5.1.1
This section presents the
assessment results of the potential water quality impact associated with the construction
and operation of the proposed Central-Wanchai Bypass (CWB) including its road tunnel and slip roads
(DP1).
Mitigation measures are also recommended to minimise potential adverse
impacts and to ensure the acceptability of any residual impact (that is, after
mitigation).
5.1.2
Temporary reclamation would be
required for construction of the proposed CWB. Assessment of the potential
water quality impacts associated with reclamation and the associated dredging
works is provided in Part C Volume 3 of this EIA report under DP3 and therefore
are excluded in this Section. Apart from the reclamation, the construction
works for the CWB would all be land-based.
5.2.1
Water sensitive receivers
(WSRs) that would potentially affected by the construction and operation of the
CWB include several seawater intakes identified at or near the Project site.
These include a number of cooling water intakes at or near the proposed
permanent reclamation (DP3) along the existing waterfront of Wan Chai, Causeway Bay and North Point. An existing WSD flushing water intake is
also located within the proposed reclamation limit at Wan Chai. Figure 5.2 shows the
locations of the existing and reprovisioned seawater intakes at or near the
Project site.
5.3.1
The criteria for evaluating
water quality impacts in this EIA Study include:
Environmental
Impact Assessment Ordinance (EIAO)
5.3.2
The Technical Memorandum on
Environmental Impact Assessment Process (Environmental Impact Assessment
Ordinance) (EIAO-TM) was issued by EPD under Section 16 of the EIAO. It specifies the assessment method and
criteria that were followed in this Study.
Reference sections in the EIAO-TM provide the details of assessment
criteria and guidelines that are relevant to the water quality assessment,
including:
·
Annex 6 – Criteria for Evaluating Water Pollution
·
Annex 14 – Guidelines for Assessment of Water
Pollution.
Water Quality
Objectives
5.3.3
The Water Pollution Control
Ordinance (WPCO) provides the major statutory framework for the protection and
control of water quality in Hong Kong. According to the Ordinance and its
subsidiary legislation, Hong Kong waters are
divided into ten Water Control Zones (WCZs). Corresponding statements of Water
Quality Objectives (WQO) are stipulated for different water regimes (marine
waters, inland waters, bathing beaches subzones, secondary contact recreation
subzones and fish culture subzones) in the WCZ based on their beneficial
uses. The proposed Project is
located within Victoria
Harbour (Phase Three) WCZ
and the corresponding WQO are listed in Table 5.1.
Table 5.1 Summary
of Water Quality Objectives for Victoria Harbour WCZ
|
Parameters
|
Objectives
|
Sub-Zone
|
|
Offensive odour, tints
|
Not to be present
|
Whole zone
|
|
Visible foam, oil scum, litter
|
Not to be present
|
Whole zone
|
|
Dissolved oxygen (DO) within 2 m of the seabed
|
Not less than 2.0 mg/l for 90% of samples
|
Marine waters
|
|
Depth-averaged DO
|
Not less than 4.0 mg/l for 90% of samples
|
Marine waters
|
|
pH
|
To be in the range of 6.5 - 8.5, change due to human activity not
to exceed 0.2
|
Marine waters
|
|
Salinity
|
Change due to human activity not to exceed 10% of ambient
|
Whole zone
|
|
Temperature
|
Change due to human activity not to exceed 2 oC
|
Whole zone
|
|
Suspended solids (SS)
|
Not to raise the ambient level by 30% caused by human activity
|
Marine waters
|
|
Unionised ammonia (UIA)
|
Annual mean not to exceed 0.021 mg/l as unionised form
|
Whole zone
|
|
Nutrients
|
Shall not cause excessive algal growth
|
Marine waters
|
|
Total inorganic nitrogen (TIN)
|
Annual mean depth-averaged inorganic nitrogen not to exceed 0.4
mg/l
|
Marine waters
|
|
Toxic substances
|
Should not attain such levels as to produce significant toxic,
carcinogenic, mutagenic or teratogenic effects in humans, fish or any other
aquatic organisms.
|
Whole zone
|
|
Human
activity should not cause a risk to any beneficial use of the aquatic
environment.
|
Whole
zone
|
Source: Statement
of Water Quality Objectives (Victoria
Harbour (Phases One, Two
and Three) Water Control Zone).
Water Supplies Department (WSD) Water Quality
Criteria
5.3.4
Besides the WQO set under the
WPCO, the WSD has specified a set of objectives for water quality at flushing
water intakes. The list is shown in
Table 5.2. The target limit for suspended solids
(SS) at these intakes is 10 mg/l or less.
Table 5.2 WSD’s
Water Quality Criteria for Flushing
Water at Sea Water Intakes
|
Parameter (in
mg/l unless otherwise stated)
|
Target Limit
|
|
Colour (HU)
|
< 20
|
|
Turbidity (NTU)
|
< 10
|
|
Threshold Odour
Number (odour unit)
|
< 100
|
|
Ammoniacal
Nitrogen
|
< 1
|
|
Suspended
Solids
|
< 10
|
|
Dissolved
Oxygen
|
> 2
|
|
Biochemical
Oxygen Demand
|
< 10
|
|
Synthetic
Detergents
|
< 5
|
|
E. coli (no. per 100
mL)
|
< 20,000
|
Cooling Water Intake Standards
5.3.5
Based on a questionnaire survey
conducted under the approved Comprehensive Feasibility Study for Wan Chai
Development Phase II (WDIICFS) EIA (),
no information on the SS limit is available for all the identified cooling
water intakes. These findings have been confirmed by a telephone survey
conducted under the recent approved EIA for the Hong Kong Convention and
Exhibition Centre (HKCEC) Atrium Link Extension (ALE). The locations of the cooling water intakes
are shown in Figure 5.2. The SS criterion for
cooling water intakes is different from that for the WSD’s intakes as their
beneficial uses are different (the former is used for cooling water system and
the latter for flushing purpose).
Technical Memorandum
5.3.6
Discharges of effluents are
subject to control under the WPCO. The Technical Memorandum on Standards for
Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal
Waters (TM-DSS) gives guidance on the permissible effluent discharges based on
the type of receiving waters (foul sewers, storm water drains, inland and
coastal waters). The limits control
the physical, chemical and microbial quality of effluents. Any sewage from the proposed
construction and operation activities must comply with the standards for
effluents discharged into the foul sewers, inshore waters or marine waters of
Victoria Harbour WCZ, as given in the TM-DSS.
Practice Note
5.3.7
A Practice Note for
Professional Persons (ProPECC) was issued by the EPD to provide guidelines for
handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site
Drainage” provides good practice guidelines for dealing with ten types of
discharge from a construction site.
These include surface runoff, groundwater, boring and drilling water,
bentonite slurry, water for testing and sterilisation of water retaining
structures and water pipes, wastewater from building constructions, acid
cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices given in the ProPECC PN 1/94
should be followed as far as possible during construction to minimise the water
quality impact due to construction site drainage.
Marine Water Quality in Victoria Harbour
5.4.1
The marine water quality
monitoring data routinely collected by EPD in Victoria Harbour
were used to establish the baseline condition. A summary of water quality data
for selected EPD monitoring stations extracted from the EPD’s publication “20
years of Marine Water Quality Monitoring in Hong Kong”
(which is the latest version available at the time of preparing this report) is
presented in Table 5.3. The
monitoring stations in Victoria
Harbour central (VM4, VM5
and VM6) are considered
to be representative of the water quality in the marine waters of the
assessment area.
Locations of the monitoring stations are shown in Figure 5.1.
5.4.2
In the past, wastewater from
both sides of the Victoria Harbour was discharged into it after just simple
screening, leading to marine water low in DO and high in organic nutrients and
sewage bacteria. Commissioning
of HATS Stage 1 in late
2001 has brought large and sustained improvements to the water quality of the
eastern and central Victoria
Harbour. However,
improvements are less noticeable in the western harbour area which was still
subject to the sewage discharges from local PTW (Central, Wan Chai West and Wan
Chai East). As the HATS Stage 1 was commissioned in late 2001, the data for
2005 as shown in Table 5.3 represent
the situation after the commissioning of HATS Stage 1.
5.4.3
In 2005, the moderate improvements in the
mid harbour area since HATS Stage 1 was commissioned were generally
sustained. Full compliance
with the WQO (for DO, TIN and UIA) was achieved in 2005 in the central Victoria
Harbour WCZ.
Table 5.3 Summary
Statistics of 2005 Marine Water Quality in Victoria Harbour
|
Parameter
|
Victoria
Harbour
Central
|
WPCO WQO (in marine waters)
|
|
VM4
|
VM5
|
VM6
|
|
Temperature
(oC)
|
22.9
(15.8-27.8)
|
23
(15.9-27.9)
|
23
(15.9-27.8)
|
Not more
than 2 oC in daily temperature range
|
|
Salinity
|
31.8(28.9-33.2)
|
31.4
(27.3-32.9)
|
31.3
(26.8-32.8)
|
Not to cause more than 10% change
|
|
Dissolved Oxygen (DO) (% Saturation)
|
Depth average
|
75
(63-88)
|
76
(68-99)
|
77
(68-96)
|
Not available
|
|
Bottom
|
74
(51-88)
|
74
(46-99)
|
73
(45-94)
|
Not available
|
|
Dissolved Oxygen (DO)
(mg/l)
|
Depth average
|
5.4
(4.4-6.6)
|
5.5
(4.7-6.6)
|
5.5
(4.8-6.5)
|
Not less than 4 mg/l for 90% of the samples
|
|
Bottom
|
5.3
(3.6-6.5)
|
5.3
(3.3-6.6)
|
5.3
(3.2-6.5)
|
Not less than 2 mg/l for 90% of the samples
|
|
pH
|
8
(7.7-8.3)
|
8
(7.6-8.3)
|
8
(7.6-8.2)
|
6.5 - 8.5 (± 0.2 from
natural range)
|
|
Secchi disc Depth (m)
|
2.1
(1.5-3.2)
|
2.1
(1.3-3.1)
|
2.1
(1.2-3.3)
|
Not available
|
|
Turbidity (NTU)
|
9.6
(4.5-15.3)
|
9.8
(4.9-14.5)
|
9.8
(5.0-14.8)
|
Not available
|
|
Suspended Solids (SS)
(mg/l)
|
3.6
(1.3-9.8)
|
3.4
(1.7-5.3)
|
3.7
(1.3-8.2)
|
Not more than 30%
increase
|
|
5-day Biochemical
Oxygen Demand (BOD5) (mg/l)
|
0.9
(0.5-1.1)
|
1.1
(0.6-1.4)
|
0.9
(0.4-1.4)
|
Not available
|
|
Nitrite Nitrogen
(NO2-N) (mg/l)
|
0.03
(0.01-0.05)
|
0.03
(0.01-0.05)
|
0.03
(0.01-0.05)
|
Not available
|
|
Nitrate Nitrogen
(NO3-N) (mg/l)
|
0.13
(0.05-0.24)
|
0.15
(0.05-0.31)
|
0.16
(0.06-0.34)
|
Not available
|
|
Ammonia Nitrogen
(NH3-N) (mg/l)
|
0.15
(0.06-0.27)
|
0.19
(0.06-0.29)
|
0.19
(0.07-0.26)
|
Not available
|
|
Unionised Ammonia (UIA) (mg/l)
|
0.006
(0.003-0.015)
|
0.007
(0.005-0.015)
|
0.008
(0.004-0.014)
|
Not more than
0.021 mg/l for annual mean
|
|
Total Inorganic
Nitrogen (TIN) (mg/l)
|
0.31
(0.12-0.54)
|
0.37
(0.12-0.64)
|
0.38
(0.14-0.65)
|
Not more than 0.4
mg/l for annual mean
|
|
Total Nitrogen
(TN) (mg/l)
|
0.47
(0.26-0.69)
|
0.55
(0.28-0.77)
|
0.55
(0.29-0.79)
|
Not available
|
|
Orthophosphate
Phosphorus (PO4) (mg/l)
|
0.03
(0.01-0.04)
|
0.04
(0.01-0.05)
|
0.03
(0.01-0.05)
|
Not available
|
|
Total Phosphorus
(TP) (mg/l)
|
0.05
(0.03-0.06)
|
0.05
(0.03-0.07)
|
0.05
(0.03-0.07)
|
Not available
|
|
Chlorophyll-a
(µg/L)
|
2.4
(0.9-7.2)
|
2.8
(0.8-9.1)
|
2.6
(0.8-9.0)
|
Not available
|
|
E coli
(cfu/100 ml)
|
2400
(310-11000)
|
7700
(2500-23000)
|
5700
(1200-33000)
|
Not available
|
|
Faecal Coliforms
(cfu/100 ml)
|
5200
(770-33000)
|
17000
(6800-40000)
|
12000
(2300-89000)
|
Not available
|
Notes: 1. Except as
specified, data presented are depth-averaged values calculated by taking the
means of three depths: Surface, mid-depth, bottom.
2. Data presented are annual arithmetic means of
depth-averaged results except for E. coli
and faecal coliforms that are annual geometric means.
3. Data in brackets indicate the ranges.
Marine Water Quality within Causeway Bay Typhoon Shelter
5.4.4
A summary of published EPD
monitoring data (in 2005) collected from the monitoring station at the Causeway
Bay Typhoon Shelter (VT2) is presented in Table 5.4.
The data are extracted from the EPD’s publication “20 years of Marine Water
Quality Monitoring in Hong Kong”.
Table 5.4 Summary
Statistics of 2005 Marine Water Quality at the Causeway Bay
Typhoon Shelter
|
Parameter
|
EPD
Monitoring Station (Bi-Monthly)
|
WPCO WQOs (in marine waters)
|
|
|
|
Temperature (oC)
|
22.8
(15.9 – 27.3)
|
Not more than 2
oC in daily temperature range
|
|
Salinity (ppt)
|
30.2
(25.2 – 32.2)
|
Not to cause
more than 10% change
|
|
Dissolved Oxygen (DO)
(% saturation)
|
Depth average
|
68
(53 – 103)
|
Not available
|
|
Bottom
|
68
(53 – 102)
|
Not available
|
|
DO (mg/l)
|
Depth average
|
4.9
(3.6 – 7.2)
|
Not less than 4
mg/L for 90% of the samples
|
|
|
Bottom
|
4.9
(3.6 – 7.1)
|
Not less than 2
mg/L for 90% of the samples
|
|
pH value
|
8.1
(7.9 – 8.3)
|
6.5 - 8.5 (± 0.2 from
natural range)
|
|
Secchi disc (m)
|
1.9
(1.5 – 2.9)
|
Not available
|
|
Turbidity (NTU)
|
8.8
(5.0 – 9.9)
|
Not available
|
|
Suspended Solids (SS) (mg/l)
|
5.8
(3.0 – 13.8)
|
Not more than
30% increase
|
|
Silica (as SiO2)(mg/l)
|
1.0
(0.5 – 1.4)
|
Not available
|
|
5-day
Biochemical Oxygen Demand (BOD5) (mg/l)
|
1.6
(1.2 – 2.9)
|
Not available
|
|
Nitrite
Nitrogen (NO2-N)
(mg/l)
|
0.04
(0.02 – 0.05)
|
Not available
|
|
Nitrate Nitrogen (NO3-N) (mg/l)
|
0.19
(0.11 – 0.32)
|
Not available
|
|
Ammoniacal Nitrogen (NH3-N)
(mg/l)
|
0.20
(0.18 – 0.30)
|
Not available
|
|
Unionised Ammonia (UIA)
(mg/l)
|
0.011
(0.005 – 0.021)
|
Not more than 0.021
mg/L for annual mean
|
|
Total Inorganic Nitrogen (TIN) (mg/l)
|
0.43
(0.35 – 0.55)
|
Not more than
0.4 mg/L for annual mean
|
|
Total Nitrogen (TN)
(mg/l)
|
0.65
(0.56 – 0.80)
|
Not available
|
|
Ortho-Phosphate (OrthoP) (mg/l)
|
0.04
(0.02 – 0.05)
|
Not available
|
|
Total Phosphorus (TP)
(mg/l)
|
0.06
(0.05 – 0.08)
|
Not available
|
|
Chlorophyll-a
(µg L-1)
|
4.3
(0.5 – 16.5)
|
Not available
|
|
E. coli (cfu per 100 mL)
|
5,200
(2,300 –
12,000)
|
Not available
|
|
Faecal Coliform
(cfu per 100 mL)
|
17,000
(5,100 –
61,000)
|
Not available
|
Note: 1. Except
as specified, data presented are depth-averaged data.
2.
Data presented are annual arithmetic means except
for E. coli and faecal coliforms that
are geometric means.
3.
Data enclosed in brackets indicate ranges.
5.4.5
Due to the embayment form and
reduced flushing capacity of the typhoon shelter, marine water within the
typhoon shelter is vulnerable to pollution. In 2005, high levels of E.coli were recorded at the Causeway Bay
Typhoon Shelter indicating faecal contamination. The water quality level
marginally exceeded the WQO for TIN but fully complied with the WQO for DO and
UIA. Significant long-term improvements in terms of decreasing trends in TIN,
TN, OrthoP and TP were observed in Causeway Bay Typhoon Shelter.
5.5
Identification of
Environmental Impact
Operational
Phase
5.5.1
It is considered that impacts
resulting from the operation of CWB, in terms of water quality, will be minimal
and similar for both the elevated and tunnel sections of the route. Surface runoff from slip-roads and
elevated structures may be contaminated by oils leaked from passing vehicles,
and tunnel seepage would potentially be contaminated to the same extent. It is considered that impacts upon water
quality will be minimal provided that the tunnel and elevated sections of the
CWB are designed with adequate drainage systems and appropriate oil
interceptors, as required.
Construction Phase
5.5.2
Potential sources of water quality
impacts arising from the construction of CWB will be similar to those of
general land-based construction activities.
Stormwater Discharges
5.5.3
Stormwater and drainage discharges
from the construction sites may contain considerable loads of SS and
contaminants during construction activities. Potential water quality impact includes
run-off and erosion of exposed bare soil and earth, drainage channels, earth
working area and stockpiles. Minimum distances of 100 m shall be maintained
between the existing or planned stormwater discharges and the existing or
planned WSD flushing water intakes during construction and operation phases.
5.5.4
Local and coastal water pollution
impact may be substantial if the construction site run-off is allowed to
discharge into the storm drains or natural drainage without mitigation.
Construction Runoff and
Drainage
5.5.5
Surface runoff generated from the
construction site may contain increased loads of SS and contaminants. Potential pollution sources of site run-off
may come from:
·
contaminated ground water from any dewatering
activities as a result of excavation
·
release of any bentonite slurries and other grouting
materials with construction run-off, storm water or ground water dewatering
process
·
wash water from dust suppression sprays and wheel
washing facilities
·
fuel, oil and lubricants from maintenance of
construction vehicles and equipment.
General Construction Activities
5.5.6
The general construction works that
will be undertaken for the roads and infrastructure will be primarily
land-based and may have the potential to cause water pollution. These could result from the accumulation
of solid waste such as packaging and construction materials, and liquid waste
such as sewage effluent from the construction work force, discharge of bilge
water and spillage of oil, diesel or solvents by vessels and vehicles involved
with the construction. If
uncontrolled, any of these could lead to deterioration in water quality. Increased nutrient levels result from
contaminated discharges and sewage effluent could also lead to a number of
secondary water quality impacts including decreases in DO concentrations and
localised increase in NH3-N concentrations which could stimulate algal growth
and reduction in oxygen levels.
5.5.7
Sewage will arise from sanitary
facilities provided for the on-site construction work force. It is characterised by high level of
BOD, NH3-N and E.coli counts. For
some of the works areas, there will be no public sewers available for domestic
sewage discharge on-site.
5.6.1
The assessment of the
potential impact of land-based construction activities on water quality has been undertaken in a
qualitative manner. Proposed construction activities were reviewed to assess the land-based water quality impact upon
the nearby water bodies. Practical
water pollution control measures / mitigation proposals (Section 5.8) have been
subsequently recommended to prevent local flooding and to ensure that effluent
discharged from the construction site will comply with the WPCO criteria. Consideration has been given to
controlling potentially harmful impacts from site works and to the use of
‘best’ practice measures to minimise the potential for discharges of pollutants
to the nearby waters of the Victoria
Harbour.
Operational Phase
5.7.1
It
is considered that impacts resulting from the operation of CWB, in terms of
water quality, will be minimal and similar for both the elevated and tunnel
sections of the route. Surface
runoff from slip-roads and elevated structures may be contaminated by oils
leaked from passing vehicles, and tunnel seepage would potentially be
contaminated to the same extent. It
is considered that impacts upon water quality will be minimal provided that the
tunnel and elevated sections of the CWB are designed with adequate drainage
systems and appropriate oil interceptors, as required.
Construction Phase
General Construction
Activities
5.7.2
The effects on water quality from
general construction activities are likely to be minimal, provided that site
drainage is well maintained and good construction practices are observed to
ensure that litter, fuels, and solvents are managed, stored and handled
properly.
5.7.3
Based on the Sewerage Manual,
Part I, 1995 of the Drainage Services Department (DSD), the global unit
flow factors for employed population of 0.06 m3 per worker per day
and commercial activities in year 2016 of 0.29 m3 per worker per day
have been used to estimate the sewage generation from the construction
site. The total sewage production
rate is estimated at 0.35 m3 per worker per day. Therefore, with 100 construction workers
working simultaneously at the construction site, a total of about 35 m3
of sewage will be generated per day.
The sewage should not be allowed to discharge directly into the
surrounding water body without treatment.
Chemical toilets and subsequently on-site sewer should be deployed at
the construction site to collect and handle sewage from workers (see Section
5.8 for recommended mitigation measures).
Construction Runoff and Drainage
5.7.4
Construction run-off and
drainage may cause physical, chemical and biological effects. The physical effects could arise from any
increase in SS from the construction site that could cause blockage of drainage
channels and associated local flooding when heavy rainfall occurs, as well as
local impact on water quality. High
SS concentrations in marine water could lead to associated reduction in DO
levels.
5.7.5
It is important that proper
site practice and good site management be strictly followed to prevent run-off
water and drainage water with high level of SS from entering the surrounding
waters. With the implementation of
appropriate measures to control run-off and drainage from the construction
site, it is considered that disturbance of water bodies will be localised and
deterioration in water quality will be minimal. Thus, unacceptable impacts on the water
quality are not expected provided that the recommended measures described in
Section 5.8 are properly implemented.
Operational Phase
5.8.1
For the operation of CWB, a
surface water drainage system would be provided to collect road runoff. The following operation stage mitigation
measures are recommended to ensure road runoff would comply with the TM under
the WPCO:
·
The drainage from tunnel sections shall be directed
through petrol interceptors to remove oil and grease before being discharged to
the nearby foul water manholes.
·
Petrol interceptors shall be regularly cleaned and
maintained in good working condition.
·
Oily contents of the petrol interceptors shall be
properly handled and disposed of, in compliance with the requirements of the
Waste Disposal Ordinance.
·
Sewage arising from ancillary facilities of CWB (for
examples, car park, control room, ventilation and administration buildings and
tunnel portals) shall be connected to public sewerage system. Sufficient capacity in public sewerage shall
be made available to the proposed facilities.
·
Road drainage should also be provided with
adequately designed silt trap to minimize discharge of silty runoff.
5.8.2
The design of the operational
stage mitigation measures for CWB shall take into account the guidelines
published in ProPECC PN 5/93 “Drainage Plans subject to Comment by the EPD.”
All operational discharges from the CWB into drainage or sewerage systems are
required to be licensed by EPD under the WPCO.
Construction
Phase
5.8.3
It is important that appropriate
measures are implemented to control runoff and drainage and prevent high
loading of SS from entering the marine environment. Proper site management is essential to
minimise surface water runoff, soil erosion and sewage effluents.
5.8.4
Any practical options for the
diversion and re-alignment of drainage should comply with both engineering and
environmental requirements in order to ensure adequate hydraulic capacity of
all drains.
5.8.5
Construction site runoff and
drainage should be prevented or minimised in accordance with the guidelines
stipulated in the EPD's Practice Note for Professional Persons, Construction
Site Drainage (ProPECC PN 1/94).
Good housekeeping and stormwater best management practices, as detailed
in below, should be implemented to ensure that all construction runoff complies
with WPCO standards and that no unacceptable impact on the WSRs arises due to
construction of the CWB. All
discharges from the construction site should be controlled to comply with the
standards for effluents discharged into Victoria Harbour WCZ under the TM-DSS.
Construction Runoff
5.8.6
Exposed soil areas should be
minimised to reduce the potential for increased siltation, contamination of
runoff, and erosion. Construction
runoff related impacts associated with the above ground construction activities
can be readily controlled through the use of appropriate mitigation measures
which include:
·
use of sediment traps
·
adequate maintenance of drainage systems to prevent
flooding and overflow.
5.8.7
Construction site should be provided
with adequately designed perimeter channel and pre-treatment facilities and
proper maintenance. The boundaries
of critical areas of earthworks should be marked and surrounded by dykes or
embankments for flood protection.
Temporary ditches should be provided to facilitate runoff discharge into
the appropriate watercourses, via a silt retention pond. Permanent drainage channels should
incorporate sediment basins or traps and baffles to enhance deposition
rates. The design of efficient silt
removal facilities should be based on the guidelines in Appendix A1 of ProPECC
PN 1/94.
5.8.8
Ideally, construction works
should be programmed to minimise surface excavation works during the rainy
season (April to September). All
exposed earth areas should be completed as soon as possible after earthworks
have been completed, or alternatively, within 14 days of the cessation of
earthworks where practicable. If
excavation of soil cannot be avoided during the rainy season, or at any time of
year when rainstorms are likely, exposed slope surfaces should be covered by
tarpaulin or other means.
5.8.9
Sediment tanks of sufficient
capacity, constructed from pre-formed individual cells of approximately 6 to 8 m3 capacity, are
recommended as a general mitigation measure which can be used for settling
surface runoff prior to disposal.
The system capacity is flexible and able to handle multiple inputs from
a variety of sources and particularly suited to applications where the influent
is pumped.
5.8.10 Open stockpiles of construction materials (for examples, aggregates,
sand and fill material) of more than 50 m3 should be covered with tarpaulin or
similar fabric during rainstorms.
Measures should be taken to prevent the washing away of construction
materials, soil, silt or debris into any drainage system.
5.8.11 Manholes (including newly constructed ones) should always be
adequately covered and temporarily sealed so as to prevent silt, construction
materials or debris being washed into the drainage system and storm runoff
being directed into foul sewers.
5.8.12 Precautions to be taken at any time of year when rainstorms are
likely, actions to be taken when a rainstorm is imminent or forecast, and
actions to be taken during or after rainstorms are summarised in Appendix A2 of
ProPECC PN 1/94. Particular attention
should be paid to the control of silty surface runoff during storm events.
5.8.13 Oil interceptors should be provided in the drainage system and
regularly cleaned to prevent the release of oils and grease into the storm
water drainage system after accidental spillages. The interceptor should have a bypass to
prevent flushing during periods of heavy rain.
5.8.14 All vehicles and plant should be cleaned before leaving a
construction site to ensure no earth, mud, debris and the like is deposited by
them on roads. An adequately
designed and located wheel washing bay should be provided at every site exit,
and wash-water should have sand and silt settled out and removed at least on a
weekly basis to ensure the continued efficiency of the process. The section of access road leading to,
and exiting from, the wheel-wash bay to the public road should be paved with
sufficient backfall toward the wheel-wash bay to prevent vehicle tracking of
soil and silty water to public roads and drains.
Drainage
5.8.15 It is recommended that on-site drainage system should be installed
prior to the commencement of other construction activities. Sediment traps should be installed in
order to minimise the sediment loading of the effluent prior to discharge into
foul sewers. There shall be no direct
discharge of effluent from the site into the sea.
5.8.16 All temporary and permanent drainage pipes and culverts provided to
facilitate runoff discharge should be adequately designed for the controlled
release of storm flows. All
sediment control measures should be regularly inspected and maintained to
ensure proper and efficient operation at all times and particularly following
rain storms. The temporarily
diverted drainage should be reinstated to its original condition when the
construction work has finished or the temporary diversion is no longer
required.
5.8.17 All fuel tanks and storage areas should be provided with locks and
be located on sealed areas, within bunds of a capacity equal to 110% of the
storage capacity of the largest tank, to prevent spilled fuel oils from
reaching the coastal waters of Victoria Harbour WCZ.
Sewage Effluent
5.8.18 Construction work force sewage discharges on site are expected to be
connected to the existing trunk sewer or sewage treatment facilities. The construction sewage may need to be
handled by portable chemical toilets prior to the commission of the on-site
sewer system. Appropriate numbers
of portable toilets shall be provided by a licensed contractor to serve the
large number of construction workers over the construction site. The Contractor shall also be responsible
for waste disposal and maintenance practices.
Stormwater Discharges
5.8.19 Minimum distances of 100 m shall be maintained between the existing
or planned stormwater discharges and the existing or planned WSD flushing water
intakes.
Debris and Litter
5.8.20 In order to maintain water quality in acceptable conditions with
regard to aesthetic quality, contractors shall be required, under conditions of
contract, to ensure that site management is optimised and that disposal of any
solid materials, litter or wastes to marine waters does not occur.
Construction
Phase
5.9.1
General construction activities
associated with the construction of CWB could lead to site runoff containing elevated
concentrations of SS and associated contaminants that may enter into the marine
water. However, it is anticipated
that the above water quality impacts will generally be temporary and localised
during construction. Therefore, no
unacceptable residual water quality impacts are anticipated during the
construction of the proposed infrastructure, provided all of the recommended
mitigation measures are implemented and all construction site / works area
discharges comply with the TM-DSS standards.
Operational Phase
5.9.2
Adverse water quality impacts
associated with the operation of CWB are not expected. Thus, there would be no unacceptable
residual impact associated with the operation of the Project.
Construction Phase
5.10.1
The potential water quality impacts
arising from the proposed land-based construction works for the CWB have been
assessed. It is predicted that
there will not be any insurmountable adverse impacts on marine water quality. With the implementation of the recommended
mitigation measures, it is anticipated that the identified water quality
impacts would be kept to within acceptable levels. Monitoring of marine water quality
during the construction phase is therefore not considered necessary for this
site.
5.10.2
Regular site
audit shall be carried out during construction stage to ensure that no adverse
impact on the marine waters and seawater intakes in the vicinity; and to
scrutinize timely implementation of the recommended mitigating measures.
Operational
Phase
5.10.3 No unacceptable water quality
impacts would be expected from the Project. No monitoring programme specific for
operational water quality would be required.
Construction
Phase
5.11.1 Water quality impacts from land-based construction are associated
with the surface runoff, effluent discharge from the site, and sewage from
on-site construction workers.
Impacts can be controlled to comply with the WPCO standards by
implementing the recommended mitigation measures. No unacceptable residual impacts on
water quality are anticipated.
Operational Phase
5.11.2 It is considered that impacts
resulting from the operation of CWB, in terms of water quality, will be minimal
and similar for both the elevated and tunnel sections of the route. Surface runoff from slip-roads and elevated
structures may be contaminated by oils leaked from passing vehicles, and tunnel
seepage would potentially be contaminated to the same extent. It is considered that impacts upon water
quality will be minimal provided that the tunnel and elevated sections of the
CWB are designed with adequate drainage systems and appropriate oil
interceptors, as required.