents were reminded that the flood protection level of existing Pak Kong River could not meet the current flood protection standard during the public consultation. Flooding may occur during heavy rainstorm. Nevertheless, the public insisted to reduce the scope works to avoid resuming existing lands. The scope of works was therefore reduced from full sale training to removal of two bottlenecks at Pak Kong River.
Ho Chung VR and Villagers	The existing overflow should be re-provided due to the fung shui reason.	The general layout was revised to re-provide the overflow weir at Ho Chung Channel.
Green groups	(a) Existing riverbed should be untouched as far as possible. (b) More ecological friendly design should be included. (c) Provide fish-ladder over the weir.	To enhance the ecological conditions, the general layout was revised according to the recommendation/ comments from green groups. (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) Fish-ladder will be provided at the downstream side of the weirs.

Concerned Parties

Major Comments/Recommendations

Responses/Outcomes

Tai Chung Hau VR and villagers

The proposed works at Pak Kong should be reduced from full-scale training to widening of 2 existing bottlenecks.

The local residents were reminded that the flood protection level of existing Pak Kong River could not meet the current flood protection standard during the public consultation. Flooding may occur during heavy rainstorm. Nevertheless, the public insisted to reduce the scope works to avoid resuming existing lands. The scope of works was therefore reduced from full sale training to removal of two bottlenecks at Pak Kong River.

Ho Chung VR and Villagers

The existing overflow should be re-provided due to the fung shui reason.

The general layout was revised to re-provide the overflow weir at Ho Chung Channel.

Green groups

(a) Existing riverbed should be untouched as far as possible.

(b) More ecological friendly design should be included.

To enhance the ecological conditions, the general layout was revised according to the recommendation/ comments from green groups.

(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.

(d) Fish-ladder will be provided at the downstream side of the weirs.

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 also presented to the green groups. The green groups considered that the revised proposal and justification of project need of drainage improvement works were acceptable.

2.3.4 In June 2004, the public consultation was completed and the channel design details were finalised. Relevant government departments, Rural Committee, District Council, local residents, green groups and relevant interested parties had no objection to the revised channel design.

2.4 Alternative Drainage Options

2.4.1 The following drainage options have been considered for relieving flooding at Sai Kung, Pak Kong and Ho Chung before concluding the current design.

Option A Maintenance of existing rivers - Carry out more frequent maintenance works such as desilting, removal of overgrown vegetation and obstructions in the three watercourses to improve the drainage capacity of the existing rivers.

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 peak flow downstream. At some location, floodwalls will also be required along the existing rivers to eliminate the overspill of water as the river bank levels are lower than the extreme high tide level.

Option D Floodwall and Pumping System – Construct floodwall along the river banks 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 – Widen and/or deepen the existing rivers in order to increase the drainage capacity of the drainage system.

Option F Vegetation Enhancement of the catchments – Planting of vegetation in the catchment in order to reduce the surface runoff.

2.4.2 The considerations, environmental benefits and dis-benefits for various drainage options are reviewed and shown in Table 2.6.

2.4.3 After the consideration of all options detailed in Table 2.6, 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 aspects 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 and house 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 Ho Chung. However, the downstream section of Ho Chung Channel will not be deepened.

2.4.5 To avoid causing major disturbance to the Sai Kung town centre and affecting the existing river, the by-pass culvert option is recommended for the Sai Kung River. During normal condition, the flow to the downstream of Sai Kung River will be maintained by the diversion pipes. In the events of heavy rainstorms, the floodwater will be conveyed to the diversion box culvert and discharged to the sea.

2.4.6 For Pak Kong, river widening option is considered the most appropriate drainage option. During the public consultation, the public was advised that flooding might occur during heavy rainstorms if the proposed river training works were not carried out. However, the people affected by the flooding agreed to accept a higher flooding risk in order to avoid resumption of their houses. They also requested to carry out minor improvement works to improve the drainage capacity of the existing river, including re-construction of the existing crossings and stabilisation of a section of river bank.

Table 2.6 Comparison of the Alternative Drainage Options

Drainage Options	Environmental Benefits	Environmental Dis-benefits	Recommendations	Reasons & Considerations
Maintenance of existing rivers	(a) The area to be affected by proposed drainage improvement works will be the least. (b) The existing condition of rivers will be improved. For example, the soil and wastes 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. (c) The aim of flooding protection cannot be achieved. The public still faces the risk of lives and economic loss during flooding.
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.	Recommend to construct by-pass box culvert at Sha Ha to collect runoff from the Sai Kung River and discharge to the downstream box culvert being constructed by CEDD. It is not recommended for Ho Chung and Pak Kong.	(a) The capacities of the existing rivers are low and can only handle a storm of design return period of 5 - 10 years. If the existing rivers are kept to convey flow during rainstorm, the size of the additional by-pass channel or box culvert is quite large, about 8m to 20m wide. (b) The construction of by-pass channel or box culvert will require extensive land resumption and will impose adverse impacts to other areas further away from the flooding spots, particularly the existing village houses and building lots at Pak Kong and Ho Chung. (c) For the Sai Kung River, as there are abandoned agricultural lands available for the construction of the by-pass box culvert, the diversion scheme is considered feasible and acceptable. Besides, there are not adequate spaces for the widening of existing drainage system in the Sai Kung town.
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. (c)	Not recommended	(a) It is estimated that a detention pond of size of 200m long by 140m wide by 2m deep is required for the Sai Kung River in order to intercept the surface runoff during the extreme rainstorm events. Even larger detention ponds are required for the Pak Kong River and Ho Chung Channel. (b) There are inadequate government lands for the implementation of this option and large-scale land resumption is required. This large-scale land resumption will attract great public objection, particularly at Pak Kong and Ho Chung where land is used for village houses or intended for village development. (c) This option is also not a cost-effective solution given the large volume of runoff needed to be stored for which will result 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 objection from the public cannot still be avoided. (d) 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. In particular, there are a lot of village houses along the Pak Kong River and Ho Chung Channel. Resumption of these village houses is not acceptable. (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	The existing condition of the rivers will be maintained 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 river habitats; the soil and wastes settled in the riverbed will be removed, eroded river bank will be re-constructed. 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 and deepening ü Sai Kung River ü Bottlenecks at Pak Kong ü River Upstream section of the Ho Chung Channel. River widening Downstream section of Ho Chung Channel.	(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 a 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 could 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 Kong River, only section of the existing rivers/bottlenecks will be widened and no deepening is required. (h) As agreed with green groups, the riverbed of existing Ho Chung Channel should remain untouched as far as possible. As such, the downstream section (approximately 300m) of existing Ho Chung Channel will not be deepened and the existing riverbed will be retained. Only widening works will be carried out.
Vegetation Enhancement	(a) The environment (visual and ecological etc.) of the catchment will be improved due to the vegetation planting. (b) Existing rivers will not be disturbed and there is no disturbance to the ecological system and water quality in existing rivers.	(a) No major environmental dis-benefits are anticipated.	Not recommended	(a) The upland areas of all three catchments are already heavily vegetated, supporting extensive areas of shrubland/woodland. Planting of additional trees in these areas would not significantly reduce the runoff and resolve flooding problems.

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 of 1 in 50 years and should contain floodwater under a design return period of 1 in 200 years.

(c) Examples of Environmental 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 the Continuous Public Involvement.

Channel Alignment

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

(a) The horizontal alignments of the proposed Sai Kung River and Ho Chung Channel have been designed to follow the existing rivers in order to minimise the total area of the land resumed for the drainage channels. In other words, the total area of existing farmland and nurseries to be used for channel construction can be minimised. Besides, it can minimise the resumption of the village houses.

(b) At Sha Ha, a site formation project is underway and a diversion culvert is being constructed by the Civil Engineering and Development Department (CEDD). The proposed Sha Ha Diversion Culvert will connect the Sai Kung River to the downstream box culvert constructed by CEDD. The alignment of the box culvert shall avoid resumption of agricultural land and dwellings as far as possible.

(c) The northern side of Ho Chung Channel is the Ho Chung Road and the Ho Chung Village. Therefore, the river could only be widened toward the southern side. However, there are a number of buildings, building lots, committed and planned houses in the southern side. This constrains the widening of the channel and in some locations, only a rectangular drainage channel is feasible and 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 immediately upstream of the channel. At the downstream, the existing riverbed will match the level of the existing rivers. 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 following have been considered:

(a) Open concrete channel

(b) Open channel with natural bed and gabion walls

(c) Rectangular or trapezoidal drainage channel

(d) Box culvert

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 To enhance the ecological conditions, natural bedding and gabion walls with planting pits will be provided for the rivers. 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 to encourage the recolonisation of improved river sections by aquatic communities. The existing riverbed will remain untouched as far as possible. The excavated materials will be re-used as the natural substrates in the riverbed.

2.5.7 Rectangular open channels have limited scope for improving the aesthetic 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 Ho Chung Channel at which the site is bounded by village houses and Ho Chung Road.

2.5.8 For the Sha Ha Diversion Box Culvert, it will connect the upstream Sai Kung River to downstream box culvert constructed by CEDD. The existing stream downstream area beyond Tai Mong Tsai Road will be filled for development and access road will be constructed by others in future. Therefore, open channel is not recommended for the diversion drain.

Dry Weather Flow Channel

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

2.5.10 Following consultation with green groups and local residents in Ho Chung, in addition to the existing Fung Shui Weir and WSD water intake weir in Ho Chung Channel, an extra weir will be provided at the downstream area in order to maintain the Ho Chung Channel in wet condition. Fish ladders will also be provided at the Fung Shui Weir to allow river habitats to connect between the rivers.

2.5.11 The conceptual design of weirs, fish ladders, pools and riffles are shown below. Rocky material will be placed between steps of the fish-ladder in order to blend in with the natural stream-bed.

Section of River Channel

Plan of River Channel

Plan View of River Channel

Conceptual Design of Weir, Fish Ladder, Pools & Riffles

Operation and Maintenance Requirement

2.5.12 To minimise the footprint of the drainage improvement works, no maintenance access will be provided alongside with the Sai Kung River, Pak Kong River and Ho Chung Channel. Only two 0.5m wide concrete access tracks will be provided along the channel bed of Sai Kung River and Ho Chung Channel to facilitate maintenance. Access points to the channel bed will be provided at specified locations of the drainage channels for desilting purpose. For the Sha Ha Diversion Box Culvert, a 3.5m wide grass concrete slab will be provided along the top of the box culvert as the maintenance access to the box culvert and Sai Kung River.

Collection of Local Runoff

2.5.13 At some locations, the proposed channels will deviate from and cut off the existing watercourses. It is proposed to connect the existing watercourses to the proposed channel through either a pipe or a culvert such that the flow can be maintained.

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

2.5.15 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

2.5.16 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 re-provided under the channel upgrading works.

2.6 Construction Methods and Engineering Requirements

2.6.1 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 either side of the river banks. Gabion blocks will be applied to the river banks to form the permanent lining. Natural substrates (e.g., sand, gravel and cobbles) will be placed on the bottom to form the channel bed. The substrates will be carefully arranged so as to re-create the pool-riffle sequence found in natural river sections. 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.

2.6.2 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 along the river is not recommended as the construction activities will adversely affect water quality and aquatic ecology. Dredging activity will adversely affect the water quality and is not recommended. Temporary culverting the rivers to form a dry area for construction is also not suggested as the river will be temporarily destroyed during the construction period.

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

2.6.4 Construction of the three-cell Sha Ha Diversion box culvert will be carried out by conventional methods that generally involve site clearance, excavation, installation of temporary supports to excavated face, erection of formwork, reinforcement fixing and concreting followed by backfilling and compaction.

2.6.5 Vehicular and pedestrian crossings will be constructed in Ho Chung Channel and Pak Kong River. 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. For crossings that need a piled foundation, bored piles are recommended such as to avoid noise problems during construction.

2.6.6 Since some of the existing pedestrian and vehicular accesses and crossings within the proposed channel alignment will be demolished under the proposed channel upgrading works, temporary vehicular and pedestrian diversions will be implemented such that the vehicular and pedestrian flows will not be affected during construction. In addition, reprovision of the demolished accesses and crossings will be carried out.

2.6.7 Temporary support for the drainage channel during construction stage shall be provided in order to minimise disturbance and ground movement to the adjacent road and structures. Ground movement monitoring should be carried out during construction stage. If there is any adverse ground movement to the adjacent road or structures, mitigation measures such as suspension of works and stabilisation works will be implemented.

2.6.8 Construction activities for the cross road drain will involve the following sequential works: concrete and road breaking, sheet piling, excavation of a trench of about 2m - 3m deep, removal of existing pipes and installation of new larger diameter pipes, removal of sheet piles, backfilling, compacting and road resurfacing.

2.6.9 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 are summarised in Table 2.7. The impacts due to the construction method will be mitigated by appropriate measures which are discussed in the relevant sections of the EIA Report.

Major Works	Locations	Major Activities	Reasons & Constraints of alternative method
Widening and deepening of existing rivers	River widening and deepening ü Sai Kung River ü Bottlenecks at Pak Kong ü River Upstream section of the Ho Chung Channel River widening Downstream section of Ho Chung Channel	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 to minimise the adverse effect on the rivers. Working in wet 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 Sha Ha Diversion Box Culvert ü Crossings at Pak Kong and Ho Chung Floodwall Northern bank of Ho Chung 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.
Retaining Walls, Box Culvert and Floodwall		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.
Bridges	Footbridges at Ho Chung	Piling works for foundation include installation of steel casings, reinforcement fixing and concreting.	Shallow foundation will not be adopted as it requires extensive area for the construction of base slab. At Ho Chung, lands are not available as the site is bounded by existing road and village houses. Using bored piles for the bridge foundation is a common construction technique. Percussion and driven piles are not recommended due to noise generation during construction.
Bridges	Footbridges at Ho Chung	Bridge structure works include the construction of falsework, formwork erection, reinforcement fixing and concreting.	Similar to retaining wall construction, there is inadequate working space for transporting, handling and installation of precast unit. In-situ concreting is more suitable.

Likely Concurrent Projects in the Vicinity	Construction Programme	Potential Cumulative impact
(i) Proposed Sai Kung River and Sha Ha Diversion Box Culvert	Dec 2005 to Apr 2008	--
Dualling of Hiram’s Highway from Marina Cove to Sai Kung Town (HyD)	N/A	The project implementation is currently under review.
Contract No. CV/2001/15 “Roads and Drains in Sai Kung Area 4”(CEDD)	Sep 2002 to Dec 2004	Not expected (completed before this Project)
(ii) Proposed Pak Kong River	Dec 2005 to Apr 2008	--
RPIS Project No. SK015 – Improvement to Access Road at Ta Ho Tun and Footpath of Hiram’s Highway, Sai Kung (HyD)	Completed in end 2002	Not expected (completed before this Project)
Dualling of Hiram’s Highway from Marina Cove to Sai Kung Town (HyD)	N/A	The project implementation is currently under review.
Proposed Junction Improvement at Hiram’s Highway/Tai Chung Hau Road/New Ta Ho Tun Road (HyD)	Sep 2003 to Mid 2004	Not expected (completed before this Project)
Design & Construction for the Eastern Transmission Pipeline along Hiram’s Highway (HKCG)	Jun 2002 to 2005	Not expected (completed before this Project)
(iii) Proposed Ho Chung Channel	Dec 2005 to Apr 2008	--
RPIS Project No. SK069 – Improvement of Ho Chung Road (HyD)	Mar 2005	Expected and to be addressed in this Report.
Dualling of Hiram’s Highway between Clear Water Bay Road and Marina Cove and Improvement to Local Access to Ho Chung (HyD)	N/A	The project implementation is currently under review.
Design & Construction for the Eastern Transmission Pipeline along Hiram’s Highway (HKCG)	Jun 2003 to 2005	Not expected (completed before this Project)