5                     WATER QUALITY

 

Introduction

 

5.1               This section evaluates the potential water quality impacts that are likely to be generated during construction phase and operation phase of the proposed Project. Appropriate mitigation measures were identified, where necessary, to mitigate the potential water quality impacts to acceptable levels.

 

Environmental Legislation, Policies, Plans, Standards and Criteria

 

Environmental Impact Assessment Ordinance (EIAO), Cap.499, S.16

 

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

Ÿ             Annex 6 Criteria for Evaluating Water Pollution

Ÿ             Annex 14 Guidelines for Assessment of Water Pollution

 

Marine Water Quality Objectives

 

5.3               The Water Pollution Control Ordinance (WPCO) Cap.358 provides Water Control Zones (WCZ).  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. A summary of WQOs for Victoria Harbour WCZ is given 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).

 

Hong Kong Planning Standards and Guidelines (HKPSG)

 

5.4               The HKPSG, Chapter 9 (Environment), provides additional information on regulatory guidelines against water pollution for sensitive uses such as aquaculture and fisheries zones, bathing waters and other contact recreational waters.

 

Water Supplies Department (WSD) Water Quality Objectives

 

5.5               The Wan Chai saltwater pumping station to the east of the ALE sea channel (Figure 5.2) would be potentially affected by the Project.  Besides the WQO set under the WPCO, the Water Supplies Department (WSD) has also specified a set of seawater quality objectives for water quality at their seawater intakes.  The list is shown in Table 5.2. 

.

 

 

 

 

Table 5.2          WSD Standards at Sea Water Intakes

 

Parameter (in mg/L unless otherwise stated)

WSD 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. / 100 ml)

< 20,000

 

Cooling Water Intake Standards

 

5.6               Cooling water intakes that would potentially be affected by the Project are shown in Figure 5.2.  Under the Comprehensive Feasibility Study for Wan Chai Development Phase II, questionnaires were sent to the owners or operators of individual cooling water intakes in Victoria Harbour requesting their operation requirements in terms of water quality.  Based on the responses received, no specific requirement on seawater quality at the cooling water abstraction point was identified for these cooling water intakes.

 

5.7               To assess the potential water quality impact on the cooling water systems under the approved EIA for Wan Chai Development Phase II, a limit for SS concentration of 40 mg L-1 at MTRC South Intake was adopted as the assessment criterion.  This criterion had been confirmed with the MTRC Property Management Office.   

 

Technical Memorandum

 

5.8               Besides setting the WQOs, the WPCO controls effluent discharges into any WCZ through a licensing system.  The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS), issued under Section 21 of the WPCO, gives guidance on 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 effluent.  Any sewage from the proposed construction activities should comply with the standards for effluent discharged into the foul sewers, inshore waters or marine waters of the Victoria Harbour WCZ, shown in Table 1, Table 9a and Table 9b, respectively, of the TM-DSS.

 

Practice Note

 

5.9               A practice note for professional persons has been 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 sterilization of water retaining structures and water pipes, wastewater from building construction, 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 minimize the water quality impact due to construction site drainage.  For operational stage effluent handling, treatment and disposal, reference should be made to ProPECC PN 5/93.

 

Description of the Environment

 

5.10            The EPD monitoring stations of most relevance (that is, in the vicinity of the Study Area) include VM4, VM5 and VM6 (Figure 5.1).  A summary of the published EPD monitoring data (in 2003) collected at these stations is presented in Table 5.3. As the Harbour Area Treatment Scheme (HATS) Stage I was commissioned in late 2001, the data shown in Table 5.3 represent the situation after the commissioning of HATS Stage 1.

 

5.11            Based on EPD’s publication “EPD Marine Water Quality in Hong Kong 2002, the implementation of HATS Stage I in 2001 has resulted in a moderate improvement in the central Victoria Harbour (VM4 and VM5).   The zone of peak E.coli and ammonia nitrogen level shifted from the central part of the harbour to the western side after commissioning of the HATS Stage 1. As shown in Table 5.3, all the three stations complied with the WQO for DO, TIN and UIA. 

 

Table 5.3          Baseline Water Quality Condition for Victoria Harbour WCZ in 2003

 

Parameter

Victoria Harbour Central

WPCO WQO (in marine waters)

VM4

VM5

VM6

Temperature (oC)

23.3

(17.0 – 27.2)

23.4

(17.1 – 27.5)

23.5

(17.3 – 27.6)

Not more than 2 oC in daily temperature range

Salinity

32.4

(31.0 – 33.5)

32.1

(29.4 – 33.4)

32.0

(29.8 – 33.3)

Not to cause more than 10% change

Dissolved Oxygen (DO) (% Saturation)

Depth average

76

(61  85)

75

(61 – 88)

74

(58 – 85)

 

Bottom

75

(51 – 90)

71

(53 – 88)

69

(44 – 85)

 

Dissolved Oxygen (DO)

(mg/L)

Depth average

5.4

(4.2 – 6.8)

5.3

(4.1 – 6.9)

5.3

(3.9 – 6.7)

Not less than 4 mg/L for 90% of the samples

Bottom

5.4

(3.6 – 7.2)

5.0

(3.7 – 6.9)

4.9

3.1 – 6.7

Not less than 2 mg/L for 90% of the samples

pH

8.1

(8.0 – 8.2)

8.1

(8.0 – 8.2)

8.0

(7.8 -8.2)

6.5 - 8.5 (± 0.2 from natural range)

Secchi disc Depth (m)

2.3

(1.5 – 3.2)

2.3

(1.5 – 4.1)

2.3

(1.5 – 4.0)

-

Turbidity (NTU)

8.5

(6.3 – 13.1)

(8.6

(5.4 – 11.0)

8.8

(5.4 – 12.2)

-

Suspended Solids (SS) (mg/L)

4.9

(2.5 – 11.3)

4.7

(2.5 – 7.5)

5.1

(2.4 – 9.8)

Not more than 30% increase

5-day Biochemical Oxygen Demand (BOD5) (mg/L)

1.1

(0.6 – 1.8)

1.4

(0.7 – 2.2)

1.1

(0.5 – 2.0)

-

Nitrite Nitrogen (NO2-N)  (mg/L)

0.02

(0.01 – 0.05)

0.03

(0.01 – 0.05)

0.03

(0.01 – 0.05)

-

Nitrate Nitrogen (NO3-N) (mg/L)

0.09

(0.03 – 0.15)

0.11

(0.04 – 0.21)

0.11

(0.04 - 0.19)

Not more than 0.021 mg/L

Ammonia Nitrogen (NH3-N) (mg/L)

0.16

(0.05 – 0.28

0.20

(0.07 – 0.34

0.20

(0.09 - 0.34

-

Unionised Ammonia (UIA) (mg/L)

0.007

(0.003 – 0.014)

0.009

(0.005 – 0.014)

0.009

(0.005 – 0.015)

-

Total Inorganic Nitrogen (TIN) (mg/L)

0.27

(0.18 – 0.42)

0.33

(0.22 – 0.50)

0.34

(0.25 – 0.50)

Not more than 0.4 mg/L

Total Nitrogen (TN) (mg/L)

 

0.44

(0.28 – 0.61)

0.52

(0.33 – 0.65)

0.52

(0.35 – 0.64)

-

Orthophosphate Phosphorus (OrthoP) (mg/L)

0.030

(0.02 – 0.05)

0.036

(0.02 – 0.05)

0.037

(0.02 – 0.05)

-

Total Phosphorus (TP) (mg/L)

0.05

(0.03 – 0.07)

0.06

(0.03 – 0.08)

0.06

(0.04 – 0.07)

-

Chlorophyll-a

(µg/L)

3.9

(0.4 – 18.2)

4.0

(0.3 – 22.7)

3.5

(0.4 – 16.7)

-

E coli

(cfu/100 mL)

2700

(580 – 23000)

5200

(640 – 42000)

3000

(250 – 14000)

-

Faecal Coliforms

(cfu/100 mL)

4800

(1100 – 42000)

12000

(2500 – 100000)

7000

(1100 – 26000)

-

Note:  

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.

 

Water Sensitive Receivers

 

5.12            The existing marine sensitive sites that may be affected by the Project include the WSD saltwater pumping station and other cooling water intakes in the vicinity of Phase I and Phase II of the HKCEC.  Figure 5.2 shows the locations of these intake points.

 

 

 

Identification of Environmental Impacts

 

General Construction Activities

 

5.13            Potential water quality impacts may arise from the construction activities due to their close vicinity to the marine water. Construction works would involve construction and demolition of temporary footbridge and temporary working platform(s), demolition of the existing Atrium Link, excavation and piling for permanent foundation of the ALE.  The excavation and demolition works may pollute the nearby storm system and marine waters due to potential release of construction wastes. Construction wastes are generally characterized by high concentration of SS and elevated pH. Adoption of good house keeping and mitigation measures would be required to reduce the generation of construction wastes and potential water pollution. In addition, the construction activities including barging activities would be conducted in close proximity to a number of cooling water intakes and WSD saltwater pumping station. Implementation of suitable pollution control measures would be required to prevent site runoff and contaminated drainage water, sediment and other pollutants from discharging into the sea.

 

Wastewater and Sewage Effluent

 

5.14            Various types of construction activities may generate wastewater. These include general cleaning and polishing, wheel washing, dust suppression and utility installation.  These types of wastewater would contain high concentrations of suspended solids.  Impacts could also result from the accumulation of solid and liquid waste such as packaging and construction materials, and sewage effluent from the construction work force involved with the construction.  If uncontrolled, these could lead to deterioration in water quality.  Contaminated discharges and sewage effluent could also lead to localized increase in ammonia and nitrogen concentrations in the marine environment.

 

Accidental Spillage

 

5.15            A large variety of chemicals may be used during construction activities. These may include surplus adhesives, spent paints, petroleum products, spent lubrication oil, grease and mineral oil, spent acid and alkaline solutions/solvent and other chemicals. Accidental spillage of chemicals may be washed away by construction site runoff or storm runoff causing water pollution.

 

Marine Construction Works

 

5.16            Potential water quality impacts may occur from installation of temporary marine piles for construction of the temporary footbridge and temporary working platform(s). The marine piles for supporting the temporary working platform(s) would be in place in the waterway between the Phase I and Phase II of the HKCEC for about 2 years during the construction period from 2006 to 2008. The marine piles for temporary footbridge will be in place in the waterway for about 3 years from 2006 to 2009.  The temporary piles may obstruct the flow and reduce the flushing capacity of the water channel.  The changes in the flushing capacity may affect the dispersion of pollutants discharged from the nearby stormwater culverts and may adversely affect the water quality at the cooling water intakes and saltwater pumping station.

 

5.17            Marine piling will be conducted for installation of the temporary cylindrical piles.  These cylindrical piles will be driven into position and internal space will not be excavated, i.e. left as soil.  No soil/sediment excavation would be carried out.  Marine piles would be removed by reverse driving and it is envisaged that hydraulic pile driving equipment would be used. All reverse driving equipment would be mounted on barges with no temporary support required off the sea bed. Should difficulties be encountered during the removal of a marine pile, the pile top would be moved from side to side by the driving equipment to loosen the side friction and facilitate upward driving.  In case if there is any hole left in the seabed after the pile is driven out, the hole would be filled with clean marine sand.  As the sand would be pumped via the 150 mm delivery pipes and discharged inside the hole at levels below the seabed, sediment loss due to the filling operation, if required, is expected to be low.Double layers of silt curtain will be deployed around the pile removal works to mitigate potential water quality impact. No dredging would be required for marine construction works. The marine piling and pile extraction may cause some disturbance to the sediments and potentially affect the nearby seawater intakes.

 

5.18            Barging activities may cause adverse impact on the water quality if not handling properly. Mitigation measures are recommended to control any pollutant discharge into the sea due to the barging activities.  Impact due to the barging activities is expected to be insignificant provided that all the recommended measures are properly implemented.

 

Operational Phase Sewage Impact

 

5.19            Additional sewage flow would be generated as a result of the ALE.  Additional storm flow would also be generated from the Project as the footprint of the new atrium link would be larger.  Impact on the existing public sewerage system and the existing public drainage system due to these additional flows would need to be addressed. Appropriate mitigation measures would need to be identified, where necessary, to mitigate the potential impacts.

 

Assessment Methodology

 

Hydrodynamic Modelling Tools

 

5.20            A detailed 3-dimensional detailed model, namely Wan Chai Model, has been developed under the present Study using the Delft3D package and is used to simulate the change in the flushing capacity of the sea channel between Phase I and Phase II of the HKCEC due to the installation of the temporary marine piles.  The grid layout and bathymetry schematization of the Wan Chai Model are shown in Appendix 5.1 and Appendix 5.2 respectively. 

 

5.21            The Wan Chai Model was linked to a regional Update Model ([1]). The hydrodynamic model of the Update Model was fully calibrated and verified and has been used to provide boundary and initial hydrodynamic conditions to the Wan Chai model. The Update Model covers the outer regions of Pearl River Estuary, Macau, Lamma Channel and Deep Bay. All major influences on hydrodynamics in the outer regions were therefore incorporated.  The performance of the Wan Chai Model has been checked against that of the approved Update Model as shown in Appendix 5.2a.

 

Modelling Scenarios

 

5.22            The layout of the temporary piles could not be confirmed at the time when this EIA was prepared. The detailed design would be conducted by the “design and build” contractor at a later stage.  The detailed design of the temporary piles should meet the requirement stipulated in the gazette notice issued under the Foreshore and Sea-bed Ordinance (i.e. there would be approximately 80 marine piles of 800mm in diameter each installed in the sea channel).  Hydrodynamic modelling was conducted for two alternative options (see Figure 2.6 and Figure 2.7 respectively) to investigate the sensitivity of different pile layouts on the flushing effects. 

 

5.23            Both scenarios cover a temporary footbridge connecting Phase I and Phase II of the HKCEC.   The layout of the temporary footbridge is provided in Appendix 2.1.  The footbridge consists of 8 pairs of tubular marine piles (total 16 piles).  The nominal diameter of the marine pile is 800 mm.

 

Option 2

 

5.24            Option 2 includes three separate temporary working platforms as shown in Figure 2.6 supported on 63 nos. of marine piles.  The nominal diameter of each marine pile is 800mm.  Including the temporary footbridge, there would be total 79 marine piles.

 

Option 3

 

5.25            Option 3 includes one temporary working platform as shown in Figure 2.7.  The platform is located on the eastern side of the sea channel and is supported by a total of 54 marine piles. The nominal diameter of each marine pile is 800 mm.  Including the temporary footbridge, there would be total 70 marine piles installed in the sea channel under this scenario.

 

5.26            Modelling was also conducted for an additional “baseline” scenario without the temporary piles for comparisons.  The existing coastline including the Central Reclamation Phase III (CRIII) was used for model simulations under baseline condition, Option 2 and Option 3 scenarios.  Wan Chai Reclamation Phase II is still subject to planning review and is therefore excluded.  The coastline configurations adopted for water quality modelling are shown in Figure 5.2a.  Existing structures including the piers of East Bridge, West Bridge and Seafront Promenade were also included in all model runs.  The positions of East Bridge, West Bridge and Seafront Promenade with respect to the model grid are shown in Appendix 5.3. 

 

5.27            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 about 914 mm.

 

5.28            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.

 

5.29            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.  

 

5.30            For each of the baseline and assessment scenarios, the simulation period of the hydrodynamic model covered two 15-day full spring-neap cycles (plus spin-up periods) for dry and wet seasons respectively.  Model test runs were conducted to check whether the spin-up periods are sufficient.  It was confirmed that a spin-up period of 8 days would be sufficient for the dry season simulations. For wet season simulations, a longer spin-up period of 23 days was provided in order to produce acceptable modelling results for impact assessment.

 

5.31            There are two existing storm outfalls in the vicinity of the Study Area, namely Culvert L and Culvert M, as shown in Figure 5.2. The pollution flows and loads from these two culverts were estimated for year 2003 and 2011 under the approved EIA for Wan Chai Development Phase II based on actual survey data as shown in Table 5.4 and Table 5.5.  Conservative tracers with zero decay were input into the Wan Chai Model at the discharge points of Culvert L and Culvert M as continuous source throughout the simulation periods including the spin up periods for both dry and wet seasons.  The discharge flows and loads of the tracers were assumed to be proportional to the flows and BOD loads conveyed by the two culverts as shown in Table 5.4 and Table 5.5.  The proportions of the estimated flows and loads conveyed by the two culverts are the same for both 2003 and 2011.  Comparison of Option 2 and Option 3 was made against the baseline scenario, in terms of the tracer concentrations predicted in the sea channel.

 

Table 5.4          Estimated Pollution Flows and Loads at Culvert L and Culvert M for 2003

 

Culvert

Flow (m3/day)

BOD (kg/day)

SS (kg/day)

TKN (kg/day)

Dry Season

L

21186

1479.543

2261.488

138.255

M

9345

523.577

591.809

95.292

Wet Season

L

55580

1678.529

3048.576

147.497

M

24516

593.994

797.782

101.662

 

Table 5.5          Estimated Pollution Flows and Loads at Culvert L and Culvert M for 2011

 

Culvert

Flow (m3/day)

BOD (kg/day)

SS (kg/day)

TKN (kg/day)

Dry Season

L

22222

1551.948

2372.160

145.021

M

9802

549.200

620.771

99.956

Wet Season

L

58300

1760.672

3197.767

154.715

M

25716

623.063

836.824

106.637

 

Pile Friction

 

5.32            The presence of the marine piles may reduce the flushing of the water channel and thus impact upon the water quality.  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 will be taken account.  This overall influence was modelled by a special feature of the Delft3D-FLOW model, namely porous plate.  Porous plates represent transparent structures in the model and are 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 ([2]) and the Delft 3D-FLOW module developed by Delft Hydraulics. 

 

5.33            For each grid cell where the piles will be located, two loss coefficients have to be specified in the model for two different flow directions respectively (i.e. the two directions perpendicular to the gridline, namely u-direction and v-direction respectively).  Details of the equations and arrangement of the porous plates used in the modelling are contained in Appendix 5.3. 

 

Uncertainties in Assessment Methodology

 

5.34            Quantitative uncertainties in the hydrodynamic modelling were considered when making an evaluation of the modelling predictions.  The purpose of this hydrodynamic modelling exercise is to investigate the overall flushing impact on the ALE sea channel.  The proposed modelling approach was not intended to assess the detailed water quality impact at specific water sensitive receivers. The following tasks have been undertaken to enhance the model performance:

Ÿ             The computational grid of the local Wan Chai Model was refined to provide representative simulations results;

Ÿ             Use of a fully calibrated and validated regional model to provide boundary and initial conditions to the local Wan Chai Model;

Ÿ             The performance of the local Wan Chai Model has been checked against the calibrated and validated regional model to ensure that reliable predictions of hydrodynamics are provided for the Study area.

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

 

Evaluation of Environmental Impacts

 

Impact on Flushing Capacity of the Sea Channel

 

5.35            Cross sections were defined in the hydrodynamics model through which the flux is determined and stored by the model as a function of time.  Three cross sections, namely ALE1, ALE4 and ALE7 respectively, were defined at the sea channel of the HKCEC and their locations are shown in Figure 5.3.  Appendix 5.4 and Appendix 5.5 shows the time series comparison plots of momentary flow and accumulated flow at the selected cross sections for dry season and wet season respectively.   Each appendix contains two figures for momentary flow and accumulated flow respectively.  Momentary flow represents the instantaneous flow rate at a specific time in m3/s whereas accumulated flow represents the total flow accumulated at a specific time in m3. Each figure contains three plots for the three cross sections respectively.  In each plot, the baseline condition was compared against Option 2 and Option 3 conditions.  As shown in the appendices, small decreases in momentary flow and accumulated flow across sea channel were observed due to the temporary piles. Based on these plots, there was no significant change in the degree of impact between Option 2 and Option 3 for dry season.  The deviations between Option 2 and Option 3 were observed to be larger during the wet season.  The degree of impact for Option 2 would be smaller as compared to Option 3.

 

5.36            Appendix 5.6 and Appendix 5.7 shows the contour plots of surface tracer contents for dry season and wet season respectively.  Each appendix contains four figures for Option 2 mid-flood, Option 2 mid-ebb, Option 3 mid-flood and Option 3 mid-ebb respectively.  Each figure contains two plots. The upper plot shows the baseline condition without the temporary marine piles whereas the lower plot shows the impact scenario with the temporary marine piles.  It can be seen from these contour plots that the patches of tracer plume near the discharge locations were slightly larger under Option 2 and Option 3 as compared to the baseline conditions.

      

5.37            The surface and depth-averaged tracer concentrations for the whole sea channel are shown in Table 5.6. The data presented in Table 5.6 represent the average concentration over the 15-day simulation period.

 

Table 5.6          Predicted Tracer Contents at the Sea Channel of HKCEC

 

Season

Scenario

Depth-average

Surface Water

Tracer Content (mg/L)

% difference compared to the Baseline

Tracer Content (mg/L)

% difference compared to the Baseline

Dry Season

Baseline

0.1582

-

0.2429

-

Option 2

0.1651

4.4

0.2639

8.7

Option 3

0.1673

5.8

0.2699

11.1

Wet Season

Baseline

0.2004

-

0.2173

-

Option 2

0.2054

2.5

0.2232

2.7

Option 3

0.2068

3.2

0.2248

3.5

 

5.38            Impacts due to the installation of temporary marine piles were found to be in general less significant in wet season.  The increases in depth-average tracer contents in the sea channel for Option 2 and Option 3 are 2.5% and 3.2% respectively for wet season.  The increases were higher during the dry season (4.4% and 5.8% for Option 2 and Option 3 respectively).

 

5.39            The degree of impact for the whole ALE sea channel was also found to be more significant for surface tracer contents.  The largest increase in the surface tracer contents was predicted to be 11.1% for Option 3 during dry season (Table 5.6).  All other increases were less than 9%. 

 

5.40            As shown in Table 5.6, the impacts would be larger under Option 3 as compared to Option 2.  The main flow directions in the sea channel would be from the east to the west and from the west to the east.  There are nine rows of piles installed across the sea channel from south to north under Option 3 (see Figure 2.6) as compared to only three rows under Option 2 (see Figure 2.7). Although the total number of marine piles under Option 3 is smaller, the effective flow area in the main flow directions is smaller due to the proposed pile layout.  It is therefore suggested that the temporary pile layout for Option 2 should be adopted.  Option 3 is not recommended.

 

5.41            In summary, hydrodynamic modelling results indicated that the installation of temporary piles in the sea channel would inevitably reduce the flushing capacity of the sea channel. The impact is however considered only short term and localized. The overall influence on the flushing capacity of the whole sea channel during the construction period was predicted to be insignificant (less than 5%) under the recommended option. Given that the marine piles would last for a maximum of three years and would be removed after construction of the ALE, it is anticipated the overall impact on the water quality inside the channel would be insignificant.

 

Impact Due to Marine Piling and Pile Removal Works

 

5.42            Marine piling will be carried out to install the temporary marine piles. The pile will be driven into position and internal space will not be excavated.  No dredging or soil/sediment excavation would be required. The marine piling works may cause some disturbance to the sediments in the immediate vicinity of the piles.  It is however expected that the disturbance would be localized and of small scale.

 

5.43            Marine piles would be removed by reverse driving. Typically there would be little potential for disturbance to the marine sediments during removal as piles would be driven straight out and removed with no equipment touching the sea bed. Should difficulties be encountered during the removal of a marine pile, the pile top would be moved from side to side by the driving equipment to loosen the side friction and facilitate upward driving. This side to side movement would increase the potential for disturbance to the marine sediments but in no instance would excavation of the sea bed be undertaken.

5.44            No significant physical disturbance to seabed sediment is expected during the installation and removal of the marine pile.  In case if there is any hole left in the seabed after the pile is driven out, the hole would be filled with clean marine sand.  As the sand would be pumped via the 150 mm delivery pipes and discharged inside the hole at levels below the seabed, sediment loss due to the filling operation, if required, is expected to be low.  Double layers of silt curtain will be deployed around the marine piling and pile removal works to mitigate potential water quality impact.  Reclamation works for CRIII would still be on-going during construction phase of this Project.  It is anticipated that the construction impacts from this Project would be localized and would not contribute any cumulative water quality impacts with the CRIII.  No other concurrent marine work is expected in the vicinity of the Project site during the construction period. The marine construction works would unlikely cause unacceptable impacts to the nearby water sensitive receivers provided that good site practice, mitigation measures and water quality monitoring programme are properly implemented. 

 

Barging Activities During Construction Phase

 

5.45            During the construction period, two new marine access points would be located on the western and eastern sides of the HKCEC Phase II respectively. Locations of the proposed marine access points are shown in Figure 5.4. Mitigation measures are recommended to control any pollutant discharge into the sea due to the barging activities. Impact due to the barging activities is expected to be insignificant provided that all the recommended measures are properly implemented.

 

Operational Phase Sewerage Impact

 

Background

 

5.46            The Project is to expand the existing exhibition facilities in the HKCEC by extending the existing Atrium Link between the Phase I and Phase II buildings.  The existing Atrium Link with a total floor area of 9,500m2 will be replaced by the proposed Atrium Link Extension (ALE) providing a net exhibition space of 19,400m2 with a total floor area of 49,100m2.

 

5.47            This section is to address the impacts on the existing and committed public sewerage infrastructure, if any, as a result of the sewage flows generated by the proposed ALE.

 

Existing and Committed Sewerage Infrastructure

 

5.48            The proposed ALE falls within the existing Wan Chai East sewerage catchment. Originally, the sewage flows from existing HKCEC was conveyed to Wan Chai West Preliminary Treatment Works (PTW), however, upon the completion of the Central, Western and Wan Chai West (CW3) Sewerage Improvement project in 2000, 1,800mm diameter deep trunk sewer was constructed along Harbour Road and it conveys the sewage flows arising from the HKCEC to Wan Chai East PTW.

 

5.49            Moreover, according to the Review of Hong Kong Island Sewerage Master Plan (HKI SMP), Wan Chai West sewerage catchment was scheduled to merge with the Wan Chai East sewerage catchment by July 2003.  Upon merging of the two catchments, the combined flows will be treated at the Wan Chai East PTW whilst the Wan Chai West PTW will be decommissioned. The sewage flows to the Wan Chai West PTW will be diverted to Wan Chai East PTW via the 1,800mm diameter deep trunk sewer along Harbour Road.  However, at present, Wan Chai West PTW is still in operation and schedule of decommissioning is uncertain. For the purpose of this assessment, it is assumed that the Wan Chai West PTW will be decommissioned by 2011 and all the sewage flows from the existing Wan Chai West sewerage catchment will be diverted to the Wan Chai East PTW via the 1,800mm diameter deep trunk sewer.

 

5.50            The existing Wan Chai East PTW has been upgraded under CW3 Project.  This Wan Chai East PTW is already commissioned in June 2002 and has a design capacity of 4.61m3/s.

 

Population and Flow Projections

 

5.51            According to the implementation programme, the year of occupancy of the proposed ALE is Year 2009.  Projections for population and sewage flow estimates are therefore considered for Year 2011 to access the sewerage impacts arising from the proposed ALE.

 

5.52            The proposed development schedule and flow projection for the proposed ALE are presented below:

 

Table 5.7          Proposed Development Schedule and Flow Projection for the Proposed ALE

 

Description

Exhibition Area (m2)

Support and Circulation Area (m2)

Proposed Extension

 

 

            L2 Hall 1 Extension

7,200

7,550

            L3

 

5,600

            L5 Hall 2 Extension

7,200

6,650

            L6

 

4,700

            L7 Hall 3 Extension

5,000

5,200

Sub-total

19,400

29,700

Occupancy (m2/person)

2.8

(visitor)

15

(employment)

No. of person

6,929

(Visitor)

1,980

(Employment)

Global Unit Flow Factor

0.08

0.28

Average Dry Weather Flow, ADWF (m3/d)

554

554

Global Peaking Factor

6

6

Design Flow (m3/d)

3,324

3,324

Total ADWF (m3/d)

1,108

Total PWWF (m3/d)

6,648

Notes:

1.       Unit flow factor in accordance with the Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning (March 2005), EPD. (GESF)

2.       Peaking factor including stormwater allowance with GESF.

3.       PWWF = ADWF x Global peaking factor

 

5.53            In the estimation of baseline population from both Wan Chai East and Wan Chai West sewerage catchments in 2011, reference is made to the planning data of the 2003-based Territory Population and Employment Data Matrices (TPEDM) Scenario II. While the baseline flows are estimated in accordance with the Guidelines for Estimating Sewage Flows for Sewage Infrastructure Planning (March 2005), EPD. (GESF).

 

5.54            Table 5.8 summarizes the baseline population and flow projection in both Wan Chai West and Wan Chai East sewerage catchments. The detailed breakdown is shown in Appendix 5.8.

 

Table 5.8    Baseline Population and Flow Projections in Wan Chai West and Wan Chai East Sewerage Catchments in 2011

 

Development Type

Population

Average Dry Weather Flow (ADWF) (m3/d)

Wan Chai West Sewerage Catchment

 

 

            Residential

62,246

14,202

            Commercial (J2 – J12)

176,628

40,510

            Industrial (J1)

1,864

466

            School

17,196

688

Sub-Total

257,934

55,866

Wan Chai East Sewerage Catchment

 

 

            Residential

88,119

20,111

            Commercial (J2 – J12)

196,963

55,605

            Industrial (J1)

2,218

555

            School

26,019

1,041

Sub-Total

313,319

77,312

Wan Chai Sewerage Catchment = Wan Chai West Sewerage Catchment + Wan Chai East Sewerage Catchment

            Residential

150,365

34,313

            Commercial (J2 – J12)

373,591

96,115

            Industrial (J1)

4,082

1,021

            School

43,215

1,729

Total

571,253

133,178

 

Sewerage Impact Assessment

 

5.55            It is proposed that the sewage arising from the proposed ALE to be discharged to the existing 400mm and 450mm diameter sewers along Convention Avenue shown in Appendix 5.9. Trunk sewers with diameter ranging from 400mm to 1,800mm assessed under this SIA is also shown in Appendix 5.9.  As shown in Appendix 5.10, both baseline and SIA condition in 2011 revealed that no sewers are identified as hydraulic inadequate in the existing sewers assessed.

 

5.56            With the proposed ALE, the total peak flows arriving at the Wan Chai East PTW are projected to be 4.05 m3/s in 2011 (see Table 5.9 below).  Therefore, the Wan Chai East PTW is capable of handling the projected 2011 flows. (c.f. capacity = 4.61m3/s)

 

Table 5.9          Peak flows to Wan Chai East PTW

 

Description

 

Baseline ADWF for Wan Chai Sewerage Catchment (m3/d)

133,178

Additional ADWF from proposed ALE (m3/d)

1,108

Total ADWF (m3/d)

134,286

Contributing Population, N (1)

497,356

Peaking Factor for PTW, PF (2)

2.60

Total peak flow = ADWF x PF

350,487m3/d or 4.05m3/s

Notes:

Contributing population = calculation total average flow (m3/day) / 0.27 (m3/person/day) in accordance with GEFS.

Peaking factor for PTW refer to Table T-5 of GEFS

 

Operational Phase Drainage Impact

 

Existing Drainage System

 

5.57            The footprint of the proposed ALE will extend to the west of the existing Atrium Link over government’s planned future Road P2 and the Central-Wan Chai Bypass by about 1,880m2.  To the east, it will cover the same roads by about 6,620m2, resulting in the total increase of footprint by 8,500m2.

 

5.58            According to the DSD drainage record plan, runoff from HKCEC is presently discharged to two drainage systems located on the western side and eastern side of HKCEC.  On the western side of HKCEC, drain pipes ranging 600mm to 1,200mm is laid along Expo Drive and eventually discharging the stormwater flows to the sea via a 1,200mm outfall.  While on the eastern side of HKCEC, there is a 3,200mm (W) x 3,200mm (H) box culvert running along Expo Drive East and discharging the stormwater flows to the sea through the 4 cells x 3,200mm (W) x 3,200mm (H) outfall near Hong Kong Tourist Harbour.

 

Runoff Estimates

 

5.59            Based on the Stormwater Drainage Manual (DSD, 2000), a design storm with a 200 years return period is adopted for estimation of the storm runoff.  Assuming a time of concentration of 5 minutes, the rainfall intensity based on the Intensity-Frequency-Duration relationship provided in the Stormwater Drainage Manual is given by:

                       

i =

a

(td + b)c

 

      ,where   i                       = extreme mean intensity (mm/hr)

                              td                      = duration (minutes)

                              a, b, c   = storm constants

 

5.60            For a design return period of 200 years, the following combination of rain and tide events are assessed.

 

Return Period:

Case I:

Case II:

200 years

200-year rain + 10-year sea level

10-year rain + 200-year sea level

 

5.61            The proposed ALE results in an additional covered area of 8,500m2 (or approximately 0.009km2) and with an assumed runoff coefficient of 1.  Therefore, in accordance with the Rational Method in Stormwater Drainage Manual, the additional runoff from the proposed ALE is 0.75 m3/s and 0.56m3/s for Case I and Case II respectively.

 

Table 5.10        Additional Runoff Estimates

 

Description

Case I

Case II

Additional catchment area from the proposed ALE, A (km2)

0.009

0.009

Rainfall intensity, i (mm/hr)

317

225

Runoff coefficient, C

1

1

Peak stormwater runoff, Qp = 0.278CiA (m3/s)

0.75

0.56

 

Drainage Impact Assessment

 

5.62            The proposed ALE lies within the Study Area of “Stormwater Drainage Master Plan Study in Northern Hong Kong Island” (NHKIDMP).  According to NHKIDMP, no improvement works for the existing drainage system in close proximity of the proposed development area are planned.

5.63            It is proposed to discharge the additional stormwater runoff to the existing 3,200mm (W) x 3,200mm (H) box culvert along Expo Drive East.  Table 5.11 summarizes the result of the drainage impact assessment. Details can be referred to Appendix 5.11.

 

Table 5.11         Summary of the Drainage Impact Assessment

 

Description

Case I

Case II

(1) Catchment Area

 

 

            Original Catchment Area of HKCEC (km2)

0.035

0.035

            Additional Catchment Area due to ALE (km2)

0.009

0.009

            Total Catchment Area, A (km2)

0.044

0.044

(2) Design Peak Flow

 

 

            Original peak flow from HKCEC (m3/s)

3.08

2.16

            Additional peak flow due to ALE (m3/s)

0.75

0.56

            Total design peak flow, Q (m3/s)

3.83

2.72

(3) Capacity of existing box culvert, (m3/s)

21.4

21.4

(4) Design sea level (mPD)

3.05

3.65

(5) Water level at most upstream of the box culvert (mPD)

3.08

3.67

(6) Ground level at most upstream of the box culvert (mPD)

5.00

5.00

(7) Freeboard (m)

1.92

1.33

 

5.64            It is concluded that the capacity of the box culvert along Expo Drive East is adequate to cater for the additional storm runoff resulting from the proposed ALE and no adverse impact is anticipated.  A properly prepared detailed drainage design during detailed design stage of the project will be sufficient to ensure the stormwater runoff be collected and conveyed to drainage system.

 

Mitigation of Adverse Environmental Impacts

 

Marine Piling and Pile Extraction Works

 

5.65            Installation of temporary marine piles is required for supporting the temporary working platforms and temporary footbridge during the construction period. These piles should be driven into position and internal space should not be excavated, i.e. left as soil.  No dredging or soil/sediment excavation should  be carried out.  Marine piles would be removed by reverse driving.  Based on the hydrodynamic modelling results, it is recommended to adopt the temporary marine pile layout under Option 2 as shown in Figure 2.6.  Option 2 includes a total of 79 nos. of marine cylindrical piles for supporting three temporary working platforms and 1 temporary footbridge.  The nominal diameter of each marine pile is 800mm.

 

5.66            In view of the close vicinity of the seawater intakes to the work site, two layers of silt curtain would be installed around each of the marine piling and pile extraction locations to minimize the potential for water quality impacts due to any unforeseen sediment release during the pile extraction or accidental release of excavated sediment during the marine piling.  The proposed silt curtain should be extended to seabed with sinker blocks and regularly inspected and maintained to ensure that it is serviceable. All marine works should be carried out in a controlled manner such that release of sediments into the marine environment would be minimized.  All wastewater generated from the piling activities should be collected and be treated before controlled discharge. Spoil should also be properly collected for proper disposal.

 

Construction Site Runoff and General Construction Activities

 

5.67            In view of the close vicinity of the seawater intakes to the work site, silt screens are recommended to be deployed at all the seawater intakes as shown in Figure 5.2 during the whole construction period. Silt screens to be provided at seawater intakes should be regularly checked and maintained to ensure that they are serviceable.

 

5.68            Refuse collection vessel should be mobilized on a need basis to collect any floating refuse lost from/ trapped at the work site during the construction period

 

5.69            To minimize the potential water quality impacts from construction site runoff and various construction activities, the practices outlined in ProPECC PN 1/94 Construction Site Drainage should be adopted.  Details of the good site practice and water pollution control measures are given in Appendix 5.12.

 

Construction Works at Storm Culvert or in Close Proximity of Seafront

 

5.70            To minimize the potential water quality impacts from the construction works located at or near the storm system or seafront, the following mitigation measures should be adopted:

 

Ÿ             The use of less or smaller construction plants may be specified to reduce the disturbance to the seabed.

Ÿ             Temporary sewerage system should be designed to prevent wastewater from entering the storm system and sea.

Ÿ             Temporary storage of materials (e.g. equipment, filling materials, chemicals and fuel) and temporary stockpile of construction materials should be located well away from any water courses during carrying out of the construction works.

Ÿ             Stockpiling of construction materials and dusty materials should be covered and located away from any water courses.

Ÿ             Construction debris and spoil should be covered up and/or disposed of as soon as possible to avoid being washed into the nearby water receivers.

Ÿ             Construction activities, which generate large amount of wastewater, should be carried out in a distance away from the waterfront, where practicable.

Ÿ             Mitigation measures to control site runoff from entering the nearby water environment should be implemented to minimize water quality impacts.  Surface channels should be provided along the edge of the waterfront within the work sites to intercept the runoff.

Ÿ             Construction effluent, site run-off and sewage should be properly collected and/or treated.

Ÿ             Proper shoring may need to be erected in order to prevent soil/mud from slipping into the storm culvert/sea.

Ÿ             Supervisory staff should be assigned to station on site to closely supervise and monitor the works.

 

 

 

Barging Activities

 

5.71            All barges should be fitted with tight seals to their bottom opening to prevent leakage of materials.

 

5.72            The decks of all vessels should be kept tidy and free of oil or other substances that might be accidentally or otherwise washed overboard.

 

5.73            Loading of barges should be controlled to prevent splashing of materials to the surrounding environment and barges should under no circumstances be filled to a level which would cause overflowing of material or sediment laden water during loading and transportation.

 

5.74            All barges should maintain adequate clearance between vessels and the seabed at all states of the tide and should operate at a reduced speeds to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash.

 

Handling of Site Drainage and Effluent

 

5.75            Connection of sewage generated from the ALE will be connected to the existing public sewer.  For handling, treatment and disposal of other operational stage effluent, the practices outlined in ProPECC PN 5/93 should be adopted where applicable. The contractor should seek consensus from DSD on the technical details of the drainage and sewerage proposals.

 

Evaluation of Residual Impacts

 

5.76            The construction phase water quality impact would generally be temporary and localized during construction.  No unacceptable residual water quality impacts would be expected, provided that all the recommended mitigation measures are properly implemented.

 

5.77            The sewerage and drainage impact assessment concluded that no adverse impact to the existing public sewerage and drainage system is anticipated due to the proposed ALE.  Therefore, no unacceptable residual water quality impacts would be expected during the operational phase.

 

Environmental Monitoring and Audit

 

5.78            If monitoring of the treated effluent quality from land-based construction sites is required during the construction phase of the Project, the monitoring should be carried out in accordance with the WPCO license which is under the ambit of regional office (RO) of EPD. Marine water quality monitoring should also be carried out at the seawater intakes within the ALE sea channel before the construction period and during the installation and removal of temporary marine piles. A more detailed description of the water quality monitoring requirements is specified in the separate EM&A Manual.

 

Conclusion

 

Construction Phase

 

5.79            Short-term water quality impact could be associated with the proposed construction works. Impacts may result from the surface runoff from construction sites, sewage from on-site construction workers, wastewater from general construction activities and seabed disturbance from marine piling and marine pile extraction.  Impacts could be controlled to comply with relevant standards in the Water Pollution Control Ordinance (WPCO) standards by implementing the recommended mitigation measures.  Installation of temporary marine piles is required for supporting the temporary working platforms and temporary footbridge during the construction period. These piles will be driven into position and internal space will not be excavated, i.e. left as soil.  No soil/sediment excavation would be carried out. Marine piles would be removed by reverse driving. Typically there would be little potential for disturbance to the marine sediments during the pile installation and removal. Double layers of silt curtain are recommended to be installed around the marine piling and marine pile removal works.  Good site practices and water pollution control measures are also recommended to minimize the water quality impacts.  Therefore, unacceptable residual impacts on water quality would be unlikely. 

 

5.80            Hydrodynamics modelling was conducted to evaluate the flushing impact on the ALE sea channel due to the installation of marine piles for supporting the temporary working platform(s) and temporary footbridge in the sea channel between Phase I and Phase II of the HKCEC during the construction period.  The modelling exercise was carried out based on a fully calibrated and verified model to ensure the model performance.  Alternative layouts of temporary working platform(s) were considered in the hydrodynamics modelling.  The pile layout under Option 2 as shown in Figure 2.6 is recommended. Option 2 includes a total of 79 nos. of marine cylindrical piles for supporting three temporary working platforms and 1 temporary footbridge.  The nominal diameter of each marine pile is 800mm.  Hydrodynamic modelling results indicated that the installation of temporary piles in the sea channel would inevitably reduce the flushing capacity of the sea channel. The impact is however considered only short term and localized. The overall influence on the flushing capacity of the whole sea channel during the construction period was predicted to be insignificant (less than 5%) under the recommended option. Given that the marine piles would last for a maximum of three years and would be removed after construction of the ALE, it is anticipated the overall impact on the water quality inside the channel would be insignificant. Refuse collection vessel is recommended to be mobilized on a need basis to collect any floating refuse trapped at the ALE sea channel during the construction period.  The recommended option for temporary marine pile layout should be incorporated into the construction contract for the contractor to follow.

 

Operational Phase

 

5.81            It is proposed that the sewage arising from the proposed ALE to be discharged to the existing 400mm and 450mm diameter sewers along Convention Avenue and eventually to Wan Chai East PTW.   Sewerage impact assessment was conducted to assess the potential impact on the existing public sewerage system due to the additional sewage flow.  The findings of the sewerage impact assessment indicate that the Project would not cause any adverse impact on the existing public sewers.  Moreover, there would be no adverse sewerage impact on the Wan Chai East PTW.

 

5.82            It is proposed to discharge the additional stormwater runoff arising from the proposed ALE to the existing 3,200mm (W) x 3,200mm (H) box culvert along Expo Drive East.  Drainage impact assessment was conducted to assess the potential impact on the existing public drainage system due to the additional stormwater runoff.  The drainage impact assessment concluded that no adverse impact on the existing public drainage system is anticipated as a result of the additional stormwater runoff.

 


 



([1])      Developed under Agreement No. CE42/97 Update on Cumulative Water Quality and Hydrological Effect of Coastal Developments and Upgrading of Assessment Tool.

([2])  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.