12. CONCLUSIONS AND RECOMMENDATIONS
Secondary drainage channel 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
12.2.1 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.
12.2.2 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, depth of 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.
12.2.3 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 egretry and associated habitat.
12.2.4 The consequences of not proceeding with the project are that flooding will persist in the flood-prone areas of Ma On Kong and Ho Pui and the development potential of the study area will be limited.
12.3.1 The alignment of the proposed bypass box culvert is constrained to the east and the southeast by the Ho Pui Village and Ho Pui Egretry. To the west and to the north, the alignment is constrained by existing 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 Ho Pui Egretry, 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
12.3.2 Options for channel lining for banks and base include natural bed (gravel or mud), gabions, concrete or grasscrete. While the hydraulic performance of concrete and grasscrete lined channels are best for smooth flow and easy maintenance, they will remove the ecological values of the land on which they occupy. Also, the rate of transport of sediments downstream, whether clean or contaminated, will also increase, resulting in increase in suspended solids and biological oxygen demand (BOD) in water.
Gabions and natural stream beds offer
surfaces for retaining sediments and establishment of vegetation on banks. This
would also reduce the rate of sediment loss from the catchment and enter into
12.3.4 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 streamcourse. This will sustain the ecological value of the watercourse and provide feeding for the egrets from the adjacent roost.
12.4.1 The construction of the secondary drainage channel KT13 is scheduled to commence in mid 2007, and is expected to be completed by mid 2010.
12.4.2 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.
12.5.1 The key environmental issues under this project include potential impacts on the Ho Pui egretry, its associated stream side habitats, construction waste, contaminated sediments from stream bed and water quality. A Qing dynasty grave is located at very close distance to the proposed bypass culvert and will require protection during construction phase. Other environmental issues include potential impacts from construction noise, fugitive dust, visual and landscape impacts.
12.5.2 Predicted significant ecological impacts and proposed mitigation measures to eliminate or reduce these to acceptable measures are summarised below:
Construction Phase Impacts
Disturbance to existing stream habitats and fauna due to channelisation.
12.5.3 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 75 m length at Section A and Section B of the Channel except for the 100 m sections at the extreme upstream and down stream limits of the project area. As the two work fronts will be constructed in one direction and since there are more than 300 m 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.
12.5.4 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
12.5.5 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.
12.5.6 Because the proposed by-pass 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.
12.5.7 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
12.5.8 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 75 m length at both Sections of the Channel (except for the 100 m 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
12.5.9 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
12.5.10 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
12.5.11 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
12.5.12 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 at the location shown in Figure 4.13.
Disturbance to egretry
12.5.13 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 potential 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.
12.5.14 However, in the long term, any adverse impacts to the egretry site will be eliminated once the recommended screen planting of bamboo 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 or in the wider area.
12.5.15 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.
12.5.16 Approximately 80,293 m3 of construction and demolition material is likely to arise from this project. As this project involves the widening and channelization of the existing channels its potential for reusing the excavated material is limited. Suitable methods and responsibilities for waste minimization, reuse, recycling and disposal have been identified according to each type of waste.
12.5.17 It is expected that some of the sediments to be excavated from the stream bed will be contaminated. Sediment quality assessment has been conducted in accordance with ETWB TCW No. 34/2002 Management of Dredged / Excavated Sediment and appropriate marine disposal sites have been obtained from the relevant authority.
12.5.18 A Construction and Demolition Material Management Plan (C&DMMP) was prepared according to the ETWB TCW No. 33/2002 Management of Construction and Demolition Material Including Rock and the information in this EIA. The Contractors will have to prepare and implement an Environmental Management Plan which shall include among others waste management measures for Engineer’s approval. With the implementation of the required measures the pollution impacts from inappropriate management of construction waste will be prevented.
12.5.19 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.
12.5.20 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.
12.5.21 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.
12.5.22 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.
12.5.23 This 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 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.
12.5.24 Potential construction noise impacts are expected during site clearance, channel excavation, form work construction and concreting. The exceedance range from 1 to 15 dB, depending on the relative distances between the relevant noise sensitive receivers and the area where the construction activity is taking place. The contractors are required to adapt their construction equipment schedule so that the noise levels are sufficiently reduced, as well as installing temporary noise barriers to screen the proposed works from direct line of sight of the potential noise sensitive receivers at specific locations.
12.5.25 The minimum density requirement for temporary noise barriers is 10 kg/m², with a minimum height of 3 m. The material for temporary noise barriers should be gap free. It is predicted that the proposed mitigation measures will sufficiently reduce the potential noise impact to an acceptable level and residual noise impacts are not expected.
12.5.26 Monitoring and audit requirements have been identified for several representative noise sensitive receivers, to ensure the effectiveness of the proposed mitigation measures and to report and investigate any complaints associated with construction noise impacts which may arise.
12.5.27 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. Fugitive dust may also arise from any uncovered excavated material stockpile or fall out from haulage traffic. The potential impact will be more pronounced if works are undertaken during dry season. 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.
12.5.28 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, should be contained in covered skips. The material must be removed from site within 1 day. Any leachate from the skips should be stored in covered buckets or tanks and regularly collected by licensed contractors with the toilet waste for discharge to government sewer.
Landscape and Visual
12.5.29 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.
12.5.30 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.
12.5.31 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.
12.5.32 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. It is concluded that Landscape and Visual Impact of this Project is considered acceptable with implementation of mitigation measures.
12.6.1 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. Appendix J shows the mitigation measures implementation schedule for secondary drainage channel KT13. Overall this EIA concludes that there is 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.