6.1.1
The ecological impact assessment in
the EIA Report identified that the key issues in terms of ecology would be potential
direct impacts to Pak Ngan Heung (PNH), Tai Tei Tong (TTT), Luk Tei Tong (LTT)
and Luk Tei Tong (2) (LTT 2) rivers resulting from drainage channel
construction, and loss of marsh habitat at Luk Tei Tong as a result of LTT
By-pass Channel improvements. 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.
6.1.2
Measures to avoid, minimise and
compensate for identified ecological impacts are recommended in the EIA Report.
These measures include the use of natural materials to line the base of newly
created drainage channels, recreation of marsh habitat within LTT By-pass
Channel, and planting along the banks of the new channels to compensate for the
loss of small areas of woodland habitat. 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.
6.2.1
The scale of impacts to river habitats
would be determined largely by the design of drainage channels built to replace
existing rivers. Ecological requirements have been considered during the design
of drainage improvement works, and are described in the following sections.
Pak
Ngan Heung River
6.2.2
The proposed improvement works to PNH
River have been considered to avoid major impacts to natural sections of the
River. Drainage improvements would be implemented by constructing an
underground channel that would bypass a sharp bend in the river close to
Butterfly Hill. Under normal conditions, flow would be maintained within the
existing river channel. During high flow events, excess floodwaters would be
conveyed through the Bypass Channel, thus avoiding the sharp bend and
bottleneck in the River and alleviating flooding risk.
6.2.3
Under this design, modification to the
existing PNH River would be limited to two short sections upstream and
downstream of the proposed Bypass Channel. At the downstream section, the river
channel would be slightly widened, with new vertical concrete retaining walls
constructed along the banks of the channel. The channel widening would result
in some new sections of riverbed being created, which would be lined with
natural materials such as small cobbles and boulders. The existing riverbed and
substrates would be retained.
6.2.4
The upstream section of PNH River
would be widened and slightly re-aligned. The new channel banks would be
trapezoidal in cross-section, and lined with rock-filled gabion. Pits would be
provided in the gabion bank to allow for planting and establishment of riparian
vegetation. The depth of the new channel bed would be varied to re-create the
pool-riffle sequence found in the existing River, and a small weir at the
upstream end of the existing river currently maintained for crop irrigation
would be re-instated. Similar to the downstream section, the newly constructed
sections of riverbed would be lined with natural materials such as cobbles and
boulders.
Tai
Tei Tong River
6.2.5
The original design of drainage
improvement works at TTT River included the widening and straightening of
approximately 370m of the existing river, leading to the disturbance of much of
the slow-flowing lowland section of the river, and the loss of a substantial
amount of riparian habitat including numerous mature trees. Following a review
of drainage improvement requirements and public consultation, the proposed
drainage improvements at TTT River would now be limited to the widening of
three small bottlenecks along the existing river, with the total length of the
river affected by the works reduced by almost half to 160m. The bottleneck at
the downstream end of TTT River (close to the confluence of the three rivers)
would be widened along one bank only, with a new concrete retaining wall
constructed along this bank. At the two upstream bottlenecks, the newly widened
banks would be stabilised with rock-filled gabion.
Luk
Tei Tong River
6.2.6
The lower section of LTT River would
be slightly widened and deepened (total length 225m). Ecological enhancement
features to be included along the improved river section would include:
·
The channel bed would be constructed using natural
materials such as small cobbles and boulders.
·
The depth of the channel would be varied to re-create
the pool-riffle sequence found in the existing river.
·
The new channel banks would be trapezoidal in
cross-section, and lined with rock-filled gabion. Pits would be provided in the
gabion bank to allow for planting and establishment of riparian vegetation.
Luk
Tei Tong (2) River
6.2.7
The design of drainage improvement
works for LTT (2) River has been extensively revised taking into account public
consultation during the EIA Study. The proposed works would largely avoid
direct impacts to the river by creating a flood bypass channel, with just small
sections of the existing river connecting to the upstream and downstream ends
of the Bypass Channel affected. Similar to the PNH River, existing flow within
the flow LTT (2) River would be maintained under normal conditions, with excess
floodwaters diverted to the Bypass Channel during high flow events. Unlike the
PNH River Bypass Channel, the LTT Bypass Channel would be open (not routed
underground) due to the hydraulic constraints imposed by the high water table
in the area.
General
Measures to Minimize Ecological Impacts
6.2.8
The works programme has been
considered to avoid impacts to breeding wildlife where possible. For example,
works within Luk Tei Tong Marsh and the confluence of the three rivers would be
restricted to the dry season, thereby avoiding impacts to White-shouldered
Starlings (Sternus sinensis) that have been recorded breeding in the
Disused Watchtowers close to the river confluence, and also to other avifauna
and herpetofauna species that may breed within Luk Tei Tong Marsh and adjacent
areas.
6.2.9
All excavation works carried out
within or close to water bodies should be carried out in the dry season where
possible, with construction carried out by land-based plant. Excavation works
within river/stream channels should be restricted to an enclosed dry section of
the river, with containment measures such as bunds and barriers used within the
river to minimize 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 minimize 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.
6.2.10
Noise mitigation measures including
the use of quiet construction plant and temporary noise barriers (Section 3 of
the reports refers) should be implemented to minimise disturbance to habitats
adjacent to the works areas.
6.2.11
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 habitats and
vegetation, particularly mature trees.
·
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.
6.2.12
Potential operational phase activities
in newly constructed river channels 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).
·
For maintenance desilting of the re-profiled river
channels, temporary barrier walls shall be used to provide a dry zone for
desilting work.
·
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.
Luk
Tei Tong Bypass Channel
6.2.13
A standard design feature of most
engineered drainage channels in Hong Kong is the inclusion of a footpath or
access road along one or both banks of the channel. To minimise impacts to Luk
Tei Tong Marsh, no footpath would be constructed along the by-pass channel.
6.2.14
Construction of the Bypass Channel
would be limited to one dry season only, thereby minimising disturbance impacts
to wetland fauna potentially breeding in the Marsh Area.
6.2.15
Operation phase impacts resulting from
desilting works within the LTT Bypass Channel are expected to be very minor in
scale, given that water flow (and therefore sediment deposition) within the
channel would be minimal for much of the year, except during spate events.
Vegetation control would be required 1-2 times per year at most to prevent the
excessive growth of (especially woody) vegetation that would impede the
drainage capacity of the channel. These measures do not necessarily conflict
with the maintenance of the channel as marshland, as the removal and control of
vegetation would be required anyway to maintain the sub-climatic vegetation
community found in marsh habitats. It is recognised, however, that regular
cutting of vegetation could prevent the re-establishment of wetland
communities, with potential disturbance to breeding fauna of particular
concern. It is therefore recommended that vegetation control is undertaken
outside of the main breeding season for wildlife (April-August) to avoid
potential impacts.
Ling
Tsui Tau U-Channel
6.2.16
The construction of the Ling Tsui Tau
U-Channel is expected to cause minor impacts to woodland habitats close to
Butterfly Hill. To minimise these impacts, the results of the Tree Survey
Report for this Project should be considered when finalising the alignment of
the proposed works. The U-Channel alignment should be ‘fine-tuned’ to avoid
nearby mature trees.
Impacts
to Fish and Amphibian Species of Conservation Interest
6.2.17
As discussed in Section 7.7.9, proposed
works at the rivers have the potential to impact fish species of conservation
interest. To minimize these potential impacts, it is recommended that
capture-surveys of the proposed works areas are conducted prior to the
commencement of construction works in the channel. These surveys should include
fish species of conservation interest recorded during the surveys (Flagtail, Kuhlia
marginata & Predaceous Chub, Parazacco spilurus), with
particular attention paid to the Flagtail. Either of these species caught
during the surveys should be re-located to areas of the watercourse upstream of
the proposed works areas, or to aquaria set up close to the works areas.
Maintenance of the fish prior to translocation would be undertaken by an
ecologist appointed by the Project Proponent. The capture surveys should be
conducted in the dry season by a suitably qualified ecologist(s) appointed by
the Project Proponent. For the Flagtail (which will probably occur in low
densities at PNH River), further surveys should be conducted to confirm the
presence and numbers of fish in the streams closer to the time of construction.
A detailed methodology for fish translocation (based in part upon the Flagtail
survey results) should be formulated during the detailed design stage of the
Project.
6.2.18
In addition to fish species of
conservation interest, the proposed drainage improvement works could also
result in direct impacts to two amphibian species of conservation interest, RTF
and Chinese Bullfrog. It is therefore recommended that additional capture
surveys are conducted prior to the commencement of works to locate any
individuals of these two species within the works areas. The surveys should be
conducted in the wet season (March/April-October), when these species are
calling and are generally more conspicuous. Any individuals captured during the
survey should be relocated to similar nearby habitats outside of the works
areas. The capture surveys should be conducted by a suitably qualified
ecologist(s) appointed by the Project Proponent. A detailed methodology for
faunal translocation should be formulated during the detailed design stage of
the Project.
6.2.19
As the new drainage channels would be
approximately the same width as the existing watercourses, it is anticipate
that most of the substrates used to line the new channel beds can be taken from
the existing rivers. Rocks unearthed during excavation works for this Project
or material purchased from local quarries would be used as material to fill
gabion bank linings and as additional substrate to line the riverbeds if
required
6.2.20
In time, it is expected that
vegetation will naturally re-colonise newly constructed channel sections. To
expedite this process, planting of suitable aquatic, emergent and riparian
plant species within the beds and gabion-lined sections of channels will be
undertaken following the completion of works. Additionally, the gabion lining
will be hydroseeded. Planting design will be finalised in the detailed design
stage.
6.2.21
The newly created PNH River drainage
channel is expected to provide a suitable habitat for the fish species of
conservation interest (Flagtail, Kuhlia marginata) previously recorded
from the river. To further increase the value of the PNH River for this and
other fish species, it is recommended that a small fish ladder is constructed
at the upstream end of the new channel. The existing agricultural weir at this
location currently impedes the movement of fish between upstream and downstream
sections of the river. The fish ladder would facilitate movement past this
obstacle, extending the range of fish communities in both upstream and
downstream areas. A conceptual layout of the proposed fish ladder is given in
Figure 7.3 of the EIA Report, showing a basic design of rock-ramp fishway
commonly adopted in Australia and other countries to facilitate the passage of
various fish species over low obstructions. The detailed design of the ladder
would be finalised at a later stage of this Project.
Luk
Tei Tong Marsh
6.2.22
The proposed Luk Tei Tong River
drainage improvement works and Luk Tei Tong By-pass Channel would result in the
permanent loss of approximately 0.8ha of habitat within the Luk Tei Tong Marsh
Area. Although the affected area was found to be degraded and of less
ecological interest than more established areas of the Marsh, the proposed Luk
Tei Tong Bypass Channel has been designed to incorporate wetland habitats,
providing compensation and enhancement of habitats within the channel
alignment. The following key features would be included in the Channel design:
·
The Bypass Channel bed would be approximately 800mm
below the existing ground level, and would be lined with turf taken from areas
of Luk Tei Tong Marsh that fall within the footprint of the proposed Channel.
This area would be maintained as compensatory marshland habitat. So as to
maintain the flood protection function of the channel, vegetation within the
Channel would be maintained at a height of about 300mm.
·
A low weir would be constructed at the downstream end
of the Bypass Channel, serving to retain rainwater and some diverted
floodwaters in the Channel. As the channel would be slightly lower than the
existing ground level, and water within the channel would be retained, it is
anticipated that habitats within the channel provide a more valuable wetland
habitat than adjacent areas of Luk Tei Tong Marsh, where wetland vegetation is
not particularly well established.
·
The banks of the proposed Channel would be lined with
rock-filled gabion, and topped with small (200mm high) earth bund. The gabion
lining is required to stabilize the banks of the Channel, and the earth-bund
would prevent surface water from surrounding areas of the marsh from draining
into the Channel.
6.2.23
The intention of re-using turf from
the channel alignment is to provide suitable soil conditions for wetland
habitats within the Bypass Channel. Additionally, roots and seeds of some plant
species within the turf would speed up the establishment of wetland vegetation
within the Channel. It should be noted though that parts of the Channel
alignment are currently dominated by the weed Mikania micrantha, which
would require removal prior to the construction phase. Reference should be made
to Nature Conservation Practice Note No. 01/2003 for clearance of Mikania.
6.2.24
The construction of the LTT By-pass
Channel is expected to take approximately 5 months. There is some concern that
during this period, vegetation in sods of turf removed from the channel
alignment would suffer some die-back before being replaced within the channel.
To minimize potential die-back, it is recommended that turf removal (and
channel construction) is limited to sections no more than 100m in length. The
construction sequence would involve excavating the first 100m of the channel
(at the downstream end of the alignment). Turf from this section would be moved
to a temporary storage area. To minimize potential damage to this vegetation,
the turf would not be stacked in layers during storage, and would be adequately
watered for the duration of the storage period (i.e., 1-2 times daily).
Following removal of the first 100m of turf, the second 100m of the channel
would be excavated, and turf from this section immediately installed in the
first excavated section of the channel. This process would be repeated until
the last section of the channel is excavated. Turf removed from the first 100m
section and stored during the construction would be used to re-vegetate this
area. Even if some die-back occurs, it is expected that the roots and seed-bank
within the turf will provide adequate material for the re-generation of
vegetation following re-introduction into the LTT Bypass Channel. Furthermore,
vegetation from adjacent areas of the marsh would also be expected to
re-colonise substrates within the Channel, which would provide a suitable
growing medium for vegetation recorded from the marsh area. A programme would
be established to monitor the ecology (including vegetation communities) of the
LTT By-pass Channel during the operation phase (section 6.2.31-2 refers). If
there appears to be any problem with vegetative re-establishment, remedial
measures (including the planting of riparian/emergent /aquatic vegetation)
would be implemented (section 6.2.33 refers).
6.2.25
The new wetland area within the Bypass
Channel would cover an area of approximately 0.6ha. Although this would be
slightly smaller than the 0.8ha of existing habitat affected by the improvement
works, wetlands provided within the channel alignment would be of higher
ecological value than the habitats they would replace, providing adequate
compensation for habitat loss.
6.2.26
Temporary works areas adjacent to the
Bypass Channel alignment would be re-instated with the planting of appropriate
wetland plant species following the completion of construction works. No
long-term impact to these areas are expected.
Woodland
Habitat
6.2.27
Planting of trees and other vegetation
along the banks of the newly created channels would provide compensation for
unavoidable tree-felling and loss of small areas of open and secondary woodland
habitat resulting from the proposed TTT and PNH River Channel and Ling Tsui Tau
U-Channel works. Compensatory planting would make use of native plant species
with flowers/fruits attractive to wildlife. Compensation in terms of woodland
area and trees planted would be on at least a 1:1 ratio, covering an area of at
least 0.13ha.
6.2.28
The implementation schedules of the
recommended mitigation measures is presented in Appendix A. The proper implementation of mitigation
measures recommended in Sections 6.2.1-6.2.27 should be audited monthly by a
suitably qualified ecologist(s) with over 6 years local experience.
6.2.29
A specific monitoring programme of
improved sections of PNH and LTT Rivers is recommended. 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 on at least one occasion
2-3 months before the commencement of the construction phase, with monitoring
conducted on a monthly basis during the construction phase, and every two
months a year for 4 years after the completion of works. For the purposes of
baseline, construction and operation phase monitoring, the improved sections of
the PNH and LTT rivers should be divided into 50m long sections, and data
collected from each of the sections should include the following parameters:
·
Water quality (e.g., dissolved oxygen, pH,
conductivity, salinity, BOD, suspended sediments, ammonia, nitrate and
phosphate concentrations);
·
Sediment characteristics;
·
Water flow;
·
Avifauna species and densities;
·
Aquatic macroinvertebrate community species composition
and abundance;
·
Fish community species composition and abundance;
·
Adult odonate community species composition and
abundance; and
·
Aquatic, emergent and riparian vegetation community
species composition and abundance.
6.2.30
Reports on the findings of each
monitoring trip should be submitted to Agriculture, Fisheries and Conservation
Department (AFCD) and the Environmental Protection Department (EPD) for review.
A final report summarising the monitoring results over the entire monitoring
period should be prepared to serve as a reference for future projects of this
kind. Monitoring should be conducted by a qualified ecologist(s) with over 6
years relevant experience.
6.2.31
Monitoring of the LTT Bypass Channel
would be required to confirm that the proposed Channel design has provided suitable
compensation for impacts to Luk Tei Tong Marsh. It is suggested that ecological
monitoring of habitats within the Channel should be undertaken for a period of
at least 4 years. Data collected should include the following key parameters:
·
Water quality (e.g., dissolved oxygen, pH,
conductivity, salinity, BOD, suspended sediments, ammonia, nitrate and
phosphate concentrations);
·
Sediment characteristics;
·
Water depth and flow;
·
Avifauna species and densities;
·
Aquatic macroinvertebrate community species composition
and abundance;
·
Fish community species composition and abundance;
·
Herpetofauna community species composition and
abundance;
·
Adult odonate community species composition and
abundance; and
·
Aquatic, emergent and riparian vegetation community
species composition and abundance.
6.2.32
To provide baseline data, it is
suggested that the same parameters are measured along a strip of existing marsh
habitat adjacent to the Channel alignment. A comparison of species diversity
and densities along the Bypass Channel alignment and at the reference site
would allow the effectiveness of habitat creation to be assessed. Monitoring of
both the Channel and Reference Site should be conducted 6 times a year for 4
years after the completion of works. For the purposes of monitoring, the
Channel and Reference site should be divided into 50m long sections, with the majority of biotic and abiotic parameters
listed in section 6.2.31 monitored in each of the 50m sections. However, water
quality (e.g.,
dissolved oxygen, pH, conductivity, salinity, BOD, suspended sediments,
ammonia, nitrate and phosphate concentrations) would be measured at four locations along
the channel and reference site alignments only, as there would be little
expected variation in water quality along the length of the channel. The
proposed location of water quality monitoring stations for ecological EM&A
are shown in Figure 6.1. Reports on the findings
of each monitoring trip should be submitted to AFCD and EPD for review. A final
report summarising the results of monitoring over 4 years should be prepared to
serve as a reference for future projects of this kind. Monitoring should be
conducted by a qualified ecologist(s) with over 6 years relevant experience.
6.2.33
The newly constructed PNH and LTT
River Channels, and LTT Bypass Channel are expected to provide a similar
habitat to the habitats they would replace. It is therefore anticipated that
wildlife found in the existing rivers and marsh 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/reference data 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
baseline/reference 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 the channels should be implemented. Such measures could
include:
-
Minor re-profiling of the channel bed(s) could be
implemented to increase micro-habitat diversity within the channels. By
excavating small areas of the bed, areas of slow flowing water would be created
within river habitats, and small pools of very slow-flowing or standing water
would be created within the marsh.
-
Increasing heterogeneity of substratum would again
increase micro-habitat diversity: providing substrates of different sizes would
provide niches for organisms preferring small sediments (e.g., burrowing
dragonfly larvae) and larger cobbles/boulders (e.g., Heptageniid mayfly
nymphs).
-
Planting of suitable aquatic, emergent and riparian
vegetation could serve to increase vegetative cover.
-
Management of water levels within the LTT Bypass
Channel could be implemented by modifying the height of the weirs at either end
of the Channel; thus optimizing conditions for wetland habitat establishment
within the Channel.
·
Further monitoring of abiotic and biotic parameters
should be conducted to determine the relative success of the contingency
habitat rehabilitation measures.
6.2.34
A specific monitoring programme for the
White-shouldered Starlings is recommended. A qualified ecologist with over 6
years relevant experience should conduct the monitoring. Monthly baseline
surveys of the Disused Watchtowers should be undertaken in the wet season prior
to the commencement of construction phase activities. The aim of baseline
surveys should be to establish whether or not any breeding Starlings are still
present. If breeding activities are identified, the number of breeding pairs,
the success of breeding and responses to any disturbance should be recorded.
During the construction phase, monthly surveys of the Watchtowers are
recommended. Information should be collected on behaviour, breeding activity,
and any response to disturbance. The source of any disturbance impacts should
be identified. A simple Event and Action Plan is recommended in Table 6.1.
Table 6.1 Monitoring of White-shouldered Starlings:
Event and Action Plan
|
Event
|
Action
|
|
Environmental Team Leader
|
Contractor
|
|
Identification of
disturbance to breeding White-shouldered Starlings
|
1.
Increase
frequency of monitoring to twice weekly.
|
1.
Check
all construction actions and working methods.
|
|
2.
Notify
Site Engineer
|
2.
Submit
proposals for remedial action to prevent abandonment of the breeding site.
|
|
3.
Review
construction activities of pervious week.
|
3.
Implement
remedial action.
|
|
4.
Identify
any changes in construction activities in previous week.
|
4.
Liaise
with ET regarding effectiveness of remedial actions.
|
|
5.
Discuss
remedial actions with Site Engineer.
|
|