7.1.1
The ecological impact assessment in
the EIA Report identified that the key issues in terms of ecology would be
potential direct impacts to moderate ecological value rivers as a result of drainage
improvement works. Other identified impacts included potential increases in
sedimentation during construction phase affecting aquatic communities
downstream of works areas, and increased disturbance to wildlife during the
construction phase resulting from noisy construction phase activities and
increased human activity.
7.1.2
Measures to minimise and compensate
for identified ecological impacts are recommended in the EIA Report. These
measures include the design of Sai Kung and Ho Chung River drainage improvement
works to incorporate ‘ecologically friendly’ features, and extensive
compensatory planting along the banks of the new channels to compensate for the
loss of riparian vegetation. Further measures to control potential increases in
sedimentation, minimise construction phase disturbance and other identified
impacts were also recommended. Ecological mitigation measures and monitoring
requirements are detailed in the following sections.
Ecological Mitigation Measures
7.2.1
The most substantial identified
impacts would be expected to occur at Sai Kung and Ho Chung Rivers, where extensive modification
of the existing river channel would be undertaken to improve drainage capacity.
The design of the improved drainage channels has been considered to minimise
and compensate for these impacts, as described below:
-
Sai Kung River
For the Sai Kung River, a short section of the river (approximately
150m) would be deepened and slightly widened. The base of the improved channel
would be lined with natural substrates (e.g., gravel, cobbles and boulders). To
re-create existing conditions as closely as possible, substrates found in the
existing river section would be used to line the new channel. Substrates lining
the improved channel would be arranged to re-create the existing riffle-pool
sequence found in the river. The banks of the improved channel would be lined
with rock-filled gabion to improve stability. Pits would be provided in the
gabion to allow planting of herbaceous riparian vegetation.
- Ho Chung River
Approximately 650m of the Ho Chung River would be subject to
drainage improvement works. Along most of the affected river section, the
existing riverbed and substrates would be retained, and works would be limited
to widening the river along the southern bank. Newly widened sections of
riverbed would be lined with natural substrates (e.g., cobbles and boulders),
with the size and arrangement of these substrates selected to complement
adjacent areas of the existing riverbed. The substrates would be sourced as far
as practicable from materials excavated during the river widening. The majority
of the southern bank of the widened channel would be lined with rock-filled
gabion. Pits would be provided in the gabion to allow planting of herbaceous
riparian vegetation.
7.2.2
The excavation works for channels
should be carried out in the dry condition where possible, with construction
carried out by land-based plant. Excavation works should be restricted to an
enclosed dry section of the river, with containment measures such as bunds and
barriers used within the river to minimise the impacts upon the downstream
water body. Site runoff should be directed towards regularly cleaned and
maintained silt traps and oil/grease separators to minimise the risk of
sedimentation and pollution of river water. The silt and oil/grease separators
should be appropriately designed for the local drainage and ground conditions.
To minimise leakage and loss of sediments during excavation in narrow channels,
tightly sealed closed grab excavators should be deployed where material to be
handled is wet.
7.2.3
Noise mitigation measures including
the use of quiet construction plant and temporary noise barriers should be
implemented to minimise disturbance to habitats adjacent to the works areas. In
particular, measures such as noise barriers should be used to minimise
disturbance to the bat roost identified close to the Pak Kong works area.
7.2.4
The proposed drainage improvement
works would unavoidably impact several individuals of the rare tree species Ehretia
acuminata recorded at Ho Chung. It is recommended that the trees and
saplings be transplanted to another area within the Ho Chung site boundary
before the commencement of construction phase activities (Figure 7.1 gives a
suggested location for the receptor site. The exact location for
transplantation would be determined by the
Landscape Architect/Others who conduct the transplantation)..
Fencing would be erected around the transplanted trees/seedlings to minimise
potential construction phase disturbance.
7.2.5
It is also recommended that additional
measures are undertaken to mitigate for impacts to E. acuminata: seeds
from the existing trees should be collected and propagated. The seeds should be
collected from mature fruits (the fruiting period of E. acuminata is
from May to September) prior to transplantation, and germinated in containers
under suitable conditions. Following germination and growth, the seedlings
should be planted at suitable locations within the Ho Chung Works Area. The
survival of newly planted seedlings should be monitored. Seed collection,
propagation, planting and monitoring of E. acuminata should be
undertaken by a suitably qualified botanist/horticulturalist appointed by the
Project Proponent. A detailed methodology for these works should be formulated
during the detailed design stage of the Project, in consultation with AFCD.
7.2.6
Potential operational phase activities
would be limited to regular channel maintenance such as de-silting. Impacts to
aquatic communities resulting from these activities are expected to be minor.
Nevertheless, the following measures are recommended to minimise potential
impacts resulting from operational phase activities:
·
To minimise sedimentation, de-silting
should be limited to the dry season (November-March).
·
The implementation of de-silting and
other activities that could disturb aquatic fauna should be phased to ensure
some areas of relatively undisturbed habitat remain available for resident
aquatic fauna at all times.
·
Waste material produced during
de-silting should be disposed of in a timely and appropriate manner.
7.2.7
Standard good site practice measures
should be implemented throughout the construction phase. The measures should
include:
·
Placement of equipment or stockpile in
designated works areas and access routes selected on existing disturbed land to
minimise disturbance to natural or moderate-high ecological value habitats.
·
Construction activities should be restricted
to work areas that should be clearly demarcated. The work areas should be
reinstated after completion of the works.
·
Waste skips should be provided to
collect general refuse and construction wastes. The wastes should be disposed
of timely and properly off-site.
·
General drainage arrangements should
include sediment and oil traps to collect and control construction site
run-off.
·
Open burning on works sites is
illegal, and should be strictly prohibited.
7.2.8
The improved Ho Chung River is
expected to provide a suitable habitat for fish communities previously recorded
from the river. To further increase the value of the River for fish
communities, it is recommended that small fish ladders are constructed over the
weirs along the river channel. Existing weirs within the river currently impede
the movement of fish between upstream and downstream sections of the river. The
fish ladders would facilitate movement past these obstacles, extending the
range of fish communities in both upstream and downstream areas. The detailed
design of the ladders would be submitted to AFCD for comment and approval
before finalisation.
7.2.9
The banks of the newly created
channels would be extensively planted with trees and other vegetation to
provide compensation for unavoidable tree-felling and loss of riparian
vegetation resulting from the proposed works. Where possible, compensatory
planting should make use of native plant species with flowers/fruits attractive
to wildlife.
Environmental Monitoring and Audit Requirements
7.2.10
The proper implementation of
mitigation measures recommended in Sections 7.2.1-7.2.9 should be audited.
7.2.11
A specific monitoring programme of
channels constructed to replace Sai Kung and Ho Chung Rivers shall be carried
out by a suitably qualified ecologist(s) (with at least 7 years’ experience in
ecological monitoring) appointed by the Project Proponent or ER. The programme
should provide data on the re-establishment of aquatic/riparian communities in
the channels, and allow an assessment of the relative success of mitigation
measures to be made. Baseline data should be collected several months before
the commencement of the construction phase, with monitoring conducted 3 times a
year during the construction phase, and 3 times a year for 3 years after the
completion of works. Data collected should include abiotic (e.g., water
quality, sediment characteristics) and biotic (e.g., avifauna,
macroinvertebrate and fish populations, aquatic, emergent and riparian
vegetation) parameters. Reports on the findings of each monitoring trip should
be submitted to AFCD for review/discussion. A final report summarising the
results of monitoring over 3 years should be prepared to serve as a reference
for future projects of this kind.
7.2.1
The newly constructed Sai Kung and Ho
Chung River Channels are expected to provide a similar habitat to the rivers
they would replace. It is therefore anticipated that wildlife found in the
existing rivers would successfully re-colonise the new channels. However, if
the post-construction phase monitoring demonstrates that re-colonisation is
unsuccessful the following contingency plan should be implemented:
·
Baseline data collected prior to the
construction phase should be compared with post-construction phase data. Any
substantial differences in habitat characteristics (e.g., water quality,
aquatic/emergent vegetation) should be noted.
·
The potential cause of differences
between pre- and post-construction phase habitat characteristics (e.g., changes
in water depth, sediment characteristics) should be identified.
·
Measures to further rehabilitate the
habitat characteristics of river channels should be implemented (e.g., minor
re-profiling of riverbed, planting of emergent/aquatic vegetation, increasing
heterogeneity of substratum).
·
Further monitoring of abiotic and biotic
parameters should be conducted to determine the relative success of the
contingency habitat rehabilitation measures.
7.2.2
Following transplantation of the two Ehretia acuminata trees and six
seedlings, and the planting of seedlings germinated from seeds collected from
the two trees, regular monitoring of the trees and seedlings should be
conducted to check on the health and condition of the plants. Monitoring should
cover the 12-month period following transplantation and planting, and be
conducted at least twice a month for the first four months after
transplantation/planting, and once a month for the remaining eight months.
Monitoring should be conducted by a suitably qualified
botanist/horticulturalist appointed by the Project Proponent.