5 HYDRODYNAMICS AND WATER QUALITY

5.1 Introduction

5.2 Water Sensitive Receivers

5.3 Environmental Legislation, Policies, Plans, Standards And Criteria

5.4 Description of The Environment

5.5 Identification of Environmental Impact

5.6 Assessment Methodology

5.7 Prediction And Evaluation Of Environmental Impacts

5.8 Mitigation of Adverse Environmental Impacts

5.9 Evaluation of Residual Impacts

5.10 Environmental Monitoring And Audit

5.11 Conclusion

5 HYDRODYNAMICS AND WATER QUALITY

5.1 Introduction

5.1.1 This section presents the assessment results of the potential water quality impact associated with the proposed cross-harbour water mains (DP6). Mitigation measures are also recommended to minimise potential adverse impacts and to ensure the acceptability of any residual impact (that is, after mitigation).

5.2 Water Sensitive Receivers

5.2.1 In order to evaluate the potential water quality impacts from the Project, water sensitive receivers (WSR) in Victoria Harbour and its adjacent waters were considered. Major water sensitive receivers identified include:

· WSD Flushing Water Intakes;

· Cooling Water Intakes; and

· Corals.

5.2.2 Water sensitive receivers identified outside the Project site boundary in farther field within Victoria Harbour and its adjacent waters are shown in Figure 5.1. No sensitive coral sites were identified in the Victoria Harbour. The Green Island and Junk Bay coral communities are located more than 5.5 km west and 6.5 km east of the proposed work site, respectively. These ecological sensitive receivers are included for water quality assessment as they are potentially affected during the construction phase of the Project due to the sedimentation of suspended solids in the water column. Potential adverse impacts on the coral communities, in terms of sedimentation rate, are addressed in Section 5.7. Further discussions are included in the marine ecological impact assessment (Section 9).

5.2.3 A number of cooling water pumping stations and intakes are located within the proposed WDII permanent reclamation limit along the existing waterfront of Wan Chai. These intakes supply cooling water to the air conditioning systems of various commercial buildings in the Wan Chai area including:

· Hong Kong Convention and Exhibition Centre (HKCEC) Phase 1

· Shui On Centre

· Telecom House

· Government Buildings (Wan Chai Tower/Revenue Tower/Immigration Tower)

· China Resources Building

· Hong Kong Exhibition Centre

· Great Eagle Centre

· Sun Hung Kai Centre.

5.2.4 Cooling water intake for Sun Hung Kai Centre will be reprovisioned to the new waterfront of Wan Chai during operational phase of the Project. The rest of the above listed cooling water intakes will be reprovisioned to the intake chambers to the north of HKCEC Extension.

5.2.5 An existing WSD flushing water intake is also located within the proposed WDII reclamation limit at Wan Chai which will be uprated and reprovisioned to Wan Shing Street under this Project.

5.2.6 Figure 5.2 shows the locations of the existing and reprovisioned seawater intakes within the Project site boundary. Cooling water intakes for some potential future developments are also included in Figure 5.2 for reference. Further description of these cooling water intakes are provided in Section 5.6.

5.2.7 It should be noted that the MTRC South Intake previously situated at the Wan Chai waterfront between Central Reclamation Phase III (CRIII) and HKCEC Extension has been relocated to the Central waterfront as shown in Figure 5.1.

5.3 Environmental Legislation, Policies, Plans, Standards and Criteria

5.3.1 The criteria for evaluating water quality impacts in this EIA Study include:

Environmental Impact Assessment Ordinance (EIAO)

5.3.2 The Technical Memorandum on Environmental Impact Assessment Process (Environmental Impact Assessment Ordinance) (EIAO-TM) was issued by EPD under Section 16 of the EIAO. It specifies the assessment method and criteria that were followed in this Study. Reference sections in the EIAO-TM provide the details of assessment criteria and guidelines that are relevant to the water quality assessment, including:

· Annex 6 – Criteria for Evaluating Water Pollution

· Annex 14 – Guidelines for Assessment of Water Pollution.

Water Quality Objectives

5.3.3 The Water Pollution Control Ordinance (WPCO) provides the major statutory framework for the protection and control of water quality in Hong Kong. According to the Ordinance and its subsidiary legislation, Hong Kong waters are divided into ten Water Control Zones (WCZs). Corresponding statements of Water Quality Objectives (WQO) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZ based on their beneficial uses. The proposed Project is located within Victoria Harbour (Phase Three) WCZ and the corresponding WQO are listed in Table 5.1.

Table 5.1 Summary of Water Quality Objectives for Victoria Harbour WCZ

Parameters	Objectives	Sub-Zone
Offensive odour, tints	Not to be present	Whole zone
Visible foam, oil scum, litter	Not to be present	Whole zone
Dissolved oxygen (DO) within 2 m of the seabed	Not less than 2.0 mg/l for 90% of samples	Marine waters
Depth-averaged DO	Not less than 4.0 mg/l for 90% of samples	Marine waters
pH	To be in the range of 6.5 - 8.5, change due to human activity not to exceed 0.2	Marine waters
Salinity	Change due to human activity not to exceed 10% of ambient	Whole zone
Temperature	Change due to human activity not to exceed 2 ^oC	Whole zone
Suspended solids (SS)	Not to raise the ambient level by 30% caused by human activity	Marine waters
Unionised ammonia (UIA)	Annual mean not to exceed 0.021 mg/l as unionised form	Whole zone
Nutrients	Shall not cause excessive algal growth	Marine waters
Total inorganic nitrogen (TIN)	Annual mean depth-averaged inorganic nitrogen not to exceed 0.4 mg/l	Marine waters
Toxic substances	Should not attain such levels as to produce significant toxic, carcinogenic, mutagenic or teratogenic effects in humans, fish or any other aquatic organisms.	Whole zone
	Human activity should not cause a risk to any beneficial use of the aquatic environment.	Whole zone

Source: Statement of Water Quality Objectives (Victoria Harbour (Phases One, Two and Three) Water Control Zone).

Water Supplies Department (WSD) Water Quality Criteria

5.3.4 Besides the WQO set under the WPCO, the WSD has specified a set of objectives for water quality at flushing water intakes as listed in Table 5.2 which shall not be exceeded at all stages of the Project.. The target limit for suspended solids (SS) at these intakes is 10 mg/l or less.

Table 5.2 WSD’s Water Quality Criteria for Flushing Water at Sea Water Intakes

Parameter (in mg/l unless otherwise stated)	Target Limit
Colour (HU)	< 20
Turbidity (NTU)	< 10
Threshold Odour Number (odour unit)	< 100
Ammoniacal Nitrogen	< 1
Suspended Solids	< 10
Dissolved Oxygen	> 2
Biochemical Oxygen Demand	< 10
Synthetic Detergents	< 5
E. coli (no. per 100 mL)	< 20,000

Cooling Water Intake Standards

5.3.5 Based on a questionnaire survey conducted under the approved Comprehensive Feasibility Study for Wan Chai Development Phase II (WDIICFS) EIA ^([1]), a SS limit of 40 mg/L was adopted as the assessment criterion for Admiralty Centre intake and MTRC South intake. No information on the SS limit is available for other cooling water intakes. These findings have been confirmed by a telephone survey conducted under the recent approved EIA for the Hong Kong Convention and Exhibition Centre (HKCEC) Atrium Link Extension (ALE). The locations of the cooling water intakes are shown in Figure 5.1 and Figure 5.2. The SS criterion for cooling water intakes is different from that for the WSD’s intakes as their beneficial uses are different (the former is used for cooling water system and the latter for flushing purpose).

Technical Memorandum

5.3.6 Discharges of effluents are subject to control under the WPCO. The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) gives guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters). The limits control the physical, chemical and microbial quality of effluents. Any sewage from the proposed construction and operation activities must comply with the standards for effluents discharged into the foul sewers, inshore waters or marine waters of Victoria Harbour WCZ, as given in the TM-DSS.

Practice Note

5.3.7 A Practice Note for Professional Persons (ProPECC) was issued by the EPD to provide guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with ten types of discharge from a construction site. These include surface runoff, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building constructions, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices given in the ProPECC PN 1/94 should be followed as far as possible during construction to minimise the water quality impact due to construction site drainage.

Assessment Criteria for Corals

5.3.8 Potential impacts on benthic organisms, including corals, may arise through excessive sediment deposition. The magnitude of impacts on marine ecological sensitive receivers was assessed based on the predicted elevation of SS and sedimentation rate.

5.3.9 According to the WQO criteria, elevation of SS less than 30% of ambient level, which is set for among other reasons, to offer protection for marine ecological resources, is adopted in this assessment for coral protection. This criterion is more stringent than that previously adopted in other EIA study for assessing SS impact on hard corals in eastern Hong Kong waters (i.e. SS elevation less than 10 mg/L, ERM 2003 ^([2])).

5.3.10 According to Pastorok and Bilyard ^([3]) and Hawker and Connell ^([4]), a sedimentation rate higher than 0.1 kg/m²/day would introduce moderate to severe impact upon corals. This criterion has been adopted for protecting the corals in Hong Kong under other approved EIAs such as Tai Po Sewage Treatment Works Stage 5 EIA ^([5]), Further Development of Tseung Kwan O Feasibility Study EIA, Wan Chai Reclamation Phase II EIA, Eastern Waters MBA Study ^([6]), West Po Toi MBA Study ^([7]) and Tai Po Gas Pipeline Study ^([8]). This sedimentation rate criterion is considered to offer sufficient protection to marine ecological sensitive receivers and is anticipated to guard against unacceptable impacts. This protection has been confirmed by previous EM&A results which have indicated no adverse impacts to corals have occurred when this assessment criterion has been adopted.

5.3.11 The assessment criteria used in this Project for protection of corals identified at Green Island, Junk Bay and Cape Collinson is also based on the WQO for SS established under the WPCO, i.e. the SS elevations should be less than 30% of ambient baseline conditions. The WQO for SS has also been adopted under the approved Tai Po Sewage Treatment Works Stage 5 EIA as one of the assessment criteria for evaluating the water quality impact from the sewage effluent on corals identified at Tolo Harbour, Green Island and Junk Bay.

5.3.12 The above assessment criteria would be used to assess water quality impact to coral habitats (i.e. the far field ecological sensitive receivers) as identified and indicated in Figure 5.1.

5.4 Description of the Environment

Marine Water Quality in Victoria Harbour

5.4.1 The marine water quality monitoring data routinely collected by EPD in Victoria Harbour were used to establish the baseline condition. A summary of water quality data for selected EPD monitoring stations extracted from the EPD’s publication “20 years of Marine Water Quality Monitoring in Hong Kong” (which is the latest version available at the time of preparing this report) is presented in Table 5.3 for Victoria Harbour WCZ (VM1 VM2, VM4-VM8, VM12 and VM15). Locations of the monitoring stations are shown in Figure 5.1.

5.4.2 In the past, wastewater from both sides of the Victoria Harbour was discharged into it after just simple screening, leading to marine water low in DO and high in organic nutrients and sewage bacteria. Commissioning of HATS Stage 1 in late 2001 has brought large and sustained improvements to the water quality of the eastern and central Victoria Harbour. However, improvements are less noticeable in the western harbour area which was still subject to the sewage discharges from local PTW (Central, Wan Chai West and Wan Chai East).As the HATS Stage 1 was commissioned in late 2001, the data for 2005 as shown in Table 5.3 represent the situation after the commissioning of HATS Stage 1.

5.4.3 In 2005, the marked improvements in eastern Victoria Harbour (VM1 and VM2) and moderate improvements in the mid harbour area (VM4 and VM5) and northern part of Rambler Channel (VM14) since HATS Stage 1 was commissioned were generally sustained. Several monitoring stations in the WCZ are located close to sewage outfalls, including VM5 (Wan Chai East and Wan Chai West PTW outfall), VM6 (Central PTW outfall), VM4 (North Point PTW outfall) and VM8 (SCISTW – HATS Stage 1 outfall). The water quality at these stations was inevitably subject to the direct impact of sewage discharge from these outfalls. The WQO compliance in 2005 was 83%, slightly lower than that in 2004 (87%). Full compliance with the WQO (for DO and UIA) was achieved in 2005 in the Victoria Harbour WCZ. However, the WQO compliance for TIN was only 50% in 2005.

Table 5.3 Summary Statistics of 2005 Marine Water Quality in Victoria Harbour

Parameter		Victoria Harbour East		Victoria Harbour Central			Victoria Harbour West		Stonecutters Island	Rambler Channel		WPCO WQO (in marine waters)
		VM1	VM2	VM4	VM5	VM6	VM7	VM8	VM15	VM12	VM14
Temperature (^oC)		22.6 (15.7-27.9)	22.9 (15.8-28.0)	22.9 (15.8-27.8)	23 (15.9-27.9)	23 (15.9-27.8)	23.1 (15.8-27.9)	23.1 (15.6-27.7)	23 (16.0-27.8)	23.1 (15.8-27.7)	23.4 (15.9-27.9)	Not more than 2 ^oC in daily temperature range
Salinity		32.3 (30.4-33.4)	31.9 (28.5-33.3)	31.8(28.9-33.2)	31.4 (27.3-32.9)	31.3 (26.8-32.8)	30.9 (26.3-32.8)	31.1 (27.4-32.9)	31.3 (26.6-32.9)	31(27.7-33.0)	29.6 (23.0-33.0)	Not to cause more than 10% change
Dissolved Oxygen (DO) (% Saturation)	Depth average	79 (59-94)	78 (66-92)	75 (63-88)	76 (68-99)	77 (68-96)	78 (72-99)	80 (61-108)	77 (64-105)	75 (54-94)	80 (68-105)	Not available
	Bottom	78 (46-93)	77 (54-90)	74 (51-88)	74 (46-99)	73 (45-94)	75 (54-94)	78 (35-108)	74 (43-101)	74 (42-92)	79 (52-103)	Not available
Dissolved Oxygen (DO) (mg/l)	Depth average	5.7 (4.2-6.9)	5.6 (4.4-6.8)	5.4 (4.4-6.6)	5.5 (4.7-6.6)	5.5 (4.8-6.5)	5.6 (4.9-6.6)	5.8 (4.3-7.1)	5.5 (4.5-7.0)	5.4 (3.8-6.4)	5.7 (4.8-6.9)	Not less than 4 mg/l for 90% of the samples
	Bottom	5.6 (3.3-6.9)	5.6 (3.8-6.8)	5.3 (3.6-6.5)	5.3 (3.3-6.6)	5.3 (3.2-6.5)	5.4 (3.8-6.5)	5.6 (2.5-7.1)	5.3 (3.1-6.7)	5.3 (2.9-6.2)	5.6 (3.7-6.9)	Not less than 2 mg/l for 90% of the samples
pH		8.1 (7.8-8.3)	8.1 (7.7-8.3)	8 (7.7-8.3)	8 (7.6-8.3)	8 (7.6-8.2)	8 (7.7-8.2)	8.1 (7.7-8.2)	8 (7.6-8.2)	8 (7.7-8.2)	8.1 (7.7-8.2)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc Depth (m)		2.3 (1.5-2.8)	2.2 (1.2-3.5)	2.1 (1.5-3.2)	2.1 (1.3-3.1)	2.1 (1.2-3.3)	1.8 (0.9-3.2)	1.9 (1.2-2.5)	1.9 (1.2-2.7)	1.7 (1.2-2.5)	1.8 (1.5-2.3)	Not available
Turbidity (NTU)		10 (5.1-16.2)	9.8 (4.8-15.8)	9.6 (4.5-15.3)	9.8 (4.9-14.5)	9.8 (5.0-14.8)	10.8(5.9-16.1)	11.9 (5.4-22.0)	10.7 (5.8-16.2)	14.4 (6.4-22.1)	11.3 (5.4-17.1)	Not available
Suspended Solids (SS) (mg/l)		4.5 (0.9-10.8)	3.6 (1.3-8.5)	3.6 (1.3-9.8)	3.4 (1.7-5.3)	3.7 (1.3-8.2)	4.1 (2.1-8.7)	5.2 (1.8-16.3)	5.1 (2.1-10.3)	7.2 (3.1-15.7)	4.7 (2.6-10.7)	Not more than 30% increase
5-day Biochemical Oxygen Demand (BOD5) (mg/l)		0.8 (0.5-1.2)	0.9 (0.4-1.5)	0.9 (0.5-1.1)	1.1 (0.6-1.4)	0.9 (0.4-1.4)	1 (0.6-1.4)	0.8 (0.5-1.4)	0.8 (0.5-1.2)	0.7 (0.4-1.2)	0.8 (0.4-1.6)	Not available
Nitrite Nitrogen (NO2-N) (mg/l)		0.02 (0.01-0.05)	0.02 (0.01-0.05)	0.03 (0.01-0.05)	0.03 (0.01-0.05)	0.03 (0.01-0.05)	0.03 (0.01-0.06)	0.04 (0.01-0.07)	0.03 (0.02-0.06)	0.04 (0.02-0.07)	0.05 (0.01-0.09)	Not available
Nitrate Nitrogen (NO3-N) (mg/l)		0.1 (0.04-0.17)	0.12 (0.03-0.23)	0.13 (0.05-0.24)	0.15 (0.05-0.31)	0.16 (0.06-0.34)	0.19 (0.08-0.45)	0.18 (0.08-0.49)	0.16 (0.09-0.31)	0.2 (0.09-0.45)	0.27 (0.09-0.67)	Not available
Ammonia Nitrogen (NH3-N) (mg/l)		0.09 (0.05-0.16)	0.13 (0.04-0.21)	0.15 (0.06-0.27)	0.19 (0.06-0.29)	0.19 (0.07-0.26)	0.21 (0.12-0.32)	0.18 (0.09-0.30)	0.23 (0.08-0.32)	0.2 (0.14-0.25)	0.17 (0.10-0.25)	Not available
Unionised Ammonia (UIA) (mg/l)		0.004 (0.002-0.010)	0.006 (0.002-0.015)	0.006 (0.003-0.015)	0.007 (0.005-0.015)	0.008 (0.004-0.014)	0.009 (0.004-0.018)	0.009 (0.003-0.022)	0.009 (0.005-0.014)	0.008 (0.005-0.012)	0.008 (0.004-0.013)	Not more than 0.021 mg/l for annual mean
Total Inorganic Nitrogen (TIN) (mg/l)		0.22 (0.11-0.32)	0.28 (0.08-0.46)	0.31 (0.12-0.54)	0.37 (0.12-0.64)	0.38 (0.14-0.65)	0.43 (0.28-0.83)	0.4 (0.22-0.76)	0.42 (0.19-0.63)	0.44 (0.31-0.71)	0.49 (0.29-0.91)	Not more than 0.4 mg/l for annual mean
Total Nitrogen (TN) (mg/l)		0.34 (0.23-0.47)	0.43 (0.22-0.63)	0.47 (0.26-0.69)	0.55 (0.28-0.77)	0.55 (0.29-0.79)	0.58 (0.47-0.93)	0.59 (0.34-1.16)	0.58 (0.36-0.76)	0.63 (0.43-1.31)	0.66 (0.40-1.02)	Not available
Orthophosphate Phosphorus (PO4) (mg/l)		0.02 (0.01-0.03)	0.03 (<0.01-0.04)	0.03 (0.01-0.04)	0.04 (0.01-0.05)	0.03 (0.01-0.05)	0.04 (0.02-0.05)	0.03 (0.01-0.05)	0.04 (0.02-0.05)	0.03 (0.02-0.04)	0.03 (0.02-0.04)	Not available
Total Phosphorus (TP) (mg/l)		0.03 (0.02-0.05)	0.04 (0.02-0.06)	0.05 (0.03-0.06)	0.05 (0.03-0.07)	0.05 (0.03-0.07)	0.05 (0.04-0.06)	0.05 (0.03-0.17)	0.05 (0.03-0.07)	0.06 (0.04-0.17)	0.05 (0.03-0.11)	Not available
Chlorophyll-a (µg/L)		2.5 (0.9-6.0)	2.4 (0.8-6.0)	2.4 (0.9-7.2)	2.8 (0.8-9.1)	2.6 (0.8-9.0)	2.2 (0.8-7.6)	2 (0.9-6.4)	3.2 (0.7-12.3)	1.8 (0.9-4.8)	2.8 (0.8-11.8)	Not available
E coli (cfu/100 ml)		640 (88-4500)	1600 (120-31000)	2400 (310-11000)	7700 (2500-23000)	5700 (1200-33000)	9100 (1200-35000)	4900 (790-40000)	5400 (490-22000)	4000 (1200-17000)	2100 (520-8700)	Not available
Faecal Coliforms (cfu/100 ml)		1300 (300-9100)	3600 (340-50000)	5200 (770-33000)	17000 (6800-40000)	12000 (2300-89000)	21000 (2700-130000)	12000 (1500-140000)	13000 (1800-97000)	9700 (2600-35000)	4700 (1500-31000)	Not available

Notes: 1. Except as specified, data presented are depth-averaged values calculated by taking the means of three depths: Surface, mid-depth, bottom.

2. Data presented are annual arithmetic means of depth-averaged results except for E. coli and faecal coliforms that are annual geometric means.

3. Data in brackets indicate the ranges.

Marine Water Quality within Causeway Bay Typhoon Shelter (CBTS)

5.4.4 A summary of published EPD monitoring data (in 2005) collected from the monitoring station at the CBTS (VT2) is presented in Table 5.4. The data are extracted from the EPD’s publication “20 years of Marine Water Quality Monitoring in Hong Kong”.

Table 5.4 Summary Statistics of 2005 Marine Water Quality at the Causeway Bay Typhoon Shelter

Parameter		EPD Monitoring Station (Bi-Monthly)	WPCO WQOs (in marine waters)
		VT2
Temperature (^oC)		22.8 (15.9 – 27.3)	Not more than 2 ^oC in daily temperature range
Salinity (ppt)		30.2 (25.2 – 32.2)	Not to cause more than 10% change
Dissolved Oxygen (DO) (% saturation)	Depth average	68 (53 – 103)	Not available
	Bottom	68 (53 – 102)	Not available
DO (mg/l)	Depth average	4.9 (3.6 – 7.2)	Not less than 4 mg/L for 90% of the samples
	Bottom	4.9 (3.6 – 7.1)	Not less than 2 mg/L for 90% of the samples
pH value		8.1 (7.9 – 8.3)	6.5 - 8.5 (± 0.2 from natural range)
Secchi disc (m)		1.9 (1.5 – 2.9)	Not available
Turbidity (NTU)		8.8 (5.0 – 9.9)	Not available
Suspended Solids (SS) (mg/l)		5.8 (3.0 – 13.8)	Not more than 30% increase
Silica (as SiO2)(mg/l)		1.0 (0.5 – 1.4)	Not available
5-day Biochemical Oxygen Demand (BOD5) (mg/l)		1.6 (1.2 – 2.9)	Not available
Nitrite Nitrogen (NO2-N) (mg/l)		0.04 (0.02 – 0.05)	Not available
Nitrate Nitrogen (NO3-N) (mg/l)		0.19 (0.11 – 0.32)	Not available
Ammoniacal Nitrogen (NH3-N) (mg/l)		0.20 (0.18 – 0.30)	Not available
Unionised Ammonia (UIA) (mg/l)		0.011 (0.005 – 0.021)	Not more than 0.021 mg/L for annual mean
Total Inorganic Nitrogen (TIN) (mg/l)		0.43 (0.35 – 0.55)	Not more than 0.4 mg/L for annual mean
Total Nitrogen (TN) (mg/l)		0.65 (0.56 – 0.80)	Not available
Ortho-Phosphate (OrthoP) (mg/l)		0.04 (0.02 – 0.05)	Not available
Total Phosphorus (TP) (mg/l)		0.06 (0.05 – 0.08)	Not available
Chlorophyll-a (µg L^-1)		4.3 (0.5 – 16.5)	Not available
E. coli (cfu per 100 mL)		5,200 (2,300 – 12,000)	Not available
Faecal Coliform (cfu per 100 mL)		17,000 (5,100 – 61,000)	Not available

Note: 1. Except as specified, data presented are depth-averaged data.

2. Data presented are annual arithmetic means except for E. coli and faecal coliforms that are geometric means.

3. Data enclosed in brackets indicate ranges.

5.4.5 Due to the embayment form and reduced flushing capacity of the typhoon shelter, marine water within the typhoon shelter is vulnerable to pollution. In 2005, high levels of E.coli were recorded at the CBTS indicating faecal contamination. The water quality level marginally exceeded the WQO for TIN but fully complied with the WQO for DO and UIA. Significant long-term improvements in terms of decreasing trends in TIN, TN, OrthoP and TP were observed in CBTS.

Sediment Quality

5.4.6 The results of marine sediment quality analysis from the marine ground investigation works at the Project site are presented in Section 6. A review of the sediment quality data from the marine ground investigation indicated that the marine sediments to be dredged at the Project area were classified as contaminated. Details of the sediment quality criteria and guidelines are given in Section 6.

5.5 Identification of Environmental Impact

5.5.1 Dredging of marine mud would be required for construction of the water mains. Key water quality concerns during the construction works are related to the dredging works will disturb the marine bottom sediment, causing an increase in SS concentrations in the water column and forming sediment plume along the tidal flows.

5.5.2 Potential impacts on water quality from dredging will vary according to the quantities and level of contamination, as well as the nature and locations of the WSR at or near the dredging sites. These impacts are summarised as follows:

· Increased suspension of sediment in the water column during dredging activities, with possible consequence of reducing DO levels and increasing nutrient levels.

· Release of previously bound organic and inorganic constituents such as heavy metals, polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and nutrients into the water column, either via suspension or by disturbance as a result of dredging activities, or depositing of fill materials.

· Release of the same contaminants due to leakage and spillage as a result of poor handling and overflow from barges during dredging and transport.

5.5.3 All of the above may result in deterioration of the receiving marine water quality and may have adverse effects on WSR. They are elaborated in the following paragraphs.

Suspended Sediment

5.5.4 As a result of dredging and filling activities during the construction phase, fine sediment (less than 63 µm) will be lost to suspension. The suspended sediment will be transported by currents to form sediment plumes, which will gradually resettle. The impact from sediment plumes is to increase the suspended sediment concentrations, and cause non-compliance in WQO and other criteria.

5.5.5 Any sediment plume will cause the ambient suspended sediment concentrations to be elevated and the extent of elevation will determine whether or not the impact is adverse or not. The determination of the acceptability of any elevation is based on the WQO. The WQO of SS is defined as being an allowable elevation of 30% above the background. EPD maintains a flexible approach to the definition of ambient levels, preferring to allow definition on a case-by-case basis rather than designating a specific statistical parameter as representing ambient. As adopted in the approved WDIICFS EIA for assessing the environmental impacts of released SS, the ambient value is represented by the 90th percentile of baseline (pre-construction) concentrations.

Release of the Contaminants due to Leakage and Spillage

5.5.6 Release of the same contaminants due to leakage and spillage as a result of poor handling and overflow from barges during dredging and transport can be addressed by proper implementation of recommended mitigation measures in Section 5.8.

General Construction Activities

5.5.7 The general construction works that will be undertaken for the Project may have the potential to cause water pollution. These could result from the accumulation of solid waste such as packaging and construction materials, and liquid waste such as sewage effluent from the construction work force and spillage of oil, diesel or solvents by vessels and vehicles involved with the construction. If uncontrolled, any of these could lead to deterioration in water quality. Increased nutrient levels result from contaminated discharges and sewage effluent could also lead to a number of secondary water quality impacts including decreases in DO concentrations and localised increase in NH₃-N concentrations which could stimulate algal growth and reduction in oxygen levels.

5.5.8 Sewage will arise from sanitary facilities provided for the on-site construction work force. It is characterised by high level of BOD, NH₃-N and E.coli counts. For some of the works areas, there will be no public sewers available for domestic sewage discharge on-site.

5.6 Assessment Methodology

5.6.1 To assess the potential water quality impacts due to the construction and operation of the Project, the sources and natures of water pollution to be generated during construction and operation phases have been identified and their impacts are quantified where practicable.

Sediment Plume Modelling

General Description of WDII Reclamation Works

5.6.1 The proposed marine construction works will involve:

· Permanent reclamation at Hong Kong Convention and Exhibition Centre (HKCEC)

· Permanent reclamation at Wan Chai (WCR)

· Permanent reclamation at North Point (NPR)

· Temporary reclamation at Public Cargo Working Area (TPCWA) and Causeway Bay (TCBR) for construction of the CWB tunnel

· Temporary reclamation at Wan Chai (TWCR4)

5.6.2 The proposed construction method adopts an approach where permanent and temporary seawalls will first be formed to enclose each phase of the reclamation. Bulk filling will be carried out behind the completed seawall. Demolition of temporary reclamation will involve excavation of bulk fills and dredging to the existing seabed level which will be carried out behind the temporary seawall. Temporary seawall will be removed after completion of all excavation and dredging works for demolition of the temporary reclamation. Therefore, the sediment plume can be effectively contained within the permanent and temporary reclamation area. Demolition of temporary seawall will involve removal of rock fill and seawall blocks only, which would not create significant SS impact. Fines content in the filling materials for seawall construction would be negligible and loss of fill material during seawall construction is therefore not expected. Thus, potential water quality impact of SS will only arise during the dredging for seawall foundation.

5.6.3 There will be a total of five main reclamation areas, namely HKCEC, WCR, NPR, TPCWA and TCBR respectively. Each of these five reclamation areas is subdivided into different stages for different engineering and environmental constraints as shown in Figure 2.7. Within the same reclamation area, seawall dredging will be performed in sequence instead of operating concurrently. Thus, dredging along the seawall will be undertaken for only one stage at a time to minimize the potential water quality impacts. The sequencing of the reclamation stages are presented in the construction programme in Appendix 2.1 (as discussed in Section 2).

5.6.4 Temporary reclamation of Causeway Bay will be divided into four stages (Figure 2.7). Construction of TCBR1W and TCBR1E will be undertaken at the first stage with seawall foundation to be constructed in sequence. Thus, dredging along the seawall of TCBR1W will not be carried out simultaneously with the dredging along the seawall of TCBR1E to minimize the dredging impact. At Stage 2, dredging at seawall of TCBR2 will take place when TCBR1W and TCBR1E are in place. Demolition of TCBR1E will then proceed and the whole TCBR1E will be removed before the commencement of TCBR3. Thus, during the third stage, dredging for seawall foundation and seawall trench filling at TCBR3 will take place when both TCBR1W and TCBR2 are in place at the same time. Subsequently, TCBR1W will be removed before the TCBR4 commences. Therefore, water body behind temporary reclamation area will not be fully enclosed, which minimise water quality impacts (also refer to Figure 2.10 to Figure 2.14).

5.6.5 After the construction of the western seawall of HKCEC Reclamation Stage 1 (HKCEC1) is completed in early 2009, a temporary embayment will be formed between the existing eastern seawall of CRIII and the HKCEC Extension. This embayment will be a particular cause of concern as a storm outfall (Culvert L) is currently discharging pollutants into this area. Locations of existing storm outfalls within the Project site are shown in Figure 5.3B. The potential water quality impact within this embayment will last for more than 2 years until the reclamation of HKCEC Stage 2 where the new Culvert L extension can be constructed via a new land formed under HKCEC2W (Figure 2.18). The delay in filling of this embayment arises due to the restriction of piling, dredging and reclamation works in the vicinity of the existing cross harbour water mains, which must be diverted first before any disturbance of the seabed in this area can take place.

5.6.6 As a mitigation measure, to avoid the accumulation of water borne pollutants within this embayment, an impermeable barrier, suspended from a floating boom on the water surface and extending down to the seabed, will be erected by the contractor before the HKCEC1 commences. The barrier will channel the stormwater discharge flows from Culvert L to the outside of the embayment. The contractor will maintain this barrier until the reclamation works in HKCEC2W are carried out and the new Culvert L extension is constructed.

5.6.7 Other storm outfalls, located at the reclamation area, will be temporarily diverted to the adjacent reclamation site before completion of seawall construction, in order to prevent discharging into temporary embayment and this minimise potential water quality impacts. In addition, storm outfalls will be diverted into the area with no nearby seawater intakes to avoid adverse impacts. In case storm outfalls and cooling water intakes are at the same area, water quality impacts have to be modelled to assess whether the impacts would be acceptable. The sequences of temporary diversion of storm outfalls are shown in Figure 2.8 to Figure 2.19.

5.6.8 Diversion of seawater intakes will be undertaken at an early stage of WCR. The existing cooling water intake of Sun Hung Kai Centre (namely 6) and the WSD flushing water intake (namely a) at the Wan Chai seafront will be reprovisioned to the new waterfront (Figure 5.2). These two seawater intakes will be diverted across the new land formed under Wan Chai Reclamation Stage 1 (WCR1) before commencement of Wan Chai Reclamation Stages 2 to 4 (WCR2, WCR3 and WCR4).

5.6.9 The existing cooling water intakes (namely 2, 3, 4 and 5) along the HKCEC water channel will be reprovisioned to the intake chambers to the north of the HKCEC Extension (as shown in Figure 5.2). These intakes will be diverted via the new land formed under HKCEC Reclamation Stage 1 (HKCEC1). According to the construction programme, these existing intake points will remain in operation during the seawall construction in HKCEC1 and therefore would be potentially affected by the dredging operations. The potential impact during the dredging works at HKCEC2E is considered less critical as these intakes would be diverted to the north of the HKCEC Extension before commencement of this reclamation stage.

5.6.10 There are two cooling water intakes (namely 8 and 9 respectively) in Causeway Bay Typhoon Shelter for Windsor House, Excelsior Hotel and World Trade Centre (as shown in Figure 5.2). Intake 9 is located within the reclamation site of TCB4 and thus will be temporarily diverted, in order to ensure continuous operation during the construction (Figure 2.13). No temporary diversion will be implemented for Intake 8. Construction of new cross-harbour water mains from Wan Chai to Tsim Sha Tsui and submarine wastewater outfall will also be included in this Project, which will require dredging along the proposed pipelines.

Other Concurrent WDII Activities

5.6.11 Since the construction of the CWB tunnel will involve temporary reclamation works in the Causeway Bay Typhoon Shelter, it will be necessary to temporarily relocate the existing moorings for those private and operational vessels during construction period for these works. The proposed temporary typhoon shelter (TBW) will require dredging for construction of a 400 m long rubble mound breakwater some 180 m offshore and parallel to the existing Causeway Bay Typhoon Shelter breakwater, together with 120 m and 130 m lengths of piled wave walls at the eastern and western ends of the sheltered mooring area respectively. The layout of the proposed temporary typhoon shelter is shown in Figure 5.5.

5.6.12 The primary wave and physical protection will be provided by the conventional rubble mound breakwater, which will be of similar construction to the existing breakwater. The piled wave walls will comprise vertical concrete downstands, supported on tubular steel piles at 8 to 10 m spacing. The down stands will extend down below the surface of the water to reduce wave transmission through the typhoon shelter entrances from the north-easterly and north-westerly direction. Typical details of the breakwater and the piled wave walls are shown in Figure 5.6.

Dredging Scenarios

5.6.13 With reference to the construction programme, one worst-case construction phase scenario, namely Scenario 2A, was selected for modelling. The proposed scenarios represent the realistic worst cases, including all the potentially concurrent dredging activities, envisaged during the water mains construction. For WDII reclamation activities, impact from the seawall dredging is considered to be the most critical. Scenario 2A assumes that the following marine works will take place concurrently in early 2009.

a. Dredging for seawall foundation at HKCEC Stage 1 (HKCEC1)

b. Dredging for seawall foundation at WCR Stage 1 (WCR1)

c. Dredging for seawall foundation at PCWA East (TPCWAE)

d. Dredging for seawall foundation at NP Stage 1 (NPR1)

e. Dredging at temporary breakwater (TBW)

f. Dredging along the proposed alignment of the WSD cross harbour water mains from Wan Chai to Tsim Sha Tsui.

5.6.14 Five reclamation areas within the WDII are to be dredged at the same time under this scenario. To compare with other construction periods, no more than five reclamation areas will be dredged or constructed simultaneously. Thus, this scenario is considered the worst case during early stage of construction phase before any new land is formed within the WDII site. The coastline configuration for Scenario 2A is the same as the existing baseline condition. The dredging locations assumed under this scenario are given in Figure 5.7.

5.6.15 The existing cooling water intakes will have to be reprovisioned to the new water front during the WDII construction. As previously pointed out, diversion of the existing cooling water intakes along the HKCEC water channel has to be conducted through the new land formed under the HKCEC1. Thus, these cooling water intakes cannot be diverted until the reclamation works of HKCEC1 has been completed as the first stage. Based on the findings of the recent marine site investigation works conducted in 2006, dredging is required for the construction of the temporary seawalls at either side of the HKCEC water channel. Therefore, SS generated from the seawall dredging phase of HKCEC1 may affect the nearby cooling water intakes, which is taken into account in Scenario 2A. These cooling water intakes will be diverted to the intake chambers to the north of the HKCEC Extension before the seawall of HKCEC2W and HKCEC3 is completely constructed. In addition, HKCEC2E will not be carried out before the diversion of these cooling water intakes. Dredging at HKCEC1 will not be carried out concurrently with dredging at HKCEC3. Impact on the cooling water intake between CRIII and HKCEC1 due to the seawall dredging at HKCEC2W is assessed under Scenario 2C.

5.6.16 Scenario 2A also covers the impact during seawall dredging at WCR1 which could potentially affect the existing cooling water intake of Sun Hung Kai Centre and the WSD Wan Chai flushing water intake. As pointed out before, these two intakes are located within the site boundary of WCR2 and cannot be diverted before the reclamation works at WCR1 have been completed.

5.6.17 Dredging for the temporary seawall in PCWA will be performed within the existing breakwater. Therefore, lesser impacts are expected from this area. Nevertheless, this potential impact is also covered under Scenario 2A for cumulative assessment.

5.6.18 Details of sediment loss rates assumed in the modelling assessment for Scenario 2A are summarized in Table 5.10 below.

Other Concurrent External Projects

5.6.19 Dredging for the proposed Kai Tak Development, Western Cross Harbour Main, Submarine Gas main relocation at Kowloon Bay and Tseung Kwan O reclamation are also considered in the sediment plume modelling.

KTD - Proposed Dredging Works for Cruise Terminal

5.6.20 Development of the proposed cruise terminal at Kai Tak would require dredging at the existing seawall at the southern tip of the former Kai Tak Airport runway for construction of a berth structure for two berths, and dredging the seabed fronting the new berth structure to provide necessary manoeuvring basin. It is planned to implement the cruise terminal in two phases. Phase I Berth for the initial phase is scheduled for operation by 2012. Phase II Berth for the longer term is currently scheduled for operation after 2015. Dredging required for operation of the Phase I Berth is currently scheduled to be carried out during the period from later half of 2008 to 2011 as Stage 1 dredging. The programme for Stage 2 dredging is unconfirmed at this stage but its completion can be extended up to 2020 and the earliest possible time for the Stage 2 dredging would be 2013 to 2014 after the Stage 1 dredging and decommissioning and removal of the existing submarine gas pipelines currently located to the west of the former Kai Tak Airport runway within the required manoeuvring space and the dredging zone of the Phase II Berth.

5.6.21 It is assumed that the dredging for construction of the water mains, namely Scenario 2A, will be undertaken concurrently with the Stage 1 dredging for construction of the manoeuvring basin (also in the open harbour) to investigate the cumulative impact. The rate of Stage 1 dredging from existing seabed within the proposed manoeuvring area is assumed to be 4,000m³ per day (by two closed grab dredgers). The dredging at or near the seawall for berth construction is also assumed to be conducted at a maximum rate of 4,000m³ per day (by another two closed grab dredgers) concurrently with the Stage 1 dredging.

5.6.22 As the water mains would be completed in 2009, the cumulative impact from dredging at the water mains was only assessed for the Stage 1 cruise terminal dredging.

KTD - Public Landing Steps cum Fireboat Berth

5.6.23 A section of the existing seawall at the former Kai Tak Airport runway will need to be re-constructed for the proposed public landing steps cum fireboat berth (Figure 5.7a) under the Kai Tak Development. Seawall reconstruction would involve excavation and dredging at and near the existing seawall of the runway. It is assumed that the dredging at and near the seawall area will be carried out at a maximum dredging rate of 1,000m³ per day concurrently with the cruise terminal dredging and the water mains dredging for cumulative assessment.

Submarine Gas Main Relocation

5.6.24 Twin 400mm diameter steel submarine gas pipelines are currently aligned 235m west of and parallel to the former Kai Tak Airport runway. The pipelines serve as a strategic gas supply to Hong Kong Island and is covered under an existing wayleave agreement. They run between a gas offtake and pigging station at Ma Tau Kok (MTK) and a gas pigging station at Quarry Bay. As mentioned before, the existing pipeline is located within the manoeuvring space and the dredging zone of the Phase II Berth for the cruise terminal. Hence, the pipeline would need to be reprovisioned before dredging can commence for the Phase II cruise berth.

5.6.25 The possible alignment for the new gas main crossing of 2.8km in a straight line from Ma Tau Kok to North Point is assumed as indicated in Figure 5.7a. The alignment is indicative only and will be subject to detailed design being conducted by the Hong Kong and China Gas Company Limited (HKCGCL).

5.6.26 The dredging associated with removal of the existing submarine gas mains will be incorporated into the Stage 2 dredging works for cruise terminal construction after the dredging activities for this Project are completed. Construction of the new gas main may involve dredging and backfilling activities. Backfilling of rock and armour would not be a water quality issue of concern. Only the dredging and sand filling, if any, would cause potential water quality impact. It is expected that backfilling would be carried out after the dredging and laying of the new gas mains is completed. As the possible dredging and backfilling activities would be conducted in sequence rather than concurrently, the worst-case impacts would be during the dredging of seabed as the dredged sediment might be contaminated. Furthermore, the rate of dredging would be larger than the rate of sand filling.

5.6.27 It is assumed that dredging of seabed for construction of the new gas main would be conducted concurrently with the water mains dredging under Scenario 2A to investigate the worst-case cumulative impact. It is also assumed that under the base case scenario the dredging for gas main construction would be conducted at a maximum rate of 1,000m³ per day, using small trailer hopper dredger in the fairway and grab dredger at the remaining areas. The trailer hopper dredger is required in the fairway as it is more manoeuvrable and self-powered. Grab dredgers are assumed elsewhere as a worst case for water quality impact. It should be noted that construction of the new gas main is a designated project and will be subject to detailed assessment under separate EIA study.

5.6.28 The dredging rate of 1,000m³ per day was calculated based on the best available information obtained at the time when the sediment plume model for this EIA was being set up. According to HKCGCL, the dredge volume will be approximately 54 m³/m run. Assuming 2.8 km of gas mains will give a total dredge volume of approximately 150,000 m³.  It is further assumed that the dredging rate will be relatively slow due to: the need for tight control on the grab to create the relatively narrow trench; the need for accurate alignment; and limited access/working hours when crossing the fairway in the Victoria Harbour. Allowing approximately 6 months to complete this dredging and working 6 days per week gives the assumed dredging rate of approximately 1,000m³ per day.

5.6.29 However, after the sediment plume modelling exercise for the base case scenarios was completed for this EIA, latest construction information for the new gas main was available from the Project Profile submitted by the HKCGCL in September 2007 under the EIAO for application of EIA study brief. Based on the Project Profile for the new gas main, the alignment option (from Ma Tau Kok to North Point) would be adopted but the latest alignment will be laid within a 500m corridor in Victoria Harbour and the exact alignment of the new gas main will be determined during the feasibility study and detail design stage. Under this EIA, the sediment spill location for the gas main construction is assumed at a point close to the WSD flushing intake at Tai Wan as shown in Figure 5.7a (Source ID: A7) which represents a worst case cumulative impact for the Tai Wan intake. Based on the latest alignment corridor provided in the Project Profile for the new gas main, the shortest distance between the new gas pipeline and the Tai Wan WSD intake is similar to that assumed under this EIA. Therefore, the dredging location (Source ID: A7) assumed in this EIA is still considered representative, considering that the Tai Wan intake was also identified in the Project Profile for the new gas main as one of the nearest water sensitive receivers. Besides, a sensitivity test was also conducted under this EIA using a higher dredging rate of 5,000 m³ per day to address the possible change of dredging rate for the gas main construction.

5.6.30 To investigate the worst-case impact on the WSD flushing intake at Quarry Bay, another sensitivity test was conducted using an alternative source point for the new gas main near the pipeline landing point at North Point with a dredging rate of 5,000 m³ per day based on the latest information provided by the HKCGCL and the indicative alignment provided in the Project Profile for the new gas main. As the landing point of the new gas main at North Point would be located in close proximity of the Quarry Bay intake, it was predicted that the SS limit for the WSD flushing intake would be exceeded at the Quarry Bay. Sensitivity analysis indicated that, under the case when dredging is conducted near the gas main landing point at North Point, the change of WDII activities would have minimal effect on the SS compliance level at the Quarry Bay intake. As indicated by the sensitivity modelling conducted under this EIA, feasible mitigation measures such as installation of silt curtains around the gas main dredging work near the North Point or reduction of the dredging rate for gas main construction for the dredging activities near the landing point at North Point would effectively eliminate the SS exceedance and achieve full compliance at all the WSD flushing water intakes. Under the base case scenario assuming that dredging for the new gas main would be located away from North Point, the SS levels predicted at the Quarry Bay intake were well below the SS limit. The Project Profile for the new gas main has only indicated an envelope alignment of about 500 metres wide across the Victoria Harbour and the exact alignment had to be determined after a feasibility study and an EIA study. In the EIA Study Brief issued to HKCGCL, the project proponent of the new gas main was requested to consider other feasible alternatives/options for the pipeline alignment. For the purpose of this EIA, full assessment results for this sensitivity analysis (assuming a source point near the pipeline landing point at North Point) are therefore not presented. However, a summary of the results for this sensitivity analysis and an additional assessment to distinguish the impacts due to the WDII activities and those due to the gas main relocation and other sediment sources are given in Section 5.8 for reference.

Western Cross Harbour Main

5.6.31 A new cross-harbour water main would be constructed concurrently with this Project. This water main will provide security of water supply from West Kowloon to Sai Ying Pun. According to the EIA report “Laying of Western Cross Harbour Main and Associated Land Mains (Western Cross Harbour Main)” (EIAO Register No.: AEIAR-109/2007), construction of the water main is currently scheduled for completion in 2009 and the dredging works would be conducted at a maximum dredging rate of 4,000m³ per day, using one grab dredger.

Further Development of Tseung Kwan O

5.6.32 Based on the approved EIA for Further Development of Tseung Kwan O Feasibility Study (TKOFS), the worst-case construction impacts would occur during the seawall construction for Phase I reclamation when dredging and filling operations are carried out concurrently at the southern area of the TKO reclamation site. According to the reclamation programme given in the approved EIA, these dredging and filling operations would commence in 2010. Based on the latest information obtained from CEDD, the Phase 1 seawall construction would likely to commence in early 2012, and in-situ soil improvement measures would be explored under the detailed design to avoid dredging. Therefore, it is possible that no dredging would be carried out for the TKO reclamation works.

5.6.33 Nevertheless, the cumulative effects of the possible dredging and filling works for TKO reclamation have been considered in this modelling exercise.  The modelling works aimed to investigate whether the WDII and CWB works, including the water mains construction, would contribute any cumulative water quality impacts with the TKO reclamation works. The TKO works have been included under Scenario 2A for worst case cumulative assessment. It is assumed that one close grab dredger would be used for dredging and one pelican barge would be used for sand filling under the TKO works. The production rates for dredging and filling would be 1400m³ per day and 3000m³ per day respectively according to the approved EIA for TKOFS.

Other Concurrent Projects

5.6.34 It should be noted that no dredging activity is anticipated for the HATS Stage 2A and HKCEC Atrium Link Extension. All the marine activities for CRIII will be completed before the construction of water mains.

Suspended Solids

Sediment Plume Modelling

5.6.35 Sediment plumes arising from the mud dredging activities will be simulated using Delft3D-PART. This model has been used for sediment plume modelling in a number of previous reclamation studies in Hong Kong including the approved WDIICFS EIA, Northshore Lantau Development Feasibility Study ^([9]) and the Theme Park Development at Penny’s Bay EIA Study ^([10]).

5.6.36 The loss of fines to the water column during dredging operations is represented by discrete particles in the model. These discrete particles are transported by advection, due to the tidal flows determined from hydrodynamic simulation, and turbulent diffusion and dispersion, based on a random walk technique. The detailed Victoria Harbour (VH) Model adopted under the approved WDIICFS EIA was used to provide the hydrodynamic information for particle tracking. The VH model developed under the approved WDIICFS EIA is considered acceptable for modelling of the construction phase impacts where the effect would be temporary only.

5.6.37 The Delft3D-PART model takes into account the sedimentation process by means of a settling velocity, while erosion of bed sediment, causing resuspension of sediment, is governed by a function of the bed shear stress. The SS elevation caused by the proposed dredging activities is predicted by the Delft3D-PART. The model results will also be presented in terms of the sedimentation rate which represents the net effect from both sediment erosion and deposition. The parameters adopted in the present study are summarised in Table 5.9. Each construction scenario was simulated with three typical spring-neap tidal cycles for spin-up and one cycle for actual simulation in both dry and wet seasons following the approach adopted under the WDIICFS EIA.

Table 5.9 Summary of Parameters for Sediment Plume Model (Delft3D-PART)

Sediment Plume Model Parameters
Horizontal Dispersion Coefficient DH (m^2s^-1)	a = 0.003 b = 0.4	DH = a t ^b, Where t is the age of particle from the instant of discharge in seconds
Vertical Dispersion Coefficient DV (m^2s^-1)	5x10^-3 1x10^-5	Dry Season Wet Season
Particle Settling Velocity	0.0001 m s^-1 (Constant)	Grain size diameter of 10 mm
Critical Shear Stress	0.05 Pa 0.15 Pa	Sedimentation Erosion

Sediment Loss Rates

5.6.38 Assumptions made in the sediment plume modelling simulations for calculating the sediment loss rates for WDII and CWB activities are as follows:

· The dry density of harbour mud is 1,370 kg/m³, based on the geotechnical site investigation for the WDII and CWB marine ground investigation works conducted under this Study.

· Spill loss during mud dredging by closed grab dredger will be continuous, 16 hours a day, 7 days per week. The grab dredger is assumed to work over 16 hours per day in order to maintain the required works rates to meet the tight construction programme.

· With respect to rate of sediment loss during dredging, the Contaminated Spoil Management Study ^([11]) (Mott MacDonald, 1991, Table 6.12) reviewed relevant literature and concluded that losses from closed grab dredgers were estimated at 11 – 20 kg/m³ of mud removed. Taking the upper figure of 20 kg/m³ to be conservative, the loss rate in kg/s was calculated based on the daily volume rate of dredging. (Assuming a dry density for marine mud of 1,370 kg/m³, the sediment loss during dredging is equivalent to a spill amount of approximately 1.5%).

· Spillage of mud dredged by closed grab dredgers is assumed to take place uniformly over the water column.

· Dredging of contaminated and uncontaminated mud will be carried out at the same rate.

5.6.39 The calculated sediment loss rates for Scenario 2A are shown in Table 5.10. The corresponding source locations are given in Figure 5.7. The loss rates shown in Table 5.10 for KTD and TKO reclamations are the reduced loss rates under the mitigated scenarios which have considered the effect of silt curtains. On the other hand, deployment of silt curtains have not been considered in calculating the sediment loss rates from WDII and CWB dredging works and the remaining concurrent activities. These sediment loss rates represent the worst case under the unmitigated scenario. It is assumed that silt curtains will only be deployed if the water quality impacts are found to be unacceptable. Deployment of silt curtains have been considered under the mitigated scenario discussed in Section 5.8.

Table 5.10 Maximum Dredging Rates - Scenario 2A (early 2009)

Source ID	Activity	Approx. Duration ^(1) (days)	Work Hours per day	Dredging Rate		Sediment Loss Rate (kg s^-1)
				m^3 per day	m^3 per hour
WDII and CWB Dredging Activities:
HKCEC1 (Figure 5.7)
A1	Dredging (1 closed grab dredger of 8 m^3 capacity)	14	16	6000	375	2.08
WCR1 (Figure 5.7)
A2	Dredging (1 closed grab dredger of 8 m^3 capacity)	29	16	6000	375	2.08
TPCWAE (Figure 5.7)
A3	Dredging (1 closed grab dredger of 8 m^3 capacity)	16	16	6000	375	2.08
NPR1 (Figure 5.7)
A4	Dredging (1 closed grab dredger of 8 m^3 capacity)	31	16	6000	375	2.08
Water Mains from Wan Chai to Tsim Sha Tsui (Figure 5.7)
Alternative dredging locations: either A5 or A5a^(2)	Dredging (1 closed grab dredger of 8 m^3 capacity)	16	16	6000	375	2.08 (for A5 or A5a)
TBW (Figure 5.7)
A6	Dredging (1 closed grab dredger of 8 m^3 capacity)	54	16	6000	375	2.08
External Concurrent Dredging Activity in Victoria Harbour and Junk Bay:
Submarine Gas Main Relocation (Figure 5.7a)
A7	Dredging (1 closed grab dredger of 8 m^3 capacity)	12	12	1000 (or 5000**)	83 (or 417**)	0.46 (or 2.31**)
KTD – Cruise Terminal – Dredging at or near the seawall for Berth Construction (Figure 5.7a)
A8	Based on the latest information from the Kai Tak Development Engineering Study					0.23
A9						0.23
KTD – Cruise Terminal - Dredging from the seabed for Construction of the Manoeuvring Basin (Figure 5.7a)
Alternative dredging locations: either A10 and A11 or A10a and A11a	Based on the latest information from the Kai Tak Development Engineering Study					0.93 (for A10 or A10a)
						0.93 (for A11 or A11a)
KTD – Public Landing Steps cum Fireboat Berth (Figure 5.7a)
A12	Based on the latest information from the Kai Tak Development Engineering Study					0.12
Western Cross Harbour Main between West Kowloon to Sai Ying Pun
A13	Based on the EIA report for Western Cross Harbour Main					0.93
Further Development of Tseung Kwan O
D1	Based on the approved EIA for TKOFS EIA					0.44
F1						0.15

(1) The duration of each operation is based on the construction programme presented in Appendix 2.1.

(2) For the purpose of modelling, two alternative dredging locations are considered with A5 close to Hong Kong Island and A6 close to Tsim Sha Tsui. However, it should be noted that the dredging will be performed by 1 close grab dredger during the construction of the cross harbour water mains. Thus, only one dredger will operate at one location at a time.

** Values in bracket are used for sensitivity test (refer to Section 5.6.30)

Contaminant Release during Dredging

5.6.40 The loss of sediment to suspension during dredging may have chemical effects on the receiving waters. This is because the sediment may contain organic and chemical pollutants. As part of the marine site investigation works for this Project, laboratory testing of sediment samples was undertaken. A full description of the sediment quality testing and the classification of the sediment according to levels of contaminants is contained in Section 6. Oxygen depletion (due to sediment plume) was calculated using the highest level of 5-day SOD ^([12]) measured in the sediment samples collected during the marine site investigation (SI), based on the predicted increases in suspended sediment concentrations for the construction phase scenarios. The reductions were then compared with the baseline levels to determine the relative effects of the increases in SS concentrations on DO.

5.6.41 The nutrient impacts from increased SS concentrations were assessed from the sediment quality data for TIN and NH₃-N. An inactive tracer was defined in the model at the dredging locations to determine the dilution in the vicinity of the dredging site. The dilution information was then used to determine the concentrations of the concerned parameters at receiving waters and to evaluate the potential impacts to the marine environment.

5.6.42 An indication of the likelihood of release of contaminants (including heavy metals, PCBs, PAHs and TBT) from the sediment during dredging is given by the results of the elutriation tests from the site investigation works. If the contaminant levels are higher in the elutriates in comparison with the blanks (marine water from the same site), it can be concluded that the contaminants are likely to be released into the marine waters during dredging activities. As there is no existing legislative standard or guideline for individual heavy metal contents in marine waters, the UK Water Quality Standards for Coastal Surface Water ^([13]) were adopted as the assessment criteria.

Water Quality Modelling

5.6.43 As adopted in the approved WDIICFS EIA for assessing the environmental impacts of released SS, the ambient value is represented by the 90th percentile of baseline (pre-construction) concentrations. Water quality modelling was carried out using Delft3D-WAQ for the pre-construction scenario. The detailed Victoria Harbour (VH) Model adopted under the approved WDIICFS EIA was used for construction phase water quality modelling. The pre-construction scenario was simulated with three typical spring-neap tidal cycles for spin-up and one cycle for actual simulation in both dry and wet seasons following the approach adopted under the WDIICFS EIA.

Time Horizon and Coastline Configurations for Modelling

5.6.44 Based on the construction programme for WDII, the worst-case construction impact would occur at early stages of the construction period between early 2009 and 2011. It is anticipated that there would not be any significant change in the background pollution loading and coastline configurations between early 2009 and 2011. The 2011 pollution loading was adopted for modelling of the water quality for the pre-construction scenario. For areas outside the Project site boundary, the 2011 coastline configurations as shown in Figure 5.12 were assumed under all the modelling scenarios.

Pile Friction

5.6.45 Existing structures including the piers of East Bridge, West Bridge and Seafront Promenade within the proposed reclamation sites at the HKCEC water channel have been considered in the construction phase assessment. The pile layouts are shown in Appendix 5.1b.

5.6.46 East Bridge consists of 11 rows of marine steel tubular piles across the waterway from south to north with a spacing of about 7 m in between the piles. Each row consists of 4 piles from east to west with a spacing of 9 m between the piles. The diameter of each pile is 914 mm.

5.6.47 The pile arrangement for West Bridge is the same as that for East Bridge except that the spacing between the piles in the east to west direction is only 7 m.

5.6.48 The Seafront Promenade is supported by 31 marine piles. The diameter of each pile is 1 m. The spacing between the piles is different in different areas of the Seafront Promenade site. The spacing varies from 3.3 m to 9 m.

5.6.49 The presence of these marine piles may affect the flushing and dispersion of sediment and pollutants in the HKCEC water channel and were therefore incorporated in all the construction phase scenarios as appropriate. The marine piles have variable separation distance. As the dimensions of the marine piles are much smaller than the grid size, the exact pier configurations cannot be adopted in the model simulation. Instead, only the overall influence of the piles on the flow was taken account. This overall influence was modelled by a special feature of the Delft3D-FLOW model, namely “Porous Plate”. “Porous Plate” represents transparent structures in the model and is placed along the model gridline where momentum can still be exchanged across the plates. The porosity of the plates is controlled by a quadratic friction term in the momentum to simulate the energy losses due to the presence of the piles. The forces on the flow due to a vertical pile or series of piles are used to determine the magnitude of the energy loss terms. The mathematical expressions for representation of piles friction were based on the Cross Border Link Study ^([14]) and the Delft 3D-FLOW module developed by Delft Hydraulics.

5.6.50 For each grid cell where the piles will be located, two loss coefficients have been specified in the model for two different flow directions respectively (i.e. the two directions perpendicular to the model gridline, namely u-direction and v-direction respectively). Details of the equations used in the modelling are contained in Appendix 5.2.

Piled Wave Walls of the Temporary Typhoon Shelter

5.6.51 The proposed temporary moorings will require construction of piled wave walls at the eastern and western ends of the sheltered mooring area respectively as shown in Figure 5.5 and Figure 5.6. The overall influence of the piled wave walls on the flow was modelled by a special feature of the Delft3D-FLOW model, namely “Current Deflection Wall (CDW)”. The CDW is represented by an impermeable sheet with supporting piles at the bottom and is placed along the model gridline where there will be no flow exchange across the sheet at the upper vertical water layers. The dimension of the impermeable sheet in the vertical direction was defined in the model with reference to the dimension of the proposed waved walls at the temporary sheltered mooring area. Flow exchange across the supportive piles of the CDW in the lower water layers is controlled by the same quadratic friction and mathematical expressions for representation of pile friction as discussed above.

Pollution Loading Inventory

5.6.52 The pollution loading inventory was compiled for the 2011 scenario. The background pollution loading was estimated for the whole HKSAR waters by desk-top method and was input to the water quality model for cumulative impact assessment. The pollution loading inventory for individual storm outfalls within the Project site boundary in Wan Chai, Causeway Bay and North Point was further refined and updated based on desk-top calculations and pollution loading field data.

HKSAR Waters (Outside the Project Site Boundary)

5.6.53 The pollution loading inventory covers the whole HKSAR waters and was input into the Update Model and the detailed VH Model for cumulative impact assessment. The inventory has incorporated all possible pollution sources within the HKSAR waters including those from landfill sites, marine culture zones, beach facilities and typhoon shelters, non-point source surface run-off and sewage from cross connections etc. The inventory has also taken into account the removal of pollutants due to wastewater treatment facilities and the possible redistribution of pollution loads due to different sewage disposal plans and sewage export schemes. The methodologies for compiling the pollution loading are given in Appendix 5.3.

5.6.54 To take account of the background pollution loading for cumulative assessment, pollution loading from the HATS was considered. Chemically enhanced primary treatment (CEPT) with disinfection is assumed as the treatment process of HATS in this EIA study for water quality modelling which involves a discharge of effluent at the existing Stonecutters Island Sewage Treatment Works (SCISTW). The HATS loading assumed in this EIA is given in Table 5.14.

Table 5.14 Pollution Loading from Stonecutters Sewage Treatment Works under HATS

Parameters	2011 Scenario (HATS Stage 1)
	Assumed Concentration	Assumed Flow and Loads
Flow rate	-	1,638,000 m^3/day ^(1)
BOD5	68 mg/l ^(2)	107188400 g/day
SS	42 mg/l ^(2)	66204600 g/day
Organic Nitrogen	9.93 mg/l ^(2)	15652659 g/day
NH3-N	17.43 mg/l ^(2)	27474909 g/day
E. coli	200,000 no./100ml (2 log bacterial kill) ^(2)	3.15E+15 no./day
Total Phosphorus	3 mg/l ^(2)	4728900 g/day
Ortho-Phosphate	1.8 mg/l ^(2)	2837340 g/day
Silicate	8.6 mg/l ^(2)	13556180 g/day
Total nitrite and nitrate	0 mg/l ^(2)	0 g/day
Total Residual Chlorine	0.2 mg/l ^(2)	315260 g/day

Notes:

(1) The projected flow rate for 2011 was estimated using the latest planning and employment statistics as detailed in Appendix 5.3.

(2) Based on the “Environmental and Engineering Feasibility Assessment Studies in Relation to the Way Forward of the Harbour Area Treatment Scheme (HATS EEFS) Final Study Report”.

5.6.55 The sewage flows generated from Wan Chai East (WCE) and Wan Chai West (WCW) catchments would be discharged via the submarine outfalls of Wan Chai East preliminary treatment works (WCEPTW) and Wan Chai West preliminary treatment works (WCWPTW) under the 2011 construction phase scenarios. The locations of catchments WCE and WCW is shown in Table A5-3-1 of Appendix 5.3.

Storm Outfalls within the Project Site Boundary

5.6.56 Pollution loading discharged from the existing storm system of the Wan Chai, Causeway Bay and North Point catchments was quantified. The storm pollution within the catchments is mainly caused by polluted stormwater runoff or street washing to the drainage system; and expedient connections from trade and residential premises and integrity problems of aged drainage and sewerage systems in the catchment areas. The pollution loading inventory for individual storm outfalls along the coastline of Wan Chai, Causeway Bay and North Point was compiled by a combination of desk-top calculations and field surveys.

Wan Chai and Causeway Bay Area

Loading Growth Ratios by Sewage Catchment Area

5.6.57 The 2003-based Territorial Population and Employment Data Matrices (TPEDM) provided by Planning Department (PlanD), which were the latest planning data available at the time when this EIA was conducted, were used to compile the pollution loads from domestic, commercial and industrial activities. The TPEDM provides the projected population breakdown by Planning Vision and Strategy (PVS) zones. To facilitate the estimation of pollution loading, the population and employment data are required to be presented at the level of sewage catchment areas. The catchments of concern would be the Wan Chai East (WCE) and Wan Chai West (WCW) sewage catchments. However, the projected population from PlanD is provided in a smaller scale at PVS zones. Population and employment data for each of the WCE and WCW catchments were estimated by overlaying the PVS zones on top of the layout of the sewage catchment area for allocating the appropriate PVS zones to the catchment area.

5.6.58 The modeling work was carried out for the 2011 scenario where the projected population data provided by PlanD at PVS zones are available for 2006, 2011 and 2016. Relevant per head flow and load were then assigned to residential, transient, commercial and industrial population to obtain the quantity and quality of total untreated wastewater by individual catchments. Further elaboration of the methodologies for compiling the pollution loading is given in Appendix 5.3.

5.6.59 The pollution loading generated within the WCE and WCW sewage catchments was calculated for 2006 and 2011. The growth ratios between 2006 and 2011 were calculated with reference to the projected loads (calculated by desk-top method) for 2006 and 2011.

Loading Inventory by individual Storm Culverts

5.6.60 An expedient connection survey and a stormwater flow and pollutant survey were conducted in 2000 ^{([15], [16])} under the WDIICFS to estimate the pollution loading discharged via the major storm outfalls along the coastline of Wan Chai and Causeway Bay. The corresponding 2000 dry weather loading results for these storm outfalls, namely Culverts L, M, N, O, P, Q, R and S, are presented in Table 5.15. The locations of these storm outfalls are shown in Figure 5.3B. The pollution loading discharged via individual storm culverts for future scenarios was estimated with reference to the 2000 survey data, taking account of the loading growth ratios compiled by desk-top approach as discussed in previous sections.

5.6.61 Based on the review of the population data for Wan Chai District, which covers the storm catchments in Wan Chai and Causeway Bay, released by Census & Statistics Department (C&SD), there was a slight reduction in the population size from mid-2000 to mid-2006 in Wan Chai District. It is therefore assumed that there would be no significant change in the pollution loading discharged via the concerned storm systems as a result of expedient connection or cross connections between drainage and sewerage system assuming that the percentage of sewage lost to the storm water system remains unchanged between 2000 and 2006. The loading due to storm water runoff or street washing to the drainage system can also be assumed to remain the same between 2000 and 2006 as there is no significant change in the land use within the concerned catchments.

5.6.62 The dry weather loading inventory for 2011 was thus compiled by applying the 2000 field data with the loading growth ratios between 2006 and 2011 to take account of the population growth between the time horizons. The same loading growth ratios were applied to the storm culverts within the same sewage catchments. As there was only trace rainfall recorded during the 2000 survey period, the loading inventory compiled for 2011 is treated as dry weather load.

5.6.63 The rainfall related pollution loads were calculated theoretically for WCW and WCE catchments and were added on top of the dry weather loading inventory to estimate the wet weather loads for conservative predictions. It was assumed that the rainfall related load would be evenly distributed amongst all the storm outfalls within the same sewage catchment. Calculations of the rainfall related loads are given in Appendix 5.3.

5.6.64 The pollution loading discharged from the vessels in CBTS due to domestic activities was taken into account in the pollution load inventory. Data on marine population for the whole territory are available from C&SD for years 1986, 1991, 1996 and 2001 which show significant decline in the total marine population between 1986 and 2001 from 37,280 to 5,895. The annual vessel count in typhoon shelters conducted by Marine Department would provide information on the distribution of marine population between different typhoon shelters. The vessel count data for CBTS as reported in the EPD Update Study also indicated a trend of decline in the vessel number between 1986 and 1997. The pollution loading in CBTS was compiled using the marine population estimated for 1997 available from the EPD Update Study. Total pollution from marine population is expected to decrease in future as a result of continued reduction in marine population. So adopting pollution loadings for year 1997 for model input would represent a worst-case scenario.

North Point Area

5.6.65 It should be highlighted that no permanent marine embayment would be created along the coastline of North Point area as a result of the WDII reclamation and therefore the polluted storm water generated from the North Point catchment would be discharged into the open water and can be easily dispersed by the fast moving tidal currents. In addition, there would be no WSD flushing water intake located close to these storm water discharges. Thus, the level of pollution loading discharged via the storm system of North Point catchment will not be a critical water quality issue of concern.

5.6.66 The same desktop methods for compiling the total loading generated in WCW and WCE catchments were used to estimate the loading inventory for North Point area except that the population data were refined to a smaller scale at the level of the catchments for individual stormwater outfalls, namely T, U, V and W as shown in Figure 5.3B, rather than at the level of sewage catchment areas. Population and employment data for each of the catchments of Culverts T, U, V and W were estimated by overlaying the PVS zones on top of the boundaries of the storm catchments for allocating the appropriate PVS zones to the catchment area. Per capital load factors were applied to the population to estimate the total sewage load generated in each storm catchment. It is assumed that 10 percent of the total load generated within the catchment would be lost to the storm water due to expedient connections or cross connections. Rainfall related load was also calculated theoretically as detailed in Appendix 5.3 for compiling the wet season loading inventory. Table 5.16 and Table 5.17 show the pollution loading results for 2011 scenario for model input.

Table 5.15 Locations and Pollution Loadings Survey Results (in 2000) of Wan Chai Stormwater Outfalls

Outfall (Figure 5.3B)	Location		Flow rate (m^3 per day)	Pollution Loadings
	Easting	Northing		BOD (kg per day)	Suspended Solids (kg per day)	Total Kjeldhal Nitrogen (kg per day)	Organic Nitrogen (kg per day)	Ammoniacal Nitrogen (kg per day)	E. coli (no. per day)
L	835467	815848	2743	1337.73	2144.12	129.86	106.70	23.16	7.889E+14
M	836000	815889	13775	514.28	581.30	93.60	58.58	35.02	1.84E+14
N	836397	815977	1761	18.80	11.37	5.76	2.69	3.07	1.86E+12
O	836551	816059	3500	378.87	346.35	53.09	33.29	19.80	3.078+14
P	836921	815940	127	84.19	50.92	7.97	2.93	5.04	6.41E+12
Q	837139	816106	13302	372.54	464.28	161.56	126.39	35.17	4.08E+13
R	837551	816230	1197	105.25	362.21	15.82	9.81	6.01	9.71E+12
S	837595	816322	1030	5.86	3.10	1.32	0.64	0.68	1.93E+12

Sources: (1) EGS (Asia) Limited (2000). Wan Chai Development Phase II, Comprehensive Feasibility Study, Section I, Stormwater Flow and Pollutant Survey of Outfalls Entering Victoria Harbour of Outfalls Entering Victoria Harbour, Final Report.

(2) EGS (Asia) Limited (2000). Wan Chai Development Phase II, Expedient Connection Survey, Supplementary Report for Section I of Works.

Table 5.16 Pollution Loading Inventory for Wan Chai, Causeway Bay and North Point - Year 2011 Dry Season

Outfall (Figure 5.3B)	Location		Flow rate (m^3 per day)	Pollution Loadings
	Easting	Northing		BOD (kg per day)	Suspended Solids (kg per day)	Total Kjeldhal Nitrogen (kg per day)	Organic Nitrogen (kg per day)	Ammoniacal Nitrogen (kg per day)	E. coli (no. per day)
L	835467	815848	2793	1360.00	2176.44	133.12	109.19	23.77	8.10E+14
M	836000	815889	14007	523.19	590.40	96.22	60.07	36.07	1.90E+14
N	836397	815977	1782	18.99	11.47	5.85	2.73	3.12	1.89E+12
O	836551	816059	3523	382.05	348.72	54.0	33.80	20.17	3.14E+14
P	836921	815940	128	84.76	51.19	8.08	2.97	5.12	6.51E+12
Q	837139	816106	13500	376.25	468.15	165.36	129.02	36.08	4.19E+13
R	837551	816230	1217	106.39	365.50	16.22	10.03	6.18	9.98E+12
S	837595	816322	1061	5.95	3.14	1.37	0.66	0.71	2.01E+12
T	837588	816609	1109	294.63	260.34	37.56	16.77	20.80	1.72E+14
U	837889	816838	788	219.09	192.11	27.51	12.30	15.21	1.26E+14
V	837975	816937	164	46.17	40.44	5.71	2.56	3.15	2.60E+13
W	838226	817085	388	93.29	82.35	13.99	6.06	7.93	6.62E+13

Table 5.17 Pollution Loading Inventory for Wan Chai, Causeway Bay and North Point - Year 2011 Wet Season

Outfall (Figure 5.3B)	Location		Flow rate (m^3 per day)	Pollution Loadings
	Easting	Northing		BOD (kg per day)	Suspended Solids (kg per day)	Total Kjeldhal Nitrogen (kg per day)	Organic Nitrogen (kg per day)	Ammoniacal Nitrogen (kg per day)	E. coli (no. per day)
L	835467	815848	12121	1983.60	4280.58	160.01	151.73	25.01	8.10E+14
M	836000	815889	66108	816.03	1278.17	120.09	88.58	38.39	1.90E+14
N	836397	815977	7300	26.17	20.93	6.88	3.65	3.26	1.89E+12
O	836551	816059	16329	579.71	732.93	67.19	49.40	21.46	3.14E+14
P	836921	815940	567	123.51	101.97	9.85	4.20	5.41	6.51E+12
Q	837139	816106	56940	520.33	855.20	196.35	174.80	37.84	4.19E+13
R	837551	816230	5321	165.88	741.93	20.89	15.09	6.68	9.98E+12
S	837595	816322	4039	7.46	4.85	1.55	0.83	0.73	2.01E+12
T	837588	816609	2885	334.55	337.14	40.05	18.90	21.15	1.72E+14
U	837889	816838	1631	238.05	228.58	28.69	13.31	15.38	1.26E+14
V	837975	816937	333	49.97	47.74	5.95	2.76	3.19	2.60E+13
W	838226	817085	709	100.49	96.21	14.44	6.45	7.99	6.62E+13

Uncertainties in Assessment Methodology

Marine-based Construction and Operational Phase Impacts

5.6.67 Quantitative uncertainties in the modelling were considered when making an evaluation of the modelling predictions.  The following approach has been adopted to enhance the model performance:

· The computational grid of the detailed Victoria Harbour (VH) Model was refined along the coastline of Wan Chai, Causeway Bay and North Point to represent the coastal features under different interim construction and operational scenarios;

· Use of a fully calibrated and validated regional Update Model to provide boundary and initial conditions to the detailed VH Model;

· The performance of the detailed VH Model was extensively calibrated and validated with reference to the field data to ensure that reliable predictions of hydrodynamics are provided for the Study area.

· The simulation comprises a sufficient spin up period so that the initial conditions do not affect the results.

5.6.68 The level of uncertainties on the water quality predictions inside the temporary embayment areas would also depend on the accuracy of the pollution loading input into the embayment areas. The storm pollution loading discharged into the embayment areas along the coastline of Wan Chai and Causeway Bay including the CBTS was derived from detailed field investigation to provide accurate information for model input. The loading input to the water quality model under various future assessment scenarios has also taken into account the future development and population growth in order to provide conservative predictions.

5.6.69 It should be noted that all the predictions made in this water quality impact assessment were based on the latest available information and assumptions discussed in this section. If there are any major changes to the key assumptions during the actual implementation of the Project in the future, including those for the concurrent projects, the prediction and assessment findings presented in this EIA report should be reviewed accordingly.

5.7 Prediction and Evaluation of Environmental Impacts

Sediment Plume Impacts

Suspended Solids

5.7.1 One sediment dispersion scenario was modelled, as defined in Table 5.10. Absolute maximum and tidal-averaged SS concentrations predicted at mid-depth for a spring-neap cycle for each seawater intake, taking into account the background SS concentration, are presented in Table 5.22 for the base case scenarios and Table 5.22a for the sensitivity test. The 90 percentile SS level predicted at the corresponding indicator points under the pre-construction scenario is used as the background SS concentrations for conservative predictions. The modelling scenario was simulated with three typical spring-neap tidal cycles for spin-up and one cycle for actual simulation in both dry and wet seasons.

5.7.2 The results shown in tables indicate exceedances (highlighted in bold) of WSD water quality (SS) criterion and target SS level of Admiralty Centre intake. Mitigation measures are therefore required to minimise the impact.

5.7.3 The construction contours presented in Appendix 5.9a, Appendix 5.9b, Appendix 5.9g and Appendix 5.9h show the extent of mid-depth SS elevations over a spring-neap cycle, during wet and dry seasons for the base case scenarios and the sensitivity test. The tidal-averaged sedimentation rate of SS during dry and wet seasons is also presented in Appendix 5.9a, 5.9b, 5.9g and 5.9h. As shown in the appendices, the sedimentation rates at waters near the Green Island and within Junk Bay would be lower than 0.1 kg m^-2 per day (Section 5.3.10). Table 5.41 and Table 5.42 in Section 5.8 also summarise the predicted SS elevation at the coral site in Junk Bay under the mitigated scenario. The coral sites at Green Island and Junk Island were found not be impacted by marine works from WDII and are therefore not included in the tables. With the recommended measures, the SS elevation predicted at the Junk Bay would fully comply with the WQO. It is therefore predicted that that the WDII development would not adversely impact the coral communities at waters near the Green Island and within Junk Bay in terms of both sedimentation rate and SS elevation.

Table 5.22 Scenario 2A – Suspended Solids Concentrations at Sensitive Receivers (Base Case Scenario)

Sensitive Receiver	SS concentration (absolute value) in mid-depth (mg/l)
	Criterion	Dry season			Wet season
		Mean	Maximum	% time in compliance	Mean	Maximum	% time in compliance
Cooling Water Intakes
Prince's Building Group	-	11.0	80.9	-	10.4	65.9	-
Queensway Government Offices	-	12.3	69.3	-	11.4	62.3	-
Admiralty Centre	< 40	12.9	100.8	95.3%	11.1	50.6	98.9%
HSBC	-	12.1	70.4	-	11.0	78.7	-
Excelsior Hotel & World Trade Centre	-	7.3	7.6	-	7.6	61.1	-
Great Eagle Centre / China Resources Building	-	15.3	69.8	-	12.7	75.7	-
Hong Kong Convention and Exhibition Centre Extension	-	9.9	66.5	-	10.7	54.1	-
Hong Kong Convention and Exhibition Centre Phase I	-	25.5	465.7	-	27.4	461.8	-
MTRC South Intake	< 40	7.1	40.4	99.7%	8.5	44.1	99.7%
Sun Hung Kai Centre	-	8.5	77.4	-	13.2	61.6	-
Telecom House / HK Academy for Performing Arts / Shun On Centre	-	19.7	122.5	-	14.6	94.9	-
Wan Chai Tower / Revenue Tower / Immigration Tower	-	16.5	93.7	-	14.2	92.4	-
Windsor House	-	7.7	7.8	-	8.2	52.7	-
Government Premises	-	5.9	15.7	-	9.6	36.4	-
City Garden	-	12.6	59.9	-	13.0	48.2	-
Provident Centre	-	11.6	66.6	-	12.9	45.3	-
WSD Saltwater Intakes
Kennedy Town	< 10	6.6	12.5	98.9%	7.1	7.6	100.0%
Kowloon South	< 10	7.2	7.4	100.0%	7.3	22.0	98.6%
Quarry Bay	< 10	8.1	43.3	82.3%	7.2	31.8	86.7%
Sai Wan Ho	< 10	6.5	51.1	88.9%	6.6	37.7	88.4%
Sheung Wan	< 10	9.2	42.4	77.0%	8.7	38.3	89.2%
Siu Sai Wan	< 10	4.8	5.6	100.0%	5.0	8.8	100.0%
Wan Chai	< 10	6.6	47.6	89.8%	10.1	38.7	82.8%

Notes:

(1) The water quality modelling results for 90 percentile SS predicted under the pre-construction scenario (Scenario 3A) at the corresponding indicator points is adopted as the ambient SS levels.

- Other WSR, including WSD Tai Wan intake, WSD Cheung Sha Wan intake, WSD Cha Kwo Ling intake, Kau Yi Chau Fishery, PLA Headquarters intake, Queen Mary Hospital intake, Stage 1 Phase 1 intake and Wah Fu Estate intake were found not be impacted by the proposed marine works.

- Bold and shaded number indicates exceedence of criterion.

Table 5.22a Construction Scenario 2A – Suspended Solids Concentrations at Sensitive Receivers (Sensitivity Test using Higher Dredging Rate for Gas Main Construction)

Sensitive Receiver	SS concentration (absolute value) in mid-depth (mg/l)
	Criterion	Dry season			Wet season
		Mean	Maximum	% time in compliance	Mean	Maximum	% time in compliance
Cooling Water Intakes
Prince's Building Group	-	11.0	80.9	-	10.6	66.4	-
Queensway Government Offices	-	12.3	69.3	-	11.5	62.3	-
Admiralty Centre	< 40	12.9	100.8	95.3%	11.2	50.6	98.6%
HSBC	-	12.1	70.4	-	11.2	78.7	-
Excelsior Hotel & World Trade Centre	-	7.3	7.6	-	7.6	61.1	-
Great Eagle Centre / China Resources Building	-	15.3	69.8	-	12.9	75.7	-
Hong Kong Convention and Exhibition Centre Extension	-	10.0	66.5	-	10.9	57.2	-
Hong Kong Convention and Exhibition Centre Phase I	-	25.5	465.7	-	27.6	461.8	-
MTRC South Intake	< 40	7.1	40.4	99.7%	8.5	44.1	99.7%
Sun Hung Kai Centre	-	8.5	77.4	-	13.3	61.6	-
Telecom House / HK Academy for Performing Arts / Shun On Centre	-	19.8	122.5	-	14.7	94.9	-
Wan Chai Tower / Revenue Tower / Immigration Tower	-	16.6	93.7	-	14.4	93.2	-
Windsor House	-	7.7	7.8	-	8.2	52.7	-
Government Premises	-	5.9	15.7	-	9.6	36.4	-
City Garden	-	12.6	59.9	-	13.0	48.2	-
Provident Centre	-	11.6	66.6	-	12.9	45.3	-
WSD Saltwater Intakes
Kennedy Town	< 10	6.6	12.5	98.9%	7.2	7.6	100.0%
Kowloon South	< 10	7.2	7.4	100.0%	7.3	22.0	98.6%
Quarry Bay	< 10	8.1	43.3	82.3%	7.4	32.0	86.7%
Sai Wan Ho	< 10	6.5	51.1	88.9%	6.8	38.3	88.4%
Sheung Wan	< 10	9.2	42.8	77.0%	8.8	38.3	88.6%
Siu Sai Wan	< 10	4.8	5.6	100.0%	5.0	8.9	100.0%
Wan Chai	< 10	6.6	47.6	89.8%	10.1	38.7	82.8%

Notes:

(1) The water quality modelling results for 90 percentile SS predicted under the pre-construction scenario (Scenario 3A) at the corresponding indicator points is adopted as the ambient SS levels.

- Other WSR, including WSD Tai Wan intake, WSD Cheung Sha Wan intake, WSD Cha Kwo Ling intake, Kau Yi Chau Fishery, PLA Headquarters intake, Queen Mary Hospital intake, Stage 1 Phase 1 intake and Wah Fu Estate intake were found not be impacted by the proposed marine works.

- Bold and shaded number indicates exceedence of criterion.

Compliance with WQO for SS Elevation

5.7.4 Non-compliance with the WQO for SS (i.e. elevation of less than 30% of ambient baseline level) is predicted in the Victoria Harbour channel under the dredging scenario as shown in Appendix 5.9a and Appendix 5.9b. Each figure attached in these appendices contains two contour plots where the upper plot shows the unmitigated scenarios and the lower plot shows the mitigated scenarios. As shown in the contour plots, the sediment plume would be relatively large under the unmitigated scenario and become localized after implementation of the mitigation measures as recommended in Section 5.8. The general compliance for DO, nutrients in Victoria Harbour is discussed in later sections.

Potential Contaminant Release During Dredging

Elutriate Test Results

5.7.5 An indication of the likelihood of release of contaminants from the marine mud during dredging is given by the results of the elutriation tests from the laboratory testing conducted under the Phase I and Phase II marine site investigation (SI) works. Phase I SI covers the waters at HKCEC water channel (with vibrocore sampling at V06-2W), within CBTS (with vibrocore sampling at V06-6W, V06-7W, V06-8W), North Point (with vibrocore sampling at V06-9W) and outside CBTS (with vibrocore sampling at V06-10W). Phase II SI covers the waters to the west of HKCEC Extension (with vibrocore sampling at V06-1W), Wan Chai (with vibrocore sampling at V06-3W and V06-4W), and within the PCWA (with vibrocore sampling at V06-5W). The locations of vibrocore samplings are shown in Figure 6.1. Permission to sample in the WSD prohibition zone and MTR protection zone in the area to the west of HKCEC was not obtained from WSD and MTRC for the marine site investigation. In addition, MTRC advised that anchoring is not permitted within 20m of their protection zone and hence it was not possible to collect sufficient elutriate samples within the marine embayment to the west of HKCEC Extension. Therefore, reference was made to the elutriate test results available from the approved WDIICFS EIA for two locations (namely MV1 and MV4) in the marine embayment to the west of HKCEC to supplement the elutriate test results obtained under the present Study. The locations of stations MV1 and MV4 are given in the approved EIA for WDIICFS (http://www.epd.gov.hk/eia/register/report/eiareport/eia_0582001/eia/Volume%20II/00000089.GIF).

5.7.6 As there is no existing legislative standard or guideline for individual heavy metal contents in marine waters, the UK Water Quality Standards for Coastal Surface Water ^([17]) have been adopted as the assessment criteria.

5.7.7 As shown in Table 5.27 to Table 5.29 below, the metal concentrations (other than silver at vibrocore V06-8W at sampling depth 3.0-4.0m and mercury at vibrocore MV4 at sampling depth 1.0-1.9m) in the elutriate samples from the Phase I SI fall within the UK Water Quality Standards. The maximum levels of silver and mercury measured in the elutriate samples collected at Stations V06-8W and MV4 are 2.8mg/l and 0.4mg/l respectively which only marginally exceeded the water quality standard of 2.3mg/l and 0.3mg/l respectively. Although exceedence of UK standards are predicted in the elutriate tests, it is expected that any release of heavy metals during dredging will be quickly diluted by the large volume of marine water within the construction site. Based on the detected highest concentrations, the required dilution to meet the assessment criteria for silver and mercury were calculated to be 1.5 only. The release of pollutants will also be minimised by the use of closed grab dredger and the dispersion of pollutants will be confined within the construction site by silt curtains (Section 5.8). Thus, it is considered that long-term off-site marine water quality impact is unlikely and any local water quality impact will be transient.

5.7.8 Elutriation tests were also conducted to assess the likelihood of release of organic compounds, such as total polychlorinated biphenyls (PCBs) and total polyaromatic hydrocarbons (PAHs), and tributyltin (TBT) from the marine mud during dredging activities. As there are no existing legislative standards or guidelines for the contaminants total PCBs and total PAHs in marine waters, reference was made to the Australia water quality guidelines ^([18]) and USEPA water quality criteria^([19]). The levels of total PCBs and total PAHs in the elutriate samples are all below the detection limit and well comply with the relevant water quality criteria except for the PCBs level at vibrocore V6-10W at sampling depth 1.9 - 2.4m. However, the high PCBs level measured at depth 1.9 -2.4 m of V6-10W is doubtful because all the rest of the levels measured at vibrocore V6-10W comply well with the assessment criterion and all the remaining contaminant levels were under the detention limit. The potential impact is therefore considered isolated and limited. In addition, V6-10W is located in open water in North Point, any release of PCBs during dredging at North Point water will be quickly dispersed by the fast moving current and diluted by the large volume of marine water. The release of PCBs, if any, will also be minimised by the use of closed grab dredger and the dispersion of pollutants will be confined within the construction site by silt curtains (Section 5.8). Thus, it is considered that long-term off-site marine water quality impact is unlikely and any local water quality impact will be transient.

5.7.9 The elutriate test results of TBT do not indicate any levels higher than the blank results nor the threshold concentration recommended by Salazar and Salazar (1996) ^([20]). It is therefore concluded that adverse water quality impacts due to the potential release of TBT from the sediment are not expected during the dredging activities.

Table 5.27 Comparison of Phase I Marine Site Investigation Sediment Elutriate Test Results with the Water Quality Standards

Vibrocore

Sampling Depth (m)

Metal content (mg/L)

Organic Compounds Content (mg/L)

Ag

Cd

Cu

Ni

Pb

Zn

Cr

As

Hg

Total PCBs

Total PAHs

TBT

V06-2W

Surface Grab

<1

0.61

<1

<1

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

2.1

1.0

<1

<10

<1

1.1

<0.1

<0.01

<0.2

<0.015

V06-6W

0.0 – 0.9

<1

<0.2

<1

1

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

<0.2

<1

3.2

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

1.9 – 2.9

<1

<0.2

<1

1.6

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

3.0 – 4.0

<1

<0.2

<1

6.3

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

5.5

<1

2.1

<10

<1

1.4

<0.1

<0.01

<0.2

<0.015

V06-7W

0.0 – 0.9

<1

<0.2

<1

2.9

<1

<10

<1

1.1

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

<0.2

<1

1.6

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

1.9 – 2.9

<1

<0.2

<1

<1

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

7.2

<1

<1

<10

<1

1.4

<0.1

<0.01

<0.2

<0.015

V06-8W

0.0 – 0.9

<1

0.27

<1

1.8

1.1

<10

<1

4.9

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

0.7

<1

1.9

18

11

<1

9.9

<0.1

<0.01

<0.2

<0.015

1.9 – 2.9

<1

0.43

<1

2.7

9.5

<10

<1

18

<0.1

<0.01

<0.2

<0.015

3.0 – 4.0

2.8

0.48

<1

2.5

1.5

<10

<1

13

<0.1

<0.01

<0.2

<0.015

6.0 – 7.0

<1

<0.2

<1

1.5

3.5

<10

<1

8.0

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

4.6

<1

<1

<10

<1

1.3

<0.1

<0.01

<0.2

<0.015

V06-9W

0.0 – 0.9

<1

<0.2

<1

2.1

<1

<10

<1

4.5

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

<0.2

<1

2.1

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

1.1

1.2

<1

<10

<1

1.5

<0.1

<0.01

<0.2

<0.015

V06-10W

0 – 0.9

<1

<0.2

<1

1.1

<1

<10

<1

3.2

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

<0.2

<1

<1

<1

<10

<1

5.3

<0.1

<0.01

<0.2

<0.015

1.9 – 2.4

<1

<0.2

<1

<1

<1

<10

<1

<1

<0.1

0.17

<0.2

<0.015

2.9 – 3.9

<1

<0.2

<1

<1

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

4.4 – 5.4

<1

<0.2

<1

1

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

1.6

1.1

<1

<10

<1

1.2

<0.1

<0.01

<0.2

<0.015

Water Quality Standards

2.3^(2)

2.5 ^(2)

5 ^(2)

30 ^(2)

25 ^(2)

40 ^(2)

15 ^(2)

25 ^(2)

0.3 ^(2)

0.03 ^(3)

3.0 ^(4)

0.1 ^(5)

Notes:

(1) Value in bold indicates exceedance of the Water Quality Standard.

(2) UK Water Quality Standard.

(3) USEPA salt water criterion.

(4) Australian water quality guidelines for fresh and marine waters.

(5) Michael H. Salazar and Sandra M. Salazar (1996). “Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation, and Growth under Natural Conditions” in Organotin, edited by M. A. Champ and P. F. Seligman. Chapman & Hall, London.

Table 5.28 Comparison of Phase II Marine Site Investigation Sediment Elutriate Test Results with the Water Quality Standards

Vibrocore

Sampling Depth (m)

Metal content (mg/L)

Organic Compounds Content (mg/L)

Ag

Cd

Cu

Ni

Pb

Zn

Cr

As

Hg

Total PCBs

Total PAHs

TBT

V06-1W

0.0 – 0.9

<1

<0.2

2.0

1.0

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

0.20

2.2

2.1

<1

<10

<1

3.1

<0.1

<0.01

<0.2

<0.015

1.9 – 2.9

<1

0.27

<1

<1

1.1

<10

<1

25

<0.1

<0.01

<0.2

<0.015

2.9 – 3.9

<1

<0.2

<1

1.1

1.2

<10

<1

21

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

3.1

1.3

1.2

<10

<1

1.7

<0.1

<0.01

<0.2

<0.015

V06-3W

0.0 – 0.9

<1

<0.2

1.1

1.2

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

<0.2

1.1

1.8

13

15

<1

6.0

<0.1

<0.01

<0.2

<0.015

1.9 – 2.9

<1

0.28

<1

2.0

1.4

<10

<1

27

<0.1

<0.01

<0.2

<0.015

2.9 – 3.9

<1

0.78

1.1

2.0

<1

<10

<1

16

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

2.0

1.4

<1

10

<1

<1

<0.1

<0.01

<0.2

<0.015

V06-4W

0.0 – 0.9

<1

<0.2

2

<1

<1

<10

<1

2

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

<0.2

<1

<1

<1

<10

<1

1.2

<0.1

<0.01

<0.2

<0.015

1.9 – 2.9

<1

<0.2

1.1

1.8

<1

<10

<1

1.8

<0.1

<0.01

<0.2

<0.015

2.9 – 3.9

<1

<0.2

<1

<1

<1

<10

<1

2.3

<0.1

<0.01

<0.2

<0.015

6.0 – 7.0

<1

<0.2

1

<1

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

8.9 – 9.9

<1

<0.2

<1

<1

<1

<10

<1

2.3

<0.1

<0.01

<0.2

<0.015

Blank

<1

<0.2

3.5

1.5

<1

<10

<1

<1

<0.1

<0.01

<0.2

<0.015

V06-5W

0.0 – 0.9

<1

<0.2

<1

2.4

<1

<10

<1

1.3

<0.1

<0.01

<0.2

<0.015

0.9 – 1.9

<1

<0.2

2.1

2.9

<1

<10

<1

2

<0.1

<0.01

<0.2

<0.015

1.9 – 2.9

<1

<0.2

<1

<1

2.8

<10

<1

15

<0.1

<0.01

<0.2

<0.015

2.9 – 3.9

<1

<0.2

<1

3.4

<1

<10

<1

3

<0.1

<0.01

<0.2

<0.015

Blank

1.0

<0.2

2.7

1.0

<1

10.0

<1

1.1

<0.1

<0.01

<0.2

<0.015

Water Quality Standards

2.3^(1)

2.5 ^(1)

5 ^(1)

30 ^(1)

25 ^(1)

40 ^(1)

15 ^(1)

25 ^(1)

0.3 ^(1)

0.03 ^(2)

3.0 ^(3)

0.1 ^(4)

Notes:

(1) UK Water Quality Standard.

(2) USEPA salt water criterion.

(3) Australian water quality guidelines for fresh and marine waters.

(4) Michael H. Salazar and Sandra M. Salazar (1996). “Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation, and Growth under Natural Conditions” in Organotin, edited by M. A. Champ and P. F. Seligman. Chapman & Hall, London.

Table 5.29 Comparison of WDIICFS Marine Site Investigation Sediment Elutriate Test Results with the Water Quality Standards

Vibrocore

Sampling Depth (m)

Metal content (mg/L)

Organic Compounds Content (mg/L)

Ag

Cd

Cu

Ni

Pb

Zn

Cr

As

Hg

Total PCBs

Total PAHs

TBT (mg-Sn L^-1)

MV1

0.55 - 0.9

<2

<0.5

<2

<5

<2

8

<10

6

<0.2

< 0.03

< 0.3

< 0.05

1.0 - 1.9

<2

<0.5

<2

<5

<2

<5

<10

3

<0.2

< 0.03

< 0.3

< 0.05

2.0 - 2.9

<2

<0.5

2

<5

<2

10

<10

3

<0.2

< 0.03

< 0.3

< 0.05

MV4

0.25 - 0.9

<2

<0.5

<2

<5

<2

<5

<10

6

<0.2

< 0.03

< 0.3

< 0.05

1.0 - 1.9

<2

<0.5

<2

<5

<2

5

<10

9

0.4

< 0.03

< 0.3

< 0.05

2.0 - 2.9

<2

<0.5

<2

<5

<2

<5

<10

6

<0.2

< 0.03

< 0.3

< 0.05

Blank

-

< 2

< 0.5

< 2

< 5

< 2

14

< 10

< 3

< 0.2

< 0.03

< 0.3

< 0.05

Water Quality Standard

2.3^(1)

2.5 ^(1)

5 ^(1)

30 ^(1)

25 ^(1)

40 ^(1)

15 ^(1)

25 ^(1)

0.3 ^(1)

0.03 ^(2)

3.0 ^(3)

0.1 ^(4)

Notes:

(1) Value in bold indicates exceedance of the Water Quality Standard.

(2) UK Water Quality Standard.

(3) USEPA salt water criterion.

(4) Australian water quality guidelines for fresh and marine waters.

(5) Michael H. Salazar and Sandra M. Salazar (1996). “Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation, and Growth under Natural Conditions” in Organotin, edited by M. A. Champ and P. F. Seligman. Chapman & Hall, London.

Oxygen Depletion During Dredging

5.7.10 An assessment of dissolved oxygen depletion during dredging has been made in relation to the results of the sediment plume modelling of dredging activities (unmitigated scenario) and the sediment quality data for the study area. The predicted maximum elevations in SS concentrations at various indicator points were used to estimate the effects of increased SS concentrations on DO. Seawater intakes along the waterfront were selected as reference points for presentation of the assessment results. In the calculation, it was assumed that all of the chemical oxygen demand is exerted. These are conservative assumptions and will likely result in an over-prediction of the potential impacts. The calculation was performed using the highest levels of 5-day SOD measured in the sediment samples collected during the SI for conservative predictions. The highest 5-day SOD level was recorded at station V06-2W inside the HKCEC water channel. The 10 percentile DO predicted under the pre-construction scenario at the corresponding indicator points were used as the background levels for reference. The results of DO depletion are given in Table 5.30 and Table 5.30a.

Table 5.30 Calculation of the Effects of Increased Suspended Sediment Concentrations on Dissolved Oxygen Concentrations under Scenario 2A

Indicator Point	Maximum Predicted SS Elevation (mg/l)	SOD5 in Sediment (mg/kg)	Maximum DO depletion (mg/l)	Background DO (mg/l)	Resultant DO (mg/l)
Cooling Water Intake within the Project Site
Hong Kong Convention and Exhibition Centre Extension	61.73	6100	0.377	4.58	4.20
Telecom House / HK Academy for Performing Arts / Shun On Centre	117.50	6100	0.717	6.08	5.36
Hong Kong Convention and Exhibition Centre Phase I	460.97	6100	2.812	6.08	3.26
Wan Chai Tower / Revenue Tower / Immigration Tower	88.98	6100	0.543	6.08	5.53
Great Eagle Centre / China Resources Building	68.66	6100	0.419	4.64	4.22
Sun Hung Kai Centre	72.66	6100	0.443	6.08	5.63
Excelsior Hotel & World Trade Centre	53.91	6100	0.329	5.26	4.93
Windsor House	45.21	6100	0.276	5.25	4.98
WSD Saltwater Intake within the Project Site
Wan Chai	42.90	6100	0.262	6.08	5.81
Cooling Water Intake outside the Project Site
Admiralty Centre	96.08	6100	0.586	6.08	5.49
WSD Saltwater Intake outside the Project Site
Kennedy Town	6.17	6100	0.038	6.08	6.04
Quarry Bay	38.57	6100	0.235	6.08	5.84
Sai Wan Ho	46.33	6100	0.283	6.08	5.80
Sheung Wan	36.40	6100	0.222	6.08	5.85
Tai Wan	12.65	6100	0.077	4.58	4.50

Table 5.30a Calculation of the Effects of Increased Suspended Sediment Concentrations on Dissolved Oxygen Concentrations under Scenario 2A (Sensitivity Test using Higher Dredging Rate for Gas Main Construction)

Indicator Point	Maximum Predicted SS Elevation (mg/l)	SOD5 in Sediment (mg/kg)	Maximum DO depletion (mg/l)	Background DO (mg/l)	Resultant DO (mg/l)
Cooling Water Intake within the Project Site
Hong Kong Convention and Exhibition Centre Extension	61.73	6100	0.377	4.58	4.20
Telecom House / HK Academy for Performing Arts / Shun On Centre	117.50	6100	0.717	6.08	5.36
Hong Kong Convention and Exhibition Centre Phase I	460.97	6100	2.812	6.08	3.26
Wan Chai Tower / Revenue Tower / Immigration Tower	88.98	6100	0.543	6.08	5.53
Great Eagle Centre / China Resources Building	68.66	6100	0.419	4.64	4.22
Sun Hung Kai Centre	72.66	6100	0.443	6.08	5.63
Excelsior Hotel & World Trade Centre	53.91	6100	0.329	5.26	4.93
Windsor House	45.21	6100	0.276	5.25	4.98
WSD Saltwater Intake within the Project Site
Wan Chai	42.90	6100	0.262	6.08	5.81
Cooling Water Intake outside the Project Site
Admiralty Centre	96.08	6100	0.586	6.08	5.49
WSD Saltwater Intake outside the Project Site
Kennedy Town	6.17	6100	0.038	6.08	6.04
Quarry Bay	38.57	6100	0.235	6.08	5.84
Sai Wan Ho	46.33	6100	0.283	6.08	5.80
Sheung Wan	36.40	6100	0.222	6.08	5.85
Tai Wan	12.69	6100	0.077	4.58	4.50

5.7.11 Scenario 2A represents 6 concurrent dredging activities at HKCEC1, WCR1, TPCWAE, TBW, NPR2W and WSD cross harbour water mains respectively. As presented in Table 5.30, the maximum DO depletion was predicted to be 2.8 mg/l at the HKCEC water channel which was mainly contributed by the dredging activities at HKCEC1. Therefore, non-compliance of DO level would be expected inside the HKCEC water channel during seawall dredging at HKCEC1. The impact from the dredging at WRC1 under the same scenario is considered less significant as the maximum DO depletion predicted at the WSD Wan Chai flushing water intake closest to the WRC1 would be less than 0.3 mg/l. The cumulative impact from these concurrent dredging activities would cause a DO depletion of less than 0.4 mg/l at the CBTS.

Release of Nutrients During Dredging

5.7.12 An assessment of contaminant release for nutrients has been made in relation to the sediment quality results as presented in Appendix 6.1. Inert tracers (with zero decay) were introduced into the Delft3D-WAQ model for Scenario 2B model runs to represent the release of these contaminants during dredging. Discharge of inert tracers was assumed at the source points (discharge locations). In the calculation of the contaminant loss rate for model input, it was assumed that all of the contaminants in the sediment would be released to the water. The assessment conducted under this EIA on the potential release of nutrients focused on the impact from the WDII activities alone.

5.7.13 Tracer simulations were performed for Scenario 2A (including source points A1, A2, A3, A4, A5, A6, A7)which covered two model runs for dry and wet seasons respectively. Under each modelling scenario, the highest nutrient levels measured under the marine SI were used to calculate the nutrient loss rate at all the source points for cumulative predictions. The highest levels of TIN and NH₃-N recorded from the marine SI were 300 mg/kg and 6.9mg/kg respectively which were measured in the sediment sample collected at Station V06-6W (inside CBTS).

5.7.14 The calculated NH₃-N released from the sediment will result in a concentration of total NH₃-N in the receiving waters. The levels of NH₃-N were converted to unionized NH₃-N which is a more critical parameter of concern. The data at EPD monitoring station VM5 indicates that on average the unionised NH₃-N constitutes 2.3% of the total NH3-N concentration.

5.7.15 Table 5.33 summarizes the maximum elevations of nutrient levels estimated at the indicator points. All the maximum elevations for UIA were negligible as compared to the WQO of 0.021mg/l. The maximum elevations for TIN were also small as compared to the WQO of 0.4mg/l. It is therefore not anticipated that the dredging work would cause any unacceptable nutrient impact upon the receiving water and any elevations of nutrients caused by the dredging works would be transient only. As there is no significant difference in the overall SS impacts between the base case scenario and the sensitivity test under Scenario 2A (refer to Appendix 5.9a, 5.9b, 5.9g and 5.9h), the nutrient impact was only assessed for the base case scenario.

Table 5.33 Maximum Elevations of Nutrient Concentrations under Scenario 2A

Indicator Point	Maximum level of TIN in Sediment (mg/kg)	Maximum level of NH3-N in Sediment (mg/kg)	Maximum Increase in TIN (mg/l)	Maximum Increase in UIA (mg/l)
WQO:			0.4	0.021
Cooling Water Intake within the Project Site
Hong Kong Convention and Exhibition Centre Extension	300	6.9	0.018	0.00001
Telecom House / HK Academy for Performing Arts / Shun On Centre	300	6.9	0.036	0.00002
Hong Kong Convention and Exhibition Centre Phase I	300	6.9	0.034	0.00002
Wan Chai Tower / Revenue Tower / Immigration Tower	300	6.9	0.034	0.00002
Great Eagle Centre / China Resources Building	300	6.9	0.034	0.00002
Sun Hung Kai Centre	300	6.9	0.087	0.00005
Excelsior Hotel & World Trade Centre	300	6.9	0.019	0.00001
Windsor House	300	6.9	0.020	0.00001
WSD Saltwater Intake within the Project Site
Wan Chai	300	6.9	0.055	0.00003
Cooling Water Intake outside the Project Site
Admiralty Centre	300	6.9	0.018	0.00001
WSD Saltwater Intake outside the Project Site
Kennedy Town	300	6.9	0.002	0.000001
Quarry Bay	300	6.9	0.006	0.000003
Sai Wan Ho	300	6.9	0.003	0.000002
Sheung Wan	300	6.9	0.005	0.000002
Tai Wan	300	6.9	0.002	0.000001

Release of PCBs During Dredging

5.7.16 As discussed above, exceedance of water quality standard for PCBs was only recorded in one isolated elutriate sample collected in the proposed North Point reclamation area (PCBs were not detected in all the remaining elutriate samples and all the blank “ambient water” samples). Thus, release of PCBs during dredging at the new water mains would not be expected.

General WQO Compliance in Victoria Harbour

5.7.17 As shown in the SS elevation contour plots presented in Appendix 5.9a, 5.9b, 5.9g and 5.9h, it is considered that the potential impact of dredging will be confined near the marine works site and will have significantly smaller impact to open waters in Victoria Harbour.

5.7.18 In addition, the dredging works will only increase the local background concentrations during the construction works and will thus be of short duration, and will not prevent recovery of the water body in the future. It is therefore concluded that the proposed dredging works will not cause adverse impacts to water quality in Victoria Harbour.

5.7.19 DO exceedance was only predicted at only a localized area within the HKCEC water channel. Full compliance with the WQO for depth-averaged and bottom DO of 4 mg/l and 2 mg/l respectively is predicted outside the Project site boundary in the Victoria Harbour under the dredging scenario. As exceedence of the WQO for DO is only expected within the WDII reclamation site (i.e. within the HKCEC water channel only), no mixing zone for DO can therefore be identified in the Victoria Harbour. No adverse impacts on the DO levels in the Victoria Harbour would be expected from the dredging works. Mitigation measures have been proposed in Section 5.8 to minimize the DO impact at the HKCEC water channel.

General Construction Activities

5.7.20 The effects on water quality from general construction activities are likely to be minimal, provided that site drainage is well maintained and good construction practices are observed to ensure that litter, fuels, and solvents are managed, stored and handled properly.

5.7.21 Based on the Sewerage Manual, Part I, 1995 of the Drainage Services Department (DSD), the global unit flow factors for employed population of 0.06 m³ per worker per day and commercial activities in year 2016 of 0.29 m³ per worker per day have been used to estimate the sewage generation from the construction site. The total sewage production rate is estimated at 0.35 m³ per worker per day. Therefore, with 80 construction workers working simultaneously at the construction site, a total of about 28 m³ of sewage will be generated per day. The sewage should not be allowed to discharge directly into the surrounding water body without treatment. Chemical toilets and subsequently on-site sewer should be deployed at the construction site to collect and handle sewage from workers (see Section 5.8 for recommended mitigation measures).

5.8 Mitigation of Adverse Environmental Impacts

Construction Design

5.8.1 The following measures have been implemented in the design of the WDII reclamation phasing to ensure the continuous operation of the existing waterfront facilities and, simultaneously, to minimise the cumulative impacts on water quality:

· a number of small and confined areas of land formation are planned

· containment of fill within each of these areas by seawalls is proposed, with the seawall constructed first (above high water mark) with filling carried out behind the completed seawalls. Any gaps that may need to be provided for marine access will be shielded by silt curtains to control sediment plume dispersion away from the site. Filling should be carried out behind the silt curtain

5.8.2 Maximum dredging rates for the construction of seawall foundation are defined for five distinctly identifiable shoreline zones where reclamation will take place:

· The North Point shoreline (NPR) - the area to the east of CBTS

· The Causeway Bay shoreline - temporary reclamations within the CBTS (TCBR) and temporary typhoon shelter (TBW)

· The PCWA shoreline (TPCWA) – temporary reclamations within the PCWA

· The Wan Chai shoreline (WCR) - from the eastern boundary of HKCEC Extension to western boundary of the PCWA

· The HKCEC shoreline (HKCEC) - the area to the west of the HKCEC Extension.

5.8.3 Maximum dredging rates are also defined for two other distinctly identifiable marine works zones including:

· Dredging along the proposed alignment of the WSD cross harbour water mains from Wan Chai to Tsim Sha Tsui (Water Mains zone)

· Dredging along the proposed alignment of the submarine sewage pipeline of the Wan Chai East Sewage Screening Plant (Sewage Pipelines zone).

5.8.4 The definition of these marine works areas will ensure easier contract monitoring and control of production rates, removing possible ambiguity of interpretation even in the event of modification of the currently envisaged staging and programme by the contractor, and will maintain flexibility in respect of possible division of the reclamation into two or three different contract packages. The maximum dredging rates for seawall construction defined for the reclamation zones and the maximum dredging rates for construction of the water mains and sewage pipelines are consistent with the impact assessment modelling approach.

5.8.5 Dredging will be carried out by closed grab dredger for the following works:

· Seawall construction in all the reclamation shoreline zones including the TBW

· Construction of the proposed water mains

· Construction of the proposed sewage pipelines.

5.8.6 The total dredging rate in each of the reclamation shoreline zones would not be more than 6,000m³ per day. No more than one closed grab dredger would be operated at the same time for seawall construction in each of the reclamation shoreline zones.

5.8.7 The total dredging rate in each of the two marine works zones (namely the water mains zone and the sewage pipelines zone respectively) would not be more than 6,000 m³ per day. No more than one closed grab dredger would be operated at the same time in each of these two marine works zones.

5.8.8 In addition, dredging for the sewage pipelines would not be carried out concurrently with the following activities to minimize the potential impacts:

· Dredging along the water mains; and

· Dredging along the seawall in the WCR zone.

5.8.9 The water body behind the temporary reclamations within the CBTS should not be fully enclosed. The current construction programme indicated that:

· TCBR3 and TCBR4 will not be implemented during the period when both TCBR1W and TBCR1E are in place at the same time.

· TCBR4 and TCBR1E will not be implemented during the period when both TCBR2 and TCBR3 are in place at the same time.

· TCBR1E, TCBR1W and TCBR2 will not be implemented during the period when TCBR3 and TCBR4 are in place at the same time.

· TCBR1W will not be in place together with TCBR4.

· TCBR1E will not be in place together with TCBR3 or TCBR4.

· TCBR2 will not be in place together with TCBR4.

Specific Mitigation Measures

5.8.10 No unacceptable impact in terms of contaminant release from the dredging operation is predicted under the unmitigated scenarios. No specific mitigation measure would be required for control of contaminant release. Also, non-compliance for DO is only predicted in a localized area within the WDII reclamation site (i.e. within HKCEC channel only). Specific mitigation measures have been recommended as discussed in later section to minimize the DO impact in the HKCEC water channel. Full compliance for DO is predicted in the Victoria Harbour.

5.8.11 As indicated in Table 5.22, exceedence of target SS levels at the Admiralty Centre and MTRC cooling water intakes and WSD salt water intakes are predicted during the construction of the water mains. To minimise the potential SS impact, deployment of silt curtains around the closed grab dredgers is recommended as an appropriate mitigation measure to minimize the SS impact due to the dredging activities. However, silt curtains should not be used in areas where current speeds are higher than 1.0 m s^-1, and the effectiveness of the silt curtains will be reduced in areas of current speeds greater than around 0.5 m s^-1. Thus, silt curtains are recommended for seawall dredging and seawall trench filling near the existing coastline where current speeds are less than 0.5 m s^-1.

5.8.12 For the dredging works to be carried out at the sewage pipelines zone, water mains zone and TBW, the associated sediment plume can easily be transported to farther field by the fast moving tidal currents and thus would potentially affect the sensitive use on both sides of the Victoria Habour. As silt curtains are considered ineffective to mitigate the SS impacts in such areas, reduction of the maximum dredging rate from 6,000 m³ per day to 1,500 m³ per day in each of these works zones is recommended to reduce SS impacts.

5.8.13 Based on the current programme, dredging along the sewage pipelines would be carried out after the seawall of WCR1 is completed. As a result of the proposed reduction of the dredging rate, the required dredging period would be longer but the dredging duration would not be extended beyond the planned seawall dredging at WCR2. Thus, dredging along the sewage pipelines would not be carried out simultaneously with the seawall dredging in WCR even with the extended dredging period. As a result, no extra SS impact would be induced by the reduced dredging rate. Similarly, at the TBW, the dredging duration would not be extended beyond the planned dredging at the TCWBR site as a result of the reduced dredging rate. Also, dredging along the water mains would not be extended beyond the planned commencement of the sewage pipelines construction due to the reduced dredging rate. The worst-case dredging scenarios modelled under this EIA take into account all potential concurrent dredging activities. The proposed reduction of the dredging rate would not result in any change in the worst-case dredging scenarios.

5.8.14 Deployment of silt curtains around the closed grab dredgers to contain SS within the construction site during seawall dredging and seawall trench filling is recommended for the areas of HKCEC, WCR, TCBR and NPR where the current speeds are expected to be less than 0.5 m s^-1. Based on the water quality modelling and assessment result, deployment of silt curtains is considered not necessary for the dredging works within the PCWA provided that the maximum dredging rate within the PCWA can be reduced from 6,000 m³ per day to 5,000 m³ per day to minimize the SS impacts.

5.8.15 Based on the modelling results, residual SS impacts were still predicted at some of the cooling water intakes and WSD flushing water intakes after the deployment of the silt curtains and reduction of the dredging rate as recommended above. Thus, deployment of silt screens is also proposed at selected cooling water intakes and WSD salt water intakes as shown in Table 5.39 to further minimize the residual impact. Table 5.39 summarises the application of silt screens under the modelling scenario (i.e. Scenario 2A (and Sensitivity Test)).

Table 5.39 Application of Silt Screens at Interim WDII Construction Stages

Interim Construction Stage	Location of Applications
Scenario 2A in early 2009 with concurrent dredging activities at HKCEC, WCR, TPCWA, TBW, NPR and Water Mains Zone	· WSD saltwater intakes at Sai Wan Ho, Quarry Bay, Sheung Wan, Kowloon South, Wan Chai · Cooling water intakes for Hong Kong Convention and Exhibition Centre Extension, Hong Kong Convention and Exhibition Centre Phase I, Telecom House / HK Academy for Performing Arts / Shun On Centre, Wan Chai Tower / Revenue Tower / Immigration Tower and Sun Hung Kai Centre.

5.8.16 According to the Contaminated Spoil Management Study ^([21]), the implementation of silt curtain around the closed grab dredgers will reduce the dispersion of SS by a factor of 4 (or about 75%). Similarly, the implementation of silt screen at the intake could reduce the SS level by a factor of 2.5 (or about 60%). This SS reduction factor has been established under the Pak Shek Kok Reclamation, Public Dump EIA (1997) and has been adopted in a number of recent studies, including the Western Coast Road EIA study. Figure 5.15 shows typical configuration of silt curtains and silt screens, design and set-up of silt curtain ^([22]).

5.8.17 Table 5.40 and Table 5.40a summarise the predicted SS levels at the intakes after the implementation of all the mitigation measures as recommended above. With the recommended measures, all sensitive receivers would fully comply with the relevant water quality criteria.

Compliance with WQO for SS Elevation

5.8.18 The sediment plumes (SS elevation) under mitigated scenarios are shown in Appendix 5.9a, 5.9b, 5.9g and 5.9h. Each of the figures attached in these appendices contains two contour plots where the upper plot shows the unmitigated scenarios and the lower plot shows the mitigated scenarios. Non-compliance with the WQO for SS (i.e. elevation of less than 30% of ambient baseline level) is predicted to be localized and acceptable after implementation of the recommended mitigation measures.

5.8.19 Table 5.41 and Table 5.42 summarise the predicted SS elevation at the coral site in Junk Bay after the implementation of all the mitigation measures as recommended above. The coral sites at Green Island and Junk Island were found not be impacted by marine works from WDII and are therefore not included in the tables. With the recommended measures, the SS elevation predicted at the Junk Bay would fully comply with the WQO.

Table 5.40 Scenario 2A –Predicted SS levels at the Seawater Intakes after the Implementation of Mitigation Measures (Base Case Scenario)

Sensitive Receiver	SS concentration (absolute value) in the mid-depth (mg/l)
		Dry season	Wet season
	Criterion	Maximum ^(1)	Maximum ^(1)
Cooling Water Intakes
Prince's Building Group	-	24.8	23.9
Queensway Government Offices	-	21.6	21.1
Admiralty Centre	< 40	29.5	18.5
HSBC	-	21.4	25.2
Excelsior Hotel & World Trade Centre	-	7.6	20.7
Great Eagle Centre / China Resources Building	-	21.3	32.2
Hong Kong Convention and Exhibition Centre Extension	-	8.1	8.7
Hong Kong Convention and Exhibition Centre Phase I	-	48.0	48.4
MTRC South Intake	< 40	13.8	16.3
Sun Hung Kai Centre	-	9.2	15.0
Telecom House / HK Academy for Performing Arts / Shun On Centre	-	13.8	11.9
Wan Chai Tower / Revenue Tower / Immigration Tower	-	10.8	15.0
Windsor House	-	7.8	18.8
Government Premises	-	9.4	15.1
City Garden	-	18.4	17.3
Provident Centre	-	20.1	18.2
WSD Saltwater Intakes
Kennedy Town	< 10	8.4	7.4
Kowloon South	< 10	3.0	4.4
Quarry Bay	< 10	5.9	5.4
Sai Wan Ho	< 10	6.6	6.1
Sheung Wan	< 10	6.6	6.0
Siu Sai Wan	< 10	5.0	6.1
Wan Chai	< 10	6.2	9.7

Notes:

(1) The water quality modelling results for 90 percentile SS predicted under the pre-construction scenario at the corresponding indicator points are adopted as the ambient SS levels.

- Other WSR, including WSD Tai Wan intake, WSD Cheung Sha Wan intake, WSD Cha Kwo Ling intake, Kau Yi Chau Fishery, PLA Headquarters intake, Queen Mary Hospital intake, Stage 1 Phase 1 intake and Wah Fu Estate intake were found not be impacted by the proposed marine works.

Table 5.40a Scenario 2A –Predicted SS levels at the Seawater Intakes after the Implementation of Mitigation Measures (Sensitivity Test using Higher Dredging Rate for Gas Main Construction)

Sensitive Receiver	SS concentration (absolute value) in the mid-depth (mg/l)
		Dry season	Wet season
	Criterion	Maximum ^(1)	Maximum ^(1)
Cooling Water Intakes
Prince's Building Group	-	24.8	24.4
Queensway Government Offices	-	21.6	21.1
Admiralty Centre	< 40	29.5	19.2
HSBC	-	21.4	25.2
Excelsior Hotel & World Trade Centre	-	7.6	20.7
Great Eagle Centre / China Resources Building	-	21.7	32.2
Hong Kong Convention and Exhibition Centre Extension	-	8.1	10.0
Hong Kong Convention and Exhibition Centre Phase I	-	48.0	48.4
MTRC South Intake	< 40	13.8	16.3
Sun Hung Kai Centre	-	9.2	15.0
Telecom House / HK Academy for Performing Arts / Shun On Centre	-	13.8	11.9
Wan Chai Tower / Revenue Tower / Immigration Tower	-	10.8	15.3
Windsor House	-	7.8	18.8
Government Premises	-	9.4	15.1
City Garden	-	18.4	17.3
Provident Centre	-	20.1	18.2
WSD Saltwater Intakes
Kennedy Town	< 10	8.4	7.4
Kowloon South	< 10	3.0	4.6
Quarry Bay	< 10	6.0	6.2
Sai Wan Ho	< 10	6.6	6.4
Sheung Wan	< 10	7.4	6.1
Siu Sai Wan	< 10	5.0	6.2
Wan Chai	< 10	6.2	9.7

Notes:

(2) The water quality modelling results for 90 percentile SS predicted under the pre-construction scenario at the corresponding indicator points are adopted as the ambient SS levels.

- Other WSR, including WSD Tai Wan intake, WSD Cheung Sha Wan intake, WSD Cha Kwo Ling intake, Kau Yi Chau Fishery, PLA Headquarters intake, Queen Mary Hospital intake, Stage 1 Phase 1 intake and Wah Fu Estate intake were found not be impacted by the proposed marine works.

Table 5.41 Predicted SS Elevations at Corals for Construction Scenario 2A - Mitigated

Corals	Background SS Level (mg/l)	SS Elevation in Bottom Layer (mg/l)
		Criterion (30% of Mean SS Level)	Mean	Maximum
Wet Season
Junk Bay ( CR27 )	4.75	< 1.10	0.03	0.53
Dry Season
Junk Bay ( CR27 )	3.93	< 1.06	0.07	1.03

Remark: The coral sites at Green Island and Junk Island were found not be impacted by marine works from Wan Chai Development Phase II

Table 5.42 Predicted SS Elevations at Corals for Construction Scenario 2A - Mitigated (Sensitivity Test using Higher Dredging Rate for Gas Main Construction)

Corals	Background SS Level (mg/l)	SS Elevation in Bottom Layer (mg/l)
		Criterion (30% of Mean SS Level)	Mean	Maximum
Wet Season
Junk Bay ( CR27 )	4.75	< 1.10	0.03	0.54
Dry Season
Junk Bay ( CR27 )	3.93	< 1.06	0.07	1.03

Remark: The coral sites at Green Island and Junk Island were found not be impacted by marine works from Wan Chai Development Phase II

5.8.20 In recognition of the potentially higher level of impacts caused by dredging close to the seawater intakes, dredging along the seawall at WCR1 should be undertaken initially at 1,500 m³ per day for construction of the western seawall (which is in close proximity of the WSD intake), followed by partial seawall construction at the western seawall (above high water mark) to isolate the adjacent intakes as much as possible from further dredging activities. Thus, the intakes would be shielded from most of the SS generated from further dredging along the northern and eastern seawall.

5.8.21 High DO depletion was predicted inside the HKCEC water channel during the seawall dredging at HKCEC1. To minimize the potential DO depletion inside the water channel, it is recommended that the seawall trench dredging in HKCEC1 and HKCEC3 should be undertaken at no more than the maximum rate of 1,500 m³ per day.

5.8.22 For dredging within the CBTS, seawall should be partially constructed to protect the nearby seawater intakes from further dredging activities. For example, at TCBR1W, the southern and eastern seawalls should be constructed first (above high water mark) so that the seawater intakes at the inner water would be protected from the impacts from the remaining dredging activities along the northern boundary.

5.8.23 Based on the considerations above, the maximum dredging rates under different marine works zones are recommended in Table 5.43. It should be noted that the dredging rates listed in Table 5.43 have not considered the effect of silt curtains as recommended in Section 5.8.14. The equivalent sediment loss rates shown in the table below represent the values before applying the silt curtains.

Table 5.43 Recommended Maximum Dredging Rates

Reclamation Area			Maximum Dredging Rate		Maximum Dredging Rate (m^3 per week)	Equivalent Sediment Loss Rate (kg s^-1)
			m^3 per day	m^3 per hour
Dredging along seawall or breakwater
North Point Shoreline Zone (NPR)			6,000	375	42,000	2.08
Causeway Bay Shoreline Zone		TBW	1,500	94	10,500	0.52
		TCBR	6,000	375	42,000	2.08
PCWA Zone			5,000	313	35,000	1.73
Wan Chai Shoreline Zone (WCR)			6,000	375	42,000	2.08
HKCEC Shoreline Zone (HKCEC)	HKCEC Stage 1 & 3		1,500	94	10,500	0.52
	HKCEC Stage 2		6,000	375	42,000	2.08
Dredging along pipelines
Cross Harbour Water Mains			1,500	94	10,500	0.52
Wan Chai East Submarine Sewage Pipeline			1,500	94	10,500	0.52

Notes: (1) Dredging to be carried out by closed grab dredger (16 hours per day)

(2) Silt curtains to be deployed around seawall dredging and seawall trench filling in NPR, TCBR, WCR and HKCEC areas.

(3) Reduced dredging rates of 1,500 m³ per day are applicable to construction of the western seawall of WCR1 which is close to the WSD intake.

(4) Silt screens to be deployed at selected seawater intakes as recommended in Table 5.39.

5.8.24 It is expected that any water quality exceedance of action / limit levels would be readily captured by an effective site audit and water quality monitoring mechanism. The water quality monitoring frequency should be increased to once per day when dredging in the vicinity of the seawater intakes, and 24 hour monitoring of turbidity at the intakes should be implemented as and when necessary.

Cumulative Impacts from WDII and Gas Main Relocation

5.8.25 To investigate the worst-case impact on the WSD flushing intake at Quarry Bay, additional sensitivity test was conducted using an alternative source point for the new gas main near the pipeline landing point at North Point with a dredging rate of 5,000 m³ per day based on the latest information provided by the HKCGCL and the indicative alignment provided in the Project Profile for the new gas main (refer to Section 5.6.31).ad Table 5.44 below compares the potential SS impact upon the Quarry Bay intake under the basecase scenario (assuming the dredging for gas main construction is conducted near Tai Wan intake) and the additional sensitivity test (assuming the dredging for gas main construction is conducted near the Quarry Bay intake). The predicted SS levels shown in Table 5.44 represent the mitigated scenario with implementation of all the mitigation measures recommended for WDII (including the installation of silt screen at the Quarry Bay intake) as discussed above.

Table 5.48 Cumulative Impact on Quarry Bay Intake

Description	Basecase Scenario assuming the dredging for gas main relocation is conducted near Tai Wan intake		Sensitivity Analysis assuming the dredging for gas main relocation is conducted near Quarry Bay intake
Dredging Rate for Gas Main Construction (m^3 per day)	5000		5000
WSD Standard for SS at flushing water intake (mg/l)	10
Maximum SS Level Predicted at the Quarry Bay Intake under the mitigated scenario(mg/l)	Maximum	Mean	Maximum	Mean
	6.2 (see Table 5.40a)	2.4	19.9	3.1
% time in compliance	100%	-	99.4%	-
Contribution from WDII activities	35.5%	7.4%	0.0%	10.1%
Contribution from Gas Main Relocation	20.5%	3.4%	89.3%	25.4%
Contribution from other concurrent projects and background sources	43.9%	89.2%	10.7%	64.5%

Note: Shaded value indicates exceedance of the WSD standard for flushing water intake

5.8.26 It should be noted that the dispersion and movement of pollutants and sediment plume in the Victoria Harbour will be driven by the changing tidal current. Therefore, the relative SS contribution at the flushing water intake from individual projects would also be changing at different tidal status and time. The % contributions for the maximum SS levels as shown in the above table represent the relative contributions at a particular instant when the SS level predicted at the Quarry Bay intake reached the maximum value. In terms of the contribution due to the WDII activity alone, the SS impact upon the Quarry Bay intake is considered minor and acceptable. The model predicted that the WDII works would not cause any non-compliance at the Quarry Bay intake with implementation of all the recommended mitigation measures. Under the case when dredging for the gas main construction is conducted near the North Point at a rate of 5,000 m³ per day, the SS level at the Quarry Bay intake would likely exceed the WSD water quality standard. However, as indicated by the sensitivity modelling conducted under this EIA, feasible mitigation measures such as installation of silt curtains around the gas main dredging work in areas close to the North Point or reduction of the dredging rate for gas main construction for the dredging activities near the North Point would effectively eliminate the SS exceedance and achieve full compliance at all the WSD flushing water intakes. The water quality impact due to the gas main relocation and the necessary mitigation measures required for protection of the flushing water intake will be addressed under the separate EIA study for the new gas main (also refer to Section 5.6.31).

Other Mitigation Measures

5.8.27 Other good site practices that should be undertaken during sand filling, public filling and dredging include:

· mechanical grabs, if used, should be designed and maintained to avoid spillage and sealed tightly while being lifted. For dredging of any contaminated mud, closed watertight grabs must be used

· all vessels should be sized so that adequate clearance is maintained between vessels and the seabed in all tide conditions, to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash

· all hopper barges and dredgers should be fitted with tight fitting seals to their bottom openings to prevent leakage of material

· construction activities should not cause foam, oil, grease, scum, litter or other objectionable matter to be present on the water within the site or dumping grounds

· loading of barges and hoppers should be controlled to prevent splashing of dredged material into the surrounding water. Barges or hoppers should not be filled to a level that will cause the overflow of materials or polluted water during loading or transportation

· before commencement of the construction works, the holder of the Environmental Permit shall submit plans showing the construction programme, design and operation of the silt curtain.

Regular Maintenance of Silt Screens

5.8.28 Silt screens are recommended to be deployed at the seawater intakes during the marine works period. Installation of silt screens at the seawater intake points may cause a potential for accumulation and trapping of pollutants, floating debris and refuse behind the silt screens and may lead to potential water quality deterioration at the seawater intake points. Major sources of pollutants and floating refuse include the runoff and storm water discharges from the nearby coastal areas. As a mitigation measure to avoid the pollutant and refuse entrapment problems and to ensure that the impact monitoring results are representative, regular maintenance of the silt screens and refuse collection should be performed at the monitoring stations at regular intervals on a daily basis. The Contractor should be responsible for keeping the water behind the silt screen free from floating rubbish and debris during the impact monitoring period.

Construction Runoff

5.8.29 Construction site runoff and drainage should be prevented or minimised in accordance with the guidelines stipulated in the EPD's Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN 1/94). Good housekeeping and stormwater best management practices, as detailed in below, should be implemented to ensure that all construction runoff complies with WPCO standards and that no unacceptable impact on the WSRs arises due to construction. All discharges from the construction site should be controlled to comply with the standards for effluents discharged into Victoria Harbour WCZ under the TM-DSS.

5.8.30 Ideally, construction works should be programmed to minimise surface excavation works during the rainy season (April to September). All exposed earth areas should be completed as soon as possible after earthworks have been completed, or alternatively, within 14 days of the cessation of earthworks where practicable. If excavation of soil cannot be avoided during the rainy season, or at any time of year when rainstorms are likely, exposed slope surfaces should be covered by tarpaulin or other means.

5.8.31 Sediment tanks of sufficient capacity, constructed from pre-formed individual cells of approximately 6 to 8 m³ capacity, are recommended as a general mitigation measure which can be used for settling surface runoff prior to disposal. The system capacity is flexible and able to handle multiple inputs from a variety of sources and particularly suited to applications where the influent is pumped.

5.8.32 Open stockpiles of construction materials (for examples, aggregates, sand and fill material) of more than 50 m³ should be covered with tarpaulin or similar fabric during rainstorms. Measures should be taken to prevent the washing away of construction materials, soil, silt or debris into any drainage system.

5.8.33 Manholes (including newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris being washed into the drainage system and storm runoff being directed into foul sewers.

5.8.34 Precautions to be taken at any time of year when rainstorms are likely, actions to be taken when a rainstorm is imminent or forecast, and actions to be taken during or after rainstorms are summarised in Appendix A2 of ProPECC PN 1/94. Particular attention should be paid to the control of silty surface runoff during storm events.

5.8.35 Oil interceptors should be provided in the drainage system and regularly cleaned to prevent the release of oils and grease into the storm water drainage system after accidental spillages. The interceptor should have a bypass to prevent flushing during periods of heavy rain.

5.8.36 All vehicles and plant should be cleaned before leaving a construction site to ensure no earth, mud, debris and the like is deposited by them on roads. An adequately designed and located wheel washing bay should be provided at every site exit, and wash-water should have sand and silt settled out and removed at least on a weekly basis to ensure the continued efficiency of the process. The section of access road leading to, and exiting from, the wheel-wash bay to the public road should be paved with sufficient backfall toward the wheel-wash bay to prevent vehicle tracking of soil and silty water to public roads and drains.

Drainage

5.8.37 It is recommended that on-site drainage system should be installed prior to the commencement of other construction activities. Sediment traps should be installed in order to minimise the sediment loading of the effluent prior to discharge into foul sewers. There shall be no direct discharge of effluent from the site into the sea.

5.8.38 All temporary and permanent drainage pipes and culverts provided to facilitate runoff discharge should be adequately designed for the controlled release of storm flows. All sediment control measures should be regularly inspected and maintained to ensure proper and efficient operation at all times and particularly following rain storms. The temporarily diverted drainage should be reinstated to its original condition when the construction work has finished or the temporary diversion is no longer required.

5.8.39 All fuel tanks and storage areas should be provided with locks and be located on sealed areas, within bunds of a capacity equal to 110% of the storage capacity of the largest tank, to prevent spilled fuel oils from reaching the coastal waters of Victoria Harbour WCZ.

Sewage Effluent

5.8.40 Construction work force sewage discharges on site are expected to be connected to the existing trunk sewer or sewage treatment facilities. The construction sewage may need to be handled by portable chemical toilets prior to the commission of the on-site sewer system. Appropriate numbers of portable toilets shall be provided by a licensed contractor to serve the large number of construction workers over the construction site. The Contractor shall also be responsible for waste disposal and maintenance practices.

Floating Refuse and Debris

5.8.41 It is recommended that collection and removal of floating refuse should be performed at regular intervals on a daily basis. The contractor should be responsible for keeping the water within the site boundary and the neighbouring water free from rubbish.

5.9 Evaluation of Residual Impacts

5.9.1 The major water quality impact associated with dredging for construction of the proposed water mains is the elevation of SS within the marine water column. Provided the recommended mitigation measures are implemented, including restriction on the maximum dredging rates, the deployment of silt curtains at the other specified dredging and filling areas in the WDII work site, and installation of silt screens at seawater intakes, there would be no unacceptable residual water quality impact due to the proposed dredging works.

5.10 Environmental Monitoring and Audit

5.10.1 There would be potential water quality impacts upon the water sensitive receivers due to the proposed dredging works. Appropriate mitigation measures are recommended in order to minimize the potential impacts. Water quality monitoring and audit during construction phase will need to be carried out to ensure that such mitigation measures are implemented properly.

5.11 Conclusion

5.11.1 The major water quality impact associated with the new water mains would be the potential SS elevations within the water column from the dredging works during the construction phase. Provided the recommended mitigation measures are implemented, including restriction on the maximum dredging rates, the deployment of silt curtains at the other specified current dredging and filling areas activities within the WDII work site, and installation of silt screens at seawater intakes, there will be no unacceptable residual water quality impact due to the proposed dredging works.

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([1]) Territory Development Department (July 2001). Agreement No. CE 74/98, Wan Chai Development Phase II, Comprehensive Feasibility Study, Environmental Impact Assessment Report, Volume I – Text.

(2) ERM (2003). The Proposed Submarine Gas Pipelines from Cheng Tou Jiao Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production Plant, Hong Kong EIA report.

([3]) Pastorok, R.A. and Bilyard, G.R. (1985). “Effects of sewage pollution on coral-reef communities.” Marine Ecology Progress Series 21: 175-189.

([4]) Hawker, D. W. and Connell, D. W. (1992). “Standards and Criteria for Pollution Control in Coral Reef Areas” in Connell, D. W and Hawker, D. W. (eds.), Pollution in Tropical Aquatic Systems, CRC Press, Inc.

([5]) Maunsell Consultants Asia Limited (2003). Tai Po Sewage Treatment Works Stage 5, EIA Report, Drainage Services Department, 2003

([6]) Hyder (1997). Sand Dredging and Backfilling of Borrow Pits at the Potential Eastern Waters Marine Borrow Area, EIA Report, CED, 1997.

([7]) ERM-Hong Kong, Limited (2001).  Focused Cumulative Water Quality Impact Assessment of Sand Dredging at the West Po Toi Marine Borrow Area Final Report.

([8]) ERM-Hong Kong, Limited (2003). The Proposed Submarine Gas Pipelines from Cheng Tou Jiao Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production Plant, Hong Kong, EIA Report, The Hong Kong and China Gas Company Limited, 2003

([9]) Scott Wilson (Hong Kong) Ltd. (February 2000). Northshore Lantau Development Feasibility Study (Agreement No. CE 60/96), Final Environmental Impact Assessment Report.

([10]) Scott Wilson (Hong Kong) Ltd. (February 2000). Construction of an International Theme Park in Penny’s Bay of North Lantau and its Essential Associated Infrastructures, Final Environmental Impact Assessment Report.

([11]) Mott MacDonald (1991). Contaminated Spoil Management Study, Final Report, Volume 1, for EPD, October 1991.

^([12]) The rate of oxygen consumption exerted by the sediment on the overlying water at 20^oC for a period of five days.

^([13]) Environmental Quality Standards and Assessment Levels for Coastal Surface Water (from HMIP (1994) Environmental Economic and BPEO Assessment Principals for Integrated Pollution Control). (Source: Environmental Impact Assessment Study for Disposal of Contaminated Mud in the East Sha Chau Marine Borrow Pit, by ERM, January 1997).

([14]) Planning Department Agreement No. CE48/97 Feasibility Study for Additional Cross-border Links Stage 2: Investigations on Environment, Ecology, Land Use Planning, Land Acquisition, Economic/Financial Viability and Preliminary Project Feasibility/Preliminary Design Final Water Quality Impact Assessment Working Paper WP2 Volume 1 1999.

([15]) EGS (Asia) Limited (2000). Wan Chai Development Phase II, Comprehensive Feasibility Study, Section I, Stormwater Flow and Pollutant Survey of Outfalls Entering Victoria Harbour of Outfalls Entering Victoria Harbour, Final Report.

([16]) EGS (Asia) Limited (2000). Wan Chai Development Phase II, Expedient Connection Survey, Supplementary Report for Section I of Works.

([17]) ERM-Hong Kong, Ltd. (January 1997). Environmental Quality Standards and Assessment Levels for Coastal Surface Water (from HMIP (1994) Environmental Economic and BPEO Assessment Principals for Integrated Pollution Control). (Source: Environmental Impact Assessment Study for Disposal of Contaminated Mud in the East Sha Chau Marine Borrow Pit.)

([18]) Australian and New Zealand Environment and Conservation Council (1992). Australian Water Quality Guidelines for Fresh and Marine Waters.

([19]) United States Environmental Protection Agency (1998). Water Quality Standards, Establishment of Numeric Criteria for Priority Toxic Pollutants; States’ Compliance – Revision of Polychlorinated Biphenyls (PCBs) Criteria.

([20]) Salazar, M. H. and Salazar, S. M. (1996). “Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation, and Growth under Natural Conditions” in Organotin, edited by M. A. Champ and P. F. Seligman. Chapman & Hall, London.

([21]) Mott MacDonald (1991). Contaminated Spoil Management Study, Final Report, Volume 1, for EPD, October 1991.

([22]) Silt curtains should be made from impervious material such as coated nylon and primarily redirect flow around the dredging area rather than blocking the entire water column. In contrast, silt screens are made from synthetic geotextile fabrics, which allow water to flow through but retain a fraction of the suspended solids.