2.1.1
The project is entitled “Hang
Hau Tsuen Channel at Lau Fau Shan” and is referred to as “the Project”.
2.1.2
The Project was previously proposed under the “PWP Item No.
471CL – Lau Fau Shan Development – Remaining Engineering Works” managed by the
then New Territories North Development Office, Territory Development Department
(TDD). In November 2001, the then TDD applied for an Environmental Impact
Assessment (EIA) study brief to undertake the EIA Study for the Project. The
Environmental Protection Department (EPD) issued an EIA study brief (ESB-090/2001)
in December 2001. However, owing to policy changes, the engineering works project
was held in abeyance with the remaining part of the works carved out for
implementation as the first phase. Subsequently, “Hang Hau Tsuen Channel at Lau
Fau Shan” (the Project) was created under PWP Item No. 705CL for the first
phase which involves training of Hang Hau Tsuen Stream. The project proponent, Civil
Engineering and Development Department (CEDD), applied and obtained an EIA
study brief (ESB-167/2007) for carrying out the EIA Study.
2.1.3
The squatter area in the vicinity of the existing Hang Hau
Tsuen stream was cleared in 1997. The poor hydraulic performance of the Hang
Hau Tsuen stream is attributed partly to the siltation of the watercourse by
the remains of the former squatter structures, and also to the meandering
watercourse and the presence of an under-sized box culvert at
2.2
Location and Description
of the Project
2.2.1
The Project is located in Hang Hau Tsuen at Lau Fau Shan,
2.2.2
The Project comprises the construction of the Hang Hau Tsuen
Channel and associated works to improve the local drainage systems in the Hang
Hau Tsuen area.
2.2.3
The Project involves the following works:
(i)
Training of Hang Hau Tsuen stream including construction of a
370 m long drainage channel connecting to the downstream ends of Fung Kong
Tsuen Channel and San Hing Tsuen Channel to
(ii)
Construction of a 25 m long triple-cell box culvert 5.5 m (W)
x 3 m (H) beneath the
(iii)
Provision of a 3.5 m wide access road with passing bays along
the northern bank of the channel, a 3.5 m wide maintenance access road along
the southern bank of the channel and 2.0 m wide footpath on both sides of the
channel and ancillary works such as retaining walls, drainage and water works
as well as the associated landscaping works.
2.2.4
As requested by a Yuen Long District Council member, a
viewing point and car park will be provided at the downstream end of the
proposed channel to allow visitors to enjoy the scenery of
General
Description of the Project Area
Hang Hau Tsuen
2.2.5
Hang Hau Tsuen is located along the banks of Hang Hau Tsuen
stream and to the south of Lau Fau Shan. Many squatter houses used to exist
among the few legal village houses along the banks, but they have been cleared
away in 1997 as part of a rural improvement scheme. The village area is
situated in a generally flat low lying area at approximately +2.7 to +3.6 mPD. Majority
of the dwellings are tin-sheeted and/or wooden structures of 1-2 storeys (Plate
1).
Hang Hau Tsuen Stream
2.2.6
Hang Hau Tsuen stream is a natural, shallow tidal water
course polluted by wastewater effluents discharged from Hang Hau Tsuen and
other unsewered villages upstream, as well as effluents from the nearby open
storage sites and container yards. It receives upstream flow from San Hing
Tsuen Channel (Plate 2) and Fung Kong Tsuen Channel (Plate 3) via a single
cell box culvert underneath
2.2.7
Marine sediments have been trapped along the tidal portion of
the stream banks. Mangroves now colonize the intertidal downstream part of the
stream (Plate 5).
2.2.8
The land around the Project area comprises of knolls, coastal
flat land, farmland, fish ponds and village settlements in low-lying flat land.
Majority of the agricultural fields and fish ponds have been abandoned and have
been converted to open storage sites, container yards, container trailer &
tractor parks and workshops (Plate 6). This has led to a rapid
degradation of the rural environment and undesirable reduction in flood storage
capacity.
2.2.9
The Hang Hau Tsuen stream is about 400 m in length and about
10 – 40 m in width between banks. The total catchment area of the stream is about
118 ha (Figure 2.4).
Coastal Protection Area
2.2.10
The area along the
2.2.11
According to the outline zoning plan, the CPA zone is
intended to conserve, protect and retain the natural coastlines and the
sensitive coastal natural environment, including attractive geological
features, physical landform or area of high landscape, scenic or ecological
value, with a minimum of built development. It may also cover areas which serve
as natural protection areas sheltering nearby developments against the effects
of coastal erosion.
Existing Drainage Conditions
2.3.1
Hang Hau Tsuen is situated in a generally flat low-lying area
with ground levels ranging from approximately +2.7 to +3.6 mPD. The area is
subject to a large amount of runoff brought down from the upper catchments and
by downstream tidal and storm surge levels from
2.3.2
The Hang Hau Tsuen stream has a very low flood protection
standard.
2.3.3
The stream course is also influenced by downstream tidal and
storm surge levels from
2.3.4
The existing wooden footbridges are sub-standard and in poor
conditions. They have low soffit level and are often flooded posing hazard to
the villagers. One of the bridge collapsed on 16 June 2008 due to structural
failure of one of the gabion pier (Plates 10 & 11).
Flooding Incidents
2.3.5
Flooding had repeatedly occurred in the vicinity of the Study
Area. One of the main flooding incident occurred in May 1989 with the arrival
of Typhoon Brenda. In this event, the area to the east of east of
2.3.6
Heavy rain occurred in June 2001 with the Hong Kong
Observatory issuing a consecutive nine days of Rainstorm Warning Signal from 5
June – 13 June 2001. There was widespread flooding in various parts of
2.3.7
Rain bands associated a trough of low pressure caused
exceptionally heavy rainfall to the north and northwestern part of the
2.3.8
During the passage of Typhoon Neoguri on 19 April 2008
about 150 mm of rainfall was recorded in Lau Fau Shan area. As reported by
local villagers, the floodwater once rose to the waist level of an adult. The
latest flooding incident occurred in June 2008. Under the influence of an
active trough of low pressure, the Hong Kong Observatory recorded on 7 June
2008 the highest hourly rainfall since record began. About 200 mm of rainfall
was recorded in Lau Fau Shan area on that day. According to villager, flooding
was up to knee deep. The villagers avoided using the footbridge across the
stream as it was flooded and was considered too dangerous to cross. One of the
footbridge collapsed during the site meeting with Green Groups on 16 June 2008.
Nobody was injured during the incident.
2.3.9
The flooding was likely attributed to backing up in the Hang
Hau Tsuen stream, which is in poor conditions with the stream badly silted up,
blockage by squatter structures and low stream bank level. As a result,
floodwater overflowed the stream banks and flooded the low-lying areas.
Standard of Flood Protection
2.3.10
Over the past decades, there has been substantial development and
modification of the floodplain in the Study Area which has reduced the flood
storage capacity and affected floodways and watercourses. These changes have
caused substantial increase in flooding hazards.
2.3.11
The current standard of flood protection of Hang Hau Tsuen stream
is very low. The stream was highlighted in the Drainage Impact Assessment (DIA)
Study[1]
as having insufficient flow capacity leading to localized flooding and causing
danger to villagers. Flooding has occurred repeatedly in Hang Hau Tsuen leading
to property damage, economic loss and disruption to everyday life. The existing
footbridges have a low soffit level and are often flooded. Moreover, given the
grossly polluted state of the water, the floodwater also poses potential health
and hygiene hazards to the villagers.
2.3.12
The drainage channel is
classified as “Main Rural Catchment Drainage Channel” according to the DSD’s Stormwater
Drainage Manual (SDM) (2000 Edition). It will be designed to cater for a
50-year return period event in accordance with the SDM.
2.3.13
It is clear that the villages next to both embankments of the proposed
Hang Hau Tsuen Channel are subject to flooding due to tidal surge. Under a high
tide condition, the village will be flooded and the maximum flood depth will be
determined by the maximum design sea water level, i.e. +3.75 mPD. However, under a low tide condition, the
villages will be protected by the raised embankments, which confine the
stormwater collected within the largest upstream catchment. Under the existing
condition, stormwater discharged from the upstream of Hang Hau Tsuen stream course
will disperse onto the villages after the stream is flooded. Therefore, the flood risks of Hang Hau
Tsuen will be reduced under low tide conditions with implementation of the
proposed works.
2.3.14
Upon completion of the Project, the proposed Hang Hau Tsuen Channel will
provide a 1 in 50 years flood protection for village development.
Purpose and Objective of the
Project
2.3.15
The purpose of the Project is to alleviate flooding that has
repeatedly occurred in Hang Hau Tsuen by converting the existing Hang Hau Tsuen
stream to an engineered channel that will have a 50-year storm event capacity.
Scenarios With and Without
Project
Without Project
2.3.16
Consequences of not proceeding with the Project include recurring
flooding in Hang Hau Tsuen during severe rainstorms resulting in danger to
life, property damage, economic loss and inconvenience to the public.
2.3.17
A hydraulic model has been set up to assess the water level
profiles under different return periods within the existing Hang Hau Tsuen
stream. The result of the model is shown in the flood risk map (Figure
App2.1 of Appendix 2.1).
2.3.18
According to the modeling results, under the 50-year flood
level return period event, the high water level near the downstream outlet,
i.e. from chainage 0 to 300 will be governed by the conditions in a 50-year sea
level in conjunction with a 10-year rainfall. The high water profile ranges
from +3.75 mPD to +4.25 mPD.
2.3.19
As shown in Figure App2.1 of Appendix 2.1,
without the proposed channel, there would be flooding ranging from 0.3 m deep
to more than 1.5 m deep during heavy rainstorm.
With Project
2.3.20
The proposed Hang Hau Tsuen Channel will have a 50-year storm
event capacity. U-channel will be provided at the side of the embankment of the
channel to collect the surface runoff from adjacent areas. The collected runoff
will be discharged into the channel through connecting drains with flap valve.
The water within the channel will not be overtopped to the adjacent low-lying
area when the water level at the channel is high and the flap valve is closed.
Justification and Benefits
of the Project
2.3.21
The DIA Study investigated the need to improve the local
drainage system to alleviate recurring flooding problems in the study area. The
proposed Project is expected to contribute to the relief of the present
drainage problems that have repeatedly led to severe flooding in the study
area.
2.3.22
The current standard of flood protection of the Hang Hau
Tsuen stream is very low. This stream was highlighted in the DIA Study as
having insufficient flow capacity leading to recurring flooding problems. Upon
completion of the Project, the standard of flood protection will be increase to
1 in 50 years.
2.3.23
The estimated population expected to be directly benefited by
the Project will be about 270 in Hang Hau Tsuen.
2.4
Consideration of
Alternative Drainage Options, Channel Alignments and Designs
Alternative Options to Avoid
Channelization Works
2.4.1
Some alternative options to avoid channelization works
include:
Ÿ
Resumption of all high risk village houses;
Ÿ
Raising of footpaths / seawalls; and
Ÿ
Enhancement of flood warning and distribution of advisory /
educational information and pamphlets on flood prevention measures to people
living in flood-prone areas.
2.4.2
These options are only first-aid measures or short term
measures which does not tackle the cause of flooding. They are considered to be
not effective, practical and even disruptive as in the case of full resumption
of houses. Channel improvement work is therefore considered to be a more
feasible solution.
Original Channel
Design
2.4.3
The original preliminary channel design consists
of a trapezoidal channel with grasscrete banks and concrete base (Figure
2.6). Minimizing land requirements and optimizing hydraulic performance
of the channel were prioritized factors in the original design of the channel improvement
works. The channel was concrete lined and straightened as far as possible to
optimize hydraulic performance. The existing stream course would however be
completely disturbed. Further design changes has been undertaken to align with
the recommendations in the Government’s technical circulars on protection of
natural streams and rivers (ETWB TCW No. 5/2005), DSD practice note on
environmental considerations for river channel design and to take into account
comments received during public consultation.
Design
Considerations
General
2.4.4
The Hang Hau Tsuen stream is not an Ecologically
Important Stream (EIS) listed under the ETWB TCW No. 5/2005. The water quality
of the stream is very poor as it currently receives domestic and industrial
wastewater. Ecological survey reveals very little aquatic fauna especially at
the heavily modified upstream section. Testing of the stream sediment indicate
that it is contaminated with high level of heavy metals. The stream habitat is
considered to have a low ecological value. However, site surveys reveal that
the downstream portion of the stream which is affected by the tidal action of
2.4.5
Intertidal mudflats can be found along the
2.4.6
Four fishponds used to be located in the CPA zone to
the south of the stream while two fishponds are located to the north of the
stream outside the CPA zone. Site surveys reveal that two of the southern
fishponds within the CPA have been filled. All the remaining fishponds are
currently abandoned.
2.4.7
Section 3.3.2 of the EIA Study Brief has specifically
indicated that in considering alternative flood alleviation options, channel
alignments and lining design, the need to minimize negative effects on the CPA,
Hang Hau Tsuen stream, the mangrove colony, nearby fish pond habitats and the
intertidal mudflats shall be taken into account.
2.4.8
The channel design was revised taking into
consideration the need to preserve the mangroves, fishponds and mudflats. The
concerns by DSD on maintenance issues were also considered. The revised channel
design consists of preserving some of the mangroves and the stream bed at the
downstream portion. A strip of maintenance access track will be necessary to
facilitate DSD future maintenance of the channel. The potential impact on the
mangroves has been kept to a minimum. The channel will need to be widened to
ensure adequate flood conveyance capacity. A small portion of an existing
abandoned fishpond (of low ecological value) located within the CPA zone will
inevitably be affected. No intertidal mudflats will be affected.
2.4.9
Details of the design considerations are presented
below.
Relevant Technical Circulars / Guidelines
2.4.10
Reference has been made to the following Government Technical
Circulars / Guidelines in designing the proposed channels.
Ÿ
ETWB Technical Circular (Works) No. 5/2005 – Protection of
Natural Streams / Rivers from Adverse Impacts Arising from Construction Works
Ÿ
DSD Technical Circular No. 2/2004 – Protection of
Ÿ
DSD Practice Note No. 1/2005 – Guidelines on Environmental
Considerations for River Channel Design
Ÿ
DSD Report – Aesthetic, Environmental and Ecological
Considerations in the Design of Drainage Channels
2.4.11
The first two Technical Circulars provide an administrative
framework to better protect all natural streams / rivers from the impacts of
construction works, to provide guidelines for the planning and execution of
construction works and for the vetting of development proposals that affect
natural rivers and streams.
2.4.12
The DSD Practice Note presents the essential environmental
considerations that should be taken into account and incorporated wherever
practicable, in the design of river channels. The Practice Note addresses the
need for more environmentally friendly river channel design and be in-line with
the new measures being put in place by Government to strengthen the protection
of natural rivers and streams (as depicted in the above two Technical
Circulars).
2.4.13
The DSD Report prepared by Research & Development Section
of Land Drainage Division summarizes the review of the recent major drainage
improvement projects in Hong Kong and recommends good practice for drainage
channel design with due consideration on the aesthetic, environmental and
ecological aspects.
Essential Design Considerations / Principles Adopted
2.4.14
In addition to the Government guidelines, the following essential
design considerations / principles were considered in formulating the preferred
channel alignment / lining.
Ÿ
Adequate flow capacity – the size of the channel should have
adequate capacities to convey design flood flows within bank to achieve the design
flood protection standard.
Ÿ
Public safety – work is required for unstable banks and where
houses and structures are located very close to the existing stream banks.
Ÿ
Existing conditions of the stream banks – some sections of
the banks have loose surfaces and are prone to erosion and scouring.
Ÿ
Land constraints – physical features such as dwellings at one
side of the existing stream control the widening of the channel at that side.
Ÿ
Accessibility for maintenance – provision of adequate access
for future long-term maintenance of the completed channel is necessary.
Ÿ
Disturbance to the public – resumption of private domestic
houses / any commercial use lands / factories need to be minimize as much as
possible to reduce public grievances.
Alternative Channel
Alignment
2.4.15
The proposed alignment generally follow the alignment of
existing stream course in order to minimise land resumption requirements
wherever possible (thus reducing disturbance and public grievances) and to
facilitate interception of incoming flow from the upstream San Hing Tsuen Channel
and Fung Kong Tsuen Channel. The alignment is also dictated by physical
features constraint such as
2.4.16
Other alternative alignments were evaluated but are
considered not practicable, cost effective or would result in adverse
environmental impacts.
2.4.17
The upstream invert of the proposed channel has been designed
to match with the level of the existing San Hing Tsuen Channel and Fung Kong
Tsuen Channel as well as the existing box culvert, whilst the downstream is
governed by the water levels of
Alternative
Location of the Drainage Systems and Discharge Points
2.4.18
The proposed channel consists of existing box culvert and drains
to be replaced or upgraded. Other alternative locations were considered but are
not practicable, cost effective or not preferable in terms of land constraints
and hydraulic capacity.
2.4.19
The proposed channel eventually needs to discharge into
Design of
Channelisation
2.4.20
The design of channelisaton generally follows the avoidance
and minimization approach as promulgated in Government technical circulars and
guidelines. These are elaborated further below.
Alternative
Drainage Options
2.4.21
The drainage options (with reference to the DSD Practice Note
No. 1/2005) that have been considered before arriving at the current proposed
design are shown in Table 2.1. Its corresponding equivalent as promulgated in ETWB
TCW No. 5/2005 is also tabulated. The evaluation of the environmental benefits,
dis-benefits and potential impacts on the Hang Hau Tsuen stream, mangrove
colony, nearby fishpond habitats and the intertidal mudflats as well as other considerations
for the proposed channel improvement works are summarized in Table
2.2.
Alternative Drainage
Options Considered for the Proposed Hang Hau Tsuen Channel
|
Approach* |
Option* |
DSD Practice Note No.
1/2005* |
ETWB TCW No. 5/2005** |
|
Avoidance |
Option 1 |
Distant flood banks |
N/A |
|
Option 2 |
Two-stage (or
multi-stage) channels |
N/A |
|
|
Option 3 |
Relief or by-pass
channels |
N/A |
|
|
Option 4 |
Flood storage / Pumping
station |
N/A |
|
|
Minimization |
Option 5 |
Bank stabilization |
(i)
protection / stabilization of river bank at locations prone
to erosion by use of natural materials such as rock |
|
Option 6 |
Clearing and removal of
obstructions |
(ii)
removal of fallen objects / obstructions and clearing of
vegetation in a selected manner |
|
|
Option 7 |
Enlargement of channel
by widening |
(iii)
enlargement of channel by modifying one bank only (iv)
enlargement of channel by modifying / widening both banks |
|
|
Option 8 |
Enlargement of channel
by deepening |
(v)
enlargement of channel by deepening |
|
|
Option 9 |
Realignment /
straightening |
(vi)
realigning by creating a new channel |
|
|
Option 10 |
Use of artificial non-vegetative
smooth lining |
(vii)
construction of a new channel using artificial
non-vegetative smooth lining such as concrete |
Notes:
* Options as promulgated
in DSD Practice Note No. 1/2005 – Guidelines on Environmental Considerations
for River Channel Design
** Options as promulgated in
ETWB TCW No. 5/2005 – Protection of natural streams / rivers from adverse
impacts arising from construction works - Appendix C (A)
2.4.22
The various options are described briefly below.
Option 1 Distant
flood banks – Flood banks (using earth bunds as barriers) are set back
from the rivers and allow the river to meander freely within their bounds.
Option 2 Two-stage
channels – Excavating the surface section of the flood plain adjacent
to the river, with the original river channel preserved while higher flows are
contained within the newly created berms.
Option 3 Relief
or bypass channels / pipes – Constructed bypass channels or pipes with the aim
of diverting excessive water from the main natural channel during periods of
high flows.
Option 4 Flood
storage / Pumping station – Intercept the runoff at the upstream area and
temporarily store in a flood storage pond/area for release later when water
level in the downstream river recedes or using floodwater pumping station to
intercept and pump upstream water to downstream.
Option 5 Bank
stabilisation – Use of natural materials for bank stabilization in areas
prone to erosion while the entire stream is left intact.
Option 6 Clearing
and removal of obstructions – Removal of fallen trees, debris or other
obstructions and desilting in a selective manner to restore the river capacity.
Option 7 Enlargement
by widening – River channel is enlarged by widening while the stream bed
is left intact.
Option 8 Enlargement
by deepening – Channel bed is excavated to a greater depth to increase
the flow capacity.
Option 9 Realignment
/ straightening – Meandering river channel is realigned or straightened to
increase flood conveyance capacity.
Option 10 Use
of artificial non-vegetative smooth lining – Use of concrete on the
river bank and bed to reduce roughness and protect them from scouring thereby
increasing the flood conveyance capacity.
Summary of the Evaluation
of Alternative Drainage Options for the Proposed Hang Hau Tsuen Channel
|
Drainage Option |
Environmental Benefits |
Environmental |
Recommendation |
Reasons &
Considerations |
|
Option 1 - Distant flood
banks |
§ No major excavation of
the existing Hang Hau Tsuen stream will be required. The existing stream bed would
be left untouched hence the stream habitats (in particular the tidal portion)
will be preserved. § There will be minimal
ecological impacts on mangrove colony and intertidal mudflats. |
§ This option requires
substantial earth bund barrier to provide adequate flood protection which may
lead to potential visual impact to the villagers and other environmental
impacts during construction. § A small portion of a nearby
fishpond within the CPA zone will be affected. |
§ This option is only recommended for the downstream
portion of the channel to preserve the stream bed and mangrove colony. |
§ More land resumption is
required and mostly on private land. § The option is
constrained by physical features such as nearby village houses. § Potential visual impact
problem on nearby residents. § This option is more
appropriate for sensitive stream. The upstream section of Hang Hau Tsuen
stream is of low ecological value and therefore not recommended. The
downstream section is of moderate ecological value with the presence of
mangrove colony. The use of distant flood bank approach is effective to
minimize the potential ecological impact. § The area of fishpond
affected is small size and of low ecological value. § The flood protection
level provided is still very low. Local flooding will occur outside the distant
bank when the flood water retained by the bank is at high level. Further
channel works will be necessary for the upstream portion. |
|
Option 2 - Two-stage (or
multi-stage) channels |
§ No major excavation of
the existing Hang Hau Tsuen stream will be required. The existing stream bed
would be left untouched hence the stream habitats (in particular the tidal
portion) will be preserved. § There will be minimal
ecological impacts on mangrove colony and intertidal mudflats. § Different flood regimes
and water tables result in the development of different types of habitat. § Depending on the design,
excavated rocks and soils could be reuse as channel bed thereby reducing
offsite disposal. |
§ Potential environmental
impacts may arise from construction of the widened sections in particular
potential adverse construction phase impact to the nearby sensitive receivers. § There may be potential lost
of stream banks (depending on design adopted). § Nearby fishponds within
the CPA zone may be affected. |
§ This option is
considered not practicable and is therefore not recommended. |
§ More land resumption is
required and mostly on private land. § Construction works will
adversely affect the sensitive receivers. § The option is more
practical for existing natural stream or high ecological value stream. The
current state of Hang Hau Tsuen stream is grossly polluted by domestic and
industrial wastewater while the sediment is contaminated with heavy metals.
Option 1 has been considered for the downstream section. § Not suitable for small
stream or where there is land and physical features constraints. |
|
Option 3 - Relief or
by-pass channels / pipes |
§ No excavation of the
existing Hang Hau Tsuen stream will be required. The existing stream bed
would be left untouched hence the stream habitats (in particular the tidal
portion) will be preserved. § There will be minimal
ecological impacts on mangrove colony and intertidal mudflats. |
§ The bypass section may
impose environmental impacts to other areas during construction. |
§ This option (the use of
1200 mm diameter bypass pipe) is recommended
in combination with Option 1. |
§ As the bypass pipe can
be constructed underneath the maintenance access road, no addition land
resumption is required. § High flood water will be
diverted via the bypass pipe to the outfall thus minimizing works at the
downstream section. |
|
Option 4 - Flood storage
/ Pumping station |
§ No excavation of the
existing Hang Hau Tsuen stream will be required. The existing stream bed
would be left untouched hence the stream habitats (in particular the tidal
portion) will be preserved. § There will be minimal
ecological impacts on mangrove colony and intertidal mudflats. |
§ All the land is at low
level, extensive bund will be needed for the storage ponds. § There will be potential
environmental impacts associated with the construction and operation of
storage ponds and pumping station and associated drains. § Potential odour problems
from the storage of polluted water. § Potential visual impact
of the pumping station on nearby areas may arise. |
§ This option is
considered not practicable and is therefore not recommended. |
§ More land resumption is
required. § The level of the
surrounding land is at low level, substantial bunds for the storage ponds
will be needed. Structural integrity will need to be carefully considered. § The option is
constrained by availability of land and nearby village houses. § This option will incur
higher construction and maintenance costs. § Sea water will still backflow
from the existing stream during high tide causing flooding. § High capital cost for
long term running and maintenance of the pumping station is required. § Breakdown of the pumping
station during storm event may lead to flooding. § Suitable location for
the outlet drains and outfall will need to be identified. § This option is not
practical given the large volume of runoff needed to be stored and/or pumped.
Substantial land resumption for storage pond and/or pumping station will be
required. |
|
Option 5 - Bank
stabilization |
§ No excavation of the
existing Hang Hau Tsuen stream will be required. The existing stream bed
would be left untouched hence the stream habitats (in particular the tidal
portion) will be preserved. § There will be minimal
ecological impacts on mangrove colony and inter-tidal mudflats. § The use of gabion /
mattress lining will enhance the stream habitat by allowing recolonization of
riparian vegetation. It will also improve the aesthetic appearance stream. § As the works will be at
the banks, the likely impacts on the Hang Hau Tsuen stream, mangrove colony
and intertidal mudflats will be reduced. |
§ Some minor localised
environmental impacts including loss of mangroves near the banks will still
be inevitable during construction. |
§ This option is
considered not practicable and is therefore not recommended. |
§ In general, all the land
in Hang Hau Tsuen is at very low level. No clear define bank can be
identified. Some of the bank forms the existing footpath which is also at a
low level. § Suitable only for
locations with localised flooding or requiring minor bank improvement works. § This option is not
suitable for channel predicted to have insufficient hydraulic capacity and
requires widening or where bank level is at a very low level. |
|
Option 6 - Clearing and
removal of obstructions |
§ As no major construction
works are necessary, there will only be very minimal impact on the
environment and ecology of Hang Hau Tsuen stream. § The area of the stream
to be affected will be very minimum. |
§ No major environmental
dis-benefits are anticipated. § Minor disturbance of the
stream sediment may be necessary during clearance and removal. |
§ This option is not recommended but the construction
works itself will remove some of the former squatter structures remnants. |
§ Flooding is mainly due
to under sized culvert, low bank level and sediment build up. § This option can only
marginally improve the capacity of the stream and may need to tie-in with
other drainage options. § There will be less
adverse impact on ecology of the stream. § The flood protection
level provided is still very low. Flooding may still continue. |
|
Option 7 - Enlargement
of channel by widening |
§ The environmental
condition can be improved with suitable design on lining and bedding such as
gabion mattress or rip-rap lining to replace some sections of engineered
banks and beds. § As the improvement works
will be carried out along existing streams, it can minimize major resumption
of land and reduce environmental impacts imposed to other areas. |
§ Impacts on the existing sensitive
receivers and nearby fishponds are unavoidable. |
§ This option is recommended in combination with other
suitable options. |
§ Widening and deepening
is the most effective way to increase the flood conveyance of the
watercourses. § Land resumption and
generation of C&D materials will be minimized as the improvement works
will be carried out along the existing watercourse. § Ecological impacts can
be mitigated or even enhanced with environmental friendly channel design
(e.g. gabions, natural banks / beds, and riparian planting). § This option is more
reliable and cost effective when compare with other options. |
|
Option 8 – Enlargement
of channel by deepening |
§ Same as Option 7 |
§ Same as Option 7 |
§ Same as Option 7 |
§ Same as Option 7 |
|
Option 9 - Realignment /
straightening |
§ Some meanders may be
preserved but must be properly designed to ensure continuous water flow. |
§ Significant impact on
existing stream, especially at the downstream section. § The hydrology and
ecology of the whole stream will be changed. § Potential adverse impact
on the inter-tidal mudflats at the channel outfall as a result of faster
conveyance of floodwater. |
§ This option will result
in unacceptable adverse environmental and ecological impacts especially to
the mangrove colony, it is therefore not
recommended. |
§ This option can provide
fast flood flow conveyance thereby achieving the objective of flood
alleviation. § This option will have an
impact on the ecology of the whole watercourse. § Environmental impacts
outweigh engineering benefits for this option. |
|
Option 10 - Use of
artificial non-vegetative smooth lining |
§ None |
§ This option will result
in total loss of stream habitats. § There is no ecological
value and will result in high visual impact. § Potential adverse impact
on the inter-tidal mudflats at the channel outfall as a result of faster
conveyance of floodwater. |
§ This option will result
in unacceptable adverse environmental and ecological impacts especially to
the mangrove colony, it is therefore not
recommended. |
§ This option can provide
fast flood flow conveyance thereby achieving the objective of flood
alleviation. § Very little maintenance
is needed for this option. § Environmental impacts
outweigh engineering benefits for the proposed channels. |
Alternative Channel
Bed Lining / Embankment Lining Options
2.4.23
Common channel bed and embankment lining consists of:
§
Natural / unlined;
§
Rip-rap lining;
§
Gabion / mattress lining;
§
Rubbles / stones embedded in concrete / mortar;
§
Geotextile reinforced grass lining;
§
Grassed cellular concrete paving; and
§
Concrete lining.
2.4.24
The various options are described briefly below.
Natural / unlined – Channel bed is left
untouched or paved with original bed substrates to retain its natural
conditions.
Rip-rap lining – This is formed by a layer
of different-sized, angular rocks or boulders, sometimes underlaid with a layer
of filter fabric or granular materials.
Gabion / mattress lining – consists of wired gabion
baskets filled with small to medium size rock or granular material, placed over
the channel bank or bed.
Rubbles / stones embedded in concrete / mortar – This is not
much different from the concrete lining.
Geotextile reinforced grass lining – The channel bank
or bed is covered with top soil and grassed with a layer of geotextile woven
fabric provided underneath the top soil to reinforce the soil layer and to
provide erosion protection to the bedding.
Grassed cellular concrete paving – The channel bank
or bed is covered with interlocking hollow concrete panels / blocks that are
filled with topsoil and grassed.
Concrete lining – The channel bank or bed
is lined with concrete.
2.4.25
The evaluation of the environmental benefits, dis-benefits, potential
impacts on the Hang Hau Tsuen stream, mangrove colony, nearby fishpond habitats
and the inter-tidal mudflats as well as other considerations for the proposed
DP channels are summarized in Table 2.3.
Summary of the Evaluation
of Alternative Channel Bed / Embankment Lining Options
|
Lining Option |
Environmental Benefits |
Environmental |
Other Considerations /
Recommendations |
|
Natural / unlined |
§
No works will be carried out on the stream hence stream
habitats can be preserved. §
Very minimum impact during construction is expected when
compared with other lining options. §
Provides opportunity for recolonization of benthic
community thus enhancing the ecological value of the channel. §
As the channel bed will be left in its natural conditions,
it is visually more acceptable. §
Allows re-establishment of riparian vegetation thus
enhancing the landscape feature and ecological value of the channel. |
§
Prone to scouring and erosion of banks during flood flows
which may affect the structural integrity in some sections of the channel. §
A wider channel may be needed to cater for poorer hydraulic
performance which in turn may impose environmental impacts to other areas. |
§
Natural beddings have poor hydraulic performance and high
roughness. §
Suitable where the flow velocity is low. §
Recommended only for the downstream portion to avoid
impacting on the stream bed and the established mangroves. |
|
Rip-rap lining |
§
Provides moderate habitat for aquatic community. §
Accumulation of silt in the interstices allow recolonzation
of benthic community in the long term. §
Provide opportunity for riparian vegetation to establish thus
further enhancing the landscape feature and ecological value of the channel. §
As natural material will be used, it is visually more acceptable. §
Provides opportunity to reuse excavated rocks and soils
thereby reducing burden on offsite disposal sites. |
§
Prone to scouring and erosion of banks during flood flows
which may affect the structural integrity of the channel. §
A wider channel may be needed to cater for poorer hydraulic
performance which in turn may impose environmental impacts to other areas. §
Entrapment of rubbish leading to hygiene nuisance. |
§
Rip-rap linings have poor hydraulic performance and high
roughness. §
Suitable where the flow velocity is low. §
Recommended for the upstream portion of the channel. |
|
Gabion / mattress lining |
§
Provides moderate habitats for aquatic communities. §
Accumulation of silt in the interstices allow recolonzation of
benthic community in the long term. Provides ecological enhancement to the
current stream. §
As natural material will be used, it is visually more
acceptable. §
Provide opportunity for riparian vegetation to establish
thus further enhancing the landscape feature and ecological value of the channel. §
Provides opportunity to reuse excavated rocks and soils
thereby reducing burden on offsite disposal sites. |
§
Moderate impact to the stream and aquatic life during
construction. §
Prone to damage if poorly designed. §
Entrapment of rubbish leading to hygiene nuisance. |
§
Moderate hydraulic performance. §
High maintenance efforts and costs in replacement. §
Susceptible to corrosion especially at the tidal portion of
the stream which will be affected by brackish water. §
As some District Council members and local villagers expressed
concerns about the crevices of the gabion harbouring rodents and other pests,
this option is not considered further. |
|
Rubbles / stones
embedded in concrete / mortar |
§
Slightly better appearance than concrete. |
§
No ecological value. §
Offer little opportunity for riparian vegetation and
benthic community to thrive. §
Provides no landscape enhancement features. §
Potential adverse impact during construction. |
§
Better hydraulic performance. §
Little maintenance. §
Recommended for the channel bank only. |
|
Geotextile reinforced
grass lining |
§
Good aesthetic appearance. §
Allows establishment of some vegetation thus provides some
landscape enhancement and offer some ecological value. |
§
Low ecological value. §
Offer little opportunity for riparian vegetation and
benthic community to thrive. |
§
High maintenance cost §
Not recommended. |
|
Grassed cellular
concrete paving lining |
§
Better appearance than concrete. §
Allows establishment of some vegetation thus provides some
landscape enhancement and offer some ecological value. |
§
Low ecological value. §
Offer little opportunity for riparian vegetation and
benthic community to thrive. §
Potential adverse impact during construction. |
§
Fast conveyance of flood flows. §
Better structural integrity. §
Little maintenance. §
Recommended for maintenance access section. |
|
Concrete lining |
§
None |
§
No ecological value. §
Offer no opportunity for riparian vegetation and benthic
community to thrive. §
Provides no landscape enhancement features. §
Potential adverse impact during construction. |
§
Fast conveyance of flood flows. §
Little maintenance. §
Environmental impacts outweigh engineering benefits. §
Not recommended. |
Final Recommended
Drainage Improvement Design Options
2.4.26
The design of the Hang Hau Tsuen channel is evaluated with
reference to the environmental benefits and dis-benefits as well as engineering
considerations presented above. The final recommended alignment, location and design
options are selected from the options that are considered engineering feasible
and have the least impacts to the environment and the ecology of the Hang Hau
Tsuen stream, mangrove colony, nearby fishpond habitats and the intertidal
mudflats. Comments from the Green Groups, oyster farmer association and
District Council members during public consultation have also been taken into
consideration. The final recommended channel designs are presented in Figures
2.1 – 2.3 and Table 2.4 and elaborated in the
subsequent sections.
Major Design Characteristics
and Environmental Friendly Design Measures
Adopted in the Proposed Hang
Hau Tsuen Channel
|
Drainage Design Option
Adopted* |
Channel Lining Option
Adopted |
Major Design Characteristics
and Environmental Friendly Design Measures Adopted |
|
Option 1 Option 3 Option 7 Option 8 |
Stone finishing channel bank Retain existing natural stream bed Rip-rap channel bed lining with a
layer of natural stream bed material (for the low flow channel) Channel bed with original stream
material Grasscrete channel bed (for
maintenance access within channel) |
§
Bypass pipe will divert flood flow to channel outfall. About
150 m of the channelisation work at the downstream portion is thus avoided thereby
preserving the stream bed and mangrove colony. §
Existing natural stream bed to be retained and/or widened
with original stream bed material. §
Natural substrates to be used at the channel bed to allow
natural re-establishment of riparian vegetation. §
Silt will be allowed to accumulate for recolonization of
benthic community. §
|
* Refer to Table 2.2 for the detailed evaluation
of the various drainage options.
2.4.27
As the current flooding problem
of the existing watercourse is neither attributed to bank erosion nor
obstruction of debris, the options of clearing and removal of obstructions and
bank stabilization could not achieve the required flood protection system and
are therefore not recommended. Flood storage option is not feasible because sea
water will still overflow from the existing watercourse causing flooding.
2.4.28
Raising the stream banks can retain tidal flow and runoff
from upstream within the channel, hence distant flood bank option is considered
feasible.
2.4.29
Furthermore, by adopting the distant banks option, the bed of
the existing stream course at the 150 m downstream section will be retained
resulting in minimal disturbance to the mangrove colony that have established
there. Bypass pipe will be provided to divert flood flow as a result of the
reduced works at the downstream section.
2.4.30
Option of small scale widening
and deepening is also considered feasible and thus selected. The existing 300
mm wide concrete U-channels will be replaced by larger sized channels or pipes.
2.4.31
A minimum 500 mm freeboard has been allowed for the proposed box culvert
and footbridges under a 50-year rainfall event. The embankments are designed to
withstand a 200-year flood within the channel.
2.4.32
The Hang Hau Tsuen stream
receives grossly polluted upstream water and has a low ecological value. The
downstream section is affected by tidal action of
2.4.33
Based on the ecological
assessment, the channelization of the downstream portion will cause moderate
impacts due to the removal of the mangrove colony, which has a moderate to high
ecological value. Based on such findings, the proposal is revised not to
channelise the downstream portion in order to avoid the associated impacts.
Only the upstream portion which has been heavily modified will be constructed
to an engineered channel. For the upstream section, the proposed channel would
be straightened to avoid resumption of the existing fishponds and houses near
the northern embankment as well as optimization of hydraulic performance by
reducing the hydraulic head loss at the bend. Even with the avoidance approach,
a small portion of the mangroves (about 0.07 ha) will need to be removed to facilitate
the necessary works for the proposed channel.
2.4.34
The channel will need to be
widened to ensure adequate flood conveyance capacity. A small portion of an abandoned fishpond
(about 0.13 ha) of low ecological value located within the CPA zone will
inevitably be affected.
2.4.35
As assessed in the subsequent chapters of the EIA, the
construction and operation of this channel is not expected to adversely affect
the existing Hang Hau Tsuen stream, the mangrove colony, nearby fishpond
habitats and the intertidal mudflats.
Channel Lining
Evaluation
2.4.36
Channel lining is required to maintain the structural
integrity of the channel in order that it can continue to perform its hydraulic
function to contain and convey the design flood flows.
2.4.37
The choice of channel lining is a compromised solution
between various factors including hydraulic characteristics, cost, operation
and maintenance needs, environmental and ecological considerations, visual
impact and availability of materials, etc.
2.4.38
Further refinements were conducted with due regards to
hydraulic capacity, ecological impact and comments received from the Green
Groups, District Council members and other parties. The preferred solution is
discussed below.
2.4.39
Options for channel lining for stream banks and bed include
natural bed (gravel or mud), gabions, concrete or grasscrete. While the
hydraulic performance of concrete and grasscrete lined channels are the best
for smooth flow and easy maintenance, they will remove the ecological values of
the land on which they occupy. Also, the rate of transport of sediments
downstream, whether clean or contaminated, will also increase, resulting in
increase in suspended solids and biological oxygen demand in water at the
downstream.
2.4.40
Gabions and natural stream beds offer surfaces that will
sustain a bottom community and are better for retaining sediments and
establishment of vegetation on banks. This would also reduce the rate of
sediment entering
2.4.41
Some members of Yuen Long District Council – Environmental
Committee and local villagers expressed concerns about the use of gabion bank
in which the crevices may harbour rodents and other pests leading to
unacceptable health and hygiene issues. Site surveys confirm the presence of
rodents which may be attributed to the nearby Lau Fau Shan fish market and
restaurants, the surrounding squatter dwellings and the various abandoned
structures nearby. Due to District Council members’ strong preference not to
adopt gabion, further discussion with the Green Groups and the maintenance department
(DSD/MN) were carried out to consider other feasible options. The preferred
option involves using natural stone wall finishing as channel bank. Such option
would not adversely affect the ecological value of the existing highly
disturbed Hang Hau Tsuen stream (with low ecological value). Green Groups
expressed no further comments on such options.
2.4.42
Natural stream bed lining by using rip-rap, original stream
bed materials or retaining the existing muddy bottom would be applied to the
proposed channel bed lining. Grasscrete lining instead of concrete would be
applied to the proposed channel bed section where maintenance vehicles will
need to travel.
2.4.43
A minimum of 100 mm thick sediment will be allow to
accumulate at the future channel bed to permit recolonization of benthic
communities.
Low (Dry Weather)
Flow Channel
2.4.44
DSD Stormwater Drainage Manual specifies that low (dry
weather) flow channels are desirable for non-tidal sections of channel to
minimise siltation during low flow conditions. However, in nature, a certain
amount of siltation will occur in natural channels. One of the overriding
principles of this Project is that the design is working with nature without
precluding engineering solutions. A controlled amount of sediment built-up is
necessary if the channel bed is to mimic natural conditions. For the upstream
portion, low flow channel using rip-rap with a layer of natural stream material
on top will be provided. A minimum of 100 mm thick sediment will be allowed to
accumulate to permit recolonization of benthic communities in the channel. For
the downstream portion, no dry weather flow channel will be adopted in the
channel design. The water will naturally develop its own low-flow channel,
through the bed of the channel. The downstream section of the channel should be
permitted to find (and adjust) its own natural low flow channel and natural
changes in the deposition of silt, sand and rock should be tolerated.
Maintenance
Access Road
2.5.1
An approximately 370 m long 3.5 m wide access road will be constructed
along the northern bank of the channel. As agreed with Transport Department,
the road will be open for public usage. A 2 m wide footpath will be provided
along the channel to facilitate villagers’ access.
2.5.2
In addition, a 3.5 m wide maintenance access will be
constructed within the channel along the southern bank (not accessible by
public). It will be located at the channel level and will form part of the
channel during flood flow. The alignment has been carefully considered taking
into account the presence of mangroves in the downstream area. The alignment
will have the least effect on the nearby mangroves (0.07 ha inevitably
affected) and will avoid creating potential ecological barrier to the
intertidal fauna, natural stream bed and the retained mangroves. This portion
of the maintenance access will be constructed using grasscrete instead of
concrete allowing some vegetations to established thereby slightly enhancing
the ecological value and landscape feature of the channel.
2.5.3
Currently, Hang Hau Tsuen is not served by proper road
access. It is only accessible by narrow sub-standard footpaths. The proposed
access road will provide a more accessible and safe passage for the villagers.
It will also facilitate easy access by emergency vehicles to Hang Hau Tsuen.
Moreover, fire hydrants and proper road lighting will be installed along the
road according to the current standards for public road.
Pedestrian Crossings
2.5.4
Three existing pedestrian bridges affected by the proposed
channel will be reinstated as appropriate. These bridges are in poor condition
with low soffit level and are often flooded posing safety hazard to the
villagers. Four bridges in the form of box culvert will be constructed to
facilitate villagers crossing the channel. The bridge will be designed to Highways
Department’s standard. Full liaison
will be undertaken to ensure that the design of the works is to the authority's
satisfaction. Also the re-provided crossings will have soffit above the
estimated water level with adequate freeboard to allow design flow to pass
without obstruction.
Viewing Point and Carpark
2.5.5
As request by a Yuen Long District Council member, a viewing point
and carpark will be provided at the downstream end of the channel to facilitate
visitor to enjoy the scenery of
2.5.6
A minimum number of parking spaces (about 8-9 nos.) will be
provided. In addition, traffic management will be implemented to restrict
vehicles over 7 m long (with the exception of maintenance and emergency
vehicles) from using the access road in order to minimize traffic-related
environmental impacts to the villagers.
2.5.7
This facility as a public amenity area would allow the
scenery of
2.6
Construction
Related Aspects
Construction
Method
2.6.1
Construction of the proposed channel generally involves standard
construction methods such as site clearance, excavation and filling,
construction of the drainage channel, construction of the maintenance access
road and finally landscaping works.
2.6.2
The box culvert and drainage pipes will be constructed by conventional
methods which involve site clearance, excavation, installation of temporary
support, erection of formwork, construction of box culvert, laying of pipes and
drains, backfilling, and surface reinstatement.
2.6.3
Since some of the existing pedestrian crossings and footpaths
will be affected by the proposed works, it will be demolished and re-provided. Adequate
temporary provision will be provided and maintained by the Contractor
throughout the construction stage to facilitate safe passage and reduce
inconvenience to the public. Such accesses and crossings will be constructed by
typical cast in-situ construction method.
2.6.4
Natural stream bed will be left undisturbed as far as
practicable. Where widening and deepening of the stream is necessary, the
stream bed materials will be excavated and temporarily stored for reuse as the
new channel bed. Natural substrates (e.g. sand, gravel and cobbles) will be
carefully arranged so as to re-create natural stream bed.
2.6.5
Excavation works will be carried out at half-width of the
stream. Adequate width of the stream will be left intact with the flow
maintained so that disturbance to the aquatic ecosystem is kept to a minimum.
Adequate temporary diversion (if necessary) will be provided to ensure
continuous water flow to the downstream and to prevent exacerbated flooding to
the surrounding areas as a result of the construction works especially during
wet seasons. Temporary containment will be provided to maintain a dry working
condition. The contractor will be required to provide interim flood protection
measures within the contract period.
Site Access
2.6.6
The proposed channel can be readily accessed through the
existing
2.7.1
The implementation programme for the proposed works is
expected to start in end 2009 for completion by end 2012. Detailed construction programme is not
available at this current stage.
The tentative programme is tabulated in Table 2.5 below.
Tentative Programme
|
Key Milestones |
Date (month/year) |
|
Tendering of Works Contract |
mid 2009 |
|
Construction Contract Award of Works Contract |
end 2009 |
|
Completion of All Works Contracts |
end 2012 |
2.8
Consideration of
Alternative Construction Methods and Sequencing of Works
Comparison of the
Environmental Benefits and Dis-benefits of Alternative Construction Methods
U-channels / Channel
2.8.1
Potential alternative construction method includes offsite
precast structures for U-channels, mass concrete retaining wall and concrete blocks.
The benefit involves less environmental nuisance to nearby sensitive receivers during
construction. However, for the excavation and construction of the open channel,
there are no other suitable or practicable alternative construction methods.
2.8.2
In order to match with existing topography, off-site precast
structures are generally not preferable, impractical or cost effective. Although
on-site construction could pose environmental nuisance in terms of noise, dust
and water quality, the duration and magnitude of the impacts are temporary and
localized given the short section and small size U-channels to be constructed.
It can be mitigated by appropriate measures. Precast mass concrete retaining
wall is not practicable as heavy lifting appliances will be required to lift
even a small section. It will be dangerous and will hinder the construction
process. Therefore, conventional construction method of in-situ construction of
channel is considered more suitable and practical.
2.8.3
Precast concrete blocks, which are relatively smaller and
easier to handle, could employ off-site precast technique.
Pedestrian Bridges (Box Culvert)
2.8.4
Similar to the above elaboration and due to narrow access, off-site
precast structures are not preferable, impractical or cost effective. Although
on-site construction could pose environmental nuisance in terms of noise, dust
and water quality, the duration and magnitude of the impacts are temporary and
localized given the short section and small size crossings to be constructed.
It can be mitigated by appropriate measures. Therefore, conventional
construction method of in-situ construction of pedestrian bridges structure (box
culvert) is considered more suitable and practical.
Box Culvert / Drainage Pipes
2.8.5
The proposed triple cell box culvert will replace the
existing undersized single-cell culvert underneath
2.8.6
The construction of the box culvert and drainage pipes will
employ common cut and cover construction method. Possible alternative
construction methods include the use of ‘no-dig’ trenchless techniques such as
pipe jacking, micro-tunneling. Such
techniques have the advantage of less traffic disruption (for road crossings), and
less construction noise & dust arising from trenching works. However, the
trenchless techniques were considered not practicable for the Project due to
the following reasons:
§
only short section of pipes are proposed hence not cost
effective;
§
presence of sewers and other utilities which will need to be
diverted;
§
drains are close to existing road/track level and there are
inadequate cover for safe operation of jacking or tunneling;
§
drains need to connect to the invert levels of upstream and
downstream sections;
§
ground treatment such as grouting may be necessary for
stability purpose, which will generate additional wastewater;
§
need several working pits to accommodate bends and curves;
and
§
requires specialized skills during construction.
2.8.7
Although on-site construction could pose environmental
nuisance in terms of noise, dust and water quality, the duration and magnitude
of the impacts are temporary and localized given the short section and small
size cross road box culvert and drainage pipes to be constructed. It can be
mitigated by appropriate measures. Therefore, conventional construction method
of in-situ construction of box culvert / drainage pipes is considered more
suitable and practical.
Justification of
the Construction Methods
2.8.8
The construction methods to be employed are all conventional
construction method. Impacts on the environment from any construction
activities are inevitable. However, the impacts are expected to be localized
and temporary, which can be readily mitigated by well proven environmental
protection measures. As assessed in the respective assessment chapters, no
adverse environmental impact is expected during construction phase of the
Project.
Environmental Implications
of Potential Flooding at Downstream Areas during Wet Season
2.8.9
The proposed construction works will widen the existing stream
and thereby increase their capacities. In order to maintain the existing
standard of flood protection during construction, it is necessary to phase the
works so that the receiving downstream section has sufficient capacity to receive
the design flow. It is therefore necessary that construction works along a
given reach be carried out from the downstream towards upstream. However, the
contractor may start at isolated sections to suit their allocation of resources
and the availability of site access provided that adequate measures are in
place to ensure the existing flood protection level is maintained throughout
the construction of the works.
2.8.10
It will be necessary to carry out careful phasing of the
works to re-connect existing incoming watercourses to ensure satisfactory
diversion of flows without the risk of flooding to adjacent areas especially
during wet seasons. It is
recommended that the connection of existing watercourses should be carried out
during dry season to minimize flooding risks.
2.8.11
Adverse effect on hydraulic performance of the channel during
construction can be managed and controlled to a minimum by well construction
planning and adopting proper construction methodology, monitoring and
contingency measures. According to the DIA conducted under this Project, no
adverse impacts to the drainage system and catchment characteristics are
anticipated.
Release of Heavy
Metals during Excavation Works
2.8.12
The water quality impact assessment (Chapter 5) indicated the release
of sediment including contaminated sediment with high heavy metal content can
be controlled by adopting proper working practices such as provision of
containment structure surrounding the active excavation area, the use of closed
grab excavator and careful handling and storage of the excavated sediment. As
stated above, it is preferable to undertake the construction works in
particular excavation of the stream bed in the dry season to avoid inadvertent
runoff during rainstorm.
2.9
Project
Interfaces / Concurrent Projects
2.9.1
According to information provided by various works
department, only one project is planned in the Study Area.
·
PWP Item No. 235DS – Yuen Long and Kam Tin Sewage and
Sewerage Disposal (under Agreement No. CE30/2006 (DS)) – Works Package A: 2A-3T
– Lau Fau Shan Trunk Sewerage.
2.9.2
This DSD project consist of construction of a sewage pumping
station at Lau Fau Shan near Deep Bay Grove and associated laying of rising
mains along Deep Bay Road. The tentative programme is mid 2009 – 2012. As the
works involve construction of a local sewage pumping station with a site area
of about 25 m x 18 m and is about 150 m away from the proposed Project, no
adverse cumulative environmental impact is expected.
2.9.3
Notwithstanding, continuous
liaison will be conducted with all works department and utilities companies to
ensure adequate phasing with each concurrent projects is taken into
consideration, if interfacing does eventually occurs.
2.10
Scope of Works
during Operational Phase
Maintenance of
the Channel
2.10.1
Maintenance would be necessary for the proposed channel to
remove excessive silts, vegetation, debris and obstructions in order to
maintain its hydraulic performance and structural integrity. Siltation will
generally be allowed to accumulate and removal of excess silt would be carried
out at locations only if it would impede water flow. Such small scale
maintenance would require only light mechanical equipment such as a small
loader and/or a small crane truck. Hand-held equipment will be used for
vegetation removal. Little or no maintenance will be necessary for the natural
stream bed section of the channel.
2.10.2
A mangrove zone with an area of about 1,500 m2
consisting of existing retained mangroves (~800 m2) and compensated
mangroves (~700 m2) will be established at the northern downstream
portion of the channel. A strip of 600 mm wide (Grade 200) rip-rap will be used
to delineate the mangrove zone with the channel zone. The location of the
mangrove zone and rip-rap is shown in Figure 2.7. It is considered that the
mangroves within the mangrove zone will need no maintenance in the long
term. However, mangroves found
outside the mangrove zone and within the proposed channel would be removed as
they will affect the hydraulic capacity of the channel. DSD will need to replace
the rip-rap when it is found to be damaged. This can be undertaken as part of
the routine maintenance works for the channel.
Maintenance Dredging
2.10.3
The Project is not expected to severely affect the sediment
deposition pattern at the channel outfall and nearby intertidal mudflats of
Operation of the Access
Road, Viewing Point and Carpark
2.10.4
The proposed access road along the northern bank of the
channel, viewing point and carpark will be open for public usage. Vehicles
longer than 7 m will be restricted from using the access road with the
exception of maintenance and emergency vehicles. This will ensure no adverse traffic
related environmental impacts to the villagers during operation. Routine
maintenance of these facilities will be carried out as necessary or following
normal established practice by the relevant maintenance authority as describe
in the next section.
2.11
Management and Maintenance
Responsibilities
2.11.1
The works included in this Project will be handed over to
relevant Government Departments for management and maintenance upon completion
of the construction contracts. For the purpose of this EIA Study, the project
proponent (CEDD) will assume to be responsible for the works and mitigation
measures until an agreement is reached between CEDD and relevant parties on the
management and maintenance of the works and mitigation measures.
2.11.2
The scope and maintenance responsibilities of individual Government
Departments are generally well established. Table 2.6 outlines the
proposed maintenance responsibilities based on current Government practices and
relevant Technical Circulars.
Proposed Management and Maintenance
Responsibilities
|
Works |
Maintenance Party+ |
Management Party+ |
|
Drainage Systems/ Natural Watercourses |
|
|
|
1.
Stormwater
drainage systems including channel wall, channel base slab, maintenance
access ramp, maintenance road within channel, existing Hang Hau Tsuen Stream,
crossroad drains, u-channels at roadside slope toe. |
DSD |
DSD |
|
2.
U-channel
and catchpit at the roadside slope toe |
DSD |
DSD |
|
3.
Triple
cell box culvert across |
HyD |
HyD |
|
Landscaped areas |
|
|
|
1.
Landscape
softworks on roadside slope, on unallocated land and at the amenity area
adjacent to carpark |
LCSD |
DLO |
|
Footpath and carriageways |
|
|
|
1.
2 m
wide footpath at north embankment including Type 2 railing, 3.5 m wide
carriageway including vehicular parapet, carpark and amenity area. |
HyD / FEHD* |
TD |
|
2.
Footbridges
across Hang Hau Tsuen Channel including railing |
HyD / FEHD* |
TD |
|
3.
Retaining
wall adjacent to carpark and amenity area |
HyD |
HyD |
|
4.
Footpath
and village access at south embankment, and village access at north
embankment, including railing on concrete footings. |
DO |
DLO |
|
Unallocated land |
|
|
|
1.
Unallocated
land at south and north embankments |
N/A |
DLO |
|
Mangrove Zone including rip-rap |
|
|
|
1.
Mangrove
zone** |
N/A |
DSD |
|
2.
Rip-rap
to delineate mangrove zone and channel zone |
DSD |
DSD |
Notes:
1.
The departmental responsibilities for the maintenance of the
completed channel as well as unchannelised stream course including meanders
will follow:
·
The schedule of responsibilities as stipulated in Appendix A
of ETWB TCW No. 14/2004 – Maintenance of Stormwater Drainage Systems and
Natural Watercourse.
·
DSD’s Stormwater Drainage Manual Table 20 – Schedule of
Maintenance Responsibilities for a Completed Main Drainage Channel Project.
2.
The departmental responsibilities for the maintenance of the vegetation
and hard landscape features will follow the schedule of responsibilities as
stipulated in Appendix A of ETWB TCW No. 2/2004 – Maintenance of Vegetation and
Hard Landscape Features.
+ CEDD will assume to be responsible for the
works and mitigation measures until an agreement is reached between CEDD and
relevant parties on the management and maintenance of the works and mitigation
measures.
* Road Cleansing by
FEHD
** It
is expected that the retained and compensated mangroves within the mangrove
zone would need no maintenance in the long run.
2.12
Continuous Public
Involvement
2.12.1
The process of Continuous Public Involvement (CPI) through
public consultation and site visits has been undertaken as shown below (Table
2.7).
Summary of Continuous
Public Involvement
|
Relevant Parties |
Process |
Date |
|
Ha Tsuen Rural Committee |
meeting |
11 May 2007 |
|
Yuen Long District Councilor (also
Ha Tsuen Rural Committee member) |
Letter to CEDD |
22 June 2007 |
|
CEDD reply letter to District Councilor |
5 July 2007 |
|
|
Green Groups §
Conservancy Association (CA) §
Friends of the Earth (FoE) §
Green Power (GP) §
§
Kadoorie Farm and Botanic Garden (KFBG) §
World Wide Fund for Nature |
consultation paper |
5 December 2007 |
|
site visit |
18 December 2007 (attended by GP, KFBG and WWF) |
|
|
follow up meeting and site visit |
16 June 2008 (attended by GP and KFBG) |
|
|
Ha Tsuen Rural Committee |
meeting |
2 January 2008 |
|
Yuen Long District Council – Environmental
Improvement Committee |
meeting |
10 March 2008 |
|
Oyster Farmer Association and Trade
Association §
YL District Council member – Mr. Tang Ka-leung (Convener) §
Lau Fau Shan Chamber of Commerce (流浮山商會) §
New Territories Lau Fau Shan Oyster Industry Association (新界流浮山蠔業總會) §
New Territories Oyster and Aquatic Products Industries United
Association (新界蠔業水產聯合會) §
The Right & Concern Association
For Deep Bay Oystermen (后海灣蠔民權益關注會) |
consultation paper |
15 August 2008 |
|
meeting |
24 September 2008 |
2.12.2
The channel details were reviewed and revised according to
the comments and recommendations from the relevant parties. The main comments
and recommendations are summarized in Table 2.8 below.
Summary of Public
Consultation
|
Relevant Parties |
Main Comments /
Recommendations |
Responses / Outcomes |
|
Ha Tsuen Rural Committee |
Request for adequate width for
footpath and bridges. |
The proposed footpath and bridges
will adopt standard HyD design. |
|
Yuen Long District Council member
(also Ha Tsuen Rural Committee member) |
Request to provide viewing point
and carpark for visitors to enjoy the scenery of |
A viewing point and carpark has
been incorporated into the design. Restricting the number of carpark spaces
and banning vehicles longer than 7 m from using the facilities will ensure
minimal traffic-related impacts to the nearby villagers. The proposal was
circulated to the District Council, the Chairman expressed no further
comment. |
|
Yuen Long District Council –
Environmental Improvement
Committee |
The members expressed concern about
the use of gabion bank whereby the crevices could potentially harbour rodents
and other pests leading to unacceptable health and hygiene issues. They also
expressed concern that the land resumption limit should be minimal in order
to minimize the effects on existing residents. |
The latest design involves using
stone pitch finishing wall as the channel bank. Both Green Groups and DSD
have no adverse comment on such design. The channel design has essentially
followed the existing stream alignment to minimize land resumption. |
|
Green Groups |
The Green Groups emphasized the
need to preserve the mangroves stands at the downstream section of the
proposed channel. The extent of natural stream to be retained should include
the mangrove colony along both sides of the existing stream beds. |
The latest design includes a bypass
pipe to divert flood flow to the outfall thereby minimizing works at the
downstream portion to the stream. The downstream portion of the stream bed
will be retained to preserve the existing mangroves. |
|
The Green Groups have no adverse comments
on the loss of mangroves (with adequate replanting). They advised that the
substratum should be at the appropriate level depending on the species to be
compensated. |
A small portion of the mangroves
will inevitably be affected as some construction works will need to be
carried out at the stream. Compensatory mangrove planting of at least 1:1
ratio will be provided. |
|
|
The Green Groups recommended the
use of natural substrates (sand, soil or mud) as channel bed material for
parts of the upstream section to allow natural recolonization of vegetation
within the channel. No specific planting is necessary. |
Parts of the channel toe zone at
the upstream section will be filled with 200 mm thick original stream bed
material to allow natural re-establishment of riparian vegetation. The low
flow channel at the upstream section will be filled with a layer of 100 mm
thick original stream bed material on top of the rip-rap lining. |
|
|
Oyster Farmer Association |
They have no major concerns on
environmental impact arising from the Project and agreed that the Project
will not affect the oyster beds and ecology in |
Their acceptance was acknowledged.
To ensure additional protection, monitoring stations will be provided near
the oyster bed to monitor the water quality during construction. |
|
However, they expressed concerns on
the poor water quality of the upstream flow that are being discharged into |
It was explained in the meeting
that the quality of the incoming water was outside the control of the project
proponent and outside the scope of this study which is to provide flood
protection to Hang Hau Tsuen. Nonetheless, the proposed drainage system for
the Project will implement measures to screen some of the rubbish and grits
from Hang Hau Tsuen. Long term improvement measure will be gradually
undertaken by the Government such as providing proper sewerage systems to the
unsewered villages in the area. |
|
|
The YL District Council member reminded
that adequate landscaping works should be provided to the Project. |
Compensatory tree planting will be
provided for the loss of trees. Amenity planting will be provided on
embankments, roadside planters, channel bund and at the viewing point. |
2.12.3
Overall, the Ha Tsuen Rural Committee and Yuen Long District
Council members, supported the proposed Project as it would bring relieve to
the flooding in Hang Hau Tsuen. No adverse comments were received from the
Green Groups and oyster farmer association.
2.13
Summary of the
Final Channel Design Recommendations
2.13.1
The recommended channel design has essentially followed the
avoidance and minimization approach promulgated in the Government’s technical
circulars and guidelines on protection of natural rivers and streams.
Consideration and evaluation of the merits of the Project for flood
alleviation, the various alternative drainage design, the various alternative
means to achieve flood alleviation, and the various alternative construction
methods presented above have demonstrated the Project will not adversely affect
the Coastal Protection Area, Hang Hau Tsuen stream, mangrove colony, nearby
fishpond habitats and the intertidal mudflats. The detailed environmental
impact assessment presented in the subsequent chapters has also demonstrated
the adopted channel design can avoid and minimize adverse environmental
impacts.
2.13.2
Distant flood bank is proposed to minimize the disturbance to
the existing stream bed and mangrove colony at the downstream section of the
stream. For the upstream section, the proposed channel is straightened to avoid
disturbance to the existing fishponds and houses near the northern embankment.
Bypass pipe is proposed to collect and convey flood flow to the channel outfall
during heavy rainstorm, thereby channelization works for the downstream section
can be substantially scaled down.
2.13.3
The channel banks will be constructed to a higher level to
cater for the design flow. Surface runoff from Hang Hau Tsuen areas behind the
embankments will be intercepted by a system of catchpits and U-channels and
discharged into the proposed channel via pipe works through the embankment.
Flap valves will be installed at these drainage outlets to prevent back flow of
sea water during tidal surge. Channel lining include stone finishing channel
bank with natural substrates as channel bed to allow natural re-establishment
of vegetation and recolonization of benthic communities. This will enhance the
visual appearance and landscape feature as well as the ecological value of the
channel.
[1] The original DIA Study was conducted in 1995 for the whole Lau Fau Shan Development. A review of the DIA was conducted in July 2007 taking into account changes in land use, drainage strategy, environmental requirements and development in the drainage catchment of the Hang Hau Tsuen channel.