11.1
Justification and
Benefits of the Project
11.1.1
The Project is one of the works
packages recommended by the Drainage Master Plan Study in the
11.1.2
The Project is expected to contribute to the relief of the
present drainage problems that have repeatedly led to severe flooding in the
Man Uk Pin and Lin Ma Hang areas of
11.2
Population and
Environmental Sensitive Areas Protected
11.2.1
Previous studies indicated that Lin Ma Hang stream has a high
ecological value. There is currently a proposal to designate the Lin Ma Hang
stream as a Site of Special Scientific Interest (SSSI) due to the high
diversity of freshwater fish of high conservation concern. The nearby Lin Ma
Hang lead mine is a designated SSSI for bats. The DMP Study originally
recommended a bypass channel to alleviate the flooding in Lin Ma Hang. In view
of the ecological importance of the area, alternative means to alleviate
flooding were considered to avoid direct ecological impacts on the aquatic
life, wildlife and habitats that depend on the stream and its associated
wetlands. The revised design includes short section of gabion channel at the
downstream end, localized bank improvement, replacement of several access
bridges, re-provision of the boundary security grilles and installation of
flood alarm systems in the village. The existing fields on either side of the
watercourse would act as flood plain, to provide additional storage during
flood peaks. In addition, the use of gabions to replace the existing concrete
lined banks and bed would improve the ecology of the stream over existing
conditions. The planned SSSI lies upstream of the Lin Ma Hang village while the
proposed improvement works are along and at the downstream of the village.
There will be no adverse residual environmental impacts during construction and
operational phases of the Project on the planned SSSI.
11.2.2
The upstream portion of MUP04A is a stretch of semi-natural
stream. In view of the potential ecological value, this upstream portion has
been avoided and retained. Flooding will be allowed to its surrounding
abandoned agricultural land during rain storms. Only the downstream portion
that has already been disturbed by village related development would be
channelised with stepped gabion banks and mattress lining to alleviate the
flooding risk to the nearby villages.
11.2.3
MUP05 is proposed to increase the capacity of the channel by
modifying and widening of banks but maintaining the existing natural channel
bottom (two-stage channel). Several sections of the channel bank have also been
retained. A number of large trees will be preserved and protected with gabions
within the channel.
11.2.4
In addition, the proposed works for MUP05 provided adequate
flow capacity thereby allowing works for MUP01 & MUP02 (non-designated
project elements) to be substantially scaled down. Only short sections of minor
bank stabilization works would be required for MUP01 & MUP02 with most of
the stream sections being retained. Both MUP01 & MUP02 have been identified
as Ecologically Important Streams (EIS) in the ETWB and DSD circulars.
11.2.5
Resumption of private land hence disturbance to the public has
been minimized by following the existing channel and road side drains.
11.2.6
By retaining existing stream beds as well as reusing
excavated rocks for the gabions, the amount of public fill requiring disposal
to public filling facilities can be minimized. Similarly, by reusing the
excavated sediment, no marine disposal would be necessary. Hence reducing the
burden on these disposal sites.
11.2.7
The overall population protected from flooding as result of
the Project will be about 780 located in the Man Uk Pin and Lin Ma Hang areas
of
11.3
Environmental Friendly
Designs Recommended
11.3.1
The principle of the channel design is to avoid environmental
impacts while at the same time to provide adequate flow capacity to achieve the
design flood protection standard, to consider public safety near unstable
banks, to take into account land constraints, to minimize public disturbance
from land resumption, cost effectiveness and ease of maintenance. Reference has
also been made to the ETWB and DSD technical circulars on protection of natural
streams/rivers.
11.3.2
When considering environmental impacts, minimization will be the
key design principle only when avoidance is not possible. The following
environmental friendly channel designs have been incorporated into the channel
design.
§
Natural stream beds and substrates would be retained as far
as practicable. Sections of stream bed would be lined with original materials
or natural substrates instead of using concrete lining.
§ The proposed channel banks would be constructed mainly of rock filled gabion basket rather than concrete or grasscrete. Sections of the stream bank have been retained where feasible.
§ Existing road side concrete drains would be replaced by trapezoidal channel with gabion banks and mattress lining. Existing concrete stream sections would be improved with gabions.
§ Sediment would be allowed to accumulate in the channel to permit re-colonisation of aquatic organisms.
§ ‘Two-stage’ channel design with the existing stream bed remains undisturbed is adopted for ecological significant sections of the watercourses.
§ Several large trees along the stream would be preserved in-situ and protected with gabion mattress.
§
Appropriate tree planting have been recommended to mitigate for
the loss of riparian corridor and to improve the visual appearance of the
proposed channels.
11.4
Key Environmental
Problems Avoided
11.4.1
The key environmental problems involve noise, water quality,
ecological and landscape & visual impacts during the construction phase. Minor
impacts from dust, waste and cultural heritage are expected.
11.4.2
The EIA study has critically
assessed the overall acceptability of environmental impacts which are likely to
arise as a result of the construction and operation of the proposed channels.
Where necessary and practicable, the EIA study has specified the conditions and
requirements for the detailed design, construction and operation of the Project
in order to mitigate environmental impacts to an acceptable level.
11.4.3
The EIA study has demonstrated
the acceptability of any residual impacts from the Project and the protection
of the existing streams, adjacent residents, Conservation Area and
environmentally sensitive resources. Environmental monitoring and audit works
have been recommended before and during construction to verify the expectations
of the EIA study and the effectiveness of the recommended mitigation measures.
11.5
Environmental
Benefits of Environmental Protection Measures Recommended
11.5.1
With the implementation of the recommended mitigation measures,
no unacceptable residual environmental impacts are expected. Details of the
implementation schedule of the recommended mitigation measures are summarized
in Appendix G.