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
|
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 |
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 (
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
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
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
Table 5.3 Baseline
Water Quality Condition for
|
Parameter |
|
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.
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
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.
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
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
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
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
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
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
5.27
5.28
The pile arrangement for
5.29
The Seafront Promenade is supported by 31 marine
piles. The diameter of each pile is
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
|
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
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
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
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
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
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,
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
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
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 |
5.55
It is
proposed that the sewage arising from the proposed ALE to be discharged to the
existing
5.56
With the
proposed ALE, the total peak flows arriving at the Wan Chai East PTW are
projected to be
Table 5.9 Peak flows to Wan Chai East PTW
|
|
|
Peaking
factor for PTW refer to Table T-5 of GEFS
Operational Phase
Drainage Impact
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
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
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
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 |
5.62
The
proposed ALE lies within the Study Area of “Stormwater Drainage Master Plan
Study in
5.63
It is
proposed to discharge the additional stormwater runoff to the existing
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)
|
|
|
|
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
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
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.
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
Operational Phase
5.81
It is
proposed that the sewage arising from the proposed ALE to be discharged to the
existing
5.82
It is
proposed to discharge the additional stormwater runoff arising from the
proposed ALE to the existing