2 Project description

2.1 Purposes and Objectives of the Project

2.1.1.1 The purposes and objectives of the Project are to revitalise the existing Tai Wai Nullah (TWN) with the aim of enhancing the ecological value of TWN, providing a greener environment, promoting water friendliness and improving the community environment. The Project also includes provision of dry weather flow interceptors to improve the water quality.

2.2 Project Site Location and Site History

2.2.1.1 The Project site is located at the centre of Tai Wai District and covers the existing TWN. TWN is an old concrete-lined nullah of approximately 2 km long and 40 m wide. It is one of the three main tributaries of Shing Mun River. It originates from Needle Hill and flows in the southeast direction towards Shing Mun River. Figure 2.1 illustrates the location plan of the Project.

2.2.1.2 The Project site is mainly surrounded by residential developments. The current zonings of the Project site under the Approved Sha Tin Outline Zoning Plan (OZP) No. S/ST/36 include “Industrial” (“I”), “Green Belt” (“GB”), “Comprehensive Development Area” (“CDA”), “Other Specified Uses” (“OU”), “Residential (Group A)” (“R(A)”), “Residential (Group B)” (“R(B)”), “Village Type Development” (“V”), and “Government, Institution or Community” (“G/IC”).

2.3 Project Scope and Design

2.3.1.1 The Project comprises the following key components:

(a) beautification of the existing nullah (approximately 2 km long and 40 m wide) by re-surfacing, greening and modification of channel bed;

(b) provision of approximately 3 km walkways along the nullah and associated stairs and access ramps;

(d) provision of features for ecological enhancement and sustainable drainage system, and two underground water pumps installed near mid-stream of TWN and underground water pipes for associated water retention and supplement;

(e) construction of viewing decks and revamp of existing footbridges/bridges;

(f) revamp / provision of footpaths, railings, pavilions, amenity areas and public open spaces along the nullah and on the nullah bed, as well as the associated UV disinfection system for water play features;

(g) improvement / modification of existing planters; and

(h) associated works including landscaping, utility works, etc.

2.3.1.2 General layout of the Project is shown in Figure 2.2 and details of the design scheme are illustrated in Appendix 2.1. The details of the design of the revitalisation works would be subject to changes at the detailed design stage, including actual alignment of the low flow channel and dimensions of proposed above-ground structures (e.g. viewing decks, revamp of existing footbridges/bridges, revamp/provision of footpaths, pavilions (in form of sheltered benches / seating) and visitor facilities for amenity areas (e.g. riparian walkway, UV disinfection system, toilet and management office), would be subject to detailed design stage.

2.3.1.3 Subsequent to the issue of the EIA Study Brief (No. ESB-320/2019), underground water pumps installed near mid-stream of TWN (beneath Shing Mun Tunnel Road by the bankside) and underground water pipes (mainly housed within the DWFI system) have been proposed to support the provision of ecological enhancement features in the nullah in view of the insufficient baseflow in TWN (Section 2.5.1.9 refers). Under the latest revitalisation design, an UV disinfection system has also been proposed for water play features to be provided in riparian amenity areas (fully separated from waterbody of TWN) to promote water friendliness. Furthermore, the Project boundary has been altered to cover modification of channel bed and bank downstream of TWN’s broad-crested weir at its confluence with Shing Mun River near Man Lai Court, with an aim to improve the existing excessive accumulation of silts brought by tidal influence due to uneven surface and flat gradient of the existing channel in front of the broad-crested weir; the areas near the cul-de-sac of Heung Fan Liu Road due to construction site constraints; and mid-stream near Shing Mun Tunnel Road for overall planning of the mid-stream riparian public open space. In accordance with Clause 6.2 of the EIA Study Brief, it has been checked if there would be any key changes in the scope of the Project mentioned in Section 1.2 of the EIA Study Brief and in Project Profile (No. PP-586/2019). Confirmation has been sought in writing on whether or not the scope of issues covered by the EIA Study Brief can still cover the key changes, and the additional issues, if any, that the EIA report must also address. It was demonstrated that these changes would not fundamentally alter the key scope of the EIA Study Brief, the scope of issues could still be covered by EIA SB and hence no fresh EIA SB would be required (Appendix 2.3 refers).

2.3.2 Construction Activities

2.3.2.1 The scope of the abovementioned key components involves the following construction works elements:

Channel Bed Modification

· Excavation works

· Re-surfacing and greening of the nullah

· Construction of energy dissipation structures at upstream section

Improvement of Existing Walkways and Riparian Public Open Spaces

· Construction of viewing deck / pavilion, renovation of existing footbridges

· Construction of staircases and access ramps

· Construction of riparian walkway, water play features and UV disinfection system, and visitor’s facilities (toilet and management office)

· Demolition of existing footbridge

Construction of Dry Weather Flow Interceptor System

· Connection works of DWFI system to the existing sewerage system; and

· Construction of DWFI channel along both sides of the nullah bed.

Landscaping and Miscellaneous Works

· Landscaping / greening of the bankside along the nullah; and

· Utility works.

Desilting at Downstream Tidal Zone

· Desilting at downstream tidal zone.

Construction of Mid-stream Underground Water Pumping Facilities for Ecological Enhancement-Associated Water Retention and Supplement

· Construction of underground pumping facilities (mid-stream underground water pumps and waterpipe along the nullah) for water supplement.

2.3.3 Operational Activities

2.3.3.1 During the operational phase , similar to the existing practices, regular maintenance works for the drainage and sewerage systems along TWN, including desilting along the nullah and minor maintenance to the DWFI system, would be carried out by the DSD to remove excessive silts, debris and any obstructions to safeguard the hydraulic capacity of the nullah. Maintenance desilting of the nullah would tentatively be carried out on an annual basis during dry season (November to March) when the water flow is low, except during emergency situations where the accumulated silt would adversely affect the hydraulic capacity of the nullah or where flooding risk is imminent, or when complaints on environmental nuisance associated with the accumulated silt are received. Minor maintenance to DWFI system would only be undertaken on an as-needed basis and frequent maintenance (i.e. monthly) would not be required. The maintenance practices and frequency would be similar to the existing maintenance works undertaken by the DSD.

2.3.3.2 The operation of the two proposed underground water pumps near mid-stream of TWN southeast to Pok Ngar Villa (one duty and one standby) will be controlled by a level detector installed at the modified stilling basin (to be deepened and expanded as detailed in Section 2.5.1.9 ) at upstream of TWN. Water flowing from the low flow channel will be collected in a wet well by gravity through a screened inlet pipe installed at the low flow channel. The water will then be pumped upstream to the modified stilling basin through pressurised underground pipes. The on-duty pump will not operate during night-time (i.e. 2300 – 0700 hours). The stored water in the basin will be naturally discharged for recharging environmental flow in the nullah.

2.4 The Need of the Project

2.4.1 Environmental Benefits of the Project

Water Quality Improvement and Promotion of Water Friendliness

2.4.1.1 Due to ageing drainage systems and possible expedient connection made in the past, polluted discharges from the existing drainage outlets along TWN are observed at present. Complaints and concerns from the public, including Sha Tin District Council (STDC) and local stakeholders, on the long-standing water quality and odour nuisance of TWN, particular at the confluence of TWN and Shing Mun River area near Man Lai Court, were received (Section 2.9 refers). With the implementation of the proposed DWFI system and treatment wetlands, the existing polluted dry weather flows from the drainage outlets along TWN will either be intercepted and discharged to the existing sewerage system via gravity mains (i.e. no DWFI pumping station required) for conveying to Shatin Sewage Treatment Works for treatment by the DWFI system, or be discharged into the treatment wetlands installed at drainage outlets for in-situ polishing. Furthermore, with the modification of channel bed downstream of TWN’s low flow channel near its confluence with Shing Mun River near Man Lai Court, the excessive accumulation of silts brought by tidal influence caused by uneven surface and flat gradient of the existing channel in front of the broad-crested weir would be greatly reduced. With the abovementioned designs, the water quality and hygiene within the nullah would be improved and odour nuisance to the resident nearby would be alleviated.

2.4.1.2 As detailed in Table 2.1, after the revitalisation works, similar to the existing condition, the nullah bed of the engineering channel of TWN would be dry most of the time during non-rainy days, except for the low flow channel in the middle. While the main water body of TWN, i.e. the low flow channel, is not suitable to be opened up for public access / physical contact due to safety reasons, the improvement of water quality and riverside environment of the nullah due to the Project would enhance user experience and hence promoting water friendliness culture. In order to allow public to better enjoy the view of the revitalised waterscape and ecological enhancement features (e.g. treatment wetland), construction / modification of footbridges / walkways and public open spaces is proposed along the nullah. The riparian areas (nullah bed of the engineering channel of TWN outside of the low flow channel) of midstream TWN near Pok Ngar Villa would also be opened for public access and be transformed into a public open space with accesses with stepped seats, riparian walkways and other community amenities [including water play features (fully separated from the water body of TWN)] to promote water-friendliness. The riparian walkway and amenity areas will be properly designed to restrict public access / physical contact to the water body of TWN in order to ensure the public safety (e.g. elevated walkway design, fencing, provision of warnings / signage etc.). Channel management system, rain detection and safety system and flood warning will also be adopted and to be managed by DSD and access to the riparian walkway and amenity areas [provided with water play features] will be closed under extreme weather conditions.

2.4.1.3 Freshwater discharge from the existing drainage outlet of the nullah near Pok Ngar Villa, which receives mainly clean river flow from upstream natural hillside catchment north to Peak One, will be partially intercepted and conveyed to the proposed UV disinfection system for treatment prior to be used in the water play features (e.g. water spouts and fountains). Based on the preliminary design, no additional stormwater collection facilities would be required for the conveyance of freshwater supply for the proposed water play features. DSD Practice Note (PN) No. 3/2021 “Guidelines on Design for Revitalisation of River Channel” lays out the Water Quality Goals (WQGs) for primary contact recreation (type 3 designated use) at revitalised river channel, i.e. fulfilling the existing Water Quality Objectives (WQOs) adopted for respective water control zone (WCZ), i.e. Tolo Harbour and Channel WCZ (Section 5 refers), E. coli not exceeding 180 cfu/100 mL (annual geometric mean) (same as the relevant WQO for bathing beach) and having no objectionable odour. A more stringent treatment standard will be adopted for the water supply for the proposed water play features that in addition to fulfilling the relevant WQGs for type 3 designated use, E. coli bacteria should be non-detectable (in cfu/100 mL, calculated as the geometric mean of all samples collected in one calendar year) in the treated water. There would also be provision of proper signage and deployment of specific management personnel to ensure safe and appropriate usage of the water play features. The water play features will be closed when the disinfection system is not under normal operation, under maintenance or when the treatment standard as specified in the post-revitalisation water quality monitoring programme established and implemented by DSD is not compiled with (Section 5 refers). The detailed designs of the UV disinfection system and water play features would be conducted in the next stage of the Project in consultation with relevant Government authorities and reference would be made to other existing water play features in Hong Kong (e.g. splash pads, water cascade facilities, water jets and spray grounds etc.).

2.4.1.4 The riparian walkway and amenity areas would be properly designed to restrict public access / physical contact to the water body, i.e. the low flow channel, and to ensure public safety (e.g. elevated walkway design, fencing, provision of warnings / signage, deployment of specific management personnel etc.). In particular, it should be noted that neither the water from the revitalised nullah with improved water quality or the water play features utilising treated freshwater shall be used for human or animal consumption, bathing or showering, food preparation / washing. Channel management system, as well as rain detection, safety and flood warning system (e.g. water level sensor at the low flow channel) will be adopted and managed by the DSD and access to the riparian walkway and amenity areas [provided with water play features] will be closed in case of foreseeable flooding at the nullah bed or under extreme weather conditions.

Greening Enhancement

2.4.1.5 The existing concrete nullah of TWN will be resurfaced with layer of vegetation, which will enhance both the aesthetic and ecological values of TWN. Greening of the nullah and along its banksides, such as provision of emergent wetland planting (e.g. reed and other aquatic herbs), mangroves, climbing plants, tree and / or grasscrete etc, will be provided. The existing mature trees growing by the bankside along the nullah will be retained, and additional tree planting will be provided along the nullah. The existing planters will also be modified to beautify the footpath along TWN.

Ecological Enhancement

2.4.1.6 The existing TWN is of low ecological value in view of its concrete-lined channel bed and bank, as well as its limited baseflow, especially in dry season (i.e. from November to March). With the proposed modification of the existing stilling basin (part of channel bed modification works) and the underground water pumps installed by the bankside near mid-stream of TWN for associated water retention and supplement, the following ecological features will be incorporated into the nullah to improve and create aquatic and terrestrial habitats, enhancing its ecological values:

· Reprofiling of the low flow channel and construction of in-stream weirs to create a meandering alignment along the nullah to mimic natural stream pattern where practicable, providing a series of microhabitats for wildlife in surrounding areas;

· Provision of DWFI system to improve water quality and hence support ecological enhancement;

· Provision of treatment wetlands (artificial wetland created to mimic processes in natural wetland ecosystems involving vegetation, soils, and their associated microbes through engineering design) at selected drainage outlets along TWN to remove pollutants and to improve stormwater quality (in terms of biochemical oxygen demand, ammonia, total suspended solids, total phosphorus, and pathogens), which can also serve as greening and ecological enhancement measures. General constructed wetland system of smaller scale for aesthetic and ecological enhancement purposes is also proposed;

· Greening of the nullah and along its banksides, such as provision of emergent wetland planting (e.g. reed and other aquatic herbs), mangroves, climbing plants, tree and / or grasscrete;

· Incorporation of planting designed to attract wildlife species (e.g. native host plant species for butterflies) during revamp of public open space along TWN;

· Provision of nest boxes / hunting perch for wildlife (e.g. birds) along TWN; and

· Modification of channel bank and existing vehicular access at downstream section of TWN into a mix of stepped terrace and slope at water edge to enhance foraging / pre-roost habitats for waterbirds (e.g. ardeids).

2.4.2 Scenario with the Project

2.4.2.1 With the implementation of the Project, over 70,000 m² of concrete-lined channel space would be transformed into green spaces to promote greening of the local environment and the ecological value of existing TWN would be enhanced with the incorporation of the ecological features as described in Section 2.4.1.6. Furthermore, the introduction of the DWFI system and treatment wetlands to intercept/polish the existing polluted discharges at the drainage outlets along TWN, as well as modification of channel bed downstream of TWN’s broad-crested weir at its confluence with Shing Mun River near Man Lai Court to minimise excessive accumulation of silts brought by tidal influence, would improve the water quality and odour issues associated with the current polluted discharges to the nullah and with excessive siltation at downstream TWN. In order to facilitate the public to better enjoy the view of the revitalised waterscape and ecological enhancement features (e.g. treatment wetland) as well as to promote water-friendliness, certain locations of the nullah bed of TWN will be transformed into a green open space that can be accessed by the public with provision of riparian walkway and amenity areas for public enjoyment. There will be multiple large entrance points with stepped seats and decked platforms on nullah bed that can provide communal spaces for recreation and leisure activities to promote water friendliness (e.g. riparian walkway, sightseeing of greening and ecological enhancement features, water play features equipped with UV disinfection system) at midstream of TWN. Cantilevered viewing decks / sheltered walkway are also proposed by the bankside in the upstream section. No environmental disbenefits would be envisaged from the proposed revitalisation works.

2.4.3 Scenario without the Project

2.4.3.1 TWN flows through a number of large housing estates and the district’s major commercial and industrial areas. During non-rainy days, the nullah bed of the engineering channel of TWN is dry most of the time, except for the low flow channel in the middle that contains mainly dry weather flows carrying some polluted discharges. Over 70,000 m² of channel space has been left unused at present. In addition, the cracked and stained concrete-lined channel shows little sign of aquatic or terrestrial life.

2.4.3.2 Without the Project, TWN would remain as a concrete channel with little aesthetic, ecological and social values, and would occupy the vast amount of space only serving a single purpose of flood conveyance. In addition, the opportunity of resolving the existing water quality and odour issues due to the polluted discharges and excessive siltation would also be negated.

2.5 Consideration of Different Development Options

2.5.1.1 Various options in regard to the extent of DWFI system, flood prevention, water retention and replenish methods, and ecological enhancement designs have been considered with a view to optimising the revitalisation design of TWN in order to enhance the ecological value of the nullah, provide a greener environment, promote water friendliness and improve the community environment whilst avoiding, minimising and mitigating any potential adverse environmental impacts of the Project.

Extent of DWFI System

2.5.1.2 Drainage outlets with polluted discharges were identified between Sections 1 to 5 of TWN (Section 2.6.2 and Figure 2.2 refer) through the pollution source identification survey conducted under this Project. Environmental implications of the following three options regarding the extent of DWFI system for the Project have been considered with a view to striking a balance between water quality improvement and ecological enhancement in the nullah.

· Option 1 (No Interception) refers to the existing condition where all dry weather flows are discharged to the nullah directly without interception or treatment;

· Option 2 (Full Interception) refers to a full-scale DWFI system that intercepts the dry weather flows from all the drainage outlets identified with polluted discharges along the nullah within part of Section 1 (from north of Tsuen Nam Road near Tai Wai Soccer Pitch) to Section 5 of TWN. Section 6 of TWN is connected to WSD’s facilities that expedient connection/misconnection or significant polluted surface runoff due to the nature of the catchment area would be unlikely. Likewise, part of Section 1 (south to Tsuen Nam Road) of the nullah is subject to tidal influence with the outlets being submerged mostly in water that the discharges cannot be intercepted by the instream DWFI system. DWFI system is hence not considered for the part of Section 1 (south to Tsuen Nam Road) and Section 6 of the nullah.

· Option 3 (Partial Interception) refers to a more site-specific design which takes into account of the water quality condition of each outlet to provide DWFI system only to some drainage outlets with highly polluted discharges (e.g. from expedient connection) and to construct treatment wetlands at remaining outlets for in-situ polishing of the less polluted discharges (e.g. outlets connected to the urban roadside drainage system). The water treatment efficiency of the treatment wetlands would be highly dependent on their sizes and retention time of the influent (which is associated with steadiness of water flow at the nullah). Similar to Option 2, DWFI system would be provided along the nullah within part of Section 1 (from north of Tsuen Nam Road near Tai Wai Soccer Pitch) to Section 5 and not be required for outlets in part of Section 1 (south to Tsuen Nam Road) and Section 6 of the nullah.

2.5.1.3 Environmental considerations of each option and the implications of each option on the revitalisation design are summarised in Table 2.1.

Table 2.1 Comparison of Alternative Extent of DWFI System

Design Considered	Environmental Benefits	Environmental Disbenefits	Implications on Revitalisation Design
Option 1 – No Interception	· No diversion of additional pollutant loads to the existing sewerage system	· Existing water quality and odour issues due to polluted discharges from drainage outlets along TWN would remain unresolved	· Poor water quality of TWN due to polluted discharges is undesirable for ecological enhancement, promotion of water friendliness or improvement of community environment and hence not considered
Option 2 – Full Interception of polluted discharges from drainage outlets to DWFI system only	· Improvement to existing water quality and odour issues by intercepting all dry weather flows from the drainage outlets identified with polluted discharges along the nullah within part of Section 1 (from north of Tsuen Nam Road near Tai Wai Soccer Pitch) to Section 5 of TWN	· Increase pollutant loads to the existing sewerage system (comparatively more than Option 3)	· Polluted discharges would be fully eliminated to improve water quality of TWN through DWFI system. · Baseflow of nullah would be reduced and may not be able to support aquatic habitat / wetland creation for greening and ecological enhancement. · Additional water retention and replenish facilities are required to provide sufficient baseflow for in-stream ecological enhancement
Option 3 – Partial Interception of polluted discharges from drainage outlets to DWFI system and provision of treatment wetlands for the less polluted discharges (Preferred)	· Improvement to water quality and odour issue by intercepting polluted dry weather flows from some of drainage outlets identified with highly polluted discharges by DWFI and along the nullah within part of Section 1 (from north of Tsuen Nam Road near Tai Wai Soccer Pitch) to Section 5 of TWN. · Provision of treatment wetlands to the drainage outlets along the nullah within part of Section 1 (from north of Tsuen Nam Road near Tai Wai Soccer Pitch) to Section 5 of TWN for in-situ polishing of the less polluted discharge can improve water quality and odour issues whilst serving as ecological enhancement and greening	· Increase pollutant loads to the existing sewerage system (comparatively less than Option 2)	· Polluted discharges would be fully eliminated by DWFI system or polished by treatment wetlands. · Baseflow of TWN would be reduced, while to a smaller extent than Option 2 – Full Interception, still may not be able to support aquatic habitat / wetland creation. · Additional water retention and replenish method is required to provide sufficient baseflow for in-stream ecological enhancement

2.5.1.4 Considering the TWN has sufficient space with gentle gradient and hence comparatively slower flow velocity, sufficient sizes of treatment wetlands could be provided and adequate retention time of influent for pollutant removal could be achieved. As such, provision of treatment wetlands at drainage outlets would be effective in polishing the remaining non-intercepted polluted discharges.

2.5.1.5 As compared to Option 2, Option 3 not only would improve the water quality and resolve the odour problem of TWN with partial interception of the highly polluted discharges by DWFI system and provision of treatment wetland for polishing the remaining non-intercepted polluted discharges, it also would minimise the addition of pollutant loads to the existing sewerage system and enhance the ecological values of TWN by providing additional wetland habitats for wildlife. Hence, it is selected as the preferred option.

Water Retention and Replenish Methods and Ecological Enhancement Designs

2.5.1.6 While the water quality of TWN could be significantly improved with the proposed partial interception of the highly polluted discharges by DWFI system, the limited baseflow of the nullah would be further reduced that would constrain the in-channel ecological enhancement designs. As such, water retention and replenish methods have been explored to retain and supplement water to the nullah in order to provide sufficient water flow to support a healthy aquatic habitat in the channel.

2.5.1.7 With the provision of treatment wetland for in-situ polishing of non-intercepted flow with less polluted discharges as discussed in Section 2.5.1.4, the polished flow could serve as a part of the environmental flow to sustain the ecological function of the flora and fauna and habitat present within the revitalised TWN.

2.5.1.8 In-stream fixed weirs would be incorporated in the reprofiled low flow channel to retain water depth for aquatic habitats. In natural streams, rocks and boulders service as natural ‘weir’ to maintain water depth and create microhabitats. Apart from the revitalisation works, TWN would remain its primary function for flood discharge that loose natural substrates can be easily washed away and cause damages and debris to the downstream area. Fixed weirs are therefore proposed to resemble the function of natural weirs. By impounding floodwater or environmental flows, weirs are particularly useful in the dry channels to retain water for a longer period, which could promote plant growth and aquatic species survival in such an otherwise dry environment.

2.5.1.9 The existing stilling basin at upstream of TWN [40m (W) x 45m (L) x 1m (D)], which has served to store water during storm events and to recharge the flow in the nullah on dry days, would be deepened and expanded during channel bed modification to increase the water storage capacity while maintaining sufficient water depth for the wetland development in the modified basin [40m (W) x 45m (L) x 3.4m (D) + 30m (W) x 55m (L) x 1.5m (D)]. In view of the limited and inconsistent baseflow, especially in dry season, underground water pumps are proposed to be constructed near mid-stream section of TWN downstream of Pok Ngar Villa to pump water to the modified stilling basin for replenishing as detailed in Section 2.3.3.2. The stored water in the basin sourced from the stormwater from upstream TWN and mid-stream underground water pumps can be used for recharging environmental flow in the nullah, which can help maintain a healthy aquatic habitat and support the proposed emergent wetland planting for in-channel greening and ecological enhancement to mimicking the biodiversity of a natural stream environment and creating an aesthetically pleasing landscape.

2.5.1.10 As detailed in Table 2.3, in response to public comments received, construction of energy dissipation structures (e.g. baffle blocks) is also proposed in the upstream section of TWN immediately downstream to WSD’s facilities to protect the downstream aquatic organisms from being washed away due to comparatively high flow rate of water discharged from Lower Shing Mun Reservoir.

Minimisation of Siltation in Downstream Tai Wai Nullah

2.5.1.11 With the proposed DWFI system and treatment wetlands, the existing polluted dry weather flows from the drainage outlets along TWN would be intercepted/polished and the associated water quality issues and odour nuisance to the nearby resident would be improved. Apart from the environmental nuisance caused by polluted discharges from drainage outlets along the nullah, excessive accumulation of silts brought by tidal influence due to uneven surface and flat gradient of the existing channel in front of the broad-crested weir at downstream TWN near its confluence with Shing Mun River near Man Lai Court was observed to cause environmental nuisance to nearby residents. In order to tackle this existing issue, modification of channel bed downstream of TWN’s broad-crested weir has been incorporated into the revitalisation design to minimise the deposition of excessive silt and resolve the associated environmental nuisance.

2.5.1.12 Taking advantage of the modification of channel bed downstream of TWN’s broad-crested weir at its confluence with Shing Mun River near Man Lai Court, part of the existing concrete vehicular access at the nullah bank would be reconstructed into a mix of stepped terrace and slope with greenings (e.g. planter, grasscrete) at water edge to create foraging / pre-roost habitats for waterbirds (e.g. ardeids) and to enhance greening, whilst maintaining the necessary emergency access function of the existing vehicular track.

2.6 Consideration of Alternative Construction Methods and Sequences of Works

2.6.1 Construction Methods

2.6.1.1 The proposed revitalisation works would be constructed by traditional open-cut and in-situ concreting method that mainly involve site clearance, soil excavation, sheet-piling, bore piling, formwork, concreting, backfilling, and reinstatement. Desilting works at downstream tidal zone would be carried out at the commencement and prior to the completion of the construction works to remove the silting accumulated at the nullah, in addition to the routine maintenance desilting works undertaken by DSD along TWN and by CEDD along Shing Mun River under existing arrangement. Environmental considerations of various construction methods for the installation of DWFI system are discussed as follows.

Dry Weather Flow Interceptor System

2.6.1.2 There are two approaches for the DWFI system construction, including conventional cast in-situ construction method and pre-cast structure. The major parts of the DWFI system are u-channel and pipe system cast-in with the concrete blocks at nullah wall for stabilisation. The concrete mass is generally 1 m wide and 1 m deep.

2.6.1.3 In-situ concrete casting could induce environmental impacts such as construction noise, dust and water quality, yet it is expected that the duration and extent of the impacts would be temporary and localised and could be mitigated by proper measures during the construction phase. While the pre-cast structure is constructed in fabrication yard away from the Project site that its production could cause less environmental nuisance to the surroundings, the concrete mass should fit the shape of existing nullah wall and the size of the drainage outlets, which vary along the nullah that on-site modification to the pre-cast structure is inevitable due to the actual site constraints. Utilisation of powered mechanical equipment (PME) such as breaker would be required that the environmental impacts for both approaches would be similar. Alternatively, if a new precast structure is decided to be adopted instead of on-site modification, detailed and prolonged coordination between different parties including structural, architectural, etc. would be required that the whole construction period might be prolonged and would result in generation of additional construction waste from the abandoned precast structure and disturbance/nuisance from the repeated loading and unloading activities / trial installation of concrete mass. Furthermore, off-site precast units are considered not practical nor cost effective due to site constraints with limited accessibility, which hinders the transportation of the precast units.

2.6.1.4 Considering that the traditional cast in-situ construction method could allow more flexibility for on-site alteration and is the most straightforward method for this Project, prolonged construction duration and repeated construction activities would be avoided and hence reduce potential disturbance/nuisance to the environment and local public, the construction method of in-situ construction of DWFI system is considered more suitable and practical and is adopted.

2.6.2 Construction Extent and Sequence

2.6.2.1 The Project site will be divided into six sections to be constructed in sequence starting from downstream TWN (Figure 2.2 and Appendix 2.2 refer) to reduce the construction impacts on nearby sensitive receivers comparing with constructing simultaneously in adjoining sections. The construction works along the nullah within each section would last for approximately 1 year and will not be undertaken at the entire section at the same time.

2.6.2.2 For each section, the construction works within the nullah, including nullah bed modification (including construction of energy dissipation structures), construction of DWFI system along the nullah, underground water pipes along the nullah, construction of riparian walkway and amenity areas, and desilting works at downstream tidal zone (Appendix 2.1 refers), will only be undertaken during dry season to reduce the potential water quality impacts due to surface run-off. To further reduce the water quality impacts, the excavation in the nullah will be carried out in an enclosed area surrounded by concrete blocks, sandbag barriers or other appropriate physical barriers. Likewise, other construction works outside the nullah such as walkway improvement will be scheduled to avoid overlapping with works within the nullah to avoid heavy construction activities concentrating in a certain area over any period.

2.7 Construction Programme

2.7.1.1 The construction works are tentatively scheduled to commence in Q1 2024 for completion in Q4 2029. A tentative construction programme for the Project is provided in Appendix 2.2. This programme provides the basis for the assessments presented in the EIA Report.

2.8 Concurrent Projects

2.8.1.1 Table 2.2 summarises the potential concurrent projects within 500 m from the Project boundary and the designated project "Revitalisation of Fo Tan Nullah" which is under the same Assignment that involves construction works within or near Shing Mun River and its tributaries, which may contribute to the cumulative environmental impacts during construction and / or operational phase. Indicative locations of these concurrent projects are shown in Figure 2.3.

Table 2.2 Potential Interfacing Projects

Project	Tentative Construction Programme	Potential Cumulative Impacts
Project	Tentative Construction Programme	Potential Cumulative Impacts
Revised Trunk Road T4 in Sha Tin	Q4 2023 – Q3 2028	The footprint of Revised Trunk Road T4, which involves mainly road network construction and modification works for connecting Shing Mun Tunnel Road and Tsing Sha Highway in the northwest with Sha Tin Road, would partly overlap with the Project in mid-stream TWN (near Pok Ngar Villa and north of Tsing Sha Highway). Based on the approved EIA of Revised Trunk Road T4 in Sha Tin (AEIAR-231/2021). During construction phase, in view that the most dusty and noisy construction works of the Revised Trunk Road T4 within the site area of overlapping with this Project would be the foundation works for the Revised Trunk Road T4 viaducts, the tentative construction programme and works at the interface area between the two projects could be arranged efficiently, e.g. scheduling works to be carried out during dry season, through close liaison between DSD’s and CEDD’s contractors of this Project and the concurrent project respectively in order to avoid construction works of respective works contracts to be carried out concurrently at the same interfacing areas and CEDD has agreed to include this requirement in the respective works contract. Cumulative construction environmental impacts, such as air quality, noise and water quality impacts upon the downstream water body have been taken into account in this EIA Study and can be minimised with such arrangement in place. This concurrent project would also involve construction of bridge piers within Shing Mun River from November 2023 to September 2025. Potential cumulative water quality and ecological impacts to downstream areas in Shing Mun River Channel and Tolo Harbour during construction phase have also been taken into account in this EIA Study. During operational phase, the key issues of the operation of the concurrent project are non-point source surface runoff from new impervious areas as well as the potential hydrodynamic and water quality impact on Shing Mun River due to the installation of permanent structures. Potential cumulative water quality impacts from these two projects have been taken into account in this EIA Study.
Drainage Improvement Works in Sha Tin and Sai Kung (proposed box culverts in Chui Tin Street)	Q2 2026 - 2031	Drainage improvement works are proposed in various locations in Sha Tin and Sai Kung under Drainage Improvement Works in Sha Tin and Sai Kung. Based on the latest available information, the box culverts proposed in Chui Tin Street under this concurrent project would fall partly within the 500m assessment area of this EIA Study, while no detailed construction programme of the concerned box culvert was available at time of the preparation of this EIA Report. During the detailed design stage, the contractor of this Project should be requested to closely liaise the contractors of this potential concurrent DSD project in planning the interfacing works properly to minimise the potential cumulative impacts by avoiding/ minimising repeated and concurrent construction works.
Revitalisation of Fo Tan Nullah	Q1 2024 – Q4 2029	TWN is situated at approximate 2.7 km from FTN. Although the construction programmes of the two projects are very similar, given the large separation distance between the two concurrent revitalisation works of FTN and TWN, insignificant cumulative construction phase air quality and noise impacts would be expected. Potential cumulative water quality and ecological impacts to downstream areas in Shing Mun River Channel and Tolo Harbour during construction phase have been taken into account in this EIA Study. Both the Project and Revitalisation of FTN would improve water quality and hygiene of the two nullahs that no unacceptable cumulative operational phase impact would be anticipated. The cumulative sewerage impact from the proposed DWFI system of the Project and Revitalisation of FTN have been taken into account in this EIA Study.
Joint-user complex at Tsuen Nam Road, Tai Wai	2024 – 2030	The proposed Joint-user complex at Tsuen Nam Road, Tai Wai falls within the 500m assessment area of this EIA. Based on the latest available information, the site would be redeveloped for multi-purpose uses (residential use and community facilities) and is tentatively planned to be occupied in 2030. No detailed construction programme of the concurrent project was available at time of the preparation of this EIA Report. Nonetheless, relevant cumulative environmental impacts, such as noise and water quality have been taken into account in this EIA Study.

2.9 Public Consultation

2.9.1.1 Consultation exercise carried out include:

· Meeting with Sha Tin District Council in June and July 2018, March 2021

· Meeting with Mei Lam Estate in August 2018;

· Meeting with Sha Tin District Council Health and Environmental Committee in March 2019;

· Meeting with Green Groups in July 2020 and July 2022;

· A series of public engagement exercises (e.g. interviews, workshops and surveys) through the use of Design Thinking Approach between December 2020 to August 2021.

2.9.1.2 During the public inspection period of the Project Profile submitted for the Application of EIA Study Brief (No. PP-586/2019) in 2019, comments received were largely in favour of the Project, in particular, the proposed DWFI system and ecological enhancement are considered to be beneficial to the community. Members of the Sha Tin District Council, general public and green groups are supportive of the Project. Key findings and concerns obtained from public inspection period of the Project Profile and the public consultation meetings, as well as follow up actions taken are summarised in Table 2.3.

Key Concerns	Follow-up Actions
Water quality issues and odour nuisance	DWFI system and treatment wetland are proposed to intercept / polish polluted dry weather flows that are discharged from drainage outlets along both sides of TWN. Modification of channel bed downstream of TWN’s broad-crested weir at its confluence with Shing Mun River near Man Lai Court to minimise the deposition of the silt brought by tidal influence due to uneven surface and flat gradient of the existing channel in front of the broad-crested weir is also proposed. With the abovementioned designs, the water quality and hygiene within the nullah would be improved and odour nuisance to the resident nearby would be alleviated.
Promotion of water friendliness, access to nullah and public safety	After revitalisation works, during non-rainy days, similar to the existing condition, the nullah bed of the engineering channel of TWN would be dry most of the time, except for the low flow channel in the middle. The main water body of TWN, i.e. the low flow channel, is considered not suitable for public access, physical contact / recreational uses for safety reasons. However, with the improved water quality, there is an opportunity to enhance user experience along TWN and hence promoting water friendliness culture. In order to facilitate public to better enjoy the view of the revitalised waterscape and ecological enhancement features (e.g. treatment wetland) as well as promote water-friendliness, certain locations of the nullah bed of TWN will be transformed into a green space with provision of riparian walkway and amenity areas to allow public access for enjoyment . There will be multiple large entrance points with stepped seats and decked platforms on nullah bed that can provide communal spaces for recreation and leisure activities to promote water friendliness (e.g. riparian walkway, sightseeing of greening and ecological enhancement features, water play features equipped with UV disinfection system) at midstream of TWN. Cantilevered viewing decks / sheltered walkway are also proposed by the bankside in the upstream section. Since TWN would continue its primary function for flood discharge, channel management system, and rain detection and safety system will be adopted and to be managed by DSD to ensure the public safety at the riparian walkway and amenity areas. Maintenance / operation practices will make reference to guidelines and considerations on safety and risk management, flood control, water quality, and leisure and amenity as stipulated in DSD PN No. 3/2021. A flood warning system would be developed which can provide the earliest warning for approaching storms and anticipated flooding, allowing sufficient time for individuals to evacuate in a safe and orderly manner. The access points into the nullah bed will be carefully planned with some access being designated for emergency use only (for instance emergency access stairs is proposed along the Mei Lam Estate boundary with openings along the existing parapet walls) to reduce the nuisance induced by the public access on nearby residents.
Insufficient baseflow to support proposed ecological enhancement	As detailed in Sections 2.5.1.6 to Section 2.5.1.10, water retention and replenish methods including construction of in-stream fixed weirs, modification and expansion of existing upstream stilling basin, provision of treatment wetland for in-situ polishing of flow with less polluted discharges instead of full interception by DWFI system, construction of underground water pumps, would be incorporated in the revitalisation design in order to maintain sufficient baseflow to support a healthy aquatic habitat and in-channel ecological enhancement. In order to prevent channel bed erosion and to protect the downstream aquatic organisms from being washed away due to flow from Lower Shing Mun Reservoir, energy dissipation structures (e.g. baffle blocks) are proposed in the upstream section of TWN immediately downstream of the Reservoir. Other ecological enhancement features along bankside and public open space along TWN (e.g. thematic greening along banksides and public open space, provision of nest boxes / hunting perch for wildlife) will also be installed to promote ecological connectivity.
Avoidance and minimisation of impacts to roosting ardeids	Ardeid night roost was recorded along Shing Mun River nearby the Project, including the mature trees alongside Man Lai Court and Sha Tin Government Secondary School (Section 9 refers). The ardeid night roosting sites in vicinity of the Project have been avoided when considering the revitalisation design. There are existing mature trees along FTN on pedestrian road within the Project site. Although these tree species are not of conservation importance, the tree specimens are mature with good foliage and tree crown that can potentially provide habitat for fauna species or serve as ardeid night roosts. These mature trees will therefore be retained under the revitalisation design. Under the revitalisation design, existing concrete channel bank and existing vehicular access at downstream section of TWN would also be modified into a mix of stepped terrace and slope at water edge to enhance foraging / pre-roost habitats for waterbirds (e.g. ardeids). No construction works within the nullah in Section 1 (channel bed modification and desilting at downstream tidal zone) should be undertaken from 16:30 to 07:30 during dry season (October to March) and from 17:00 to 07:00 during wet season (April to September) in order to minimise the potential disturbance to pre-roosting / night-roosting ardeids. Likewise, no construction works with PME should be undertaken within 100m from any night roost should be undertaken from 16:30 to 07:30 during dry season (October to March) and from 17:00 to 07:00 during wet season (April to September) to avoid disturbance for all work sections 1 – 6 of TWN.