The Drainage Services Department (DSD) completed the “Stormwater Drainage Master Plan (DMP) Study in Tsuen Wan, Kwai Chung and Tsing Yi” in July 1999. In that Study it was identified
that the drainage systems in Tsing Yi could meet the
current flood protection standard in general, but not the drainage systems in Tsuen Wan and Kwai Chung. A series of drainage improvement works
including the “Drainage Improvement in Tsuen Wan and Kwai Chung – Tsuen Wan Drainage
Tunnel” (The Project) and the “Drainage Improvement in Tsuen
Wan and Kwai Chung – Urban Drainage Improvement
Works” were proposed, and subsequently DSD commissioned a Preliminary Project
Feasibility Study (PPFS) for “Drainage Improvement in Tsuen
Wan and Kwai Chung – Package A, Tsuen
Wan Drainage Tunnel” which was completed in April 2000.
The PPFS concluded that the construction of a tunnel of an
internal diameter of 6.5m and length 5.35km should be implemented to alleviate
the flooding risk in Tsuen Wan and Kwai Chung. At the
end of October 2002, Mott Connell Limited (MCL) was commissioned by the
Drainage Services Department under Agreement No. CE 80/2001 (DS) to carry out
the investigation and preliminary design for the “Drainage Improvement in Tsuen Wan and Kwai Chung – Tsuen Wan Drainage Tunnel” (The Project).
The primary objective of the Project is to develop an
optimum preliminary design of the proposed tunnel and associated drainage
structures based on the recommendations of the Preliminary Project Feasibility
Study Report (PPFS), the Stormwater Drainage Master
Plan Study in Tsuen Wan, Kwai
Chung and Tsing Yi (DMP) and the results of the
various investigations to be carried out under this Assignment. This Environmental Impact Assessment (EIA) is
a key submission of the project and has been prepared in conjunction with other
design teams including the drainage, geotechnical and landscape teams.
A plan showing the proposed drainage improvement works,
which comprise a tunnel and associated intakes and outfall structures
is provided as Figure 1.1. The alternative locations of the intakes I-2
and I-3 are shown in Figures 1.2 and
1.3 respectively.
The scope of construction works for the tunnel development
comprises:
·
5.13km drainage tunnel system between Kwai Chung and Yau Kom Tau (reduced from 5.35km
during preliminary design);
·
provision of three Intake locations:
i. Intake I-1: Kwai
Chung, adjacent to the junction of
ii Intake I-2: At Lo Wai,
adjacent to
iii Intake I-3: At Tso Kung Tam, about 350m off
Route Twist.
·
provision of Outfall O-1: Yau
Kom Tau, underneath the
existing
·
provision of associated
environmental mitigation measures.
In October 2002, DSD appointed Mott Connell Limited (MCL)
to carry out the investigation, preliminary design and Environmental Impact
Assessment (EIA) for the Drainage Improvement to Tsuen
Wan and Kwai Chung – Tsuen
Wan Drainage Tunnel (The Project). This report presents the findings of the
Environmental Impact Assessment (EIA) which was carried out as an integral part
of the Project and makes reference to other related documents and reports which
were also prepared under the auspices of this Project.
The justification for the project as stated in the
Approved PPFS document is reiterated below:
The study area, with a population in excess of 700,000, is
currently served by an existing stormwater drainage
system which has been in place for 30 years or more. The existing system is principally drained by
large decked nullahs which are effectively large
shallow culverts. The nullah invert at its outfall is invariably below sea level
and large lengths of the nullahs are permanently
submerged which leads to siltation and reduction of
effective cross sectional area. The nullahs not only accept large flows from the upland catchment but also the ever-increasing runoff from the
expanding urban area with increased impermeable surfaces.
Due to extensive urbanisation and rapid developments of
the area including massive reclamation, there have been numerous modifications
and extensions on a piecemeal basis to the existing stormwater
drainage system including extension and decking of existing main nullahs. The system
is unable to meet the current flood protection standards and flooding occurs
during severe rainstorms. Hydraulic
analyses undertaken in the DMP show that the trunk drains in the Tsuen Wan and Kwai Chung areas
are grossly under-capacity. As a result, the areas have a flood protection level
of between 10 to 20-year return period in general.
Intense rainfall events have led to the surcharge of the
main nullahs in the past and have resulted in
flooding. In one such storm event in the recent year (the 8th May 1997 storm
which had been defined as a 20-year return period event), the storm caused the
main nullahs to surcharge and flooding in Tsuen Wan and Kwai Chung.
Under the study, the hydraulic performance of the drainage
system in the Study Area was assessed, with drawings highlighting the parts of
the drainage system with hydraulic deficiencies under various rainstorm return
periods in accordance with the Stormwater Drainage
Manual (SDM). The DMP study revealed that under the current development levels
all the trunk drains and few branches in Tsuen Wan
and Kwai Chung do not meet the current SDM standard.
The Tsing Yi drainage system was found to meet the
current flood protection standard in general.
Subsequently, improvement options to alleviate flooding
problems in Tsuen Wan and Kwai
Chung were developed and evaluated. Based on these improvement options, flood
extent maps were compiled to determine the extent of flooding which will occur
in the existing system and compare that with the amount of flooding which will
occur if different improvement options are implemented.
A benefit-cost analysis was also undertaken to compare the
various capital schemes by defining the anticipated cost of the proposed
options and the benefit (i.e. the reduction in flood losses) which will be
derived from the proposed schemes. The key objective of the benefit-cost
analysis was to recommend the best and most cost-effective option to solve the
flood problems in the study area on a long-term basis.
The benefit-cost analysis concluded that the tunnel option
would provide the best benefit-cost of the options considered. The results of the analysis revealed that the
cost of potential damages caused by flooding would be about 1.6 times the cost
of the proposed tunnel works.
In addition, although flood damage can be quantified in
monetary terms, flooding can also result in situations involving intangible
costs. Flooding may induce potential
long-term risks or adverse effects on the life and well being of the community.
It may also tarnish Hong Kong's international image. These intangible costs concerning quality of
life, social and community benefit, environment and reputation are equally
important and must be taken into account in the decision making.
The DMP study therefore recommended a drainage tunnel
system for the study area on a long-term basis. This project has been justified
on technical and economic grounds.
The DMP study indicated that flooding of the urban
drainage system are directly related to the runoff flows from the upland catchment, and that this amount of flooding will be
significantly reduced if the above tunnel is constructed.
With the tunnel system, the flood protection level of
trunk drains in most of the Tsuen Wan and Kwai Chung areas will be improved to a 200-year return period
and it entirely avoids works in the trunk drains in developed urban areas of
heavy traffic and congested underground utilities.
A Value Management Workshop involving a cross section of
government departments has been conducted in April 1999 where a group consensus
was reached that the tunnel option was the best solution for the areas'
flooding problem.
Results of the hydraulic performance analyses conducted
under the DMP Study show that much of the urban areas in Tsuen
Wan and Kwai Chung are currently subject to risk of
flooding during very heavy rainstorms.
It should be noted that the location, extent and possible
depth of flooding will be very much dependent on many highly variable factors
such as distribution of rainfall intensity during a heavy rainstorm event;
gullies and stormwater inlets that may become blocked
by rubbish; the extent of ground saturation; silting of drains from previous
heavy rainstorms; adverse effects on drainage patterns by landslides etc. However, certain locations have been
identified as being at very high risk of flooding. For example, during a
rainstorm of about a 20-year return period, there is a possibility that
flooding up to a depth of about 0.3m may occur in area bounded by Castle Peak
Road, Tsuen Wan Road and Texaco Road North in Tsuen Wan district; and localized area in Kwai Chung. Flooding
may well be deeper in other locations along Kwai
Chung Road from Kwai Fong to Sheung
Kwai Chung.
Localised flooding may also occur at isolated locations in
other areas. In addition, stormwater overflowing from trunk drains in steep roads may
flow over the road surface with such a high velocity that it would be hazardous
to life and limb. Examples include
Cheung Wing Road and Kwok Shui Road.
During an even more severe rainstorm of, say, a 50-year
return period, both the extent and depth of flooding would obviously increase,
particularly in Cheung Wing Road towards Kwai Chung
Road down to Kwai Foo Road
and adjacent streets and in Lei Pui Street and Wai Tsuen Road where there is the
potential flood depths of about 1 metre.
In summary, with the existing drainage system at its
current capacity levels, there is the risk of flooding during very heavy
rainstorms in many of the urban areas in Tsuen Wan
and Kwai Chung. The potential consequences of such
flooding include:
1. Risk to life
and limb
2. Damage to
property
3. Nuisance to
the public
4. Disruption
to normal economic activity
5. Disruption
to traffic
6. Psychological
stress
7. Loss of
reputation
If the project does not proceed, then these risks to the
community will continue.
There are no simple, small-scale works that could be
implemented to reduce these risks. The
only effective solution to provide adequate flood relief is to implement all
the proposed works items included in this project. Even then, there may still
be some localised flooding upto about 0.1m deep in Tsuen Wan, Kwai Chung and Tsing Yi during extreme rainfall events. To eliminate this
residual flooding would require very much more substantial and disruptive works
in these areas and the benefits achieved would not be worthwhile.
From the foregoing it was concluded that the form of the
works would be less disruptive, and have less environmental impact and
disturbance. On this basis the project
proceeded as described in the following section.
In the July 1999, report on the “Stormwater
Drainage Master Plan (DMP) Study in Tsuen Wan, Kwai Chung and Tsing Yi”, it was
identified that drainage systems in Tsing Yi would be
adequate to meet current flood protection standard in general, however drainage
systems in Tsing Yi and Kwai
Chung were not capable of achieving such standards.
The DMP recommended a series of drainage improvement works
including the “Drainage Improvement in Tsuen Wan and Kwai Chung – Tsuen Wan Drainage
Tunnel” (The Project) and the “Drainage Improvement in Tsuen
Wan and Kwai Chung – Urban Drainage Improvement
Works”. A Preliminary Project Feasibility Study (PPFS) for “Drainage
Improvement in Tsuen Wan and Kwai
Chung – Package A, Tsuen Wan Drainage Tunnel” was
undertaken and completed in April 2000 which provides the justification for the
Project in its current from (i.e. tunnel and intakes).
The reasons for choosing the Recommended Option in the DMP
were:
“The proposed tunnel is considered as the preferred
option. The benefit-cost analysis
undertaken for the major improvement options considered concluded that the
tunnel option will be the cheapest to implement and the most effective in
reducing flooding and flood damage costs, and will have the best value for
money. Consequently, it was concluded
that the tunnel option would provide the highest savings in flood damages over
the construction costs of the works. The
Value Management Workshop which involved a cross section of government
departments also concluded that the tunnel option was the most cost-effective
solution of government departments also concluded that the tunnel option was
the most cost-effective solution for the areas’ flooding problem. In addition, the tunnel option would offer
the following advantages:
·
The characteristics of the
large upland catchments in Tsuen Wan and Kwai Chung makes the interception of stormwater runoffs from the areas using the tunnel
particularly effective in reducing flooding.
·
Additionally, topography of the catchment areas allows for gravity flow in the tunnel
system; therefore, no pumping is required.
·
The tunnel entirely avoids the need to enlarge
the main nullahs in urban areas; therefore, traffic
disruptions and conflicts with congested underground utilities are
significantly reduced.
·
Due to the proposed location of the tunnel and
the underground nature of the works, environmental-related issue arising from
the tunnel works will be minimized and will be confined to the excavation works
at the intake and outlet structures.
·
Land requirements for the drainage tunnel will
not be extensive and will be confined to the surface works at the intake and
outlet structures. Only government lands
are selected and no private land needs to be resumed.
The proposed urban drainage improvement works is
considered to be necessary in supplement to the tunnel to resolve local
hydraulic and structural deficiencies of the drainage system at these
locations. These works are small in scale
and will not cause excessive environmental and traffic disruptions.”
The aforementioned reports provided the basis for the
Adoptive Review which was carried out by Mott Connell Ltd, and reported upon in
2003. The Adoptive Review presented a review of the scope and recommendations
of the PPFS report as well as the findings, conclusions and recommendations of
the Final Report of the DMP study.
Following the Adoptive Review, an Options Selection Study
was conducted as part of the present commission. The Options Selection Study
investigated alternative alignments for the drainage tunnel as well as possible
locations for the associated intake/outfall structures, the objective being to determine the most
favourable tunnel/intake/outfall option in order to divert runoff into Rambler
Channel bypassing Tsuen Wan urban area through an
outfall located to the south of Yau Kom Tau.
The Options Selection Study also examined the possibility
of maximising the upstream collection of runoff to increase the flood protection
level of the urban area without the requirements of carrying out substantial
drainage upgrading works in the congested urban area.
The Options Selection Study has also investigated the
possibility of intercepting
discharges into the existing box culvert along
This Project is a “designated project” (DP) under Schedule
2 (Part 1 Q.1) of the Environmental
Impact Assessment Ordinance (EIAO) (Cap. 499) and an environmental permit (EP)
issued under the EIAO is required for Project construction and operation.
In order to apply for an Environmental Permit an EIA must
be undertaken in accordance with the requirements of the Study Brief issued by
EPD in May 2001, under reference ESB-069/2001. Reference can be made to the
full requirements of the Study Brief which is contained in Appendix A. In addition to the Study Brief the issues identified in
the DMP Study, PPFS, Adoptive Review Report and Project Profile have also been
taken into account.
The EIA has been conducted in accordance with the Study
Brief, the Project Profile (No. PP-120/2001) and the criteria in the relevant
sections of the Technical Memorandum on the EIA Process of the EIAO
(hereinafter referred to as the TM). The EIA has identified, described,
predicted and evaluated potential environmental impacts, mitigation measures
and will consider the impacts of any feasible alternatives. The alternative options for the scheme as a
whole are also contained in the report along with the justification for the
Project.
The specific objectives of the EIA Study are listed in the
Study Brief (Item 6.3.11.2) (ESB-069/2001), as follows:
(a) to
describe the proposed Project and associated works together with the
requirements for carrying out the proposed Project;
(b) to
identify and describe the elements of the community and environment likely to
be affected by the proposed Project and/or likely to cause adverse impacts to
the proposed Project, including both the natural and man-made environment;
(c) to
identify and quantify emission sources and determine the significance of
impacts on sensitive receivers and potential affected uses;
(d) to
identify any potential impacts from point and non-point pollution sources on
the identified water systems and sensitive receivers, in particular water
gathering grounds, during the construction and operation stages;
(e) to
identify and quantify any potential losses and damage to flora, fauna and
wildlife habitats;
(f) to
identify any potential impacts to the historical, archaeological and cultural
resources within the study area and propose measures to mitigate these impacts;
(g) to
propose the provision of infrastructure or mitigation measures so as to minimise
pollution, environmental disturbance and nuisance during construction and
operation of the Project;
(h) to
identify, predict and evaluate the residual (i.e. after practicable mitigation)
environmental impacts and the cumulative effects expected to arise during the
construction and operation phases of the Project in relation to the sensitive
receivers and potential affected uses;
(i) to
identify, assess and specify methods, measures and standards, to be included in
the detailed design, construction and operation of the Project which are
necessary to mitigate these environmental impacts and reducing them to
acceptable levels;
(j) to
investigate the extent of secondary environmental impacts that may arise from
the proposed mitigation measures and to identify constraints associated with
the mitigation measures recommended in the EIA Study, as well as the provision
of any necessary modification;
(k) to
design and specify the environmental monitoring and audit requirements, if required,
to ensure the implementation and the effectiveness of the environmental
protection and pollution control measures adopted; and
(l) to consider feasible alternatives with a view to avoiding or
minimising the potential environmental impacts to sensitive uses, and to
compare the environmental benefits and dis-benefits
of each of the different options and to provide reasons for selecting a
preferred option and to describe the part environmental factors played in the
selection.
Particular attention has been given to the following
documents when undertaking this EIA Assignment :
·
Final Preliminary Environmental Review,
Preliminary Project Feasibility Study for the Reconstruction and Improvement of
Tuen Mun Road (potential
interfacing project; it is required for the cumulative impact assessment);
·
Final Preliminary Environmental Review Report,
Feasibility Study on Modernization of Tsuen Wan Water
Treatment Works;
·
DMP Study in Tsuen
Wan, Kwai Chung & Tsing
Yi – Final Report; Volume 1 - Main Report & Summary of Recommendations;
Volume 2 - Hydraulic Assessments; Volume 3 - Structural and Condition
Assessments; Volume 4 - Project Appraisal; Volume 5 - Miscellaneous Items;
·
Preliminary Project Feasibility Study Report for
Drainage Improvement in Tsuen Wan, Kwai Chung and Tsing Yi – Package
A: Tsuen Wan Drainage Tunnel;
·
The computer models including the Macro-Model,
Micro-Model and Overland Flow Model produced under the DMP study;
·
Working Papers (WP) and Technical Notes for the
DMP study in Tsuen Wan, Kwai
Chung & Tsing Yi; WP1 - Hydraulic Design
Criteria; WP2 - Reservoir and Catchment Overflows
Analyses; WP3 - Model Approach and Schematics; WP4 - Asset Inventory and
Management System Functional Description; WP5 - Review of DMP Strategies; WP6 -
Structural Performance Assessment; Technical Note - Physical Hydraulic Model;
Technical Note No.2 - Model Verification; Interim Report (Feb. 1998); and
·
Supplementary Report to the Interim Report
(Flood Extent).
A number of previous, ongoing
and proposed EIAs, feasibility and engineering
studies are relevant to the Project including:
·
EIA-062/2001 Demolition of Kwai
Chung Incineration Plant Civil Engineering
Department (Atkins China Ltd., 2001); and
·
Environmental
Impact Assessment Ordinance (Cap. 499).
Maunsell Consultants Asia Ltd, (1999)
Stormwater Drainage Master Plan Study in Tsuen Wan, Kwai Chung and Tsing Yi, Unpublished Report.
Maunsell Consultants Asia Ltd
(2000) Preliminary Project Feasibility
Study for Drainage Improvement in Tsuen Wan and Kwai Chung – Package A, Tsuen Wan
Drainage Tunnel, Unpublished Report.
Mott Connell Ltd. (2003) Agreement No. CE5/2002 (DS) Drainage Improvement in the Northern New Territories – Package B Investigation, Design and Construction: Adoptive Review Report, Unpublished Report.