6A1
introduction
6A1.1
Background
Castle Peak Power Company Limited (CAPCO) proposes to
construct and operate two submarine gas pipelines connecting Black Point Power
Station (BPPS) with gas export facilities in southern Guangdong Province
(Figure 6A.1). At the BPPS two Gas Receiving Stations (GRSs) will also be constructed and operated to receive the
natural gas (Figure 6A.2). One new GRS will be located within the
site boundary of the BPPS, co-located with the existing GRS. The other new GRS will be situated on a
newly reclaimed land (approximately 0.5 ha land area) to be constructed along
the artificial seawall of the BPPS.
The site for the new reclamation will be the same as that proposed for
the GRS in the South Soko option for the HKLNG EIA ().
Figure 6A.1 Indicative
Alignment of the Cross-boundary Submarine Gas Pipelines Connecting the BPPS and
the New Gas Export Facilities in Mainland China
Figure 6A.2 Locations
of Proposed Gas Receiving Stations
6A1.2
Purpose of the
Method Statement
This Method
Statement presents information on the approach for the water quality assessment
and modelling works for the Study. It is important to note that at the time of completion of
this Method Statement the detailed
engineering information for both construction and operation activities is yet
to be confirmed and therefore a general approach as to how the modelling works
would be carried out is presented herein, with relevant assumptions provided as
appropriate.
The methodology has been based on the following three
focus areas, as follows:
·
Model
Selection;
·
Input
Data; and
·
Scenarios.
6A1.3
Interpretation of
the Requirements: Key Issues and Constraints
The objectives of the modelling exercise
are to assess:
·
Effects of construction, which comprises the study of the
dispersion of sediments released during construction, including the
installation of two submarine gas pipelines and construction of the reclamation
for one of the new gas receiving stations at BPPS;
·
Effects of operation due to reclamation, including potential
effects on flows and subsequent water quality effects due to changing flows,
and potential sedimentation/ siltation around the new reclamation;
·
Any residual impacts, which include any change in hydrodynamic
regime (e.g. flushing capacity);
·
Any cumulative impacts due to other projects or activities within
the Study Area.
The construction and operational effects have been
studied by means of mathematical modelling using the Delft 3D package which is a world leading
2D/3D modelling system to investigate hydrodynamics, sediment transport and
morphology and water quality for fluvial, estuarine and coastal environments.
6A1.4
Model Selection
A new model has been developed using the model setup
details of the 3-dimentional model that was adopted in the HKLNG EIA ().
The Delft
3D hydrodynamic (Delft3D-FLOW) and water quality (Delft3D-WAQ) suite of models
have been used to simulate potential impacts on hydrodynamics and water
quality, respectively, during construction and operation of the Project.
The original Update Model has the
required spatial extent for the proposed project facilities, ie waters 2.5 km beyond the HKSAR
Boundary (up to the west boundary of Tonggu Fairway) (Figure 6A.3).
Figure 6A.3 Model
Grid of the Original Update Model in the Vicinity of Black Point ()
6A1.5
Grid Refinement
& Subsequent Approval
As seen in Figure 6A.3, the grid size of the original model near the Black
Point Power Station (BPPS) is in the order of about 300 m. During the HKLNG EIA (), local refinement of the water
quality and hydrodynamic grids was carried out to provide improved resolution
(less than 75 m) in some of the key areas of interest (Figure 6A.4). A further
refinement of the model grids with a target resolution of 50 m ´ 50 m has been implemented by
means of the domain decomposition technique in this Study (Figure 6A.5). The
FLOW model grid has subsequently been adopted without further aggregation in
the water quality models.
In order to provide locally increased
grid resolution, a four-grid Domain Decomposition (DD) system has been setup
with the highest resolution (approximately 15 m ´ 30 m) in the immediate
vicinity of the BPPS intake and outfall structures. The existing model grid in the vicinity
of Deep Bay has been adjusted and higher resolution
DD domains have been included as follows:
·
WHM - an overall domain which is almost same as the original
model;
·
TFW - an intermediate domain provides enhanced resolution in a
wider area around the Project site and the pipeline alignments;
·
SHZ - Deep
Bay domain; and
·
BPP – local BPPS domain with the highest resolution.
Figure 6A.4 The
Existing Model Grid in the Vicinity of Black Point for HKLNG EIA ()
Figure
6A.5 Refined
Model Grid in the Vicinity of Black Point for This Project
The model used in this study is based on the Update
Model. Parts of the model were
refined to compute flow behaviour with a high resolution in the vicinity of the
Black Point Power Station. To show
that the refined model is consistent with the Update Model, computed water
levels, depth-averaged current speed and directions, and depth-averaged salinities
are compared at two locations in the model (see
Box 1).
Comparisons are made between the two models for the
wet and dry seasons. The results
for the dry season are shown in Boxes 2 and 3. For the wet season the results are shown
in Boxes 4
and 5.
Given that the models show similar results, the
models were considered as consistent.
Differences between the model solutions are caused by:
1)
the
higher resolution solution in the finer domains, causing more accurately
represented seabed variations and coastlines;
2)
the initial conditions in the runs were not exactly identical, because in
the project the single domain run was used to spin-up the salinity and
temperature fields for the refined model run.
Box 1 Model Validation Locations
6A1.6
Coastline &
Bathymetry
Hydrodynamic data have been obtained using coastline
and bathymetry for a time horizon representative of the construction of the
facility (ie 2011 onwards). Bathymetry and coastline information
have been updated in this exercise ()
to reflect the depth of Tonggu Fairway and recent changes in coastline (Figure 6A.6).
Figure 6A.6 Updated
Bathymetry and Coastline Used in the Present Model
6A1.7
Boundary
Conditions
The boundary conditions, adopted in the HKLNG EIA (), have been applied at the outer
boundaries of the WHM domain. The
background surface layer temperature is around 23 ºC in the dry season
simulations and about 28 ºC in the wet season simulations.
6A1.8
Vector
Information
The current patterns under the baseline situation
were generated as an output of the baseline hydrodynamic modelling. Another scenario was used to simulate
the tidal flow in the Project area in the presence of the reclamation (i.e.
post-project situation). The
hydrodynamic modelling results are presented in Annex 6B as vectors
showing the current direction and magnitude.
6A1.9
Information on
Model Inputs
6A1.9.1
Details of Hydrodynamic Simulations
All hydrodynamic scenarios were simulated for a
spring-neap-cycle during the dry season and a spring-neap-cycle during the wet
seasons. The
simulation periods were:
·
Dry season: simulation period from 2 February 12:00h to 22
February 12:00h, simulation period 20 days, time step 12 seconds.
·
Wet season: simulation period from 19 July 04:00h to 10 August
04:00h, simulation period 20 days, time step 12 seconds.
Adequate spin-up was provided for
salinity and temperature by using initial values derived from the baseline
simulations for the HKLNG EIA (), after interpolation on a
single domain version of the WHM. A
22 day single domain run was conducted with this model to ensure that salinity
and temperature fields were sufficiently spun up. Suitable salinity and temperature fields
were selected from the single domain runs and interpolated on the 4 domain
model. These fields were used as
initial conditions for the production runs covering a period of 22 days of
which the first 7 days were meant to spin up water levels and currents whilst
the last 15 days as production.
As explained under Section 6A.1.5, the single domain runs were used as spin-up for the
refined model runs. As the results
of the single domain runs are consistent with those of the detailed model, the
spin-up period can be considered sufficient (see Boxes 2 to 5).
The hydrodynamic model has included the
fresh water inflows from four Pear river outlets as well as from Shenzhen River in Deep Water. The salinity of the river outflows was
assumed to be 0.1 ‰ and the temperatures in the dry and wet seasons were attributed
to be 23 ºC and 30 ºC, respectively.
Table 6A.1 Typical
Seasonal River Discharge Rates
|
River
|
Dry Season (m3 s-1)
|
Wet Season (m3 s-1)
|
|
Humen
|
1248
|
7442
|
|
Jiaomen
|
527
|
4732
|
|
Hongqili
|
128
|
1535
|
|
Hengmen
|
136
|
2805
|
|
Deep Bay
(Shenzhen River)
|
2.5
|
16
|
The newly created 4 domain model was found to provide
consistent results in comparison with the unrefined WHM and the locally refined
model developed as part of the HKLNG EIA ().
6A1.9.2
Details of Sediment Dispersion / Water
Quality Modelling
The sediment dispersion and water quality
simulations were driven by the results from the hydrodynamic simulations
described in Section 6A1.9.2. In particular, the velocity components
in the three spatial directions were directly derived from the hydrodynamic
model. In addition, the vertical
dispersion coefficient was calculated from the turbulent eddy diffusivity
generated by the hydrodynamic model.
Given this quantity can approach zero, an additional value of 10-6
m2 s-1 was added to represent molecular diffusion and
turbulent diffusion that could not be resolved by the hydrodynamic model.
The horizontal dispersion coefficients in
two directions were set to 1 m2 s-1. This small value was considered to be
suitable for use in 3D modelling since all vertical and lateral velocity
gradients were already explicitly represented by the velocity components in the
3 spatial directions.
The time step for the water quality simulations was
set to 30 minutes whereas the time step for the sediment dispersion simulations
had been narrowed down to 2 minutes.
This could not only improve the model accuracy, but also provide
sufficient temporal resolution in order to accurately represent the various
moving sediment sources in the model.
Both sets of simulations used a fully implicit integration method which
provided unconditional stability.
In view of the relatively small temporal and spatial gradients, the
selected time step in combination with the locally refined grid ascertained
sufficient accuracy.
The sediment dispersion and water quality simulations
were modelled for typical spring-neap cycles in the dry and wet seasons. The time periods representing these
seasons were derived from the corresponding hydrodynamic model simulations,
where the spin-up periods of 8 days were skipped:
·
Dry season: simulation period from 10 February 12:00h to 22
February 12:00h.
·
Wet season: simulation period from 27 July 04:00h to 10 August
04:00h.
For the water quality simulations, the
relevant pollution loadings to the model area were derived from the 2012 time
horizon pollution loadings (baseline scenario). To obtain relevant boundary conditions,
the present model was “nested” in the 2012 time horizon (baseline
scenario). This implies that time
and space dependent concentrations of all state variables along the model
boundaries have been extracted from the output of the Update model, for
equivalent simulation periods. This
ascertains that the impact of pollution loadings outside the current model
domain can be correctly transferred to the current model via the boundary
conditions.
The water quality simulations have been run for 3
consecutive spring neap periods of 14 days where the last cycle is used to
produce output. It has been
verified that the simulation results for the third cycle are independent of the
initial conditions and constitute cyclical concentration patterns
representative for the dry and wet seasons.
6A1.10
Uncertainties In
Assessment Methodologies
Uncertainties in the assessment of the impacts from
suspended sediment plumes should be considered when drawing conclusions from
the assessment. In carrying out the
assessment, the worst case assumptions have been made in order to provide a
conservative assessment of environmental impacts. These assumptions are as follows:
·
The
assessment is based on the peak dredging and filling rates. In reality, these will only occur for
short period of time;
·
The
calculations of loss rates of sediment to suspension are based on conservative
estimates for the types of plant and methods of working; and,
·
The
assumptions of the dredger forward speed are made only for the modelling
purpose but the actual dredging rates may not be the same and will be subject to
the weather constraints, site conditions and continued operational
progress. In reality, the dredger
moving speed should be calculated from the result of dividing the total volume
of dredged materials (m3) by the duration of the dredging works
(day).
The
conservative assumptions presented above allow a prudent approach to be applied
to the water quality assessment.
The
following uncertainties have not been included in the modelling assessment:
·
Ad
hoc navigation of marine traffic;
·
Propeller scour of seabed sediments from
vessels;
·
Near shore scouring of bottom sediment; and
·
Access of marine barges back and forth the
site.
6A1.11
Assessment
Scenarios
The water quality modelling exercise was conducted
with regard to two main components, construction phase and operation phase as
detailed below.
·
Construction Phase: the assessment examined potential water
quality impacts arising from the following activities:
·
Dredging/
jetting for the installation of the two submarine pipelines (32” – 42”; about
813 mm – 1067 mm) in separate trenches (assuming the two pipelines are 100 m
apart). Sediment dispersion
simulations covered pipeline installation works within HKSAR waters and for
works within a set distance (ie about 2.5 km) from
the HKSAR boundary.
·
Dredging
and reclamation works for the construction of the GRS: at present it is
expected that the GRS for Pipeline 2 will be placed on a reclaimed land that
will need to be dredged to remove soft marine deposits. Two simulations for one dredged scenario
for reclamation were conducted for both the wet and dry seasons.
This Project is expected to adopt a Phased
Construction approach. First Phase
will include the installation of Pipeline 1 and Second Phase will consist of
the installation of Pipeline 2 and the construction of the GRS at the
reclamation site. At this stage the
construction of the two phases are planned to be separate. First Phase construction will commence
in 2011 to allow for First gas to arrive by 2012, while Second Phase
construction is expected to commence within 24 months following commissioning
of the first pipeline and GRS.
To assess potential project-specific impacts of this
phased construction, computational modelling was conducted separately for the First
Phase and Second Phase construction.
·
Operation Phase: the assessment examined potential water
quality and hydrodynamic impacts arising primarily from the presence of new
reclamation for the construction of the GRS. It is anticipated that the positioning
of the reclamation would have the potential to interfere with current patterns
and flow regime in the area, which could potentially cause impacts as follows:
o
Changes
to the flushing of Deep
Bay potentially leading
to a deterioration in water quality;
o
Changes
in the patterns of sedimentation around the BPPS area; and
o
Changes
in the dispersion and dilution of the cooling water effluent discharged from
BPPS.
Water quality and hydrodynamic modelling were
conducted to examine these potential changes during the operation phase.
A detailed description of the modelling scenarios for
construction phase and operation phase is presented in Sections 6A3 and 6A4,
respectively.
The water quality sensitive receivers (WSRs) have been identified in accordance with Annex 14 of the Technical Memorandum on EIA Process (EIAO, Cap.499, S.16). These WSRs are
illustrated in Figure 6A.7 and listed in Table 6A.2. The representative WSRs
are included as discrete model output points as shown in Figure 6A.8.
Table 6A.2 Water
Quality Sensitive Receivers (WSRs) in the Vicinity of
the Project Site
|
Sensitive
Receiver
|
Name
|
Water Quality
Modelling Output Location
|
|
Fisheries and Marine Ecological Sensitive Receivers
|
|
Fisheries
Sensitive Receivers
|
|
Oyster
Production Area
|
Sheung Pak Nai
|
SR2
|
|
Recognised
Spawning/
Nursery Grounds
|
Fisheries
Spawning Ground in North Lantau
|
SR8
|
|
Artificial
Reef Deployment Area
|
Sha Chau and Lung Kwu Chau
|
SR6e
|
|
Marine
Ecological Sensitive Receivers
|
|
Mangroves
|
Sheung Pak Nai
|
SR2
|
|
|
Ngau Hom Shek
|
SR2a
|
|
Marine
Parks
|
Designated Sha Chau and Lung Kwu Chau
|
SR6a,c
|
|
Intertidal
Mudflats
|
Ha Pak Nai
|
SR1
|
|
Seagrass Beds
|
Sheung Pak Nai
|
SR2
|
|
|
Ha Pak Nai
|
SR1
|
|
Horseshoe
Crab Nursery Grounds
|
Ha Pak Nai
|
SR1
|
|
Between Ngau Hom Shek
and Pak Nai
|
SR2a
|
|
Water
Quality Sensitive Receivers
|
|
Non-gazetted
Beaches
|
Lung Kwu Sheung
Tan
|
SR5a
|
|
Lung Kwu Tan
|
SR5b
|
|
Secondary Recreation
Subzone
|
NW WCZ
|
SR5b
|
|
Seawater
Intakes
|
Black Point
Power Station
|
SR4
|
|
|
Castle Peak Power Station
|
SR7a
|
|
|
Tuen Mun Area 38
|
SR7b
|
|
|
Shiu Wing
Steel Mill
|
SR7i
|
For the construction phase the WAQ model has been
used to directly simulate the
following parameters:
·
suspended
sediments (SS); and
·
sediment deposition.
It is assumed that the worst-case construction phase
impacts will be at the commencement of dredging, when there is no depression
formed to trap sediments disturbed during dredging works.
Note that Dissolved Oxygen (DO) depletion, Total
Inorganic Nitrogen (TIN) and Unionised Ammonia-Nitrogen (NH3-N) has
been calculated using the modelled maximum SS concentrations. This method has been adopted in a
recently approved EIA ().
6A3.1
Working Time
The
works programme for dredging and reclamation activities for the proposed GRS at
BPPS is based on the assumption of a 16 working hours per day with 7 working
days per week. An arrangement of 24
working hours and 7 working days is unlikely to be feasible for Black Point due
to the potential noise impact generated by barges travelling at night to the
villages located in close proximity to the route of Black Point.
As
for the works programme for submarine gas pipeline installation, it is based on
the assumption of a 12 working hours (daylight hours) per day with 7 working
days per week for grab dredging / jetting for pipeline sections outside of the
Urmston Road. An assumption of a 24
working hours per day with 7 working days per week has been adopted for grab
dredging for pipeline sections along the Urmston Road.
The
assumption of working time in the model is summarised in Table 6A.3.
Table 6A.3 A summary of working time assumed in the model for various
construction activities
|
Construction Activities
|
Locations
|
Assumption of Working Time
|
|
Dredging
and Reclamation of Gas Receiving Station
|
BPPS,
Black Point
|
16
hours per day and 7 days per week
|
|
Dredging
and Installation of Submarine Gas Pipelines in Hong Kong
waters
|
From
Black Point (BPPS) to HKSAR Boundary
|
For
grab dredging and jetting, 12 hours per day (daytime) and 7 days per week for
pipeline sections outside of the Urmston
Road
|
|
|
|
For
grab dredging, 24 hours per day and 7 days per week for pipeline sections along
the Urmston Road
|
|
Dredging
and Installation of Submarine Gas Pipelines in PRC waters
|
2.5
km beyond the HKSAR Boundary, up to the western boundary of Tonggu Fairway
|
For
both grab dredging and jetting, 24 hours per day and 7 days per week
|
6A3.2
Overview Of Dredging Plants
6A3.2.1
Grab Dredgers
The
submarine pipelines may need to be pre-trenched and this is likely to be done
utilising a grab dredger.
Grab
dredgers may release sediment into suspension by the following mechanisms:
·
Impact
of the grab on the seabed as it is lowered;
·
Washing
of sediment off the outside of the grab as it is raised through the water
column and when it is lowered again after being emptied;
·
Leakage
of water from the grab as it is hauled above the water surface;
·
Spillage
of sediment from over-full grabs;
·
Loss
from grabs which cannot be fully closed due to the presence of debris;
·
Release
by splashing when loading barges by careless, inaccurate methods; and
·
Disturbance
of the seabed as the closed grab is removed.
In
the transport of dredging materials, sediment may be lost through leakage from
barges. However, dredging permits
in Hong Kong include requirements that barges
used for the transport of dredging materials have bottom-doors that are
properly maintained and have tight-fitting seals in order to prevent
leakage. Given this requirement,
sediment release during transport is not proposed for modelling and its impact
on water quality will not be addressed under this Study.
Sediment
is also lost to the water column when discharging material at disposal
sites. The amount that is lost
depends on a large number of factors including material characteristics, the
speed and manner in which it is discharged from the vessel, and the
characteristics of the disposal sites.
It is considered that potential water quality issues associated with
disposal at the intended government disposal site(s) have already been assessed
by CEDD and permitted by EPD, hence and the environmental acceptability of such
disposal operations is proven.
Therefore modelling of impacts at disposal sites does not need to be
addressed and reference to relevant studies will be provided in the EIA for
this Project where appropriate. It
should be noted that the capacity of the existing mud disposal facilities, the
East of Sha Chau Mud Pits,
is very limited and may not be available at the time of the disposal for this
Project. As such, the Project
Proponent has, in discussion with EPD, obtained from
the MFC provisional capacities at the appropriate marine disposal site(s) for
confined marine sediment disposal (see Section
7 Waste Management Assessment).
Loss rates have been taken from previously
accepted EIAs in Hong Kong () () () and have been based on a review of
worldwide data on loss rates from dredging operations undertaken as part of
assessing the impacts of dredging areas of Kellett
Bank for mooring buoys ([14]).
The assessment concluded that for 8 m3 (minimum) grab
dredgers working in areas with significant amounts of debris on the seabed
(such as in the vicinity of existing mooring buoys) that the loss rates would
be 25 kg m-3 dredged, while the loss rate in areas where debris is
less likely to hinder operations would be 17 kg m-3 dredged. The results of the recent geophysical
surveys suggested that there are no significant quantities of debris in the
vicinity of the dredging areas (see Figure 11.6, Section
11 Cultural Heritage Assessment).
The value of 17 kg m-3, for dredgers with grab size of 12/16 m3,
was therefore used for this Study.
Generally, a split-bottom barge could have
a capacity of 900 m³. A bulk factor
of 1.3 would normally be applied, giving a dredging rate of 700 m³ per
barge. The hopper dry density for
an 800 to 1,000 m3 capacity barge is around 0.75 to 1.24 ton m-3. Assuming 12 to 24 working hours
per day for the proposed construction activities, with allowance on the
demobilisation of filled barge and remobilisation of empty barges,
approximately 5 to 10 barges could be filled per day, giving a daily dredging
rate of approximately 3,500 m3 to 7,000 m3.
The
average release rates will, in fact, be somehow less than those indicated
above. The instantaneous dredging
(and loss) rates will also decrease as the depth increases. This is because the assumed dredging
production rates are instantaneous rates that will not be maintained due to
delays for breakdowns, maintenance, crew changes and time spent relocating the
dredgers. The release rates that
are to be modelled area, therefore, considered to represent conservative
conditions that will not prevail for any great length of time.
A
review of the vector plots at the sites allowed identification of areas that
would disperse sediment further than other areas due to higher current velocities. These areas were consequently being
chosen as the locations of the sources of sediment in the model.
6A3.2.2
Jetting
Jetting would be deployed for the
installation of submarine gas pipelines in HKSAR or PRC waters. The speed of the machine is taken as 360
m day-1 for gas pipeline installation.
The
maximum burial depth is assumed to be 5 m typically. It is envisaged that it will require
three passes of the jetting machine to reach the required burial depth. The volumes of sediment disturbed will
vary depending upon whether it is the first, second or third pass. The consecutive passes may uplift the
bottom sediments in a short period of time. However, this will be temporary and
instantaneous disturbances to the seabed since the disturbed sediments are expected
to settle on the seabed in a short period after the jetting machine has passed.
The
volumes of seabed sediments to be fluidised are assumed to be:
·
First Pass = 5 m3 m-1;
·
Second Pass = 10 m3 m-1; and
·
Third Pass = 15 m3 m-1.
Assuming
a conservative case for the third pass (15 m3 per m), the rate of
disturbance will be:
Rate of Disturbance (m3 s-1)
– 24-hour Working Time in PRC Waters
= 360 m day-1 * (1/86400 s day-1)
* 15 m3 m-1
= 0.0625 m3 s-1
when the gas pipeline trench is fully
fluidised.
Rate of Disturbance (m3 s-1)
– 12-hour Working Time in HKSAR Waters
= 360 m day-1 * (1/43200 s day-1)
* 15 m3 m-1
= 0.125 m3 s-1
when the gas pipeline trench is fully
fluidised.
The
disturbed sediments will constitute a layer of fluid mud flowing across the
seabed either side of the jetting machine and only a small portion of this
sediment will enter the water column.
It
is conservatively assumed that 20% of the disturbed sediments enter suspension
and this would give a loss rate.
The loss rate used here has been used in previous projects for submarine
utility installations under the EIAO
that have been installed using jetting methods and have obtained Environmental
Permits:
·
The
Proposed Submarine Gas Pipelines from Cheng Tou Jiao
Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production
Plant, Hong Kong – EIA Study (AEIAR-071/2003).
EP granted on 23 April 2003 (EP-167/2003).
·
132kV
Submarine Cable Installation for Wong Chuk Hang -
Chung Hom Kok 132kV
Circuits
(AEP-126/2002). EP granted on 2
April 2002 (EP-126/2002).
·
FLAG
North Asian Loop
(AEP-099/2001). EP granted on 18
June 2001 (EP-099/2001).
·
East
Asian Crossing (EAC) Cable System (TKO), Asia
Global Crossing (AEP-081/2000). EP granted on 4 October 2000
(EP-081/2000).
·
East
Asian Crossing (EAC) Cable System, Asia Global
Crossing
(AEP-079/2000). EP granted on 6
September 2000 (EP-079/2000).
·
Submarine
Cable Landing Installation in Tong Fuk Lantau for Asia Pacific
Cable Network 2 (APCN 2) Fibre Optic Submarine Cable System, EGS. EP granted on 26 July 2000 (EP-069/2000).
·
Telecommunication
Installation at Lot 591SA in DD 328, Tong Fuk, South Lantau Coast and the Associated Cable Landing Work in Tong Fuk, South Lantau for the North
Asia Cable (NAC) Fibre Optic Submarine Cable System
(AEP-064/2000). EP granted in June
2000 (EP-064/2000).
To calculate the mass entrainment rate it
is necessary to apply a dry density for the material, which is conservatively
assumed to be 700 kg m-3 ([15]).
The sediments are entered into the model in the model layer closest to
the seabed because this represents the entrainment of sediments to suspension
from the layer of fluid mud flowing over the existing seabed. This approach is considered valid as the
jetting machine is fluidising the seabed sediments and not excavating the
sediments, consequently there will be little vertical entrainment of sediments
into the water column.
The
sediments are entered into the model within a series of grid cells to represent
the jetting machine moving along the pipeline route. Thus each grid cell represents a section
of the pipeline route and sediments are entered into that grid cell for the
length of time it takes the jetting machine to pass the length of that cell,
based on the jetting machine speed given above. Once the jetting machine has passed that
grid cell, sediments will then be entered in the next grid cell on the
route. The sediment release in the
bed layer (constitute 10%) of the water column is assumed in the model. These rates have been adopted in the
approved EIA ().
6A3.3
Construction
Scenarios
6A3.3.1
First Phase Construction: Scenario 1
Grab
Dredging for Sections of Submarine Gas Pipeline 1
Grab
dredger(s) will be deployed during the dredging for specific sections of
submarine Pipeline 1 in the HKSAR and PRC waters (Figure 6A.9). This includes grab dredging works
for pipeline sections 1 (Black Point Shore Approach) and 3 (across Urmston Road) in
HKSAR waters and about 2.5 km of pipeline (from the HKSAR boundary) in PRC
waters.
Following
installation, the pipelines will be protected by rock armour. Typical cross sections of the trench
designs that may be used for these sections of the submarine pipeline are shown
in Figure 6A.10.
Details of the pipeline protection designs are presented in Section 3 Project Description. Water quality impacts are not expected
from the implementation of the pipeline protection measures as the fines
content of the armour rock material is low.
Grab
Dredging
The
estimated dredged volume, the number of plant and the distance apart between
each plant at the associated pipeline section is shown in Table 6A.5.
The dredging operations for the pipeline
installation will be carried out by closed grab dredgers. For the Black Point Shore Approach
pipeline section, working hours are assumed to be 12 hours per day with a maximum
dredging rate of 4,000 m3 day-1 (ie
0.093 m3 s-1) per dredger, giving a rate of release (in
kg s-1) for the dredger as follows:
Loss
Rate (kg s-1)
=
Dredging Rate (m3 s-1) * Loss Rate (kg m-3)
=
0.093 m3 s-1 * 17 kg m-3
=
1.57 kg s-1
The maximum daily rate of production for
the Urmston Road Crossing is assumed to be 8,000 m3 day-1. Given that the working hours at that
section will be 24 hours per day, the loss rate (in kg s-1) for each
dredger will also be 1.57 kg s-1. These assumptions were also applied to
the PRC pipeline sections.
6A3.3.2
First Phase Construction: Scenario 2
Jetting
for Sections of Submarine Gas Pipeline 1
Scenario
2 assesses the impacts of sequential jetting works for installing suitable
sections of Pipeline 1 in HKSAR and PRC waters. This includes sections 2 (from Black
Point to Urmston Road)
and 4 (from Urmston Road
to HKSAR Boundary) in HKSAR waters and about 2.5 km of pipeline (from the HKSAR
boundary) in PRC waters. It is
expected that the jetting works in both HKSAR and PRC waters will take place
after the completion of Pipeline 1 grab dredging activities, i.e. completion of
works simulated under Scenario 1.
It
is expected that only one jetting machine would be used for post-trenching of
both the HKSAR and PRC pipeline sections.
Therefore under this assumption the jetting operations in different
pipeline sections in HKSAR and PRC waters will not be concurrent,
hence three separate scenarios (Scenarios 2a, 2b and 2c) were simulated.
Working
hours have been assumed to be 12 hr and 24 hr per day for works in HKSAR and
PRC waters respectively. The moving
speed of the jetting machine was assumed to be 360 m day-1. The dry density of sediment was taken to
be 700 kg m3.
6A3.3.3
Second Phase Construction: Scenario 3
Scenario
3 allows the assessment of impacts through concurrent activities in HKSAR and
PRC waters, including:
·
Reclamation
works, i.e. dredging underneath the seawall and backfilling works for the
construction of the reclamation; and
·
Pre-trenching
for the installation of specific sections of Pipeline 2, including dredging
works for pipeline sections 1 (Black Point Shore Approach) and 3 (across Urmston Road) in
HKSAR waters and about 2.5 km of pipeline (from the HKSAR boundary) in PRC
waters.
Construction
Works for Gas Receiving Station
Scenario
3 simulated the dredging and sandfilling works for
the reclamation site (Figure 6A.11).
All dredging works will be undertaken by grab dredgers while sandfilling works will be conducted by a pelican
barge. In situ dredging volume is estimated to be 0.120 Mm3.
Grab
Dredging
Dredging
works will be carried out at the northern side of the BPPS site. Working hours are assumed to be 16 hours
per day and 7 days per week at Black Point.
Assuming
the deployment of two dredgers with a maximum total dredging rate of 8,000 m3
day-1, giving a dredging rate of 0.139 m3 s-1. The rate of release (in kg s-1)
for the dredger will be:
Loss
Rate (kg s-1)
=
Dredging Rate (m3 s-1) * Loss Rate (kg m-3)
=
0.139 m3 s-1 * 17 kg m-3
=
2.36 kg s-1
Therefore a continuous release rate of 2.36 kg s-1 for each dredger
will be adopted in the model throughout the whole water column.
Two
stationary sources, BP01 and BP02, have been assumed in the model to represent
the grab dredgers.
Sandfilling
It is worth noting that backfilling of the
reclamation site has been assumed to be undertaken before the construction of a
seawall in the model simulations and there would be no sandfilling
works for the vertical seawall. Backfilling
works for the reclamation by a pelican barge have been stimulated by assuming a
filling rate of 50,000 m3 day-1 (ie 0.868 m3 s-1) with working
hours to be 16 per day. The fill
material will be marine sand which generally has a fine content ranging from 2%
to 10%. As the source of material
is not available at this stage, the upper bound of the fine content (ie 10%) with a loss rate of 10% will be assumed for the
conservative case.
With a representative dry density of the
sand fill taken as 1,938 kg m-3, the loss rate in kg s-1
(continuous release in the whole water column) is calculated as:
Loss
Rate (kg s-1)
=
Percentage Fine Content * Percentage Loss Rate * Filling Rate (m3 s-1)
* Dry Density of Sand Fill (kg m-3)
=
10% * 10% * 0.868 m3 s-1 * 1938 kg m-3
=16.8 kg s-1
Since
the reclamation area is small and close to the shore, a stationary emission
point, BP03, was chosen at a location that represents a worst-case scenario
(i.e. closest proximity to the nearest sensitive receiver etc).
Grab
Dredging for Sections of Submarine Gas Pipeline 2
This
works sequence is essentially the same as that used under Scenario 1, except
that this sequence deals with the dredging for specific sections of submarine
Pipeline 2 in the HKSAR and PRC waters (Figure 6A.9). This includes grab dredging works
for pipeline sections 1 (Black Point Shore Approach) and 3 (across Urmston Road) in
HKSAR waters and about 2.5 km of pipeline (from the HKSAR boundary) in PRC waters.
The
works assumptions for Scenario 1 regarding grab dredging are also adopted for
this Scenario.
6A3.3.4
Second Phase Construction: Scenario 4
Jetting
for Sections of Submarine Gas Pipeline 2
Scenario
4 is the same as Scenario 2 except that the sequential pipeline jetting works
are for the installation of Pipeline 2.
Impacts of post-trenching along sections 2 (from Black Point to Urmston Road) and 4
(from Urmston Road
to HKSAR Boundary) in HKSAR waters and about 2.5 km of pipeline (from the HKSAR
boundary) in PRC waters, were assessed. The works assumptions adopted in
Scenario 2 are also adopted in this Scenario.
6A3.4
Construction
Programme and Sequence
Tentative
construction programme and indicative construction sequence are shown in Figure 6A.12.
Note that jetting works in HKSAR and PRC waters will take place after
the completion of all dredging activities in HKSAR Waters.
6A3.5
Sediment Input
Parameters
For
simulating sediment impacts the following general parameters have been used:
·
Settling
velocity – 0.5 mm s-1
·
Critical
shear stress for deposition – 0.2 N m-2
·
Critical
shear stress for erosion – 0.3 N m-2
·
Minimum
depth where deposition allowed – 0.1 m
·
Resuspension rate – 30 g m-2 d-1
·
Wave
calculation method – Tamminga
·
Chezy calculation method – White/Colebrook
·
Bottom
roughess – 0.001 m ()
·
Fetch
for wave driven erosion – 35 km
·
Depth
gradient effect on waves - absent
The
above parameters have been used to simulate the impacts from sediment plumes in
Hong Kong associated with uncontaminated mud
disposal into the Brothers MBA () and dredging for the Permanent Aviation
Fuel Facility at Sha Chau () .
The critical shear stress values for erosion and deposition were
determined by laboratory testing of a large sample of marine mud from Hong Kong as part of the original Water Quality and
Hydraulic Mathematical Model (WAHMO) studies associated with the new airport at
Chek Lap Kok.
For the study of operational effects, the approach
requires several steps:
1) Adapting
the hydrodynamic model for the new conditions such as the reclamations.
3) Running
the hydrodynamic model for the specified conditions (wet/dry season) covering a
spring-neap cycle.
4) Running
the water quality model (ie Delft3D-WAQ). The objectives are:
a) to assess the changes in the dispersion and dilution of the
cooling water effluent discharged from BPPS;
b) to assess the flushing of Deep
Bay through a tracer release in Shenzhen River; and
c) to
assess the potential changes in water quality as a result of changes in the
circulation near the project sites: to this end up to 5 conservative, ie non-decayable, tracers have
been defined, which will be discharged from a number of locations representing
main pollution sources (e.g. Hong Kong as a whole, major point sources in the
vicinity of the candidate sites).
In addition, an assessment of the potential changes
in the patterns of sedimentation along the new reclamation has been
conducted. A similar approach has
been adopted in the previously approved HKLNG EIA ().
It is proposed to predict potential variation in patterns of
sedimentation around the BPPS area relative to the existing conditions, based
on an analysis of modelled shear stresses and existing data on wave conditions
and sediment concentration. The
sedimentation assessment consists of the following three phases:
1) Examine
the siltation processes by reviewing the existing sediment concentration data,
wave heights, bed sediment and bathymetric changes, if any;
2) Compile
shear stresses maps based on the hydrodynamic results; and
3) Compare
the shear stress distribution before the commencement and after the completion
of the construction works to determine whether more sedimentation and/or
erosion will be anticipated. The
magnitude of sedimentation (if any) can be estimated by simple calculations
incorporating typical critical shear stresses, sediment settling velocities and
sediment concentrations.
It is expected that the construction works of this
Project will commence in 2011 to allow for First Gas to arrive by 2012. According to publicly available sources,
the following major developments in northwestern or
western Hong Kong waters may be constructed
and/or operated concurrently with the construction works for the proposed
facilities at Black Point.
·
Hong Kong Link Road (HKLR) of the Hong Kong
– Zhuhai – Macao Bridge (HZMB), which is about 15 km
south of the pipeline corridor;
·
Hong Kong Boundary Crossing Facilities
(HKBCF) of the HZMB, which is about 12 km south of the pipeline corridor;
·
Tuen
Mun – Chek Lap Kok Link (TMCLKL), which is about 10 km from the pipeline
corridor; and
·
Contaminated Mud Pits (CMPs)
at East Sha Chau and South
Brothers, which are at least 10 km from the pipeline corridor.
A qualitative assessment of potential cumulative
impacts has been conducted and the results are discussed in the Section 6 – Water Quality Impact Assessment.
Construction phase scenarios are established in
accordance with the tentative construction programme and presented in Table 6A.6. Table
6A.7 summarises the inputs defined in the sediment dispersion
simulations. To simulate
conservative worst cases, potential concurrent activities are simulated at the
same time regardless the reality that they may not all occur simultaneously. It is important to note that the jetting
works in HKSAR and PRC waters will take place after the completion of all the
dredging operations and there will be only one jetting machine in operation at
one time.
