1.1.1
The Territorial Land Drainage
and Flood Control Strategy Study – Phase II (TELAFLOCOSS 2) recommended as part
of its findings that detailed studies were required to resolve secondary and
local stormwater drainage problems in five drainage basins in the North and
1.1.2 The YLDMP Study has identified problems in the existing drainage systems in the study area and has recommended drainage improvement proposals under a two-stage implementation programme. The Stage 1 works are to be implemented under this Assignment for mitigating major drainage problems requiring immediate attention. Stage 1 works are further divided into two phases, of which Phase 2 comprises the construction of various secondary drainage channels including the secondary drainage channel KT13 (the Project).
1.2.1
Secondary drainage channel KT13
(hereinafter also referred as KT13) is in the vicinity of a recognized egretry,
the Ho Pui Egretry. The site is located between two villages, Ma On Kong and Ho
Pui in southern Kam Tin in
1.3 Requirements under the Environmental Impact Assessment Ordinance
1.3.1 The construction and operation of secondary drainage channel KT13 (the Project) constitutes a Designated Project as defined under the Environmental Impact Assessment Ordinance (EIAO) pursuant to Item I.1(b)(vii) and Item Q.1 of Schedule 2 Part I of the Ordinance. An Environmental Permit (EP) from the Director of Environmental Protection is required for its construction and operation. In applying for the EP, the Project Proponent is required to submit an Environmental Impact Assessment (EIA) Report to the Director for consideration, in accordance with the statutory approval process in the Ordinance.
1.3.2 The purpose of the EIA is to provide information on the nature and extent of environmental impacts arising from the construction and operation of the proposed designated project and related activities taking place concurrently, and to contribute to decisions on the overall environmental acceptability of the Project.
2.1 Secondary Drainage Channel KT13
2.1.1 Secondary drainage channel KT13 has been proposed in the YLDMP to drain the areas in Ma On Kong where the existing streamcourse is insufficient, to deal with catchwater overflows for Ho Pui Reservoir to the south and to provide infrastructure for future development.
2.1.2 In order to improve the hydraulic performance of the streamcourse, the downstream section (Section A) of the streamcourse is widened and deepened to a trapezoidal channel with 5 m wide gabion base and banks, and depth 3 m. The upstream section (Section B) is also trained to a trapezoidal channel with 5 m wide gabion base and banks with a constant depth of 2 m.
2.1.3 A concrete bypass culvert will be built for the middle section of Ma On Kong streamcourse with the maintenance access to be formed above the bypass culvert. The bypass culvert is a twin cell box culvert. With the bypass culvert, the flow in the middle section of Ma On Kong streamcourse can be controlled so that there will be no overbank flow during a 1 in 50 years storm, while maintaining dry weather flow similar to existing condition in the unmodified existing middle section of the KT13 stream to ensure protection of Ho Pui Egretry and associated habitat.
2.1.4 With the widening and deepening of both upstream and downstream sections and a bypass culvert for the middle section, the Ma On Kong streamcourse will have adequate flow capacities to convey the maximum predicted flows within bank. Figures 2a and 2b show the details of KT13. Figure 3 shows the typical gabion bank construction.
2.2
Consideration of Alternative Alignments and Channel Lining Options
Alternative
Alignments
2.2.1 The design of the bypass culvert was initiated by the requirement to avoid modification of the middle section of the stream for preservation of Ho Pui Egretry and the associated habitats. The alignment of the proposed bypass box culvert is constrained to the east and the southeast by existing occupied village houses of the Ho Pui and the Ho Pui Egretry. To the west and to the north, the alignment is constrained by existing occupied village houses of Ma On Kong and grave sites. The evaluation of the alternative alignments and construction methods demonstrated that there is very little feasible variation for the design of the bypass culvert. The bypass culvert has avoided the more ecologically sensitive areas, the Ho Pui Egretry and the nearby stream habitat. However, a localized encroachment of about 420 m2 of the Conservation Area consisting of grassland habitat will inevitably be required. This encroachment has been reduced to an absolute minimum taking into account environmental impacts yet without compromising the main objective of providing flood relieve to the villagers.
Channel Lining Options
2.2.2 After considering the hydraulic performance, maintenance requirements and ecological function of the affected stream section, gabion design for the existing stream sections to be channelized is adopted, while the bypass culvert is of a twin box concrete design. This way the flood water is quickly bypassed down stream in case of rain storm, but the channelized sections using gabion will convey the dry weather flow less rapidly, retaining sediments and allow bank side vegetation to establish, and will retain some ecological value to the land occupy by the KT13 project. The bypass culvert in the middle section will convey all the peak flows while safely provide a continuous compensation flow to the existing stream course. This will sustain the ecological value of the watercourse and provide feeding for the egrets from adjacent roost.
2.3.1 The Yuen Long, Kam Tin, Ngau Tam Mei & Tin Shui Wai catchments are served by existing drainage systems. However, rapid urbanisation in the areas has resulted in a higher runoff ratio and the existing systems are insufficient to provide the required protection standard so that frequent flooding occurs in the area during heavy rainfall. In accordance with the findings of the YLDMP, 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. With the implementation of the Project, the defence of the well being and properties of local residents against flood risks can be much enhanced. In particular, the YLDMP identified that secondary channels in Tin Shui Wai, Kam Tin and Ngau Tam Mei areas are required:
(i) to drain areas where the existing secondary drainage is insufficient;
(ii) to solve known flooding problems;
(iii) to deal with catchwater overflows; or
(iv) to provide infrastructure for future development where applicable.
Consequences of Not Proceeding
2.3.2
Two
consequences of not proceeding with the project are identified as follows:
(i)
Flooding
will persist in the flood-prone areas in Yuen Long, Kam Tin, Ngau Tam Mei &
Tin Shui Wai identified in the DMP Study.
(ii)
Development
of the study area will be hindered.
2.3.3
Local
areas will still be vulnerable to flooding during heavy rainfall even after
completion of training of the primary channels because of deficiencies in the
secondary and local drainage systems. Moreover, urbanisation and changes of
land use will increase runoff and thus the flooding risk. It is therefore clear
that not proceeding with this project will leave residents exposed to
increasing and unacceptable flooding risk and will hinder development of the
NWNT.
Previous Studies (identifying need)
and Policy Acceptance
2.3.4
The
fore-runner of this project was initiated under a previous consultancy study
entitled “Territorial Land Drainage and Flood Control Strategy Study - Phase
II” (TELADFLOCOSS 2) which was conducted for Drainage Services Department to
address the flooding problems in the North-Western and
Needs and Benefits
of the Project
2.3.5 The main purpose of the Project is to bring relief to present drainage problems that have repeatedly led to severe flooding in the area. The villagers of Ma On Kong and Ho Pui will be benefited from the Project.
2.3.6 The introduction of the bypass culvert would addressed the over bank flow while at the same time avoided the middle portion of the existing stream which is in close proximity to the Ho Pui Egretry. The design would allow low flow to continue flowing in this middle section hence preserving the stream habitat next to the Egretry.
2.3.7 The Project would provide opportunity to remove odourous, unsightly and potentially contaminated sediment accumulated in the stream bed from previous livestock waste and domestic discharges. The Project also would remove some fill material that has been dumped in the nearby fishponds. With the use of gabion bank and base as well as the proposed compensatory and screen planting, the associated environmental impacts will be mitigated to an acceptable level.
2.3.8
Overall, the Project would
minimize the ecological impact while reducing the flooding risks to be
villagers.
2.4.1 Public consultation with the Green Groups, Kam Tin and Pat Heung Rural Committees, Yuen Long District Council and Village Representatives were undertaken during the course of the EIA study. Overall, there are no objections to the final design and the relevant village representatives and Rural Committees supported the proposed Project as it would bring relieve to the flooding in the area.
2.5.1 The construction of the Project is scheduled to commence as in mid 2007, and is expected to be completed in mid 2010.
2.6.1 Few concurrent projects will be present near KT13 at the construction stage as all the major civil works project such as West Rail and Kam Tin Main Drainage Channel have already been completed.
3.
SUMMARY OF THE EIA FINDINGS
3.1.1 The main ecological impacts identified include the loss of habitats and potential disturbance to the Ho Pui egretry. Practical mitigation measures have been formulated following the general policy for mitigating impacts under the Technical Memorandum on EIA Process which are Avoidance, Minimizing and Compensation. Avoidance is adopted in the channel design through careful alignment selection and the use of bypass channel to avoid the Ho Pui egretry and channelisation of the stream near the egretry, loss of CA zone and the nearby woodland. Minimization is achieved by reducing the duration & length of the construction works and restricting access to sensitive areas during construction. Screen planting and the use of gabion channel design will minimized the associated impacts during operation. Compensation planting will be provided for the habitat loss and to enhance a stream side area to become suitable habitat for egrets. Predicted ecological impacts and proposed mitigation measures (see Figure 4) to eliminate or reduce these to acceptable levels are summarized below.
Construction
Phase Impacts
Disturbance to existing stream habitats and fauna due
to channelisation
3.1.2 In the absence of mitigation measures, disturbance to stream habitats and fauna due to channelisation of sections of the stream is considered to result in a moderate adverse ecological impact. The nature of the project is such that the direct physical impacts on stream habitats and fauna cannot be eliminated or reduced; however, it is proposed to restrict the works area to a maximum of 75m length at Section A and Section B of the Channel except for the 100m sections at the extreme upstream and downstream limits of the project area. As the two work fronts will be constructed in one direction and since there are more than 300m separation between Section A and Section B, the works could be undertaken concurrently at Section A and Section B of the Channel. This will reduce the duration of construction phase impacts at any one location and will reduce the extent of habitat loss for wildlife which may use the streamcourse and surrounding riparian corridor as a feeding or roosting area.
3.1.3 It is considered that with such mitigation, the residual construction phase impact would be reduced to an acceptable level.
Disturbance to egretry due to channelisation
3.1.4 The Ho Pui Egretry was not occupied in 2005. Should the Ho Pui egretry be re-occupied, in the absence of mitigation measures, there is a significant risk that construction activity could cause it to be abandoned again, which would be a major adverse impact. However, by restricting construction works within 100m of the egretry to the non-breeding season direct disturbance impacts to the breeding egrets will be eliminated.
3.1.5 Because the proposed bypass culvert is close to the northern section of the egretry site, the project would make the egretry more vulnerable to casual disturbance in the event of its being re-established in this location. It is considered most likely that the egretry will not be re-occupied. However, in the event of re-occupation prior to the commencement of construction it is considered most probable that any egrets which are displaced from the disturbed part of the egretry will move to another part of the egretry or to another egretry in the area and it is unlikely that they will be prevented from breeding entirely. Accordingly, it is considered that the residual impact of any disturbance would, most likely, be insignificant or small and of only local importance and the risk of the residual impact being of moderate importance (birds being displaced and prevented from breeding) is very small.
Disturbance to habitats and fauna (other than the
egretry) due to construction of the bypass culvert
3.1.6 In the absence of mitigation measures, the construction of the bypass culvert would have only a low adverse impact of habitats and species other than egrets. Nonetheless, it is proposed to provide chain linked fence along the site boundary near the CA zone and Ho Pui Egretry to restrict access by the Contractor into these sensitive areas. Whilst there may be some impact on breeding habitat of White-throated Kingfisher, a sufficient unchannelised length of the stream will remain to provide breeding habitat for this species. Accordingly, it is considered that specific mitigation measures will not be required to mitigate for minor impacts on this species.
Changes to water table in adjacent wetlands
3.1.7 Construction works are predicted to result in the reduction of the water table in adjacent wetlands by up to 600 mm (both vertically and laterally). The overall area of wetlands affected would, therefore, be very small and the predicted adverse ecological impact is considered to be low. Making provision to limit the working area to a maximum of 75m length at both sections of the channel (except for the 100m extreme upstream and downstream limits) would further reduce the duration of this adverse impact at any one location. However, this does not alter the overall evaluation of the residual impact as it is already considered to be low in the absence of mitigation.
Operational Phase Impacts
Loss of stream habitat and
fauna
3.1.8 In the absence of mitigation measures, loss of stream habitats and fauna due to channelisation of sections of the stream is considered to result in a moderate adverse ecological impact. The proposed use of gabions to form the banks and bottom of the channelised sections of the stream will permit recolonisation by riparian and aquatic fauna and flora. Accordingly, it is considered that, with mitigation measures in place, operational phase impacts on stream habitat and fauna will be very low.
Loss of low-lying
grassland/fallow land of moderate ecological value
3.1.9 The proposed secondary drainage channel KT13 will result in the permanent loss of an area of low-lying grassland/fallow land. Though this habitat is considered to be of moderate ecological importance, the area affected is small and hence the scale of the impact is considered to be low. As such, no mitigation measures are considered necessary.
Loss of fishponds of
moderate ecological value
3.1.10 The proposed secondary drainage channel KT13 will result in the permanent loss of an area of fishponds. Though this habitat is considered to be of moderate ecological importance, the area affected is small and the loss of habitat and feeding areas for birds and other wildlife is also small. The scale of the impact is, therefore considered to be low and, in the absence of an alternative alignment for KT13, is considered to be acceptable without additional mitigation measures.
Loss of woodland of
moderate ecological value
3.1.11 Loss of woodland of moderate ecological value has been reduced to a minimum (only 0.008 ha) by fine-tuning the bypass channel alignment to minimize the requirement for felling of existing trees. Though this habitat is considered to be of moderate ecological importance, the area affected is very small and the ecological value of the habitat and individual trees to be lost is correspondingly small. As such the scale of the impact is considered to be low. No ecological measures to compensate for the loss of individual trees are considered necessary. However, it is proposed to compensate for the habitat loss by carrying out tree planting (Figure 4).
Disturbance to egretry
3.1.12 In the absence of mitigation measures, channelisation of the section of the stream which passes through the egretry site is predicted to result in a high likelihood of abandonment of the egretry, should it be re-established prior to the commencement of construction. This would be a major adverse ecological impact. Avoidance of channelisation of the section of stream (except for a small localized realignment of the meander) with bypass channel design which passes through the egretry site, together with the restriction of the timing of construction works is predicted to eliminate this adverse impact. The bypass channel design is such that continuous water flow similar to existing condition will be maintained at the unmodified portion of the stream.
3.1.13 However, in the long term, any adverse impacts to the egretry site will be eliminated once the recommended screen and compensation planting matures. This will screen the egretry site from human disturbance and will also provide potential nesting sites in the event that the egretry is re-established. Long term impacts on the egret population of the area are considered to be insignificant as there will be no net reduction in the available nesting, roosting and foraging habitat, either in the immediate vicinity of the egretry site or in the wider area.
3.1.14 Accordingly, there will be no significant permanent residual ecological impacts arising from this project. It is considered, therefore, that in the context of the wider benefits of the project in reducing flooding, the low to moderate impacts arising during the construction period are acceptable.
3.2.1 The use of powered mechanical equipment during the construction phase of KT13 is expected to create construction noise nuisance due to the close proximity between the works area and the sensitive receivers. Mitigation measures are available to reduce the construction noise impacts to acceptable levels. The recommended mitigation measures include use of quiet plant, the use of noise barriers and proper site practices. Provided the Contractor implements the recommended mitigation measures, the construction noise levels can be kept to reasonable levels at all times.
3.2.2 Noise monitoring is recommended at selected representative noise sensitive receivers during the construction period to ensure the effectiveness of the proposed mitigation measures and to monitor compliance with daytime construction noise levels for the construction activities being undertaken for the drainage improvement.
3.3.1 It is predicted that during the site formation and channel excavation stage of the construction phase, fugitive dust could cause potential air quality impacts at the nearby sensitive receivers. Construction dust may also arise from any uncovered excavated material stockpile or fall out from haulage traffic. A series of preventive measures have been proposed in the EIA to minimize potential fugitive dust impact. An environmental monitoring and audit programme has been designed to monitor the potential fugitive dust impacts and to audit the implementation of the dust preventive measures by construction contractors.
3.3.2
Temporary odor nuisance may
arise while excavating the polluted sediments from the sections of existing stream
to be channelized under the proposed KT13 works. Excavated sediments, while
waiting to be taken off site, will be contained in covered skips and removed
from site within 1 day.
3.4.1 The current condition of stream KT13 is observed to be free of debris or rubbish, the water is free flowing, clear but still has noticeable odour. It continues to receive some domestic wastewater from the nearby un-sewered villages houses and surface runoff from the nearby livestock farms.
3.4.2 The construction of the channel could adversely impact the water bodies through silt-laden site runoff, disturbance of stream sediment during excavation and concreting works. These impacts can be readily mitigated through the construction of a suitable drainage system with silt traps, good site management practices and careful working practices when excavating contaminated sediments and proper sewage collection and disposal system.
3.4.3 Environmental monitoring and audit requirements have been identified for water quality to establish the up to date baseline conditions of the potentially affected water bodies, and during the construction phase, which among other purposes, will check the effectiveness of the implementation of the proposed mitigation measures.
3.4.4 With the effective implementation of the mitigation measures, impacts on the water quality of receiving water bodies due to the operation of the project are expected to be negligible.
3.5.1 Construction and demolition materials will inevitably be produced during the construction phase of KT13. Construction phase wastes will include site clearance and demolition material, excavated materials, contaminated sediments, waste concrete, wooden material, chemical waste, aqueous waste, wheel wash waste, sewage, and municipal/domestic waste. The potential for re-use and recycling has been evaluated and appropriate pathways for disposal have been identified. Waste types, quantities and timing have been estimated as far as possible and mitigation measures evaluated in terms of the avoidance-minimization-recycling-disposal hierarchy. The responsibilities for recycling and disposal have also been identified.
3.5.2 Provided that there is strict control of construction and demolition materials generated from construction works and that all arisings are stored, handled, transported and disposed of in accordance with the recommended mitigation measures, potential impact is not expected. The recommended mitigation measures can be enforced by incorporating them into an Environmental Management Plan as part of the contract document. Environmental audit would be necessary to ensure the implementation of proper waste management practices during construction.
3.6.1 The EIA have reviewed the archaeological potential of all the works area which will be disturbed or excavated under this project. It concluded that no evidence was found of archaeological deposits, in situ or otherwise in the areas sampled under this EIA. A comprehensive record have been made and included as part of this EIA of the cultural heritage features which lie within 35 to 90 m from the works area of KT13. Visual, vibration and water level impacts are expected to be negligible at all identified features except a Qing dynasty grave which lie at a distance of less than 15m from the proposed bypass culvert. A condition survey before and during the construction phase of the project is required as part of the environmental monitoring and audit to ensure protection of the structural integrity of this historic feature.
3.7.1
Potential
Landscape Resource and Landscape Character Impact are assessed within the study area. Major
landscape resources identified include approximately 800m of semi-natural stream, natural
woodland and orchard/horticultural land with approximately 910 trees. There are no rare
and protected tree species, champion trees, old and valuable trees or species of conservation importance identified. Dominant tree species in natural
woodland includes Schefflera octophylla,
Pinus massoniana, Aporusa chinensis, Celtis sinensis, Bridelia tomentosa, Cinnamomum camphora, Rhus chinensis, Rhus succedanea, Ficus
hispida and Macaranga tanarius. Landscape Resource
Impact is identified as including channelization of approximately 800m semi-natural stream, filling of approximately 830m2
fish pond and approximately 134 and 59 trees being affected permanently
and temporarily.
3.7.2
Landscape
Character Areas of the study area include agricultural, woodland, stream, fish pond, industrial
and nullah. Particular sensitive Landscape Character Areas are woodland,
stream and fish pond Character Area, which range from medium to high. Moderate adverse impact
on stream Landscape Character Area is identified. Impact on other Landscape
Resources and Landscape Character Areas is considered to
be slight to
insubstantial.
3.7.3
Potential
visual
impacts are
assessed based on the Visual Sensitive Receivers (VSRs)
of significance. VSRs
identified include residents, industrial workers, motorists and
pedestrians and sitting-out area users. Particular sensitive VSRs are the
residents of village Ma On Kong, Tai Kek and north of Ho Pui, which is high in
sensitivity. During construction phase, residual visual impact upon
VSRs is predicted as moderate to slight. During operation phase, residual visual impact upon VSRs is predicted as slight to
insubstantial.
3.7.4 Mitigation measures, which include trees preservation, compensatory and buffer planting (including approximately 148 trees and bamboos), gabion embankment and substratum, aesthetical design/chromatic treatment to vehicular and pedestrian crossings, maintenance access road, and sitting-out area, etc. shall be implemented according to the Landscape Mitigation Plans (Figures 5a and 5b). It is concluded that Landscape and Visual Impacts of this Project is considered acceptable with implementation of mitigation measures.
3.8
Environmental Monitoring and Audit (EM&A) Requirements
3.8.1
Mitigation
measures have been recommended in the EIA Report to mitigate potential adverse
impacts from the construction and operation of the Project. Based on these
recommendations, EM&A requirements have been established.
3.8.2
Necessary and appropriate requirements on mitigation measures
will be included into the construction contract documents. To ensure that the environmental
performance of the works meet all relevant legal and contractual requirements,
the Contractor will be required to prepare and implement an Environmental
Management Plan, which details the approach the Contractor is to adopt in
managing and controlling potential environmental impact from construction
activities for this Project. In
addition, to verify compliance with legal and contractual requirements;
adherence to the Environmental Management Plan; and that undue impacts are not
posed to the environment especially the ecology of the Project Area, the
Contractor will be required to implement an EM&A programme.
4.1.1 With due consideration of the recommended design requirements for environmental impact mitigation, and the implementation of the required mitigation measures during construction phase, the residual impacts will be acceptable. Residual operation phase impacts in terms of ecology, landscape and visual impacts will also be acceptable. Other types of operation phase impacts are not expected. Overall the EIA concludes that there are no insurmountable environmental impacts associated with the implementation and operation of the secondary drainage channel KT13 provided that all the legislation, guidelines, administrative and technical mitigation measures are followed.
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