2.1.1
The project is entitled
“Drainage Improvement in
2.1.2
The Preliminary Project Feasibility Study (PPFS) completed in
August 2001 under the DMP Study concluded that rapid developments in the study
area have resulted in higher runoff and some existing drainage systems are
inadequate in respect to flood protection capacities. Consequently, severe
flooding occurs in these low-lying areas during heavy rainfall.
2.1.3
The proposed drainage improvements to designated project (DP)
channels under the Project involve the construction of the secondary local drainage
channels at Lin Ma Hang in the
channels. The DP channels are assessed in this EIA Study.
2.1.4
An Environmental Review (ER) was
completed in May 2000 under the DMP Study to assess potential environmental
impacts of the proposed drainage improvement works. The ER concluded that
several components of the proposed drainage improvement works are regarded as a
DP under the Environmental Impact Assessment Ordinance (EIAO).
2.2
Location and Description
of the Project
2.2.1
The drainage improvement work for Package C involves
construction of river channels in the identified flood-prone regions. The
proposed works consist of improving the river channels to convey a 10 year
return period event including freeboard within the channel banks meeting the
flood protection standard for villages. Details of the proposed works are
provided below and shown in Figures 2.2 – 2.5.
Lung Yeuk Tau and
Kwan Tei South Region
2.2.2
The proposed works within this region consist of widening and
formalizing the existing channel, LYT04.
2.2.3
The proposed channel, LYT04, was previously identified as a DP.
Subsequent to design changes, LYT04 is confirmed as not a DP. Further
elaboration is provided below.
2.2.4
LYT04 was originally proposed as a 2 m wide open concrete channel
in the DMP Study (Figure 2.6). It was considered as a DP by virtue of Item
I.1(b)(ii) of the EIAO as it discharges into an area within 300 m of an
existing site of cultural heritage (the Entrance Tower of Ma Wat Wai) and an
EIA Study Brief covering LYT04 together with other DPs in Package C works was
obtained.
2.2.5
However, during the public consultation, the villagers of
Shung Him Tong objected the previous proposal of open channel along private
lands and requested alternative options. Upgrading of the existing underground
single-cell box culvert is proposed to convey flows from upland catchment to
the tributary of Ma Wat Channel near Ma Wat Wai. The existing underground single-cell
box culvert will be replaced by a new 107 m long twin-cell box culvert of
internal cell size, 2.3 m wide by 2.3 m high underneath the access road
connecting Ma Wat Wai and Shung Him Tong area (Figure 2.7). The downstream
interfaced section of Ma Wat Channel and its tributary have already been
trained. There were no further objections by the villagers on the revised
design.
2.2.6
Based on revised scope and design of LYT04 which does not
involve drainage channel works, it was confirmed that the works is not regarded
as DP under the EIAO and hence would not be subject to the provision of the
Ordinance. Relevant correspondence pertaining to LYT04 is enclosed in Appendix
A2.
Man
2.2.7
Works within this region consist of upgrading and improving
the River Indus, its tributaries and two roadside drains within Man Uk Pin
region. The works are subdivided into five reaches, three of which are DPs as
it discharges into an area within 300 m from the nearest boundary of an
existing Conservation Area land use zoning.
2.2.8
The original preliminary channel design consists of channels
with concrete base and geotextile reinforced grass as banks (Figure 2.8).
Further design changes has been undertaken to align with the recommendations in
the Government’s technical circulars on protection of natural streams and
rivers and to take into account comments received during public consultation.
2.2.9
The proposed channel MUP03
(Figures 2.2 & 2.3) is located to the north of
2.2.10
The proposed channel MUP04B
(Figure 2.3) is located to the west of
2.2.11
Surface roadside drains (MUP04B) and a trapezoidal gabion
channel (MUP04A) are proposed to collect flow from upland and Loi Tung areas.
The existing local stream at Loi Tung is to be upgraded as the trapezoidal
channel (MUP04A) of size 4.5 m (base width) by 1.75 m (depth) which will tie
into River Indus at the downstream via three proposed 1950 mm diameter crossroad
drainage pipes underneath Sha Tau Kok Road. Surface drains (MUP04B) consist of U-channels
and carrier pipes to be constructed alongside the southern verge of
2.2.12
The proposed channel MUP05
(Figures 2.2, 2.3 and 2.5) is located at the existing River Indus at the
southwest of Loi Tung Village running along the northern verge of Sha Tau Kok
Road (Wo Hang Section). The improvement to the existing River Indus is proposed
from its confluence with MUP01 and MUP02 extending to the existing culvert
underneath
2.2.13
The other proposed channels in the region (MUP01 and MUP02)
are non-DP. These are included in the EIA for potential cumulative impacts.
Lin Ma Hang Region
2.2.14
The DMP Study recommends a flood by-pass channel of
approximate 250 m in length and average 4 m wide concrete base to alleviate
flooding at Lin Ma Hang (Figure 2.9). During the Adoptive Review Stage, an
alternative scheme of raising local access bridges and bank levels was
proposed, thereby achieving the objective of flood prevention without taking up
the potentially valuable wetland area or Lin Ma Hang stream for the
construction of the flood by-pass channel. This is discussed further in the
subsequent sections.
2.2.15
The adopted proposed drainage improvement works along the
existing Lin Ma Hang stream, LMH01
(Figure 2.4), consist of replacing the existing sub-standard screen with a
combination of flap valve and security grille underneath the
2.2.16
The general location plan of the proposed DP channels is shown
in Figure 2.1. The major items of DP channel works as assessed in the EIA are
summarized in Table 2.1 below and shown in Figures 2.2 – 2.5.
Summary of Works for the
Proposed Designated Projects of Channel Improvement Works
|
Channel |
Proposed Works |
Extent of Works |
||
|
Approximate Length |
Approximate Width |
Approximate Depth |
||
|
MUP03 |
U-channel (Diameter = 0.6m) |
150m |
- |
- |
|
Drainage pipe (Diameter = 0.6m) |
12m |
- |
- |
|
|
Box culvert connecting MUP05 |
28m |
3m |
1m |
|
|
Trapezoidal channel with gabion walls and mattress
lining |
93m |
Top = 2.4m – 2.7m |
1.2m – 1.5m |
|
|
Re-provision of crossings |
One footbridge |
|||
|
MUP04A |
Trapezoidal channel with gabion walls and mattress
lining |
163m |
Top = 6.3m |
1.75m |
|
Drainage pipes (Diameter
= 1.95m) |
21m |
- |
- |
|
|
Re-provision of crossings |
Two vehicular bridges |
|||
|
MUP04B |
U-channel (Diameter = 0.9m) |
58m |
- |
- |
|
Drainage pipes (Diameter = 1.35m) |
138m |
- |
- |
|
|
MUP05 |
Trapezoidal channel with gabion walls and mattress
lining |
287m |
2m – 3m |
2m – 2.8m |
|
Trapezoidal channel with gabion walls and mattress
lining |
54m |
Top = 15.4m |
2.5m |
|
|
Two-stage channel with existing natural river bed
retained |
554m |
Top = 15.4m |
2.5m |
|
|
Trapezoidal channel with gabion walls and mattress
lining |
69m |
Top = 20.3m |
2.2m |
|
|
Box culvert |
50m |
3m |
2m |
|
|
Re-provision of crossings |
Five footbridges, four
vehicular bridges |
|||
|
LMH01 |
Gabion channel and localised bank improvement |
190m |
5m |
1.5m |
|
Re-provision of crossings |
Three vehicular bridges |
|||
Notes:
Other ancillary works include provision of inlet/outlet
pipes, maintenance ramps & accesses and final landscaping works.
2.3
Consideration of
the Merit of the Proposed Project for Flood Alleviation
Standard of Flood Protection
2.3.1
The current standard of flood protection of the watercourses
is very low, with the majority having a standard of protection of 1 year or
less. These watercourses were highlighted in the DMP Study as having
insufficient flow capacity leading to localized flooding and causing danger to
villagers and risk of economic loss.
2.3.2
In view of the fact that most of the proposed secondary
channels under Package C project are currently located in relatively rural
areas, the sizes of these secondary channels are designed to achieve the
standards set out in the Stormwater Drainage Manual for village drainage with
the current sizes of the downstream channels under the existing scenario for
the planned development conditions (equivalent to the DMP’s Planned Scenario).
This design requirement could avoid unnecessarily large sizes of the proposed
channels due to the limitation on capacities of the downstream channels /
rivers. As such, upgrading works to the Package C channels shall be designed to
achieve a 10 year design return period flood protection standard under the
existing scenario for the planned development conditions for villages.
MUP Channels
2.3.3
Extensive hydraulic and hydrological predictions were carried
out at Man Uk Pin area. Figure 2.10 shows the flood extent map under a 10 year
return period event with and without the proposed project at Man Uk Pin. As
shown in the figure, without the proposed channels, there would be flooding
ranging from 0.1 m deep to more than 2 m deep during heavy rainstorm.
MUP04A
2.3.4
Site survey reveals flooding occurs at both the
village area and the agricultural fields further upstream. Site visit also indicates
that while the upstream portion consist of active and inactive agricultural
fields, the downstream portion has been heavily disturbed from village
development (portion of the stream has been culverted for an access road) with construction
debris found dumped at the stream banks. It is therefore proposed to
channelised the heavily modified downstream portion with gabion channel while
leaving the upstream portion untouched to minimize impact on the stream
habitats, the aquatic life therein and the wildlife that depends on the stream.
MUP05
2.3.5
To minimize ecological impact to the stream at MUP05,
‘two-stage’ channel design has been adopted whereby natural stream banks and
bed will be retained.
2.3.6
Hydraulic modelling results revealed that the extent
of improvements for the channels MUP01 and MUP02 (Non-DP elements but
identified as Ecologically Important Streams (EIS)) could be modified due to
the direct beneficial effect on flow conveyance after upgrading the channel
MUP05 at their immediate downstream. The flooding situation of MUP01 and MUP02
can be further improved by removing the existing bottleneck at their confluence
with the proposed MUP05 near
LMH01
2.3.7
Lin Ma Hang is located to the east of Wang Lek in the
2.3.8
Some past flooding records at Lin Ma Hang areas are
listed below.
-
There was a storm event on 24 May 1998, which caused
the collapse and closure of
-
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 northern
-
Rain bands associated a trough of low pressure caused
exceptionally heavy rainfall to the north and northwestern part of the
2.3.9
Previous surveys indicated that Lin Ma Hang consists
of high ecological value of lowland streams for freshwater fish, and lowland
habitats for bats. Currently, there is a proposal on designation of the Lin Ma
Hang stream as a Site of Special Scientific Interest (SSSI) due to the high
diversity of freshwater fish of high conversation concern. The nearby abandoned
Lin Ma Hang lead mine is a designated SSSI for bats.
2.3.10
Alternative means to achieve flood alleviation were
therefore considered necessary for Lin Ma Hang stream to avoid direct
ecological impacts on the aquatic life, wildlife and habitats that depend on
the stream and its associated wetlands. Lin Ma Hang is the nearest village to
Lin Ma Hang stream. According to the 2001 Population Census, the population at
Lin Ma Hang is about 144. The majority of the village houses are located on the
upland side. Preliminary assessment indicates that without constructing the
bypass channel (proposed under the DMP Study), the possible flooding extent
would only be limited to the abandoned agricultural lands in the vicinity of
Lin Ma Hang stream. Some village houses, however, could still be affected by
flood water.
2.3.11
Following extensive consultation and hydraulic
analysis, it is proposed that the stream be retained to avoid posing
unacceptable environmental impacts (especially ecological impact).
2.3.12
The hydraulic performance of the existing watercourse has
been studied. The existing fields on either side of the watercourse could act
as flood plain, providing additional storage during flood peaks (Figure 2.11).
There are local constraints at existing footbridges just upstream of the
village which have low soffits, these inhibit the hydraulic performance of the
existing channel. Whilst the main village itself is unaffected by flooding,
access is limited when these footbridges are submerged (Figure 2.11). Rising of
local footbridges and bank levels will enable stream flow to pass through to
downstream reaches without adverse effect to the village.
2.3.13
Based on site observation and discussions in joint
site visits with DSD, DO/N and the Village Representatives, the local residents
of Lin Ma Hang are more concerned about overflow at various stream crossings
and blockage of the Lin Ma Hang stream outlet, rather than the flooding in the
abandoned fallow field, which is the floodplain of the stream. Generally, most
village houses are located on sufficiently high ground free from damage in
previous flooding events. However, the houses near stream crossings may still be
affected by localised flooding when these crossings are flooded (Figure 2.11).
A pragmatic approach is therefore adopted in formulating the flood protection
measures at Lin Ma Hang.
2.3.14
The proposed drainage measures comprise raising the
existing stream crossings to allow flood flow under the design standard to pass
through, improvement to unstable stream banks, improvement to the security
grille at the outlet of Lin Ma Hang stream and installation of flood detection
& alarm systems which are elaborated further in the subsequent sections.
Justification and Benefits
of the Project
2.3.15
The DMP Study investigated the need to improve secondary and
local drainage systems 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.
2.3.16
The current standard of flood protection of the watercourses
is very low, with the majority having a standard of protection of 1 year or
less. These watercourses were highlighted in the DMP Study as having
insufficient flow capacity leading to recurring flooding problems. Upon
completion of the Project, the standard of protection will be increase to 1 in 10
years.
2.3.17
The population (based on the Population Census in 2001)
expected to be directly benefited by the proposed drainage improvement will be
about 780.
§
MUP channels – 634
§
LMH01 – 144
Consequences
of Not Proceeding, ‘No-go’ Scenario
2.3.18
Consequences of not proceeding with the Project include recurringe
flooding in the low lying areas during severe rainstorms resulting in danger to
life, property damage, economic loss and inconvenience to the public.
Previous
Studies and Policy Acceptance
2.3.19
The DMP Study was one of the seven DMP studies under the
Governor’s Policy Address of 1995 to cover all flood prone areas of the territories.
Drainage Services Department completed a PPFS for the Project in August 2001.
The Project was included in Category B as Item No. 4119CD in November 2001. The
Preliminary Environmental Review (PER) conducted for the PPFS was reviewed by
EPD and was considered satisfactory from their point of view. It was considered
necessary to carry out additional environmental studies for the project at a
later stage.
The Need for the Project
2.3.20
The drainage basins in
2.3.21
The current standard of flood protection of the watercourses
is very low, with the majority having a standard of protection of 1 year or
less. Flooding has occurred repeatedly in the study area leading to property
damage, economic loss and disruption to everyday life.
2.3.22
Hydraulic analysis indicates that backwater from the main
rivers / channels causes local flooding in their tributary drainage systems at
upstream. Also, developments within floodplains have reduced the flows and
storage capacities of the existing natural drainage systems.
2.4
Consideration of
Alternative Means and Avoidance of Environmental Impacts
Alternative
Solution to Flooding Problem at Lin Ma Hang
2.4.1
Under the DMP Study, a bypass channel of approximately
250 m in length and 13 m in width (with concrete base) was to be provided for
the prevention of flooding at Lin Ma Hang (Figure 2.9).
2.4.2
As described in the previous sections, Lin Ma Hang
area is of high ecological value due to the high diversity of freshwater fish
(of high conservation concern) found in Lin Ma Hang stream and due to the
lowland habitats for bats.
2.4.3
Based on extensive consultation and hydraulic
analysis, it is proposed that the stream be retained to avoid posing
unacceptable environmental impacts (especially ecological impact).
2.4.4
However, there are concerns about overflow at various
stream crossings and blockage of the Lin Ma Hang stream outlet. A pragmatic
approach is therefore adopted in formulating the flood protection measures at
Lin Ma Hang. The proposed measures would comprise the following:
Improvement to Stream
Crossings
2.4.5
The soffits of the existing stream crossings are too
low to allow flood flow to pass during severe rainstorms. The crossings would
be demolished and re-constructed to a higher level to allow flood flow at the
design standard to pass through. Proper safety railings would also be
installed. A total of three vehicular crossings will be demolished and raised.
Improvement to River
Banks
2.4.6
Repair and strengthening of damaged or eroded stream
banks with gabions would be implemented. It is worthy to note that some of the
banks are close to village houses and any further deterioration would pose a
safety hazard to the residents.
2.4.7
After site meetings, the Green Groups supported the
use of gabion for the bank improvement works including the downstream end of
the stream as it would enhance the ecology of the current concrete lined stream
banks and bed.
Improvement to the
Security Grille at the Outlet of Lin Ma Hang Stream
2.4.8
The existing security grille was engineered and
installed in accordance with the recommendations in the DMP Study. As advised
by DSD/MN, this design failed to resolve the problem of blockage to the flow in
the past rainstorms. The existing grille has trapped tree branches, debris, and
silt washed down from the water course, contributing to historical flood
events. Improvement to the current security grille at the outlet to
Installation of Flood
Alarm System
2.4.9
Flood alarm system would be provided for the Lin Ma
Hang villagers. The flood level detector would be located at the outlet of Lin
Ma Hang stream. Flood alarm sirens would be located at the community hall of
the village and would be activated once the flood level is reached. The alarm
signal would also be sent to DSD/MN, Ta Kwu Ling Police Station, DO/N and
Ambulance Command/NTE of FSD. The system will be further reviewed in the
detailed design stage.
2.4.10
Based on the above minimized improvement works at
LMH01, there will be no direct adverse ecological impacts on the Lin Ma Hang
stream, its tributaries and associated wetland as well as the aquatic life,
wildlife therein and habitats that depends on the stream.
Alternatives to
Avoid
2.4.11
Section 3.3.1 of the EIA Study Brief has specifically
indicated the consideration of alternative channelisation of Loi Tung stream (MUP04A).
The DMP Study originally proposed a 340 m length channel with concrete base
(Figure 2.8). Site survey reveals that flooding occurs at both the village area
and the agricultural fields further upstream. Site visit also indicates that
the downstream portion has been heavily disturbed from village development
(portion of the stream has been culverted for an access road) with construction
debris found dumped at the stream banks. The ecological value of this
downstream section is considered as very low. The upstream portion of moderate
ecological value mostly flows adjacent to abandoned agricultural fields. With
due consideration of the potential ecological value of the upstream portion of
MUP04A, the latest adopted design avoided the channelisation of this upstream portion
altogether (approximately 180 m). Only the downstream portion near the village
area would be channelised using gabions. Localized flooding of the inactive
agricultural land would remain at the upstream portion (Figure 2.10).
2.4.12
With the avoidance of channelising the upstream
portion, there will be no direct adverse ecological impacts on the Loi Tung
stream, its tributaries and associated wetland as well as the aquatic life,
wildlife therein and habitats that depends on the stream.
Alternative
Design to Channels in Man
2.4.13
The DMP Study originally proposed a 14 m wide channel
with concrete base and geotextile reinforced grass as bank (Figure 2.8). The
middle section of MUP05 consists of semi-natural stream bank with riparian
vegetation and is of moderate ecological value. The upper section and
downstream end are heavily modified and lined with concrete and is of very low
ecological value. To minimise ecological impact as a result of the proposed
channelisation works, a ‘two-stage channel’ design has been adopted. Some
section of the banks would be retained. Likewise, portion of the stream bed
would be retained or be provided with natural bedding to minimise potential ecological
and visual impacts.
2.4.14
Hydraulic modelling results revealed that the extent
of improvements for the channels MUP01 and MUP02 (Non-DP elements but
identified as Ecologically Important Streams (EIS)) could be modified due to
the direct beneficial effect on flow conveyance after upgrading the channel
MUP05 at their immediate downstream. The flooding situation of MUP01 and MUP02
can be further improved by removing the existing bottleneck at their confluence
with the proposed MUP05 near
2.4.15
With the minimisation of channelisation works and
adopting environmental friendly ‘two-stage’ channel design, there will be no
direct adverse ecological impacts on MUP05 stream, its tributaries and
associated wetland as well as the aquatic life, wildlife therein and habitats
that depends on the stream.
Relevant
Technical Circulars / Guidelines
2.5.1
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
2.5.2
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.
2.5.3
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).
2.5.4
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
2.5.5
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 river channels
should have adequate capacities to convey design flood flows within bank to
achieve the design flood protection standard.
Ÿ
Public safety – bank stabilisation work is required for
unstable banks and where houses and structures are located very close to the existing
river banks.
Ÿ
Existing conditions of the river banks – some sections of the
existing river banks have loose surfaces and are prone to erosion and scouring.
Ÿ
Land constraints – physical features such as road carriageway
at one side of the existing river 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 channels 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.
2.6
Consideration of
Alternative Drainage Design
Alternative
Alignment
2.6.1
The proposed alignment as recommended under the DMP generally
follow the alignment of existing watercourses in order to minimise land
resumption requirements wherever possible (thus reducing disturbance and
inconvenience to the public) and to facilitate interception of incoming
watercourses. The alignment is also dictated by physical features constraint
such as
2.6.2
Other alternative alignments were evaluated but are
considered not practicable, cost effective or would result in adverse
environmental impacts.
2.6.3
The upstream invert of the proposed channel has been designed
to match with the level of the existing watercourse, whilst the downstream is
governed by the primary channel and the invert levels of side connections.
2.6.4
In general, the bank level of the proposed channel will match
with the adjacent ground levels thus allowing free surface runoff by gravity
into the proposed channel. Invert levels of the proposed channels also take due
consideration of side connections.
Alternative
Location of Stormwater Drains and Discharge Points
2.6.5
The proposed stormwater drains including cross road drains
consist of existing 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.
2.6.6
The proposed MUP channels eventually need to discharge into
the recently upgraded River Indus. No alternative discharge points for the
proposed channels are therefore suitable or practical.
Design of
Channelisation
2.6.7
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
2.6.8
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 2.2. Its corresponding equivalent as promulgated in ETWB
TCW No. 5/2005 and as required under the EIA study brief is also tabulated. The
evaluation of the environmental benefits, dis-benefits and potential impacts on
the stream, aquatic life, and surrounding wildlife therein as well as other considerations
for the proposed DPs of channel improvement works are summarized in Table 2.3.
Alternative Drainage
Options Considered for the Proposed DP Channels
|
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+++ |
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)
+ Equivalent to
bunds as flood barriers design option as highlighted in EIA Study Brief
++ Equivalent to bypass
design option as highlighted in EIA Study Brief
+++ Equivalent to flood pond design
option as highlighted in EIA Study Brief
2.6.9
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 – Constructed channels with the aim of diverting excessive
water from the main natural channel during periods of high flows.
Option 4 Flood
storage – 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.
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 DP Channels
|
Drainage Option |
Environmental Benefits |
Environmental |
Recommendation |
Reasons &
Considerations |
|
Option 1 - Distant flood
banks |
§ No excavation of the
existing streams at Man Uk Pin and Lin Ma Hang will be required. The existing
streams would be left untouched hence the stream habitats will be preserved. § There will be no
ecological impacts on Man Uk Pin and Lin Ma Hang streams, aquatic life and
surrounding wildlife therein. |
§ This option requires
substantial earth bund to provide adequate flood protection which may lead to
potential visual impact to the villagers of Lin Ma Hang and Loi Tung. § The earth bund may
hinder passage of wildlife across flood banks especially in Lin Ma Hang. |
§ This option is
considered not practicable for the proposed DP channels and is therefore not recommended. |
§ More land resumption is
required and mostly on private land. § The option is
constrained by physical features such as § Potential visual impact
problem on residents at Lin Ma Hang and Loi Tung. § 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. |
|
Option 2 - Two-stage (or
multi-stage) channels |
§ No excavation of the
existing streams at Man Uk Pin and Lin Ma Hang will be required. The existing
streams would be left untouched hence the stream habitats will be preserved. § There will be no
ecological impacts on Man Uk Pin and Lin Ma Hang streams, aquatic life and
surrounding wildlife therein. § 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 sensitive Lin Ma Hang
stream. § There may be potential lost
of stream banks (depending on design adopted). |
§ This option is only recommended for the middle portion of
MUP05. § Not recommended for LMH01 due to potential adverse
ecological impact during construction. |
§ More land resumption is
required and mostly on private land. § Construction works will
adversely affect the sensitive Lin Ma Hang stream. § The option could only be
adopted at existing natural stream sections (i.e. not applicable for the
existing roadside concrete drains at MUP03 and MUP04B). § The middle section of
MUP05 is of moderate ecological value on natural stream habitat. The adoption
of two-stage channel approach is effective to minimize the potential
ecological impact. § Not suitable for small
stream or where there is land and physical features constraints such as
MUP04A which is constrained by Loi Tung village. |
|
Option 3 - Relief or
by-pass channels |
§ No excavation of the
existing Man Uk Pin and Lin Ma Hang streams will be required. The existing
streams would be left untouched hence the stream habitats will be preserved. § There will be no
ecological impacts on Man Uk Pin and Lin Ma Hang streams, aquatic life and
surrounding wildlife therein. |
§ The bypass section may
impose environmental impacts to other areas. § It may pose ecological
impact on surrounding inactive wet & dry agricultural land of low to
moderate ecological value at Man Uk Pin and Lin Ma Hang. |
§ This option is
considered not practicable for the proposed DP channels and is therefore not recommended. |
§ More land resumption is
required. § The option is
constrained by availability of land and physical features such as § A very
effective option at meander. However, it is less practicable to be adopted at
relatively straight streams like MUP05. The construction of bypass channel
will require extensive land resumption and will impose adverse impact to
other areas further away from the stream. |
|
Option 4 - Flood storage |
§ No excavation of the
existing Man Uk Pin and Lin Ma Hang streams will be required. The existing
streams would be left untouched hence the stream habitats will be preserved. § There will be no
ecological impacts on Man Uk Pin and Lin Ma Hang streams, aquatic life and
surrounding wildlife therein. |
§ There will be potential
environmental impacts associated with the construction and operation of
storage ponds and pumping station. § Potential visual impact
of the pumping station on Lin Ma Hang and Man Uk Pin areas may arise. |
§ This option is
considered not practicable for the proposed DP channels and is therefore not recommended. |
§ More land resumption is
required. § The option is
constrained by availability of land and physical features such as § This option will incur
higher construction and maintenance costs. § This option is not
practical given the large volume of runoff needed to be stored. Substantial
land resumption and construction & operation of pumping station will be
required. |
|
Option 5 - Bank
stabilization |
§ No excavation of the
existing Man Uk Pin and Lin Ma Hang streams will be required. The existing stream
bed will be left intact hence preserving the stream habitat. § The use of gabion /
mattress lining to replace existing sections of concrete lining at LMH01 will
enhance the stream habitat by allowing recolonization of riparian vegetation.
It will also improve the aesthetic appearance of the current shotcrete lined
sections. § As the works will be at
the banks, the likely impacts on the Lin Ma Hang stream, aquatic life and
surrounding wildlife therein will be reduced. |
§ Some minor localised
environmental impacts will still be inevitable during construction. |
§ This option is only recommended for the ecologically sensitive
stream LMH01, not practicable for the MUP channels. |
§ Suitable for locations
with localised flooding or requiring minor bank improvement works such as
LMH01. § This option is not
suitable for channel predicted to have insufficient hydraulic capacity and
requires widening such as MUP channels. § This option was also
recommended by the Green Groups to enhance the ecology of the existing
shotcrete banks at LMH01. |
|
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 the Man Uk Pin and Lin Ma Hang streams. § The area of the stream
to be affected will be the least. |
§ No major environmental
dis-benefits are anticipated. |
§ This option is recommended for ecologically
sensitive stream LMH01 by providing security flap valve to replace existing
security grille which is prone to obstruction, not practicable for the MUP
channels. |
§ This option can only marginally
improve the capacity of the existing streams and may need to tie-in with
other drainage options. § There will be no adverse
impact on ecology of the streams. § The flood protection
level provided is still very low and is not suitable for larger channels.
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 of Man Uk Pin. |
§ Impacts on the existing streams
such as the sensitive Lin Ma Hang unavoidable. |
§ This option is recommended in combination with other
suitable options for MUP channels only. § Not recommended for LMH01 due to potential adverse
ecological impact during construction. |
§ 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 Man Uk Pin and Lin Ma Hang streams, especially on those abandoned
sections. § The hydrology and
ecology of the whole stream will be changed. |
§ This option will result
in unacceptable adverse environmental and ecological impacts to Man Uk Pin
and Lin Ma Hang streams, 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 at Man Uk Pin and Lin Ma Hang. § There is no ecological
value and will result in high visual impact. |
§ This option will result
in unacceptable adverse environmental and ecological impacts to Man Uk Pin
and Lin Ma Hang streams, 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
2.6.10
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.
2.6.11
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.
2.6.12
The evaluation of the environmental benefits, dis-benefits, potential
impacts on the stream, aquatic life, and surrounding wildlife therein as well
as other considerations for the proposed DP channels are summarized in Table
2.4.
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 Man Uk Pin and Lin Ma
Hang streams hence stream habitats can be preserved. §
Very minimum impact during construction is expected when
compared with other lining options. §
As the channel bed will be left in its natural conditions,
it is visually more acceptable. |
§
Prone to scouring and erosion of banks during flood flows
which may affect the structural integrity of the channel in particular to
some sections of Man Uk Pin channels. §
A wider channel may be needed to cater for poorer hydraulic
performance which in turn may impose environmental impacts to other areas
such as Lin Man Hang stream. |
§
Natural beddings have poor hydraulic performance and high
roughness. §
Suitable where the flow velocity is low. §
Recommended for middle portion of MUP05 (two-stage channel). |
|
Rip-rap lining |
§
Provides moderate habitat for aquatic community. §
Accumulation of silt in the interstices allow recolonzation of
benthic community in the long term. §
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 in particular to
some sections of Man Uk Pin channels. §
A wider channel may be needed to cater for poorer hydraulic
performance which in turn may impose environmental impacts to other areas
such as Lin Ma Hang stream. |
§
Rip-rap linings have poor hydraulic performance and high
roughness. §
Suitable where the flow velocity is low. §
Recommended for middle portion of MUP05 (two-stage 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. §
As natural material will be used, it is visually more
acceptable. §
Provides ecological enhancement to the current concrete
lined section of Lin Ma Hang stream. §
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. |
§
Moderate hydraulic performance. §
High maintenance efforts and costs in replacement. §
Susceptible to corrosion. §
Recommended for MUP03, MUP04A, MUP05 and downstream section
of LMH01. |
|
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. §
Adverse impact during construction. |
§
Better hydraulic performance. §
Little maintenance. §
Not recommended. |
|
Geotextile reinforced
grass lining |
§
Good aesthetic appearance. |
§
Low ecological value. §
Offer little opportunity for riparian vegetation and
benthic community to thrive. |
§
High maintenance cost §
Not recommended. |
|
Grassed cellular
concrete paving lining |
§
Slightly better appearance than concrete. |
§
Low ecological value. §
Offer little opportunity for riparian vegetation and
benthic community to thrive. §
Adverse impact during construction. |
§
Fast conveyance of flood flows. §
Better structural integrity. §
Little maintenance . §
Applied to MUP05 as maintenance access only. |
|
Concrete lining |
§
None |
§
No ecological value. §
Offer no opportunity for riparian vegetation and benthic
community to thrive. §
Adverse impact during construction. |
§
Fast conveyance of flood flows. §
Little maintenance. §
Environmental impacts outweigh engineering benefits. §
Not recommended. |
Final Recommended
Drainage Improvement Design Options
2.6.13
The design of each channel is evaluated with reference to the
environmental benefits and dis-benefits as well as engineering considerations
presented above. The evaluation presented in Table 2.5 and paragraphs 2.6.14 to
2.6.35 details the suitability and practicability of each option to the
proposed channel. 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 streams, aquatic
life therein as well as other habitats that depend on the streams. The final recommended
channel designs are presented in Figures 2.2 – 2.5 and Table 2.6 and elaborated
in the subsequent sections.
Evaluation of the Drainage
Design Options adopted for the Proposed Channels
|
Drainage Design Options |
Proposed Channels |
|||
|
MUP03 |
MUP04A & 04B |
MUP05 |
LMH01 |
|
|
Option 1 –
distant flood bank |
û |
û |
û |
û |
|
Option 2 –
two-stage channel |
û |
û |
|
û |
|
Option 3 –
relief or by-pass channel |
û |
û |
û |
û |
|
Option 4 –
flood storage |
û |
û |
û |
û |
|
Option 5 –
bank stabilization |
û |
û |
û |
|
|
Option 6 –
clearing and removal of obstructions |
û |
û |
û |
|
|
Option 7 –
enlargement by widening |
|
|
|
û |
|
Option 8 –
enlargement by deepening |
|
|
|
û |
|
Option 9 –
realignment / straightening |
û |
û |
û |
û |
|
Option 10 –
artificial non-vegetative smooth lining |
û |
û |
û |
û |
|
Proposed Design Option(s) |
Gabion channel |
Gabion channel |
Two-stage channel (with natural bank and
bed retained) & gabion channel |
Bank stabilization, replacement of
security grille |
Notes:
- Applicable
û - Not applicable
Major Design Characteristics
and Environmental Friendly Design Measures
Adopted in the Proposed
Channels
|
Proposed Channel |
Drainage Design Option
Adopted |
Channel Lining Option
Adopted |
Major Design Characteristics
and Environmental Friendly Design Measures Adopted |
|
MUP03 |
Option 7 Option 8 |
Gabion bank and bed |
§
Existing road side concrete drains to be replaced by
trapezoidal channel with gabion bank and mattress lining. §
Gabion banks with a tilted angle of 10o for
stability purpose. §
Silt will be allowed to accumulate for recolonization of
benthic community. §
|
|
MUP04A |
Option 7 Option 8 |
Gabion bank and bed |
§
About 180 m of the original channelisation work (proposed
under the DMP) at the upstream portion is avoided. §
Highly disturbed downstream portion to be replaced by
trapezoidal channel with gabion bank and mattress lining. §
Gabion banks with a tilted angle of 10o for
stability purpose. §
Silt will be allowed to accumulate for recolonization of
benthic community. §
|
|
MUP04B |
N/A |
N/A |
§
Channel size is reduced with new underground drainage
pipes. §
Land resumption kept to a minimum. |
|
MUP05 |
Option 2 Option 7 Option 8 |
Gabion bank and bed Natural river bed Rip-rap lining Grassed cellular concrete paving |
§
Existing road side concrete drains to be replaced by
trapezoidal channel with gabion bank and mattress lining. §
Original trapezoidal (Grasscrete®)
channel design (proposed under the DMP) is replaced by an environmentally
acceptable “Two-Stage Channel”. §
Existing natural stream bed to be retained and/or widened
with natural substrate. §
Some semi-natural stream banks (and riparian vegetation) to
be retained while the others will be stabilized with gabions. §
Silt will be allowed to accumulate for recolonization of
benthic community. §
§
Some sections of the maintenance access to be provided
inside the channel above bed level with Grasscrete®
or similar design. §
Removal of bottleneck at confluence of two tributaries
(MUP01&02) to minimize the channelisation of the two upstream
Ecologically Sensitive Streams (MUP01&02). |
|
LMH01 |
Option 5 Option 6 |
Gabion bank |
§
Original channelization works (proposed under the DMP)
avoided altogether. §
Only localized stream bank stabilization (with gabions) and
re-construction of crossings & security grille are to be carried out. The
stream bed will remain untouched. §
Existing downstream concrete lined section to be replaced
by gabion bank and mattress lining allowing recolonization of riparian
vegetation and accumulation of silt for benthic community, thereby enhancing
the existing habitat. |
MUP03 & MUP04B
2.6.14
MUP03 and MUP04B involve
construction and/or reconstruction of roadside drains which are intended to
collect surface runoff from village areas on the south side of
2.6.15
As they are local drains
collecting water from small catchment areas, options like distant flood bank,
two-stage channel, by-pass channel and flood storage are not suitable. These
options are severely constrained by
2.6.16
Part of MUP03 will be upgraded
as a 1.2 m wide gabion lined channel (MUP03B). In order to enhance the ecology
at these gabion channels, their profile has been adjusted such that the channel
sides are stepped rather than vertical, whilst the base have a mattress lining
rather than being formed with concrete.
2.6.17
For the remaining part of MUP03,
due to the constraints of
2.6.18
For the downstream section of
MUP04B, 1350 mm diameter drain pipes are proposed to connect its upstream (U-channel)
to MUP04A. The pipes will run underneath a local road of Loi Tung village.
2.6.19
These channels are currently road side concrete drains or are
along the verge of
2.6.20
The construction and operation of these channels are not
expected to adversely affect the existing stream, aquatic life and surrounding
wildlife therein.
MUP04A
2.6.21
The downstream section of
MUP04A has been heavily modified and has very low ecological value. The
upstream section is a stretch of semi-natural stream which is bordered by
agricultural habitats on both sides. It has higher ecological value than its
downstream section.
2.6.22
Several options to increase the
flood protection standard of MUP04A have been considered. Distant flood bank,
flood storage, by-pass channel and two-stage channel are considered not
feasible due to the need of resuming extensive private land (and impose
potential environmental impacts) to the nearby sensitive receivers including
the newly built small houses on both sides of the stream. 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. The most appropriate
option is to widen the watercourse and transforming it to a gabion-lined
channel.
2.6.23
Based on the ecological
assessment, the channelization of the upstream portion of MUP04A will cause
moderate impacts on the semi-natural stream and the adjacent inactive
agricultural land, which is unpopulated. Based on such findings, the proposal
is revised not to channelise the upper portion in order to avoid the
associated impacts. Localized flooding of the inactive agricultural land would
remain at the upstream portion (see Figure 2.10). Only the downstream portion
which has been heavily modified will be constructed to a gabion channel.
2.6.24
The construction and operation of this channel is not
expected to adversely affect the existing stream, aquatic life and surrounding
wildlife therein.
MUP05
2.6.25
The upper watercourse of MUP05
has been heavily modified, with most of the bank and bottom concreted, and
therefore provides very few ecological resources for wildlife or aquatic life.
In contrast, the middle and lower watercourse, except a small portion at its
most downstream, is a semi-natural stream which has moderate ecological value.
2.6.26
Options such as distant flood
bank and flood storage had been considered. However they require extensive
private land resumption and clearance of adjacent house and are considered not
favourable. By-pass channel is also not suitable as the alignment of MUP05 is
generally straight. The presence of
2.6.27
In view of such, two-stage
channel is proposed for the downstream portion of MUP05 to minimize the impact
to this semi-natural stream. The upstream portion however will not be upgraded
to a two-stage channel as it has already been concrete lined and has low
ecological value. It is considered appropriate to replace this section by a
gabion lined channel.
2.6.28
The construction and operation of this channel is not
expected to adversely affect the existing stream, aquatic life and surrounding
wildlife therein.
LMH01
2.6.29
LMH01 is a stream which remains
in a largely natural state and is unpolluted. Ecological surveys have confirmed
the importance of the stream for fish and dragonflies, both pollution sensitive
groups.
2.6.30
The proposed improvement works
to be carried out at LMH01 are therefore limited to avoidance approach. The
option of providing a by-pass channel to reduce the flood risk was firstly
considered (Figure 2.9). Under such option, the existing watercourse will not
be touched except at its confluences with the by-pass channel. However it will
require large land resumption on the adjacent abandoned fallow fields.
2.6.31
During
the consultation, the Villager’s Representative indicated that local residents
of Lin Ma Hang are more concerned with overflow at various stream crossings and
blockage of the Lin Ma Hang stream outlet, rather than the flooding in the
abandoned fallow field, which is the floodplain of the stream. The abandoned fallow field has been
assessed to be of moderate environmental values. Generally, most village houses are
located on sufficiently high ground to be free from damage in previous flooding
events. Some village houses near crossings however still experience localised
flooding when the stream crossings are flooded.
2.6.32
Site
meeting had been arranged with Green Groups to discuss the proposed drainage
improvement works. During the meeting, the Green Groups requested to replace
some concrete lined stream sections by gabion in order to enhance the ecological
value of the area.
2.6.33
In view of the above, instead
of adopting by-pass channel or other options, it was opted to undertake bank
stabilization and reinstatement using gabions at the sections where erosion is
occurring or being lined by concrete, replacement of three bridges to higher
levels, and the security grille at the stream outlet. These drainage
improvement design options would result in the least environmental and
ecological impacts while achieving the project objective of flood protection.
2.6.34
The revised proposal was
presented to North District Council and Green Groups who indicated no adverse
comment. The revised proposal is thus adopted for the improvement to Lin Ma
Hang stream.
2.6.35
The construction and operation of this channel is not
expected to adversely affect the existing stream, aquatic life and surrounding
wildlife therein.
Channel
Lining Evaluation
2.6.36
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.
2.6.37
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 and
aesthetic impacts, availability of materials, etc.
2.6.38
Following comprehensive research several solutions and the
pertinent issues were discussed with interested Government departments at the
Workshop held on 10 May 2004. Further
refinement were conducted with due regards to hydraulic capacity and ecological
impact. The preferred solution for each of the proposed channels is discussed
below.
2.6.39
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.
2.6.40
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 loss from the catchment which will ultimately reach
2.6.41
Natural river bed lining would be applied to parts of MUP05.
However, to retain sufficient hydraulic capacity of flood prevention and cost
effectiveness in maintenance, it is considered not viable to adopt natural
stream bed for the smaller channels, i.e. MUP03 and MUP04A.
Proposed Gabion
Channels for MUP Channels
2.6.42
The DMP proposed deep U-shaped reinforced concrete channels and/or
trapezoidal grasscrete lined channels for MUP03, MUP04A, MUP04B and MUP05.
Whilst this offers the most efficient hydraulic solution to convey flood flows
downstream in the minimum possible time, it exacerbates peak flood flows and
has negligible ecological value and is aesthetically problematic.
2.6.43
It is proposed that the channel beds be constructed of a 300mm
thick gabion mattress; this is essentially graded stone contained in a wire
cage. The stone is graded in a manner which will allow some sedimentation and
eventually re-colonization of the channel bed by benthic organisms. Although,
in the case of the smaller channels, it is felt that the relative narrowness of
the channel in relation to depth may limit the development of natural flora and
fauna. The mattress also has the effect of holding the stone in place and
limiting migration of bed material during flood flows.
2.6.44
Whilst limited migration of bed material is preferred and
would simulate the natural ecosystem more accurately, the wholesale migration
of large quantities of bed material during a major flood, in channels in this
nature could have catastrophic consequences. The structural integrity of the
channel itself could be compromised if the banks are buffeted by large
quantities of material.
2.6.45
The channel banks are to be constructed from gabions, wire
cages filled with stone. The gabion retaining structure will be wider at the
base stepping back to a thinner structure closer to the natural ground level. It
will be tilted to an angle of about 10o for stability purpose. This
produces the ‘V’ effect in the channel. The extent to which the width of the
gabion retaining structure will vary, will need to be determined during the
detailed design stage. Typical preliminary cross-sections are shown as insets
in the figure for each channel.
2.6.46
A minimum of 75 mm thick sediment will be allow to accumulate
at the future gabion channels bed to permit recolonization of benthic
communities.
2.6.47
It is possible to introduce pre-seeded topsoil into the
gabion wall, however this may led to the introduction of invasive non-native
species to the area and thus is more preferable to let the channel ‘green-up’
naturally.
2.6.48
During detailed design the draw-down effect on the local
water table will be reviewed. If necessary an appropriate geotextile will be
recommend to line the rear of the gabion walls and control water ingress and
egress from the channel. It is more likely that in these areas a geotextile
separation layer will be employed.
Dry
Weather Flow Channel
2.6.49
DSD Stormwater Drainage Manual specifies that 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 beds of these channels are to mimic natural conditions. Accordingly, as the
proposed channels are not wide, 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 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. A minimum of 75 mm thick sediment
will be allowed to accumulate to permit recolonization of benthic communities
in the future channels.
2.6.50
Locating the channel immediately adjacent to the river bank
allows bankside vegetation to overhang the river whilst in low flow which
provides shading conditions conducive to sustaining riparian species.
Maintenance
Access Road
2.7.1
DMP proposes that a 3.5 m wide maintenance access channel be
provided on the verge of the channel, used and maintained by DSD. In order to
minimize land resumption and potential visual impact, the base of the channel
has been used as maintenance access wherever possible (downstream portion of MUP05)
for maintenance vehicles carrying out desilting and repair works. Grasscrete
verges of 1.6 m and 1.0 m wide respectively are being provided on opposite
sides of the smaller channels (MUP03, MUP04A and upstream portion of MUP05) as
delineation of the channels and providing accesses for inspection. The detailed
arrangement for maintenance access roads and ramps will be explored further
during detailed design.
Crossings
2.7.2
Existing pedestrian crossings, vehicular roads and bridges
affected by the proposed channel will be reinstated as appropriate. These
crossings and bridges will be reinstated to Highways Department’s Structures
Design Manual. The responsible
maintenance authority will be identified during the detailed design stage. 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.
2.8
Construction
Related Aspects
Construction
Method
2.8.1
Construction of the proposed channels generally involves standard
construction methods such as site clearance, excavation and filling, assembly
of gabion wall and mattress lining, construction of maintenance / access roads
and finally landscaping works.
2.8.2
The box culvert, cross road drains 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.
2.8.3
Since some of the existing pedestrian and vehicular accesses /
crossings will be affected by the proposed works, it will be demolished and
re-provided. Temporary provision will be provided during the construction stage
to facilitate passage and reduce inconvenience to the public. Such accesses and
crossings will be constructed by typical cast in-situ construction method.
2.8.4
Bank improvement works at LMH01 involves breaking and removal
of existing shotcrete banks, and replacing them with gabions. Breaking of the
shotcrete will be done using hand-held equipment. No concrete is necessary for
the construction of the gabion banks.
2.8.5
For the gabion channels, woven wire-mesh gabions will be used
for their comparatively higher flexibility than welded wire-mesh gabions as
channel lining materials. Geotextile will be provided before the gabions are
placed. Empty gabion baskets (cages) or mattress will be assembled and laced
together with connecting and binding (lacing) wires.
2.8.6
Filling of the gabions with graded stones will be carried out
either by hand or by machine depending on the accessibility of the site and the
finishing requirements. Suitable rocks / stones and fines from the excavation
of the channels will be reused as far as practicable in filling the gabions to
reduce the amount of excavated materials requiring off-site disposal.
2.8.7
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.
2.8.8
If the stream is wide enough, 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.
Site Access
2.8.9
All the proposed channels can be readily accessed
through existing roads. Temporary run-in will be provided for the construction
vehicles to enable them to merge and diverge from the normal traffic as
smoothly as possible.
2.8.10
For Lin Ma Hang, as the channel is located in Closed Area,
permits issued by the Hong Kong Police Force are required for all works-related
persons and vehicles.
Implementation Programme
2.8.11
The implementation programme for the proposed works is
tentatively expected to start in end 2007 for completion by mid 2011. Detailed construction programme is not
available at this current stage.
The tentative key milestone dates are tabulated in Table 2.7 below and
shown in Figure 2.12.
Tentative Key Milestone Dates
|
Key Milestones |
Date (month/year) |
|
Tender Gazetting of Works Contract |
08/2007 |
|
Construction Contract Award of Works Contract |
12/2007 |
|
Completion of All Works Contracts |
mid 2011 |
2.9
Consideration of
Alternative Construction Methods
Comparison of the
Environmental Benefits and Dis-benefits of Alternative Construction Methods
U-channels / Gabion Channels
2.9.1
Potential alternative construction method includes offsite
precast structures for U-channels and pre-filled gabion 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.
2.9.2
In order to match with existing topography, 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 U-channels to be constructed.
It can be mitigated by appropriate measures. Pre-filled gabion blocks are not
practicable as heavy lifting appliances will be required to lift even a half
loaded gabion. It will be dangerous and will hinder the assembly process as
well as restrict the potential reuse of excavated rocks within the site.
Therefore, conventional construction method of in-situ construction of channels
as well as assembly and filling of gabions is considered more suitable and
practical.
Crossings / Bridges
2.9.3
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 bridge/crossing structure is
considered more suitable and practical.
Box Culvert / Drainage Pipes
2.9.4
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.
2.9.5
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 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
2.9.6
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
2.9.7
The proposed construction works will widen the existing
channels 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 channel (downstream) 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.
2.9.8
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.
2.9.9
Adverse effect on hydraulic performance of the channels
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 Drainage Impact Assessment (
2.9.10
Cumulative environmental implications of potential flooding at
downstream areas during wet season are not expected.
Cumulative
Effects for the Construction of the Proposed Channels
2.9.11
Cumulative impacts associated with the construction of the
proposed channels (MUP03, MUP04A, MUP04B, MUP05 & LMH01) have been assessed
and presented in the subsequent assessment chapters. It is concluded that there
will be no adverse cumulative impacts.
2.9.12
In summary, having regard to the cumulative effects for the
construction of the proposed channels and the severity of the construction
impacts to the affected sensitive receivers along the proposed channels, the
EIA study has concluded that there will be no adverse environmental impacts.
2.10
Justification of
the Final Channel Design Recommendations
2.10.1
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 existing stream, the aquatic life and surrounding wildlife therein. The
detailed environmental impact assessment presented in the subsequent chapters
has also demonstrated the adopted channel design can avoid and minimize adverse
environmental impacts.
2.11
Project
Interfaces / Concurrent Projects
2.11.1
There are a number of projects
that are planned or on-going in the Study area (Table 2.8). Many do not have a
significant influence on the proposed channels in this Project. However, if
interfacing does occur, close liaison will be conducted to ensure adequate
phasing with each concurrent projects is taken into consideration.
2.11.2
The interfacing with the non-DP
channels has been assessed in the cumulative impact section of the relevant
assessment chapters of this EIA. According to the EIA findings, no adverse
cumulative environmental impact is expected.
On-going and Planned Projects in the Study Area
|
Concurrent Projects |
Tentative Programme |
Project Interfaces with
Proposed Channels |
|
Drainage Improvement in |
end 2007 – mid 2011 |
Interfacing with MUP05 |
|
North District Sewerage,
Stage 3 Phase 1 – Package ND1 (Sewerage to Man |
02/2008 – 02/2010 |
Very minor interfacing
with upstream portion of MUP05 |
|
North District Sewerage,
Stage 3 Phase 1 – Package ND1 (Sewerage to Loi Tung) |
02/2008 – 02/2010 |
Minor interfacing with
proposed 1350mm drainage pipes at MUP04B |
|
North District Sewerage,
Stage 2 (Sewerage to Lin Ma Hang) |
2/2008 – 10/2009 (Cat B item – under planning) |
Minor interfacing with
proposed vehicular crossings V1 & V2 at LMH01 |
|
|
2008 - 2009 |
MUP channels and LMH01,
no interfacing expected |
2.12
Scope of Works
during Operational Phase
2.12.1
Maintenance would be necessary for the proposed gabion
channels 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
‘two-stage’ channel at MUP05 and the natural stream sections of LMH01.
2.13
Maintenance
Responsibilities
2.13.1
The works included in this Project will be handed over to
relevant Government Departments for maintenance upon completion of the
construction contracts.
2.13.2
The scope and maintenance responsibilities of individual
government Departments are generally well established. Table 2.9 outlines the
proposed maintenance responsibilities based on current Government practices and
relevant Technical Circulars.
Proposed Maintenance
Responsibilities
|
Items |
Management Department |
Maintenance Department |
|
Proposed channels |
DSD |
DSD |
|
Proposed box culvert, drainage pipes, U-channel |
DSD |
DSD |
|
Maintenance access roads (not open to public) |
DSD |
DSD |
|
Local access roads / footpaths (open to public) |
DO/N |
DO/N |
|
Pedestrian and vehicular crossings |
DO/N |
DO/N |
|
Vegetation within the proposed channels |
DSD |
DSD |
|
Roadside landscape hardworks and softworks |
LCSD |
LCSD |
|
Channel verge landscape hardworks and softworks |
LandsD |
LandsD |
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.
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.
2.14
Continuous Public
Involvement
2.14.1
The process of Continuous Public Involvement (
Summary of Continuous
Public Involvement
|
Relevant Parties |
Process |
Date |
|
Sha Tau Kok Rural Committee Village Representatives (VRs) of Man
|
meeting |
4 February 2005 |
|
site visits |
24 February 2005 & 18 March 2005 |
|
|
VRs of Lin Ma Hang |
site visit |
|
|
Fanling Rural Committee VRs of Fanling Rural Committee |
meeting |
13 April 2005 |
|
site visits |
21 & 22 May 2005 |
|
|
Shung Him Tong and |
site visits |
21 & 22 April 2005 |
|
North District Council – District Development
and Environmental Improvement ( |
meeting |
23 May 2005 |
|
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 |
6 October 2005 |
|
site visits |
25 October 2005 (attended by GP, HKBWS and KFBG) 16 January 2006 (attended by KFBG and WWF) |
|
|
letter on latest progress of design |
31 July 2006 |
2.14.2
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 2.11 below.
Summary of Public
Consultation
|
Relevant Parties |
Main Comments /
Recommendations |
Responses / Outcomes |
|
Sha Tau Kok Rural Committee Lin Ma Hang VRs |
Request for river training /
additional bank improvements to the existing Lin Ma Hang stream (LMH01). |
Site visit was arranged with the
Lin Ma Hang VRs to investigate local flooding situations and identify
critical locations for improving for improving the existing river banks. The
proposed additional bank improvement works were provided to the VRs. No
further comments were received. |
|
Fanling Rural Committee Shung Him Tong VRs Lung Yeuk Tau VRs |
Shung Him Tong and Lung Yeuk Tau VRs
objected the locations of the proposed open-channel (LYT04) alongside the
existing road within private farmlands. They proposed to re-construct and
enlarge the existing under-road box culvert. |
Site visits with the VRs were
arranged. To take into account the objection raised by the VRs, the proposed
channel LYT04 was revised to a twin cell culvert under the existing access
road. The alternative proposal could reduce the resumption of private lands
but would require temporary traffic diversions during construction. No
further comments were received. (As a result of the design changes,
LYT04 is confirmed not to be a DP under the EIAO and will be excluded from
this EIA.) |
|
North District Council – District Development
and Environmental Improvement ( |
Enquired whether the Project is
governed by the EIA Ordinance, to consider the latest Government circulars
and guidelines on protection of natural rivers and the likely environmental
protection measures to be taken. |
The Project will undergo the EIA
process under the EIA Ordinance. The Project will also take into account the
latest relevant Government circulars and guidelines. Gabions and other
environmental friendly channel design will be incorporated to the proposed
channels. |
|
Green Groups |
LMH The Green Groups opined that the
downstream section currently stabilized by shotcrete is not ecological
friendly and several sections of the stream banks were not in good condition.
They suggest replacing it with natural materials. Also expressed concerns of potential
pollution during construction and request adequate supervision of the
contractor as well as to carry out works during dry season and in phases. |
The design has been reviewed and
sections of the banks will be replaced/stabilized with gabions. Stringent
mitigation measures will be proposed in the EIA to ensure no adverse impacts
on Lin Ma Hang stream during construction. Construction phase monitoring and
audit will also be recommended. |
|
MUP Provision of proper support and protection
to a big tree at MUP05. |
Several large trees within MUP05
will be preserved in-situ with proper support and protection. |
2.14.3
Overall, the relevant Rural Committee and North District
Council members, together with Village Representatives supported the proposed
Project as it would bring relieve to the flooding in the Northern New
Territories. The Green Groups appreciate the adoption of environmental friendly
designs such as two-stage channel, etc in this drainage improvement project to
minimize the potential impacts to the ecology and the environment. No adverse
comments were received from the Green Groups.