6 WATER QUALITY

6.1 Existing Water Quality

6.1.1 Ground water is not abstracted from the study area with the possible exception of some remote villages houses and thus not relevant to this assessment. Thus, the key concerns during the implementation of the scheme relate to the protection of marine and river water bodies. It should be noted, however, that much of hinterland in both the northern and southern parts of the study area forms part of the Water Supplies Department (WSD) water gathering ground, as shown in Figure 6.1. The South Lantau water is collected in the catchwater which runs east to west for much of the length of South Lantau and feeds the Shek Pik Reservoir. In the north, water in Tung Chung Stream is intercepted by a weir across the stream and collected in a tunnel for transfer to the reservoir. WSD have requested that in order to maintain the integrity of the water catchment system, any discharge into the catchwater during the construction phase must be passed through a sediment trap and oil interceptor to reduce the suspended solids and pollutant content but that no operational discharges into the catchwater will be allowed. During the operational phase, WSD have specified that no road runoff should enter the catchwater or the streams in the water gathering grounds.

6.1.2 As part of its role in monitoring and protecting the water environment in Hong Kong, EPD carries out routine water quality surveys throughout Hong Kong’s marine and river waters, although this monitoring does not cover the streams in the southern part of the study area, including Cheung Sha stream.

6.1.3 In respect of the marine environment, the study area stretches from one side of Lantau Island to the other and thus is relevant to both the North Western and Southern Water Control Zones (WCZ). Water quality in the North Western WCZ was reported as good in 1999, with full compliance with the Water Quality Objectives (WQOs) for unionised ammonia, dissolved oxygen and total inorganic nitrogen. The waters close to Cheung Sha, fall into the Southern WCZ and were reported to be in full compliance with the dissolved oxygen and unionised ammonia criteria was recorded in 1999, although exceedances in total inorganic nitrogen occurred at 90% of the stations.

6.1.4 Principal catchments in the study area drain the Tung Chung and Cheung Sha hinterland via the key water courses of Tung Chung and Cheung Sha streams. The majority of streams in both catchments are steep in the upland reaches and remain natural and unpolluted due to the lack of human habitation and pollution sources. In addition, the main Tung Chung and Cheung Sha streams, a total of 44 tributaries are present in the study area, with the proposed alignment crossing all but two of these. The streams are shown in Figure 6.1.

6.1.5 Tung Chung Stream is fed by numerous steep upland streams, which have a high degree of naturalness. Monitoring at three locations in Tung Chung, including Tung Chung Stream (monitoring point TC2) is undertaken by EPD as part of their routine water quality monitoring. The water quality index for the stream has been classed as ‘excellent’ since 1993 with high levels of dissolved oxygen and low organics and nutrients and the water course achieves full compliance with the Water Quality Objectives.

6.1.6 Tung Chung Stream has been identified as the second richest stream in Hong Kong for freshwater fish species, with 23 species being identified (Chong and Dudgeon, 1992), and has provided the first record of the fish Beijiang Thick-lipped Barb (Acrossocheilus wenchowensis beijiangensis), a species of locally restricted distribution. Recent surveys of the stream also identified the locally common Predaceous Chub (Parazacco spilurus), which is of low conservation interest. The lower reaches of the stream are however channelised, which reduces its ecological value due to the lack of suitable micro-habitat. Native fish species favour more natural environments comprising rocky streambeds, with cool, fast flowing, well-oxygenated water.

6.1.7 No EPD routine monitoring is undertaken for the streams in the southern part of the study area. Much of the southern study area of hinterland forms part of the WSD water gathering ground, and water is collected in the catchwater. This results in the majority of the watercourses below the catchwater having little or no flow, or in the case of Cheung Sha Stream, flow controlled via a weir at the junction of the existing Tung Chung Road.

6.1.8 Recent surveys of the streams have yielded some seven fish species in the Cheung Sha Stream and two species in the Cheung Sha Sheung Tsuen Stream (Stream 40, Figure 6.1), with two species of Regional Concern (Fellowes et al., in prep) being encountered. The Black-headed Thick-lipped Goby (Awaous melanocephalus) was first recorded in Hong Kong in 1999 (Chan, 1999) and has been recorded in only three other sites in Hong Kong. It is of high conservation value. The Philippine Neon Goby (Stiphodon atropurpureus) was only very recently discovered in Hong Kong (ibid.), and is known to occur in Tong Fuk and San Shek Wan in Pui O. In addition, the majority of tributary streams are steep in the upland reaches and remain natural and unpolluted due to the lack of pollution sources.

6.1.9 Based upon the quality of the key streams in the study area, particularly Tung Chung and Cheung Sha streams, the project has been designed in the northern section such that the road widening works will generally take place on the east side of the existing road, such that during construction, the existing road can be used as a buffer zone to help prevent construction site runoff from entering into the ecologically important Tung Chung Stream. In the southern section, an alignment has been selected which is the furthest from the Cheung Sha stream, thus minimising impacts on this stream.

6.2 Assessment Methodology

6.2.1 The assessment of water quality has been undertaken in accordance with Annex 6 and Annex 14 of the TMEIA. The identification and assessment of water pollution impacts has taken into account the following factors:

¨ water quality of the water bodies with the potential to be affected;

¨ the hydrology of the aquatic systems;

¨ bottom sediments;

¨ the supported aquatic ecology; and

¨ WSD requirements for the watering grounds.

6.2.2 A semi-quantitative analysis has been undertaken of the potential impacts on the sensitive receivers taking into account both point and non-point sources of potential water pollutants during the construction and operational phases of the project. The scope of the assessment extends to direct and indirect impacts on all the stream courses crossed by the proposed alignment and potential downsteam effects on Tung Chung Bay and Pui O Wan.

6.2.3 The aim of the assessment has been to maintain the balance and integrity of the water causes as far as possible through prevention and minimisation of impacts at source in order to:

¨ maintain the natural properties of the water body;

¨ maintain the hydrological factors;

¨ control any discharges to within the relevant Water Quality Objectives;

¨ maintain the physical environment as far as practicable;

¨ protect aquatic ecology; and

¨ protect beneficial uses, which comprises the streams within the Tung Chung and Cheung Sha catchment areas.

6.3 Construction Water Quality Impacts

6.3.1 General

6.3.1.1 Potential impacts during the construction phase will be relevant primarily to the inland water courses and any associated downstream effects resulting from changes in the stream water quality discharging into the Southern and North Western WCZs.

6.3.2 Potential Surface Water Quality Impacts

6.3.2.1 Areas of potential impacts to the surface waters during the construction phase will be principally related to:

¨ construction site runoff containing elevated suspended solids and possibly oils due to erosion of exposed surfaces, stockpiles and material storage areas, fuel and oil storage and maintenance areas and dust suppression sprays;

¨ disturbance to banks and stream beds during culverting, slope reinforcement, haul road use and bridge widening;

¨ litter from packaging materials and waste construction materials; and

¨ construction workforce sewage.

6.3.3 Construction Site Runoff

6.3.3.1 The northern section of the new road runs up along the east side of Tung Chung Valley above Tung Chung Stream. The distance between the road and the stream varies along the route. At Lung Tseng Tau, the stream is about 210 m away from the road. However, as the road climbs the valley towards the crest at Pak Kung Au, the road passes as close as about 30 m to the stream in several locations, whilst crossing over a number of tributaries to the stream. In the southern section of the road, the alignment also passes over tributaries of the Cheung Sha Stream.

6.3.3.2 Thus, a key issue will be the control of runoff from earthworks into Tung Chung and Cheung Sha Streams and their tributaries. Due to the ecological sensitive nature of these streams, any unmitigated influx of elevated suspended solids could have impacts on the water quality of the streams and associated flora and fauna, although even at elevated levels, by the time any sediment in suspension reaches the receiving waters, it can be expected to have been diluted significantly before reaching Tung Chung Bay and Pui O Bay marine waters where it would be further diluted and dispersed and settle out rapidly, as discussed in more detail below. Further details on the unmitigated effects of construction runoff on the aquatic ecosystem is detailed in Section 8.7 of this report.

6.3.3.3 In respect of site runoff, in order to comply with the WQO, the suspended solids concentrations due to site runoff must be reduced to such an extent that they do not exceed 25 mg/L in any of the surface waters. The implementation of good construction site practices such as restricting stock piling in areas near watercourses, covering stock piled materials, limiting the area of site clearance undertaken at any one time, minimisation of exposed areas to rain by carrying out reinstatement as soon as practicable will be required to reduce the suspended solid concentrations.

6.3.3.4 However, given the importance of the streams in the study area and the water gathering grounds, in addition to the above, as mentioned in Section 6.1.9, the new roadworks will be restricted to the east side of the existing road as far as possible, such that during construction, the existing road can be used as a buffer zone to help prevent any pollutants from running into Tung Chung Stream. The only locations where roadworks will be required on the west side of the existing road are in the vicinity of Burial Ground No. 18L and Tai Tung Shan Service Reservoir. Also, the alignment which is furthest away from Cheung Sha Stream has been selected.

6.3.3.5 In addition, a stormwater drainage system has been devised to avoid / minimise water quality impacts as far as possible. The system proposed is different for the northern and southern sections of the road, although in both cases the system capacity has been designed for a 1:10 year storm event for the construction stage. The proposed temporary drainage system for both the northern and southern sections of the road are discussed below and shown in Figures 6.2a to 6.2j.

Northern Alignment Drainage Arrangements

6.3.3.6 For the northern section of the new road, it is proposed that a drainage system be constructed ahead of the main roadworks such that it may be used to discharge the construction site runoff, thus avoid discharging construction site runoff into the streams. The same system will remain in place and form the permanent operational drainage system which will avoid discharging carriageway runoff into the streams altogether, as discussed in Section 6.4 below. Details of the combined construction and operational drainage system are provided below.

6.3.3.7 The discharge outfall for the northern section’s drainage system will be located in the “Wong Lung Hang Channel” nullah near Ha Ling Pei. From here, a special carrier pipe is proposed along the route of the new road all the way up to the crest at Pak Kung Au. The carrier pipe will be designed to ultimately convey all the carriageway, verge and footway runoff from north of the crest down to the nullah, thereby avoiding the need to discharge any of the runoff into Tung Chung Stream or its tributaries during the operational stage. A sedimentation tank is proposed during the construction phase for the treatment of all discharge into the nullah in order to protect the Tung Chung Bay from elevated suspended solids. Based upon an assumed 1:10 year rainfall event of 87mm per hour, the flow from the works area in the northern section would be 2.9m3/s. Thus, allowing for a 3 minute minimum settlement period during a 1:10 year storm event, the minimum size of the sedimentation tank will be approximately 16 x 8 x 4m, but with the exact dimensions subject to detailed design. The sedimentation tank should be inspected every day rainfall is recorded and weekly thereafter and should be cleaned out when the volume of settled sediments amounts to 10% of the total volume of the tank.

6.3.3.8 The pipeline alignment is divided into three main portions in this northern end of the project. In the first section, from Ha Ling Pei up to south of Lung Tseng Tau (Ch 1,260), the carrier pipe will be laid along the western side of the new road.

6.3.3.9 Between Lung Tseng Tau and to Shek Mun Kap Road (Ch 1,740), it is necessary to implement a dual pipe system in order to accommodate two local high points along the road. One pipeline will take an alignment away from the road and run along the hillside below the road and a secondary carrier pipe will also be laid under the new eastern southbound lane of the carriageway. The secondary carrier pipe will discharge to the main carrier pipe on the west side of the road by cross pipes located at the local low points along this section of the road.

6.3.3.10 At Shek Mun Kap Road, the carrier pipe will pass underneath the new road, and run under the eastern southbound lane of the new carriageway all the way up to the crest (Ch 4,480). The pipe will vary in size from about 1,200 mm diameter at the northern outfall down to 225mm at the crest.

6.3.3.11 Along the on-line section of the roadworks, where the new road crosses the tributaries to Tung Chung Stream, the culvert extension and bridge widening works will be designed with sufficient cover to accommodate the carrier pipe where possible. In locations where it will not be possible to provide sufficient cover, the carrier pipe will be laid under the new culvert extensions or newly widened bridge structures, as discussed in more detail below in Section 6.3.4.16. Along the off-line section of the roadworks, new culverts will be provided with sufficient cover for the carrier pipe to cross the culverts, and new bridges will be provided with a special utilities trough to accommodate the carrier pipe.

6.3.3.12 The pipeline system will comprise digging a trench in the first instance. The pipe will ultimately be placed in the trench, backfilled and covered. While along certain sections the full pipeline system will be completed in advance of the main construction works, the trench only will form the basis of the construction phase drainage system in other areas with the pipe being installed once all earthworks have been completed and exposed areas reinstated.

6.3.3.13 Construction of the northern section of the new road will commence at Lung Tseng Tau and will advance up Tung Chung Valley to the crest. The on-line section of the new road between Lung Tseng Tau and Shek Mun Kap Road will be completed first, followed by the on-line section between Shek Mun Kap Road and Tai Tung Shan Service Reservoir (Ch 3,200). This will then be followed by the construction of the off-line section of the new road between the reservoir and the crest. The construction steps associated with the advanced works for the drainage system based upon the phasing above are summarised below:

¨ On-Line Section (Ch 1,000 to 3,200):

(i) it is necessary to construct the connection to the nullah to allow for the discharge of run-off to be undertaken in the first instance. Based upon this, the trench and pipeline from Ha Ling Pei up to Shek Mun Kap Road and under Tung Chung Road to the west side of the road will be completed. This section of the drainage system is off-line, running along the west side of the road. For the construction phase, a temporary settlement tank will be provided at Lung Tseng Tau to treat the run-off prior to discharge into the nullah;

(ii) construct cross pipes at the local low points between Lung Tseng Tau and Shek Mun Kap Road from the main carrier pipe under Tung Chung Road to the east side of the road. The cross pipes will transfer the run-off from the widening works on the eastern side of the existing Tung Chung Road in to the main pipeline system which runs into the nullah;

(iii) construct temporary drainage ditches along the hillsides above the existing road, draining into the adjacent streams, to intercept the hinterland runoff before reaching the construction site. This is necessary to avoid clean water becoming contaminated by the works. The proposed extent of the hinterland drainage system can be seen in Figures 6.2a to 6.2j and further description described in Section 6.3.3.23;

(iv) construct the pipeline trench along the east side of the existing road between Lung Tseng Tau and Shek Mun Kap Road. This trench will be lined with concrete and will be connected up to the main carrier pipe at Shek Mun Kap Road and via the cross pipes at the local low points. Thus, run-off from the new retaining walls, cut slopes and associated carriageway earthworks in this section will be collected in the drainage ditch and ultimately discharged into the nullah via the completed off-line section of carrier pipe, described in (i) above;

(v) once all works in this area are completed, the trench will be further expanded to be big enough for the pipeline and the carrier pipe will be installed. Once covered the carriageway can be completed and traffic can be transferred to allow reconstruction works on the existing western northbound carriageway;

(vi) between Shek Mun Kap Road up to the southern end of the on-line section of the new road at Tai Tung Shan Service Reservoir, the same sequence as in (iv) and (v) above will be applied, with the pipeline trench being concrete lined and used to collect construction run-off until the works are and then the permanent pipeline being installed once the main works are complete. As soon as the pipeline trench has been completed to the southern end of the on-line section of the new road, the construction of the off-line section may commence as described below. The construction sequence is illustrated in Figure 6.3a to 6.3d.

¨ Off-Line Section (Ch 3,200 to 4,390):

(vii) a drainage channel will be constructed either side of the construction site for the off-line section up to the crest. The channel on the eastern side will ultimately contain the pipeline required for the operational drainage while the channel on the western side is required for the construction only. Channels are required on both sides in this location to collect the runoff from the cuttings and earthworks on either side of the new road;

(viii) in addition, a temporary carrier pipe will be laid along the east side of the existing Tung Chung Road from the southern end of the on-line section of the new road up to the crest. This pipe would drain into the drainage channel at Tai Tung Shan Service Reservoir which connects to the drainage system described above. It would also be connected to the channels detailed in (vii) above by the provision of temporary cross pipes where the existing road adjoins the construction site (ie, Ch 3,200, 3,400, 3,650, 3,780, 4,080 and 4380). This pipeline and cross pipes are required for the construction phase only and will be removed once the works are completed. This pipeline is required to enable multiple work fronts to be opened up;

(ix) as with the on-line section, temporary drainage ditches along the hillsides above the construction site, draining into the adjacent streams, to intercept the hinterland runoff before reaching the construction site will be constructed; and

(x) once the new retaining walls, cut slopes, embankments, elevated structures and associated works inside the construction site have been completed, the pipe will be installed in the eastern channel to form the permanent system and then the new off-line carriageway, gullies and outlet pipes will be connected to the main carrier pipe.

6.3.3.14 In this way, run-off during the main construction works will be collected and transported away from the sensitive inland streams, particularly Tung Chung stream, thus avoiding any discharge into these water courses. The “Wong Lung Hang Channel” nullah near Ha Ling Pei is a man-made drainage structure will negligible ecological value and as such discharge into this is not considered to result in significant impacts. Notwithstanding, any discharge would be notably diluted once entering this system and ultimately the Tung Chung Bay, as discussed below.

6.3.3.15 Based upon an assumed 1:10 year rainfall event of 87mm per hour, the flow from the works area in the northern section would be 2.9 m³/s. However, the equivalent flow from Catchment C, shown in Figure 6.4, which also discharges into the nullah, would be 67.6 m³/s and thus the percentage contribution of potentially contaminated runoff into Wong Lung Hang nullah is 4.3% or a dilution factor of over 23 times. Once this flow enters Tung Chung Bay, it will be further diluted by the 49.7m³/s flow from Catchment A, reducing the potentially ‘contaminated’ contribution from the works site to just 2.5% equating to a 40 times dilution. This represents a worst case scenario as the discharge would be further diluted by not only the marine waters in the bay but also the run-off from the catchments to the west of the valley. In addition, as the discharge would ultimately end up in Tung Chung Bay which contains the San Tau SSSI, in order to further minimise any effects in this area, all the collected construction phase run-off will be treated via a sedimentation tank before to discharge. No significant ecological impacts on Tung Chung Bay and ecological communities including the seagrass beds are predicted according to the assessment in Section 8.7.

6.3.3.16 Excavation work associated with the construction of the drainage system itself has the potential to cause run-off. However, this activity is considered to be minor works and run-off during the excavation of the drainage channel will be minimised by the contractor providing a concrete lining to each stretch of dug channel before the end of each day. Control and proper disposal of excavated material in accordance with the Waste Management Plan is also recommended.

Southern Section Drainage Arrangements

6.3.3.17 The southern section of the new road runs down along the east side of the head of Cheung Sha Valley above Cheung Sha Stream. The road then runs around the southern flanks of Sunset Peak and across the catchwater before connecting with South Lantau Road at Cheung Sha Sheung Tsuen. One of the advantages of this route is that the new road directly avoids the ecologically important Cheung Sha Stream as far as possible. Indeed, only about 0.5 km out of the 2.7km long southern section of the new road falls within the catchment of this stream. However, the route traverses other catchments, albeit of smaller streams around the southern flanks of Sunset Peak, within the water gathering grounds above the catchwater.

6.3.3.18 Given the importance of Cheung Sha Stream and the water gathering grounds, permanent and temporary drainage systems have also been devised for the southern section of the new road to avoid / minimise water quality impacts. Indeed, the proposed permanent drainage system will avoid discharging carriageway runoff into the streams altogether, similar to the proposed system for the northern section of the new road. However, since this system will need to be carried across the new elevated structures along this section of the route, it will generally not be possible to construct the system ahead of the main roadworks as is proposed in the northern section of the alignment, and use it as the basis of a temporary drainage system.

6.3.3.19 Thus, a separate temporary drainage system will generally be required for the control of the construction site runoff in the southern area, since, until such a time as the bridge structures are constructed, there is no way of using the operational drainage system. Furthermore, at the location of some of the longer bridges, a temporary off-line haul road will be required to bypass the bridge site, which will require drainage. Also, the use of a temporary carrier pipe along the existing road, as proposed for the northern off-line section of the new road, is not practical as the alignment of the southern section is too far away from the existing road.

6.3.3.20 The only practical option, therefore, is to use a series of peripheral surface channels to encase sections of the works site to intercept and divert the surface runoff from the hinterland and the construction site. The site run-off will be collected into a temporary drainage channel which will comprise a continuous ditch (minimum recommended size 400 mm wide by 500 mm deep) running along the bottom edge of the construction site. The channel will be lined with concrete to prevent erosion. A temporary headwall, comprising wooden planking and posts (minimum recommended height 250 mm) will be erected on the downhill side of the ditch to prevent the construction site runoff from overrunning the channel. The channel is proposed to drain into a settling tank at the downhill end of the channel via a short section of open stepped channel or pipe prior to the runoff being discharged into the adjacent stream courses.

6.3.3.21 The alignment will be divided up into smaller catchments in order to enhance the control of the runoff. Based upon this and owing to the number of streams in the area, as shown in Figure 6.1, the individual areas of the construction site draining into any one settling tank will be relatively small. Hence, the runoff will be relatively small and the size of the settling tanks will be manageable. Based upon an assumed 1:10 year rainfall event of 87mm per hour, the flow from a typical sub-catchment would be about 0.1m³/s. Thus, allowing for a 3 minute minimum settling period during a 1:10 year storm event, the minimum size of the settlement tanks will be approximately 3 x 3 x 2m but with the exact dimensions subject to detailed design. The settling tanks will comprise a steel plate structure with waterproof lining. The outlet of the tank will be fitted with a filter, which will require regular cleaning. Based upon the need to protect the water gathering grounds from any pollutants in the discharged water in addition to suspended solids, oil interceptors will be either combined with the settlement tanks or provided as a separate adjacent system above the catchwater, as shown in Figures 6.2f to 6.2i. The tanks will be located below the downhill end of the construction site drainage channel. The sedimentation tank/oil interceptors should be inspected every day rainfall is recorded and weekly thereafter. The sedimentation tanks should be cleaned out when the volume of settled sediments amounts to 10% of the total volume of the tank and the oil interceptors should be cleaned when the volume of oil amounts to 50% of the total volume of the oil interceptor.

6.3.3.22 Where the settling tanks are located close above the catchwater, the discharge from the tanks will be diverted across the catchwater and discharged into the adjacent streams to reduce the amount of treated discharge flowing into the catchwater. Furthermore, where the works area abuts the catchwater, the catchwater will be protected from debris falling into the catchwater by temporary decking.

6.3.3.23 The hinterland drainage will be separated from the site runoff, as recommended for the northern section of the alignment, in order to prevent it becoming contaminated and to minimise the amount of polluted runoff that has to the controlled. The hinterland drainage channel will comprise a continuous ditch (minimum recommended size (300 mm wide by 400 mm deep) running along the top edge of the construction site. The ditch will include a concrete lining and a temporary headwall, similar to the construction site drainage channel.

6.3.3.24 It is recommended that the temporary drainage system be designed as a gravity system as far as possible, and not be reliant on pumping as the performance and reliability of any pumps is likely to decrease due to the silt content of the construction site runoff. However, it will be necessary in some locations, such as excavations for bridge foundations, to pump the site runoff into the settling tanks. This system is a temporary system required for the construction period only and will be removed prior to operation.

6.3.3.25 Run-off from the works site at the new junction with South Lantau Road will also be treated via a temporary sedimentation tank prior to discharge into the proposed carrier pipe discharging on the foreshore of Pui O Wan. This part of the permanent drainage system will be constructed in advance in order for it to be used during the construction phase.

6.3.3.26 While discharge into the local streams, catchwater and sea at Pui O Wan cannot be avoided, the proposed construction drainage system minimises impacts by dividing the areas of discharge into small units, thereby reducing the amount of discharge into any one stream and also by treating the run-off prior to discharge. In terms of the effects on the water gathering ground, the flow into the catchwater from the portion of Catchment B above the catchwater, as shown in Figure 6.4, based upon an assumed 1:10 year rainfall event of 87mm per hour, would be in the region of 34.7 m³/s. This compares to the predicted discharge from the works area, only that area above the catchwater, of 2 m³/s. This equates to a 5.8% contribution of site run-off to the normal flow or a dilution rate of 17. The works area discharge predicted would be slightly higher than the actual due to the fact that some site run-off, as described in Section 6.3.3.22, will be discharged below the catchwater and thus the contribution would actually be even lower. In addition, this site run-off is also treated prior to discharge. Based upon these factors, significant effects on the water quality or the flows of the streams or water gathering grounds are not predicted. Ecological effects of the treated discharge into the streams are discussed in Section 8 of this report.

6.3.3.27 Again, as for the northern section, construction of the drainage system itself is considered to be minor works and run-off during the excavation of the drainage channel will be minimised by the contractor providing a concrete lining to each stretch of dug channel before the end of each day. Control and proper disposal of excavated material will also be recommended.

6.3.3.28 Prior to construction of the permanent works, the Contractor will first construct the haul road between the crest and the junction with South Lantau Road. In the northern section of the alignment, the existing Tung Chung Road will be utilised for both the on and off-line sections. However, in the southern section, besides the main works, run-off control is also required during the construction of the haul roads. While the haul road will generally follow the alignment of the new road, it will bypass the construction site at some stream courses where longer bridges are proposed, as shown in Figures 2.1 to 2.11. The construction of the haul road will involve some earthworks and in order to reduce the areas of exposed earthworks during this phase, the contractor will be required to pave the off-line section of haul road and associated slopes immediately on the completion of individual sections of the off-line haul road. The temporary drainage system described above would be constructed concurrently to the formation of the haul road but the contractor would be required to progress the drainage system in advance of the haul road formation to ensure proper runoff control before the road is paved. A temporary barrier will be provided on the downhill side of the haul road to prevent errant vehicles from crashing off the haul road.

6.3.4 Stream Modifications

6.3.4.1 The water quality of the majority of the streams in the Study Area is considered to be high in their upper reaches, as they are not subject to any significant pollution sources. However, the stream habitats can be judged to be of particular value based upon the fauna present, either utilizing the riparian zone (terrestrial and aquatic species) or within the stream itself. Based upon both wet and dry season field surveys undertaken by ecologists, as detailed in Section 8, numerous streams in the Study Area are apparently seasonal, or of very low base flow, some influenced by the water gathering grounds and catchwater system, and the surveys have shown these streams to be of lower ecological value than the permanent streams with reliable discharge, upon which fully aquatic fauna are dependent. Thus, the streams can be divided into two broad categories comprising those with generally higher base flow supporting species of conservation interest, and those with generally lower base flow not supporting species of conservation interest. A summary of the streams, using the numbering system provided in Figure 6.1, which fit into these two categories are as follows:

¨ Streams with high base flow supporting species of conservation interest: Tung Chung Stream, 15, 18, 19, 21, 23, 24, 25, 26, 27, 32, 35, 40, 41, 42, 43; and

¨ Streams with low base flow not supporting species of conservation interest: 1-14, 16, 17, 20, 22, 28-31, 33, 34, 36-39, 44.

6.3.4.2 Further details of the evaluation of the ecological value of the streams in the study area are provided in Section 8.5.6 and Table 8.20.

6.3.4.3 It has been necessary to provide a balance between the provision of elevated structures which span the streams and avoid direct impacts and the visual impacts of these structures. Thus, new bridge structures have been proposed for as many of the streams crossed by the new road as possible, with emphasis on the streams which has been confirmed by wet and dry season ecological surveys to be of high ecological value, as noted in the first bullet above. For all but four of these streams of high ecological value, namely Streams 15, 18, 19 and 21 (Figure 6.1), stream modifications, and thus direct impacts, have been avoided, with the new road passing over the water courses on bridge. Thus, bridges will be provided for streams 23, 24, 25, 26, 27, 32, 35, 40, 43 of high ecological value, but also for streams 22, 28, 29, 31, 33, 34, 36, 37, 38, 44 which fit into the other classification. Streams No.’s 39, 41 and 42 are not crossed by the alignment and will not be affected by the project. The bridge piers/abutments will be constructed at least 5m away from the streams and excavation works for the columns will be protected by the drainage works described above in order to protected from any runoff entering the streams.

6.3.4.4 The modifications to Streams 15, 18, 19 and 21 (Figure 6.1) cannot be avoided, as discussed further below. All other streams modification works, proposed in order to provide the balance with visual impacts appropriately, will be undertaken on streams deemed to be of low ecological value. The impacts of these works are discussed in Section 6.3.4.13 to 6.3.4.19. A summary of the streams, as shown in Figure 6.1, and the proposed works on them is provided in Table 6.1 below.

Table 6.1 Summary of Works on Streams

Stream No	Structure Type	Stream No	Structure Type
1	Existing culvert to be extended	23	New bridge
2	Existing culvert to be extended	24	New bridge
3	Existing culvert to be extended	25	New bridge
4	Existing culvert to be extended	26	New bridge
5	Existing culvert to be extended	27	New bridge
6	Existing culvert to be extended	28	New bridge
7	Existing culvert to be extended	29	New bridge
8	Existing culvert to be extended	30	New culvert
9	Existing culvert to be extended	31	New bridge
10	Existing bridge to be widened	32	New bridge
11	Existing bridge to be widened	33	New bridge
12	Existing bridge to be widened	34	New bridge
13	Existing culvert to be extended	35	New bridge
14	Existing bridge to be widened	36	New bridge
15	Existing bridge to be widened	37	New bridge
16	New culvert	38	New bridge
17	Existing culvert to be extended	39	n/a^(1)
18	Existing culvert to be extended	40	New bridge
19	New culvert	41	n/a^(1)
20	New culvert	42	n/a^(1)
21	New culvert	43	New bridge
22	New bridge	44	New bridge

Note (1): streams not crossed by the alignment

Impacts on Streams 15, 18, 19 and 21

6.3.4.5 Along the northern off-line section of the route, the new road runs along a small side‑valley in a cutting for about 50m (Ch 3,850 to 3,900), before reaching the small plateau at the head of Tung Chung Valley. One of the tributaries of Tung Chung Stream (Stream No. 21) runs along the bottom of this side-valley, and hence will require culverting for about 50 m under the road. However, the inlet and outlet of the culvert will be on the line of the existing stream.

6.3.4.6 Unfortunately, it is not possible to avoid this situation. The northern off-line section of the new road is constrained by the existing road levels at either end of the section (ie, at Tai Tung Shan Service Reservoir and at the crest at Pak Kung Au), which force the road into cutting over much of the section.

6.3.4.7 Stream 19 passes under the new road along the northern off-line section of the route. The stream will require culverting for approximately 30m under the new road, which is in cutting at this location. Unfortunately, a bridge cannot be provided for reasons similar to those given above.

6.3.4.8 Stream 15 and stream 18 already have an existing bridge and culvert respectively. Stream 15 is traversed by an existing bridge structure (STR05) along the northern on-line section of the improved road. This bridge will be widened on its eastern uphill side to accommodate the new road. The existing bridge comprises a relatively simple three-sided reinforced concrete portal frame, with a single span less than 5 m in length. The widened section will likely comprise a reinforced concrete box, with the permanent carrier pipe passing underneath the box. The construction of the pipe and box will result in local impacts to the stream. However, local steepening of the upstream bed of the stream will not be required in this location.

6.3.4.9 Stream 18 passes under the new road where it adjoins the existing road along the northern off-line section of the route. As such, the existing culvert will be extended under the new road. A bridge cannot be provided over the stream, since the road is in cutting at this location. Unfortunately, it is not possible to avoid this situation. The northern off-line section of the new road is constrained by the existing road levels at either end of the section (ie, at Tai Tung Shan Service Reservoir and at the crest at Pak Kung Au), which force the road into cutting over much of the section.

6.3.4.10 Works in these streams will result in potential impacts as follows:

¨ loss of stream bed habitat along the obsolete section of the stream;

¨ fragmentation or additional fragmentation of stream; and

¨ downstream run-off of suspended solids.

6.3.4.11 The ecological implications of these are discussed in Section 8.7 of the Ecology Impact Assessment. However, the potential for elevated suspended solids could have water quality implications on the steam itself and, also, on Tung Chung stream. In respect of streams 18, 19 and 21, it is proposed to construct the culvert section prior to its connection with the stream, thus avoiding any direct impacts on the stream course during this time. Thus, the potential for high suspended solids will be limited to the period when the culvert is connected up at either end and stream flow is directed through the culvert. This process will comprise damming the original path of flow, which is not expected to generate large amounts of suspended solids. Also the activity is short term taking a matter of days only. Coarse material will settle out rapidly, probably within the culvert itself and finer material in the water column will be progressively more diluted as it flows down stream. Based upon this, and the fact that the streams in this area are subject to suspended solids up to 500 mg/L normally (see Table 6.2, EPD monitoring of Tung Chung River) and are subject to periodic high suspended solid during storm events, the impacts are not expected to be significant.

6.3.4.12 The works in Stream 15 will comprise digging a trench for laying of the permanent drainage pipeline and then constructing a concrete base slab as the foundation for the bridge extension. The base slab will take up some 6 metres of streambed. However, prior to these works commencing, whatever flow is in the stream will be ponded by building a dam across the stream, upstream from the works. A temporary pipeline will then divert the stream water around the works area into the unaffected downstream area. Once the base slab has been completed, the dam will be removed and the stream will flow across this to the existing bridge base slab. The works will be relatively short term, taking approximately 4 weeks to construct the trench and a similar time period for the base slab. However, as all natural stream flow will be diverted, the potential for suspended solids migrating downstream is minimal. The potential is further reduced by the works being undertaken in the dry season.

Remaining Streams Not Supporting Key Species

6.3.4.13 For steams with existing bridges or culverts, it will be necessary to undertake bridge widening or culvert widening and these works cannot be avoided. However, the streams which require these works are on the whole streams classed as having low ecological value, with the exception of streams 15 and 18 as discussed above. In addition, in accordance with the proposal to protect streams of high ecological value as far as possible, but balance the visual impacts of bridge structures, new culverts have been proposed for a number of streams but these works have been restricted to streams of low ecological value, with the exception of streams 19 and 21 where it is unavoidable, as discussed above. The ecological impacts of these works are discussed in Section 8 of this report.

New Culverts

6.3.4.14 New culverts will be required for streams crossed by the off-line section of the new road, streams 16, 20, 30 as shown in Table 6.1. In all cases, the culverts will be built separately in advance of any connection with the existing stream, as described for Streams 18, 19 and 21 above. Impacts associated with suspended solids, again, will be limited to the period when the flow is diverted. As the steams being culverted already have a very low base flow and in many cases are dry for much of the year, the opportunity for suspended solids to be transported downstream is negligible. Notwithstanding, any release of suspended solids would be expected to be localised as the material settles out and as the period of works is of short duration, significant impacts on water quality are not predicted.

6.3.4.15 The bridge and culvert widening will be implemented along the on-line section of the alignment in the north and these works will have direct impacts in the streams in question and the potential for indirect impacts on Tung Chung stream.

Existing Bridges to be Extended

6.3.4.16 There are five existing small bridges (STR01 to 05 on the figures in Section 2) along the on-line section of the roadworks, crossing streams 10, 11, 12, 14 and 15, and these will require widening to accommodate both the new road and drainage carrier pipe. The impacts on Stream 15 are also discussed above. All these bridges comprise relatively simple three-sided reinforced concrete portal frames, with single spans under 5 m in length. However, to lay the pipe across the bridge deck of STR02, STR04 and STR05 (relevant to streams 11, 14 and 15), would require reconstruction of the entire bridge and lowering of the stream profile to provide sufficient depth and cover for the pipe. Thus, in order to minimise impacts on these streams, the main carrier pipe will pass underneath the foundations of the widened bridges. This has be benefit of allowing the bridges to be widened only on the east upstream sides, with the pipe passing below the underside of the box (ie, below the stream) and also allowing the carrier pipeline to be put in place and be operational in advance of the main works in order to transfer construction run-off away from the streams. Structures STR01 and STR03 have sufficient cover to accommodate the carrier pipe across the bridge deck.

6.3.4.17 To accommodate the bridge widening works, local steepening of streams 10, 11, 12 and 14 will be required. This is not required for stream 15. This will comprise a relatively small open stepped channel, together with a debris trap, at the inlet at the top of the channel, to prevent rocks and boulders from falling down the channel and damaging the bridge during heavy rainfall.

Existing Culverts to be Extended

6.3.4.18 There are also a number of existing culverts along the on-line section of the roadworks that will require extending to accommodate the new road, and the upstream bed of the streams may also require local steepening and construction of a small open stepped channel with debris trap. These works are relevant to streams 1 to 9, 13 and 17. The carrier pipe will either be constructed above or below the culvert, depending on the existing level difference between the culvert and the road.

6.3.4.19 As with the requirement for new culverts, the key potential impacts associated with these works include loss of stream bed habitat; fragmentation of stream; and downstream run-off of suspended solids. The ecological effects of the works are discussed in Section 8.

6.3.4.20 As noted above, all the streams affected by bridge or culvert widening works are of low base flow and do not support key ecological species. There will, however, be direct disturbance to the stream bed and there is the potential for suspended solids to enter the water column and reach Tung Chung stream. However, as the flow in the streams is low overall and works will be undertaken when the flow is low, the opportunity for this to occur is reduced. Coarse material will settle out rapidly, probably not reach Tung Chung stream and, as noted above, the finer material in the water column will be progressively more diluted as it flows down stream. As the water quality and ecosystem of the stream is also subject to periodic high suspended solid events, such as during severe rain fall events, without long term effects on the stream, the impacts from the works are not expected to have a significant effect on water quality. Construction of the on-line section may only commence when either the pipeline trench or carrier pipe itself has been put in place up to the Tai Tung Shan Service Reservoir.

Haul Roads

6.3.4.21 In addition, to the above works, the construction of the haul road also has the potential to have direct impacts on some stream courses. The location of the proposed haul road in shown in Figure 2.1 to 2.11. Prior to construction of the permanent works, the Contractor will construct the haul road between the crest and the junction with South Lantau Road, with the existing Tung Chung Road being used in the northern section. The haul road will generally follow the alignment of the new road, but will bypass the construction site at some stream courses where longer bridges are proposed. However, in order to avoid affecting these streams, the haulage road will be designed to pass over the stream courses on temporary steel bridges of at least twice the width of the stream thus, protecting against direct impacts to the streams.

6.3.4.22 The sequence of the construction of a temporary bridge is as follows:

(i) Construct haul road up to a point before the stream course or location at which bridging is required.

(ii) Construct suitable level conditions to support the bridge at each embankment making sure habitat along the stream is not damaged.

(iii) Crane equipment across stream course.

(iv) Construct or launch bridge over stream course depending on which method is suitable for location.

(v) Move on to next crossing.

6.3.4.23 Based upon this method, significant impacts on water quality from the construction and use of the haul road are not predicted.

6.3.5 Stabilisation of Unstable Boulders

6.3.5.1 The Natural Terrain Hazard Assessment (NTHA) has identified that some measures to protect against rockfall from the natural terrain above the road link will be required. The work will include the stabilisation of unstable boulders together with debris traps to prevent debris from passing down certain streams as described in Section 6.3.4.16 above. Details on the scope and extent of the proposed works are provided in Section 2 of this report. No stream courses will be directly affected by the boulder stabilisation works. In addition, the boulder slope works will not generate any significant exposed surfaces which could generate runoff with only small areas within about 1m of the boulder potentially being cleared and thus significant water quality impacts in adjacent streams are not predicted. The effects of the debris traps on the steams has been addressed above.

6.3.6 Detailed Ground Investigation

6.3.6.1 Prior to the main construction works, a detailed ground investigation will also be required. Details are provided in Section 2 of this report. The ground investigation will comprise a combination of boreholes and trial pits along the length of the proposed alignment, particularly at pier foundation locations. This will result in the disturbance and removal to small areas of ground along the alignment and will generate wastewater and contaminants which have the potential to affect the water courses in the vicinity.

6.3.6.2 Based upon this, the specification for the detailed ground investigation will be prepared with the aim of minimising the potential impacts. The measures to minimise the impacts will be dominated by the need for good housekeeping, control and containment of fuel and lubricants, and training and de-silting of run-off. The following good practice shall be implemented:

¨ Good housekeeping (machinery): All of the investigation machines (drilling rigs, compressors, pumps etc.) will be subject to maintenance inspections before being mobilised to site. All machinery will be required to be in good working order with no serious oil, fuel or coolant leaks. Regular inspections will be required throughout the period of the works.

¨ Good housekeeping (work site): The size of all of the working platforms will be kept to a minimum. It will be a requirement that all waste associated with the works should be properly stored and removed at the end of the investigation. All used stages, scaffolding, hoarding etc. will similarly be stored and removed at the end of the works.

¨ Control of waste fuel and lubricants: A lined sump will be provided at each site to catch all waste fuel and lubricant. The contents of the sump will be collected at the end of the investigation and disposed of off-site. The lining will be stripped off and similarly disposed of and the sump re-instated. All drilling fluid additives will be bio-degradable and such an agent is used in order to reduce the water requirements and thus wastewater generation. Details of all bio-degradable drilling fluid additives shall be submitted to WSD prior to the GI works to confirm they will not contaminate the water gathering grounds;

¨ Control of run-off: Run-off from each of the investigation locations will be controlled and soak-aways will be provided where appropriate. The amount of exposed soil at each of the locations will be minimised. Water used for drilling the boreholes sited in or near the Tung Chung and Cheung Sha Streams will need to be de-silted in sedimentation tanks before discharge.

¨ Re-instatement: The pre-existing profile will be re-instated at all investigation locations. Turfing or hydro-seeding will be specified in order to re-establish the vegetative cover at each of the sites.

6.3.6.3 The operation is small scale and of short duration lasting in the region of a week and water quality impacts are expected to be small, localised and short-term.

6.3.7 Remediation of Existing Man-Made Slopes

6.3.7.1 As obsolete sections of the existing Tung Chung Road will be retained for various reasons including use as Emergency Vehicular Access, it is necessary to maintain the slopes along sections of the road. Details of the slope remediation works are provided in Section 2 of this report, and shown in figures in Section 2. The streams in the vicinity of the upgrading works are tributaries of Stream 27 (see Figure 6.1) which has been shown to be of high ecological value. These streams pass under the existing Tung Chung Road in culvert but pass close enough to the remediation works for run-off containing sediments and oils to enter the steams if not controlled.

6.3.7.2 The works are generally small scale and will be on-going for approximately 3 months in any one location. However, in order to minimise the potential for impacts on the local streams, mitigation measures in the form of good practice shall be implemented.

6.3.8 Construction Waste Materials

6.3.8.1 The use of mechanical plant during construction will inevitably require refuelling, together with fuel and oil storage and maintenance areas. As a consequence, there is the risk of spillage and leakage of the runoff into the adjacent water bodies during rainfall and from vehicle wheel and sub-frame washing areas for vehicles leaving and entering the site. Work areas will be located at Leung Tseng Tau, Figure 2.2, a small existing area near the Tai Tung Shan header tank, Figure 2.7, and adjacent to the catchwater as shown in Figure 2.10. However, smaller areas may be designated within the works boundary along the alignment but these will be specified by the Contractor as required and have not been determined at this stage. Streams in the vicinity will be sensitive to impacts and mitigation measures will be required to limit water quality impact and maintain present conditions in the surface waters. Recommended mitigation measures are detailed in Section 6.3.10 below. It is expected that with the implementation of these measures, any adverse impacts will be reduced to within acceptable levels. The effects of litter and waste construction material can be limited by the use of good site management practices.

6.3.8.2 Notwithstanding, no chemical waste shall be stored within the water gathering grounds and all maintenance activities which could give rise to chemical wastes, as discussed in Section 8 of this report, shall be undertaken outside the water gathering grounds.

6.3.8.3 While no oil or fuel shall be stored within the Country Park or water gathering grounds, it will be necessary for the Contractor to refuel equipment along the length of the works. In order to protect against impacts to the water gathering ground int he event of an accidental spillage of fuel or oil, the Contractor will be required to prepare a spill response plan to the satisfaction of FSD, HyD, Police, TD and WSD to define procedures for the control, containment and clean-up of any spillage that could occur on the construction site.

6.3.9 Waste Water Discharges

6.3.9.1 Impacts may also result from the discharge of grey wastewater from temporary works areas, comprising canteen and washing water and sewage from toilet facilities. In respect of lavatory facilities for the workforce, chemical toilets would be required if connection to a sewer is not practicable. It is likely that, in addition to chemical toilets along the length of the alignment, sewage from both works area, together with the other grey water, will be treated using an on-site chemical treatment and storage facility with regular collection of the wastewater carried out by tanker. In order to reduce the risk of contamination of the water gathering grounds, no canteen facilities will be located within the water gathering grounds. Also, the details of the chemical toilets shall be submitted to WSD prior to the construction commencement.

6.3.10 Construction Mitigation Measures

6.3.10.1 In accordance with the EPD Practice Note for Professional Persons ProPECC PN 1/94 Construction Site Drainage and the recommendations made in the assessment above, the following measures which shall be adopted during the construction process. These measures are specified in addition to the measures required for the detailed ground investigation in Section 6.3.2 above.

¨ discharges to natural water courses shall only take place when the effluent can be shown to comply with the standards specified in the Technical Memorandum, Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters. Discharges in the water gathering grounds should meet Group A standards for inland waters;

¨ the drainage system capacity for the construction stage shall be designed for a 1:10 year storm event;

¨ for the northern section, a drainage system shall be constructed ahead of the main roadworks such that it may be used to discharge the construction site runoff, thus avoid discharging construction site runoff into the streams;

¨ a separate temporary drainage system will be provided for the control of the construction site runoff in the southern area and shall be in place prior to the main road works;

¨ the southern alignment will be divided into small catchments for the provision of the temporary drainage system in order to enhance the control of runoff;

¨ construction of the off-line section of the road shall only commence when either the pipeline trench or carrier pipe has been put in place up to Tai Tung Shan Reservoir;

¨ during excavation work for the drainage system itself, each stretch of dug channel will be concrete lined before the end of each day to minimised run-off. Control and proper disposal of excavated material in accordance with the Waste Management Plan shall be undertaken;

¨ run-off during the excavation of the drainage channel will be minimised by the provision of a concrete lining to each stretch of dug channel before the end of each day;

¨ all temporary drainage channels shall be lined with concrete to prevent erosion;

¨ all off-line sections of the haul road and associated slopes will be paved within 3 days of the individual sections having been completed;

¨ progress of the construction phase drainage system shall be made in advance of the off-line haul road formation;

¨ for all haulage roads, temporary brides shall be used to cross all streams with spans of at least twice the stream width and no contact is to be made with the stream or its riparian vegetation unless authorised by the Engineer;

¨ a temporary barrier will be provided on the downhill side of the haul road to prevent errant vehicles from crashing off the haul road;

¨ the new bridge piers/abutments will be constructed at least 5m away from the streams and excavation works for the columns will be protected by the drainage works;

¨ all culverts will be constructed separately prior to their connection with the stream to avoid direct impacts on the streams during this period;

¨ during bridge extension works, a dam shall be built across the stream, upstream from the works and a temporary pipeline shall be used to divert the stream water around the works area into the unaffected downstream area in order to prevent contamination of the stream flow;

¨ areas in which works have been completed shall be restored within 3 weeks of completion in accordance with the Landscape Proposal detailed in Appendix I;

¨ temporary cut slopes shall be covered with tarpaulin during rainfall when not being worked to prevent erosion;

¨ major stockpiles shall be sited outside the Country Park and shall avoid being located near stream courses;

¨ spoil heaps shall be covered at all times to minimise losses in the wet season and reduce the dust emissions during the dry season to minimise impacts on air quality;

¨ stock piled materials shall be contained in a designated area down gradient from any stream or up gradient with a suitably constructed barrier to reduce loss of materials to the stream;

¨ hinterland drainage shall be collected by the use of perimeter channels and diverted to the nearest stream course to prevent external run off from crossing the site in order to prevent it becoming contaminated and to minimise the amount of polluted runoff to be controlled (refer to Figures 6.2a to 6.2j);

¨ sedimentation tanks and oil interceptors shall be provided during the construction stage on the southern section of the road above the catchwater for the treatment of all site runoff prior to discharge into the streams, as detailed in Figures 6.2f to 6.2i;

¨ sedimentation tanks shall be provided during the construction stage on the southern section of the road below the catchwater for the treatment of all site runoff prior to discharge into the streams or via the Pui O Wan outfall, as detailed in Figures 6.2i to 6.2j.;

¨ where settling tanks are close to the catchwater, as shown in figure 6.2i, the discharge from the tanks will be diverted across the catchwater and discharged into adjacent streams to reduce the amount of treated discharge flowing into the catchwater;

¨ where the works area abuts the catchwater, the catchwater will be protected from debris falling into the catchwater by temporary decking;

¨ a sedimentation tank shall be provided during the construction stage for the treatment of all site runoff from the northern section of the road prior to discharge into the Wong Lung Hang nullah, as detailed in Figure 6.2a;

¨ sedimentation tank, oil interceptors and channels used during the construction phase should be inspected every day rainfall is recorded and weekly thereafter. The sedimentation tanks and channels should be cleaned out when the volume of settled sediments amounts to 10% of the total volume of the tank and the oil interceptors should be cleaned when the volume of oil amounts to 50% of the total volume of the oil interceptor;

¨ run off from the roofs of site buildings shall be conveyed in closed drains to the nearest surface water body to prevent the generation of excessive quantities of surface water run off carrying suspended solids;

¨ wastewater from the site offices and toilet facilities, if not connected to a sewerage system, shall be discharged into an on-site chemical treatment and storage facility with regular collection of the wastewater carried out by tanker. Where non-flush toilets are provided these shall be chemical toilets and the holding tanks emptied by a licensed contractor on a frequent basis;

¨ the details of the chemical toilets shall be submitted to WSD prior to the construction commencement;

¨ the wheels of all vehicles leaving and entering the construction site shall be washed to minimise the carry over of mud onto public roads and into the water gathering grounds. Wheel wash water shall be recycled and only discharged following removal of silt by sand/silt removal traps;

¨ all plant shall be in proper working order and maintained such that there is no leakage of fuel or oil. Any waste oils shall be collected in designated tanks prior to disposal off site;

¨ oils and fuels shall be stored within lined and bunded containment areas which shall have a capacity not less than 110% of the largest tank capacity and shall be regularly drained of rain water;

¨ no oil or fuel shall be stored within the Country Park or the water gathering grounds;

¨ The Contractor will be required to prepare a response plan to the satisfaction of FSD, HyD, Police, TD and WSD to define procedures for the control, containment and clean-up of any spillage that could occur on the construction site;

¨ no chemical wastes shall be stored in the water gathering grounds;

¨ no canteen facilities shall be located within the water gathering grounds;

¨ the Contractor shall comply with WSD’s General Conditions for Working within Water Gathering Grounds; and

¨ no maintenance activities which may generate chemical wastes shall be undertaken in the water gathering grounds. Vehicle maintenance shall be confined to designated paved areas only and any spillages shall be cleared up immediately using absorbents and waste oils shall be collected in designated tanks prior to disposal off site. All storm water run-off from these areas shall be discharged via oil/petrol separators and sand/silt removal traps.

6.4 Operational Water Quality Impacts

6.4.1 Operational Road Runoff

6.4.1.1 Areas of potential water quality impacts during the operational phase will largely comprise paved area run-off containing suspended solids onto which hydrocarbons and heavy metals, principally lead, copper, zinc and iron, may be adsorbed.

6.4.1.2 Potential adverse impacts from runoff from the paved road surfaces can result from storm water carrying dissolved and particulate material from degradation of the road surface and tyres and from normal operational fuel and oil loss from vehicles. The most vulnerable time will be during the first flush of rain. Contaminants present in the normal operational runoff will, by their chemical nature, be strongly adsorbed onto particulates present.

6.4.1.3 Details of rural road runoff quality are sparse and no previous water quality problems in the receiving marine waters due to rural road runoff in Hong Kong are known. Runoff quality for urban highways, however, as used in the Airport Master Plan EIA is given below in Table 6.2.

Table 6.2 Runoff Quality for Urban Highways

Biochemical Oxygen Demand (BOD) (mg/L)	Chemical Oxygen Demand (COD) (mg/L)	Suspended Solids (mg/L)	Ammoniacal Nitrogen (NH4-N) (mg/L)	Lead (µg/L)	Polyaromatic Hydrocarbons (PAH) (µg/L)
12-32	128-171	28-1178	0.02-1.8	0.15-2.9	0.36-6

Reference: [Ellis J.B. the Management and Control of Urban Runoff Quality. J. IWEM, 3 (2), 116-123.]

6.4.1.4 The urban runoff can be expected to have higher contaminant concentrations than rural road runoff and so the additional contaminant loading calculated from these figures for the proposed road should be considered to be the worst case. In addition, it has been assumed in the reference above that all contaminants which have accumulated since the previous rainfall on a mixed urban catchment would be flushed from a road during the first 10mm of rainfall. Assuming that the above concentrations would persist for the duration of a more severe storm, would also overestimate the potential contaminant load.

6.4.1.5 For comparison with the road runoff quality tabulated above, the data on river water quality given in Table 6.3 below has been taken from EPD’s river water quality monitoring programme for Tung Chung River.

Table 6.3 Water Quality in Tung Chung River

Biochemical Oxygen Demand (BOD) (mg/L)	Chemical Oxygen Demand (COD) (mg/L)	Suspended Solids (mg/L)	Ammoniacal Nitrogen (NH4-N) (mg/L)	Lead (µg/L)	Polyaromatic Hydrocarbons (PAH) (µg/L)
1-2	2-25	1-480	0.01-0.1	1-27	No Data

Reference: [EPD River Water Quality in Hong Kong in 1997]

6.4.1.6 Assuming the worst case road runoff concentrations in Table 6.1 were discharged into the existing stream courses, it can be seen in comparison with Table 6.2 that, for the relatively small percentage of the total runoff entering the streams which will be intercepted by the road, the estimated BOD, COD, suspended solids and NH₄-N concentrations are an order of ten times larger than the corresponding concentrations found in the local natural stream flows, but this is not taking into account any dilution factors.

6.4.1.7 However, in respect of lead concentrations from the highway, discharges are expected to reach a maximum of 2.9 µg/L (Table 6.2) which is similar to the lead concentrations recorded in the Tung Chung River during EPD’s routine monitoring (Table 6.3).

6.4.1.8 While the existing Tung Chung Road runoff is discharged into both the adjacent streams and associated water gathering grounds, concerns have been expressed regarding the future road run off into the sensitive water courses. In addition, WSD has advised that road runoff cannot be discharged in the water gathering grounds. Thus, in order to avoid discharge of road runoff into any of the inland waters, a specially designed pipeline has been developed to collect all road runoff along the entire alignment and discharge it at either end of the road. The pipeline capacity has been designed to a 1:50 year event. Details of the operational drainage system are provided in the sections below.

Northern Section

6.4.1.9 Operational drainage details for the northern section are described in Section 6.3 above. This section will be largely built in advance for use during the construction phase and will avoid discharge of construction and operational run-off into the inland streams. The system will also provide an oil interceptor to treat the runoff prior to discharge in order to minimise impacts on the Tung Chung Bay and particularly the San Tau SSSI. The northern outfall for the permanent drainage system is shown in Figure 6.2a.

Southern Section

6.4.1.10 The outfall for the proposed permanent drainage system for the southern section will be located on the foreshore of Pui O Wan, about 180m south-east of the new road junction with South Lantau Road. The outfall will be over 1.0 km away from the gazette boundary of Cheung Sha Beach to the west, and over 1.2 km away from the gazette boundary of Pui O Beach to the east. The outfall will likely comprise an open stepped channel, with a dispersion apron at the mouth at the bottom of the channel. The outfall into Pui O Wan is shown in Figure 6.2j.

6.4.1.11 From the inlet at the top of the channel, a special carrier pipe will be constructed up the hillside to the new road junction, and then along the new road all the way up to the crest at Pak Kung Au. The carrier pipe will be designed to convey all the carriageway, verge and footway runoff south of the crest down to the sea, thereby avoiding the need to discharge any of the runoff into Cheung Sha Stream or the other streams along the southern flanks of Sunset Peak. The carrier pipe will also convey the runoff from the bus-bay facilities at the new road junction. The pipe will vary in size from about 900mm diameter at the southern outfall down to about 225mm at the crest.

6.4.1.12 From the outfall up to the new junction (Ch 7,190), the carrier pipe will be laid along the eastern side of the small valley leading down to the sea, below South Lantau Road, thereby avoiding the burial ground and private lots to the west. A footway will be provided along the route of the pipe, from South Lantau Road down to the outfall, to provide access for future maintenance. However, a vehicular track will not be provided for maintenance to minimise the landtake and impacts of providing such a track down the steep hillside. Besides, vehicular access is considered unnecessary, since the buried pipe and outfall will require relatively little maintenance.

6.4.1.13 From the new junction with South Lantau Road, the carrier pipe will be laid along the eastern southbound lane of the new carriageway all the way up to the crest (Ch 4,480). The carriageway, verge and footway runoff will be collected by gullies located along either kerb of the new road. The gullies will be connected to the carrier pipe with outlet pipes laid under the new carriageway.

6.4.1.14 As discussed above, the pipe will need to be carried across the new bridge structures along the southern section of the new road. A dedicated service trough will be provided on the east side of the structures, under the footway, for this purpose. For the longer structures, where gullies will be required along the elevated carriageway, a secondary standard carrier pipe will be provided inside the bridge deck to convey the runoff from the elevated carriageway. The secondary carrier pipe will be connected to the main carrier pipe behind the lower southern abutment. The provision of a separate carrier pipe will avoid the need to make connections to the main carrier pipe inside the service trough. This will make the future maintenance of the main carrier pipe in the service trough easier.

6.4.1.15 The above arrangement cannot be used at the catchwater crossing. The catchwater comprises a reinforced concrete trough section, about 4m wide by 3 m deep. A vehicular maintenance track runs along the southern side of the catchwater. The new road has been designed to intersect the track, such that maintenance vehicles can access the track from the new road. As such, there is insufficient depth between the new road and the free board of the catchwater to accommodate a service trough in the bridge deck. However, since the span is relatively small, it will be possible for the carrier pipe to be self-supporting and to cross the catchwater adjacent to the bridge. The maintenance track will be raised locally to provide both sufficient depth for the bridge and allow the pipe to pass under the track.

6.4.1.16 Where the new road crosses the smallest streams along the southern section of the route, it is proposed that the streams pass through culverts under the new road. The new culverts will be provided with sufficient cover for the carrier pipe to cross above them, similar to the proposed arrangement along the northern section of the route.

Impact Assessment

6.4.1.17 Thus, all road run-off will be discharged either into Tung Chung Bay via the nullah or Pui O Wan. While worst case concentrations of BOD, COD, suspended solids and NH₄-N are predicted to be an order of ten times larger than the corresponding concentrations found in the local natural stream flows, once discharged though an outfall, a dilution factor of at least 10 would be expected relatively quickly and within a few tens of metres from the discharge point. Thus, increases in concentrations of water quality parameters in the receiving waters would be centred within a short distance of the discharge points, much as is the case for the natural streams at present.

6.4.1.18 The European Marine Environmental Quality Standard for the Protection of Marine Life sets an annual mean concentration of 25 µg/L for lead concentrations in sea water and so the worst case runoff of 2.9 µg/L road runoff should not have a significant impact on marine life in the receiving waters. No criteria are available for PAH concentrations but PAHs are not significantly soluble in water.

6.4.1.19 Thus, any road runoff into the bays at either end of the alignment would not be expected to result in a significant increase in the concentration of the water quality parameters of interest and will not result in a significant increase in the existing pollutant load to Tung Chung Bay or Pui O Wan. For example, based upon a 1:50 year peak rainfall event of 109 mm per hour, the peak discharge from the northern section of the road entering the Wong Lung Hang nullah is estimated to be 1.6m³/s, while the equivalent discharge from Catchment C, Figure 6.4, which also discharges into the nullah, is 84m³/s. This means that the peak road runoff only amounts to about 1.9% of the total peak natural runoff. When also taking the natural discharge of 61.9m³/s from Catchment A into account, the contribution further reduces to about 1% of the natural runoff. Even this figure is overstated as the natural runoff from the catchments on the western side of the valley have not been included.

6.4.1.20 Notwithstanding this dilution, assuming the northern road runoff is assimilated into the receiving waters of Tung Chung Bay within say 100m from the discharge location, the equivalent tidal volume, that is, the increase in volume between low tide and high tide, of the area inshore of this 100m line is estimated to be in the range 80,000 to 200,000m³ depending upon the tide type. The total runoff from the northern road in a 20 minute 1:50 year storm event is estimated to be in the order of 1,900m³. Thus, it can be seen that the runoff from the road would be equivalent to about 1% to 2.5% of the tidal volume depending upon the particular tide and a much smaller percentage would apply to more frequent storm events. Similar, levels of contribution and dilution would be expected for the southern section of the road and the discharge into Pui O Wan.

6.4.1.21 Bearing in mind the dilution rates which would be achieved, the road runoff should not result in a significant increase in concentrations of contaminants. The total contaminant load in the road runoff will represent a small increase in the natural contaminant load and should not result in a significant detrimental impact on marine water quality especially when the small rainfall volumes with respect to the tidal volume are taken into account. Notwithstanding, for the northern section, based upon the fact that the runoff enters the Tung Chung Bay which contains the San Tau SSSI, all road runoff will be treated via a petrol interceptor before discharge. The petrol interceptor will be cleaned on a 3 monthly basis.

6.4.2 Accidental Spillage

6.4.2.1 Under normal operating circumstances, significant impacts on water quality are not predicted. However, in the event of an accident with a tanker, there is the possibility for the spillage of a large volume of oil or chemicals. In this case, it is expected that the integrated permanent road drainage system would help to protect the inland streams by collecting the spillage. In addition, a profile barrier has been proposed along large sections of the road which will prevent vehicles from leaving the road, as shown in Figure 2.17 and described in Section 2.3.2.4. However, in the event that a major spill occurs on Tung Chung Road, a defined response plan is required in order to, not only be able to reopen the road as soon as possible to minimise distruption to Lantau traffic, but also to minimise effects on the local ecological resources and water gathering grounds. All methods of spill clearance, should be environmentally acceptable and should not lead to pollution of the catchwater system or beaches. The following sections detail the procedures and key contacts that would be applicable in this situation.

6.4.2.2 Chemical Spillages: For chemical spillages that do not pose fire, explosion or life risks, the spills should be contained, recovered and soaked-up for disposal as chemical waste. Under no circumstances should chemical spillages be washed into the natural streams, or any other natural or man-made water bodies or carrying systems.

6.4.2.3 Oil Spillages: In respect of oil spillage, the use of chemical dispersants to break up the oil is not recommended as their use could impact on the surrounding environment and compound the pollution situation. In addition, the oil spill should be contained in the location of the spill wherever possible.

6.4.2.4 For all spillages, the acceptable method of control is by “absorption” and then removal of the absorbed waste for disposal by special contractors. Absorption of the oil should be achieved by the use of sawdust or other suitable material. Advice on how to clean-up a chemical spillage if required can be sought from EPD. Contact should be made with EPD’s Chemical Waste Treatment Centre for assistance in disposing of the contaminated sawdust. Source of sawdust for use in case of emergencies can be obtained from Transport Department.

6.4.2.5 The management and maintenance authority for the venue/roads/parts would be responsible for clearing up a spillage in their responsible area, as summarised in Table 6.4.

6.4.2.6 The emergency call-out procedure in case oil/chemical spillage on roads in Lantau is as follows:

(i) Police

¨ to access the impact of incident and then immediately inform:

- FSD in case of fire hazard;

- TD in case of road closure;

- WSD who would consider whether closure of water gates of catchwaters leading to Shek Pik Reservoir is required;

¨ to set up a Mobile Command Post to co-ordinate the road closure and clearing up operations. All parties concerned might then liaise with the Command Post for updated information; and

¨ to inform EPD, FEHD and other departments to render assistance if necessary after the immediate traffic and rescue operations completed.

(ii) Transport Department

¨ to inform HyD’s Emergency Co-ordination Centre;

¨ to liaise with NLB, MTRCL, relevant ferry operators on emergency public transport arrangements;

¨ to inform DO/Is (ADO/Is or Senior Liaison Officer) of the road closure and emergency public transport arrangements;

¨ to disseminate information of emergency public transport arrangements through GIS.

(iii) DO/Is

¨ to keep local DC members and RC Chairmen informed of the updated development of the incident and any emergency public transport arrangements;

¨ to consider setting up the Islands District Emergency Control Centre (Is DECC) when deemed necessary.

6.4.2.7 A summary of the key responsibilities of relevant authorities bodies and their contact details are provided in the following Table 6.4.

Table 6.4 Spill Response Key Responsibilities

Department	Area of Responsibility	Contact Person	Phone No.	Fax No.
FSD	1. To standby at scene if there is fire hazard	Fire Services Communication Centre	2723 2233	2311 0066
HyD	1. To be responsible for clearing up chemical spillage on public roads by supplying labour and sawdust	General E&C Hotline	Office Hours 2926 4222	2714 5228
	2. To assist Police in assessing the road re-opening time.	M1/SK&Is Reporting Centre M1/SK&Is	2762 3941 After Office Hours 2926 4333 or 9185 9922	2762 2764
Police	1. To implement road closure and direct traffic 2. To coordinate the operations of clearing up	Traffic Console / Lantau DOO/Lantau (Mr. Michael Chan)	2666 4514 9380 3752
	3. To inform FSD, TD and WSD in the first instant and then other departments on an as need basis	Police Marine Console CIP E&C / NTS	2803 6267 2611 3228 or 9460 0499
Transport Department	1. To inform HyD 2. To liaise with NLB, MTRCL, ferry operators on emergency public transport 3. To update DO/Is of the development of the incident 4. To disseminate information of emergency public transport arrangements through GIS	First Contact Point	2410 0066 or 2410 0193	2428 6502
WSD	1. To close water gates of catchwaters leading to Shek Pik Reservoir when necessary 2. To monitor the water quality at Shek Pik Reservior and nearby catchment areas which could be possibly affected.	S1/HK(HW2)1	Office Hours 2980 6111 After Office Hours 9388 8441 or 7305 1273 call 5727	2984 2406

6.4.2.8 Thus, these measures will reduce the magnitude of impacts. Notwithstanding it is possible for some of the spilled material to be transferred along the operational carrier pipe and be discharged either in the nullah in Tung Chung or Pui O Wan. The spill will decay through a variety of means including evaporation, adsorption onto suspended materials and emulsification. Adsorption of oils can be effective means for breaking up the spill. However, the natural process of the spill spreading reduces its thickness and allows the process of wind and wave action in breaking up the spill to increase as well as evaporation and dissolution. Dispersion of any spill will occur more rapidly in high energy waters but with the relatively low flows of the waters in the bays at either end of the alignment, dispersion could take longer. Notwithstanding, it is likely that a spill would disperse in region of 3-4 days without any long term effects on water quality.^[1] However, based upon the fact that, for the northern section of the road, any spill would ultimately end up in Tung Chung Bay which contains the San Tau SSSI seagrasses and mangroves, an oil interceptor will be provided for treatment of the road runoff prior to its discharge into the Wong Lung Hang nullah. The largest standard Highways Department oil interceptor should be used. This will have the effect of minimising the impacts by containing the oil. The emergency response plan and the provision of an oil interceptor are considered to be sufficient to reduce impacts to acceptable levels.

6.4.3 Operational Mitigation Measures

6.4.3.1 Mitigation measures for the operational phase as discussed above are as follows:

¨ an operational drainage system shall be provided for the northern which transfers all road run off for discharge into the Wong Lung Hang nullah in Tung Chung (refer to Figure 6.2a);

¨ for the southern section of the road, a carrier pipe will be designed to convey all the road runoff south of the crest to the sea, avoiding discharge into the streams crossed by the alignment. The outfall shall be located on the foreshore of Pui O Wan and shall be over 1.0 km away from the gazette boundary of Cheung Sha Beach to the west and over 1.2 km away from the gazette boundary of Pui O Beach to the east (refer Figure 6.2j);

¨ operational drainage system capacity shall be designed for a 1:50 year storm event;

¨ all new bridges shall be provided with a special utilities trough to accommodate the carrier pipe for operational road runoff;

¨ an oil interceptor shall be provided during the operational stage for the treatment of all road runoff from the northern section of the road prior to discharge into the Wong Lung Hang nullah, as detailed in Figure 6.2a. The largest standard Highways Department oil interceptor should be used; and

¨ the oil interceptor shall be cleaned at 3 monthly intervals.

6.5 Residual Impacts

6.5.1 The recommended mitigation measures are predicted to be sufficient to control water quality impacts to acceptable levels during both the construction and operational phases. Thus, no adverse residual water quality impacts are predicted.

6.6 Environmental Monitoring and Audit

6.6.1 The assessment has concluded that water quality impacts can under normal circumstances be mitigated to acceptable levels. However, in light of the sensitive nature of many of the streams in the study area and the, albeit treated, discharge into the catchwater, it is recommended that construction phase water quality monitoring in the streams along the alignment, together with supervision of the works during the regular EM&A site inspections, is undertaken to ensure that the recommended mitigation measures are being implemented and are effective. Further details of the specific EM&A requirements are detailed in Section 13 of this report and in the EM&A Manual.

6.7 References

[1] Proposed Aviation Fuel Receiving Facility at Sha Chau: Environmental Impact Assessment, Volume 1, January 1995, ERM.