8.1.1
In
accordance with the EIA Study Brief (ESB-127/2005), a Hazard Assessment (HA)
shall be conducted to assess the hazard to life impact of the Tuen Mun Water
Treatment Works (TMWTW) to both the construction and operation stages of the
Project. Location of TMWTW is
shown in Figure 8.1.
“to identify and
quantify any potential hazard to life impacts and to propose measures to
mitigate these impacts”.
8.1.3
The
scope of the hazard to life assessment is to address the potential hazard to
life impacts from construction and operation of the Project as specified in
Section 3.4.5 of the EIA study brief as shown below.
“3.4.5.1 A hazard assessment shall be carried
out following the criteria for evaluating hazard to life as stated in Annexes 4
and 22 of the TM (Hong Kong Risk
Guidelines). The assessment shall
evaluate hazardous scenarios from Tuen Mun Water Treatment Works (TMWTW) to
both the construction and operation stages of the Project. The hazard assessment shall include the
following
(i)
Identification of all credible hazardous scenarios associated with the chlorine
storage and on-site transport in the Tuen Mun Water Treatment Works;
(ii) Execution of a Quantitative Risk Assessment
(QRA) expressing population risks in both individual and societal terms;
(iii) Comparison of individual and societal
risks with Hong Kong Risk Guidelines (Annex 4 of TM); and
(iv) Identification and assessment of
practicable and cost-effective risk mitigation measures.
3.4.5.2 The methodology of hazard assessment
shall be agreed and approved by the Director taking into account previous EIAO
hazard assessments such as Route 8 (formerly Route 16).“
General
8.2.1
A
similar Hazard Assessment study, namely the Route 8 EIA Study (formerly Route
16), has been reviewed and taken as a reference for the Project.
8.2.2
Hazardous
scenarios have been identified by reviewing hazardous scenarios developed for
similar installations.
8.2.3
Hazardous
scenarios and frequencies adopted for TMWTW has been confirmed independently
using review of historical incidents as instructed by the Court of Final Appeal
(CFA). The Major Hazard Incident Data Services (MHIDAS) accident database has
been reviewed in this study.
a)
Baseline
Validation – Validation of risk level of 2006 base case scenario generated from
BMT’s in-house RISKSUM model is comparable with other risk
summation software, such as GISRisk.
b)
Construction
Stage Case – Assessment of risk level during construction (Year 2009) of the
Project taking into account the peak construction workforce level, future
traffic population in Tuen Mun Road (TMR) and adjacent populations.
c)
Operation
Stage Case – Assessment of risk level during operation (Year 2024) of the
Project taking into account the future traffic forecast in TMR and adjacent
populations forecast.
a)
Hazard
Identification: Identification of all credible hazardous scenarios associated
with the chlorine storage and on-site transport in the TMWTW by reviewing of
historical incident database and similar previous studies.
b)
Frequency
Assessment: Assess the likelihood of occurrence of the identified hazardous
scenarios.
c)
Consequence
Assessment: Assess the consequences and impact to construction workers, road
population in TMR and adjacent population.
d)
Risk
Assessment: Evaluates the risks level, in terms of individual risk and societal
risk. The risks will be compared with the criteria stipulated in Annex 4 of the
Technical Memorandum on Environmental Impact Assessment Process issued by
Environmental Protection Department (EPD) to determine their acceptability.
e)
Identification
of Mitigation Measures: Practicable and cost-effective risk mitigation measures
will be identified and assessed as necessary. Risks of mitigated case will be
then reassessed to determine the level of risk reduction.
a)
Individual
Risk: the maximum level of off-site individual risk should not exceed 1 x 10-5
/ year, i.e. 1 in 100,000 per year.
b)
Societal
Risk is presented graphically as in Figure
8.2. The Societal Risk
Guideline is expressed in terms of lines plotting the cumulative frequency (F)
of N or more fatalities in the population from accidents at the facility of
concern.
8.2.7
Calibration
exercises have been conducted to verify the risks results generated by RISKSUM is comparable with
other software used for hazard assessment.
8.2.8
In
comparison with the Individual Contours and FN Curve generated by RISKSUM, the magnitude of the
individual risk and societal risk are found comparable with the results
generated by other risk software package, such as GISRisk. Therefore,
using the RISKSUM for the risk assessment in this Widening
of Tuen Mun Road project is considered acceptable. Figure 8.3 and Figure 8.4 presented the
comparison of the Individual Risk Contours and the FN curves generated by RISKSUM with that of the GISRisk respectively.
Population – Base Year (Year 2006)
8.3.2
Population
data considered include all the existing and potential future population that
may be affected by accidental release of chlorine gas from the TMWTW.
8.3.3
Table
8.1
presents the data sources that have been collected.
Table 8‑1 Population
and Traffic Data Sources
|
Sources
|
Details
|
|
Census
and Statistic Department
|
2006
Population Census
Summary
Statistic of Constituency Area in Tuen Mun, 2006
|
|
Planning
Department
|
Projections
of Population Distribution, 2006-2015
Territory
Population and Employment Data Matrices (TPEDM) 2021
|
|
Housing
Authority
|
Population
in Public Housing Estate
|
|
Transport
Department
|
Traffic
Census 2006
|
|
Hospital
Authority
|
Bed
spaces available and Number of Staff in Tuen
Mun Hospital
|
|
Education
Bureau
|
School
Information lists by District
|
|
Lingnan
College
|
Student
Enrolment, Staff and Hostel Occupancy Rate
|
|
Future
Traffic Forecast
|
Traffic
Forecast data 2009 and 2024, ENSR 2007
|
|
Others
|
Information
from Water Supplies Department’s (WSD) previous study of Tuen Mun Water
Treatment Works
|
8.3.4
Population
in Public Housing Estate are extracted from Housing Authority Public Housing
Estate profile, these include Prime
View Garden,
Siu Hong Court,
and Fu Tai Estate.
8.3.5
Population
in private residential developments are generated based on the number of
households in each building and the average household size defined by different
districts in Tuen Mun as specified from the Census and Statistic Department.
8.3.6
Regarding
the population in schools and colleges, relevant student enrolment information
for year 2006/07 is collected from Education Bureau for all nurseries,
kindergartens, and primary schools located within the study area. Students, staff, and hostel residents
at Lingnan College
are also taken into consideration.
8.3.7
For
a conservative estimation of population in Tuen
Mun Hospital,
it is assumed that 100% occupancy rate for all its available bed spaces. The
numbers of patients together with the number of staff working in the hospital
give the total population in the hospital for risk modelling.
8.3.8
The
population data distribution at different time periods listed below is
considered. A ‘jammed peak’ period has also been considered for traffic
standstill, ‘bumper-to-bumper’ conditions. Percentage distributions of each
time period are given in Appendix 8.1 of this report.
a)
Working Day.
b)
Weekend Day.
c)
Night.
d)
Peak hour.
e)
Jammed Peak
Hour.
8.3.10
Table
8.2
summaries the details for estimation of Tuen Mun population in 2009 and 2024.
Details of population estimation and the percentage distribution at each time
period for each population group in the vicinity of the TMWTW are consolidated
in Appendix 8.1.
Table
8‑2 Estimation
of Future Population
|
Population Sector
|
2009 (Construction Stage)
|
2024 (Operation Stage)
|
|
Residential
|
Resident
Population (2006/2009) Tuen Mun District = 8% increase
Source:
Census Department - 2009 Projected HK Resident
|
Resident
Population (2009/2021) Tuen Mun District = 11% increase
Additional
0.9% per year increment from 2021 to 2024 is assumed
Source:
Planning Department – TPEDM 2021
|
|
Transportation
Facilities
|
During
Peak Hours, LRT Station and trains reach maximum capacity
Consistent
with base case representing worst case scenario
|
During
Peak Hours, LRT Station and trains reach maximum capacity
Consistent
with base case representing worst case scenario
|
|
Schools
|
Population
by Age (2006/2009)
Age
0-4 = 3% decrease
Age
5-9 = 14% decrease
Age
10-14 = 17% decrease
Source:
Census Department - 2009 Projected HK Resident
|
For
a conservative assessment, 11% increase from 2009 is assumed (in line with
resident population growth in Tuen Mun)
Source: Planning Department –
TPEDM 2021
|
|
Hospitals
|
100%
occupancy rate for all available bed spaces
Consistent
with base case representing worst case scenario
|
100%
occupancy rate for all available bed spaces
Consistent
with base case representing worst case scenario
|
8.3.11
To
estimate transient population at each time period mentioned before, the same
methodology as adopted in the WSD previous study has been used.
8.3.12
Traffic
and transportation facilities population considered in this study includes
population on Tuen Mun Road
and Castle Peak Road
and facilities include five (5) Light Rail Train (LRT) stations, and two (2)
light rail trains route. The
locations of these populations are indicated in the figure attached in Appendix
8.1.
8.3.13
The
population associated with the road vehicles and railway has been modelled as
100% outdoor. Stationary condition of the vehicles at major highways are
assumed to account for the traffic stop due to traffic light control, peak jam
conditions or the possibility of chlorine gas leakage resulted in stand still
of the traffic in one direction. For the opposite direction of traffic,
population of vehicle travelling has been estimated based on traffic flow of
forecast.
8.3.14
Population
of traffic in standstill condition at major highway is estimated based on the
total length of road, number of traffic lanes, length of vehicle, distribution
of different vehicle classes, and occupancy rate of different vehicles.
8.3.15
The
standstill condition of Tuen Mun
Road before and after widening from a dual 2-lane to
dual 3-lane arrangement at Tsing Tin Interchange has also been included in the
transient population estimations.
8.3.17
Future
traffic forecast of TMR at operation stage (Year 2024) for the EIA study has
been conducted by traffic consultants and endorsed by Transport Department
(TD). The methodology of traffic flow forecast in 6 vehicle classes for the
project is presented below.
a)
Step
1 - The
2021 traffic model have been adopted as a background traffic model flow in term
of passenger car unit (PCU) per hour to present each road link.
b)
Step
2
- The percentage of vehicle types was derived according to the traffic
counts survey. The model traffic flows were split into different vehicle
classes based on the percentage of vehicle type obtained from traffic survey.
c)
Step
3 - The
model flows in PCU were converted to vehicle (VEH) using predefined conversion
factors derived from the TPDM recommendation. The PCU factor for car,
taxi, special purpose buses, light goods vehicles, heavy goods vehicles and
public transport are 1.00, 1.00, 2.00, 1.50, 2.25 and 3.00 respectively.
d)
Step
4 - The
year 2024 traffic flows has been developed by applying an annual growth factor
derived from the TPEDM planning data. The annual growth factor 0.9% per
year has been applied to the year 2021 traffic flow as shown in Step 3.
e)
Step 5 - The various type of 6
vehicle classes were calculated based on the result of Step 4.
8.3.18
Population
of other traffic in normal travelling condition is calculated based on the
following equation for each type of traffic mix.
|
Traffic
Population =
|
No.
of person per vehicle * No. of vehicles per hr * Road Length
|
|
Vehicle
speed
|
8.3.19
Population
in crush loading at peak hour of LRT trains are assumed to be 434 and 394 for
LRT III and LRT IV respectively.
8.3.20
For
a conservative assessment, population at each Light Rail Train Station is
assumed at its maximum capacity and similarly for passengers on light rail trains.
8.3.21
Meteorological
conditions (wind direction, speed and stability) would affect the consequence
of release. Meteorological data from the Hong Kong Observatory Tuen Mun weather
station in Year 2006 has been adopted in the model.
Review of TMWTW Facilities
8.4.1
The
TMWTW facilities have been reviewed to ensure the latest information has been
incorporated into the assessment.
8.4.2
Information
regarding latest chlorine facilities installed in the TMWTW was gathered during
site visit to the treatment work on 17 July 2007. During the visit, an
interview has been conducted with operation staff to confirm the latest
operation and safety practice in place in the treatment works.
8.4.3
The
mechanical ventilation system and the chlorine scrubbing system installed
provide controlled air circulation and treatment of air in case of chlorine
release.
8.4.4
The
scrubber system consists of an absorber tower with re-circulating sodium
hydroxide solution as a neutralizing agent. Air contaminated with chlorine gas
will be extracted from the plant room, where a chlorine leak has occurred, and
neutralized.
8.4.5
These
systems are designed to prevent chlorine gas escape from the storage area in
case of leakage.
8.4.6
Regarding
the safety provision, emergency repair and stoppage kit manufactured to the
specification of the Chlorine Institute is provided according to safety
requirements of Fire Services Department (FSD).
Review
of Identified Hazards
a)
Access
Road;
b)
Container
Handling;
c)
Containers
in Storage;
d)
Connection
and Disconnection of Chlorine Containers; and
e)
Chlorination
System.
8.4.9
Related
chlorine release scenarios have been examined and confirmed valid to use in
this study.
Review
of MHIDAS Incident Database
8.4.10
The
Major Hazard Incidents Data Service (MHIDAS) database holds details of over
seven thousand incidents which have occurred during the transport, processing
or storage of hazardous materials which resulted in or it is considered had the
potential to cause off-site impact.
8.4.11
A
review on MHIDAS database of the relevant historical incidents of the same
genus to TMWTW has been conducted to confirm if the hazardous scenarios
identified are acceptable.
8.4.12
A
search in the MHIDAS using the keywords such as “Chlorine”, “Leakage”, “Water
Treatment Works” and “Cylinders” has been conducted to identify incidents
involving in Drinking Water Treatment Process. The following
combinations of keywords search were conducted and a total of 109 records have
been reviewed.
·
Chlorine & Release;
·
Chlorine & Leakage;
·
Chlorine & Water & Treatment;
·
Chlorine & Truck & Fire;
·
Chlorine & Truck & Collision; and
·
Chlorine & Truck & Impact.
8.4.13
From the 109 records, 16 records were identified
as the same genus of Water Treatment Plant of this study. The remaining
records, which are not related to water treatment plant, such as leakage from
railcar, release of chlorine due to mixing of two chemicals, release in
chemical factory, chemical production plant etc, are not further considered in
this study.
8.4.14
A
total of 10 incidents from these 16 hazards from the MHIDAS were retained for further frequencies
analysis. Details of each incident are given in Appendix 8.2.
8.4.15
Table 8‑3 summarised the types of chlorine incidents from the
MHIDAS database.
Table 8‑3 Summary
of Chlorine Incidents of Water Treatment Plant from MHIDAS
|
Hazardous Scenario
|
No. of Cases
|
Country
|
Failure Frequency Calculation
|
|
Pipework Failure
|
7
|
France, Hong Kong, UK & USA
|
Yes.
|
|
Cylinder Failure
|
3
|
France,
UK & USA
|
Yes.
|
|
Others (Tank/Drum)
|
6
|
Puerto Rico, UK,
USA
|
No.
(No Tank and Drum installed in TMWTW)
|
8.4.16
Upon
completion of the incident review, no new hazard has been identified. For other
hazards such as earthquake, truck fire, etc., no historical incidents related
to these hazards has been found from the MHIDAS search.
8.4.17
ALL hazards identified have
been adopted in the risk model of current study.
Hazard
Associated with Construction Works
8.4.19
In
addition, there is a small hill (50m high) located southwest of the TMWTW. This
hill forms a protective barrier to prevent any construction activities or
incidents to affect the chlorine facilities.
8.4.20
Therefore,
it is considered that risk associated with the construction activities to the
chlorine storage and dosing system of the TMWTW is zero.
General
8.5.5
As noted in Table 8‑3,
ten (10) relevant incidents were identified and used for estimating the failure
frequencies.
8.5.6
To
assess the occurrence frequencies for the historical chlorine release
incidents, estimation for the number of drinking water treatment plants (WTP)
in the countries that the accidents occurred has been conducted.
8.5.7
To
estimate the total number of water treatment plant operating during 1964 to
2006 (Time span of MHIDAS data) in Europe and USA,
population and number of WTP of a range of representative cities are collected.
The number of residents that a WTP serves in the region is then calculated.
Then the total population of Europe and USA
are collected to estimate the numbers of WTP are operating in these countries.
8.5.8
Estimated
number of water treatment plants in Europe and USA
based on the total country population is presented in Table 8‑4 and the average number of served population per
plant is given in Table 8‑5.
Table 8‑4 Estimation of Served Population per Plant
|
Country
|
Locations
|
No. of WTP
|
Served Population
|
Average Served Population per plant
|
|
UK
|
England and Wales
|
1,386
|
52,710,000
|
38,030
|
|
USA
|
8
Cities /District
|
47
|
8,525,143
|
181,386
|
Table 8‑5 Estimation
of Total WTP in Europe and USA
|
Region
|
Total Population
|
Population serve per plant
|
Estimate No. of WTP
|
|
Europe
|
728,501,000
|
38,030
|
19,156
|
|
USA
|
281,421,906
|
181,386
|
1,552
|
|
Total:
|
20,708
|
8.5.9
Considering
there was a release case occurred in Hong Kong in 1992, the 21 Water Treatment
Works serving Hong Kong residents are included
in the calculation (i.e. 20,708 + 21 = 20729).
8.5.10
From
1964 to 2006, a total of 42 years of records registered in MHIDAS database.
Therefore, it is estimated that 870,618 plant-year of operation. From this
figure, the occurrence frequencies of pipework leakage and cylinder leakage in
WTW are calculated to be 8.04E-06 per year and 3.44E-06 per year respectively.
8.5.11
In
comparison with the frequency of container failure (1.5E-4 per year) to be
adopted, the occurrence frequency (3.44E-06) from historical incidents is
lower. Therefore, it is conservative to adopt the container failure frequency
of 1.5E-4 per year.
8.5.12
With
regard to accident of delivery vehicle, no incident record related to truck
fire, rollover and impact was found. To estimate a future potential failure
frequency of these events, an expected number of 0.7 incident is obtained from
Poisson distribution with an assumption of 50% probability of occurrence.
8.5.13
From
the above discussion, occurrence frequency of truck incident lead to chlorine
gas leakage is estimated to be 8.04E-7 per year and is the same order of
magnitude with or smaller than the frequencies proposed to be adopted.
Therefore, it is considered that adoption of truck accident frequencies is
acceptable.
8.5.14
Table 8‑6 presents the comparison of frequencies estimated from
historical accident data and that adopted in this study.
Table 8‑6 Comparison of Event Frequencies
|
Scenarios
|
Event Frequency per year
|
|
(Adopt in this study)
|
(Historical incidents)
|
|
Cylinder
failure
|
1.5E-4
|
3.44E-6
|
|
Truck
Rollover
|
9.9E-6
|
8.04E-7
|
|
Truck
Impact
|
2.0E-5
|
|
Truck
Fire
|
2.0E-7
|
8.5.15
For
external events which are earthquake and aircraft crash, the frequencies of
occurrence were determined using accident data in previous study. It is confirmed that adopting these
frequencies in current study are acceptable. Table 8‑7 provides the details.
Table 8‑7 External Event Frequencies
|
Scenarios
|
Event Frequency
|
Source
|
|
Aircraft
Crash
|
1.2E-08
per landing
|
Based
on US National Transportation Safety Board aircraft crash data
|
|
Earthquake
0.4g ground acceleration
|
1.00E-05
|
Cook
et al (1993)
|
|
Earthquake
0.6g ground acceleration
|
1.00E-06
|
General
8.6.2
Lethal
Dose (LD) contours provided by WSD from a wind tunnel test have been adopted in
this study.
8.6.3
The
advantage of wind tunnel testing over other consequence modelling tools is that
it can simulate the dense gas dispersion due to the effects of buildings and
complex terrain under different wind speed and direction.
8.6.4
Therefore,
adopting the results from the wind tunnel testing conducted can provide a best
estimation of consequences of chlorine gas release.
8.6.5
The
following subsections summarise the assumptions and other calculations used in
the consequence assessment.
Table 8‑8 Summary of Source Term Modelling
Details
|
Release case
|
Hole size (mm)
|
Phase
|
Mode of release to
atmosphere (internal release cases only
|
Release rate to
atmosphere or instantaneous release quantity
|
Release duration
|
|
External release (50kg
cylinder)
|
|
Small leak
|
2
|
Vapour
|
-
|
0.005 kg/s
|
167 min
|
|
|
|
Liquid
|
-
|
0.09 kg/s
|
9 min
|
|
Medium leak
|
5
|
Liquid
|
-
|
0.55 kg/s
|
1.5 min
|
|
|
|
Two-phase
|
-
|
0.14 kg/s
|
6 min
|
|
Multiple medium leaks
|
35 x 5
|
Two-phase
|
-
|
5.0 kg/s
|
6 min
|
|
Rupture
|
-
|
Liquid
|
-
|
50 kg
|
-
|
|
Internal releases (50kg
cylinder or chlorine pipework)
|
|
Pigtail-guillotine failure
|
2.5
|
Vapour
|
Normal
ventilation remains on
|
0.0016
kg/s
|
10 min
|
|
|
|
|
Door
left open
|
0.007 kg/s
|
10 min
|
|
Small leak from cylinder
|
2
|
Liquid
|
Normal
ventilation remains on
|
0.018
kg/s
|
10 min
|
|
|
|
|
Door
left open
|
0.008 kg/s
|
10 min
|
|
Medium leak from cylinder
|
5
|
Liquid
|
Normal
ventilation remains on
|
0.028 kg/s
|
10 min
|
|
|
|
|
Door
left open
|
0.013 kg/s
|
10 min
|
|
Rupture
(1 cylinder)
|
-
|
Liquid
|
Pressurisation
of chlorine store – release via weak points
|
0.06 kg/s
|
2s
|
|
Rupture
(5 cylinders)
|
-
|
Liquid
|
Pressurisation
of chlorine store release via weak points
|
1.5 kg/s
|
2s
|
Toxic Impact Assessment
8.6.7
Chlorine
Probit equation, shown below, was used for estimating the fatality rate of
human expose to chlorine gas, and hence the LD contours.
Pr
= -14.3 + ln C 2.3t
Pr
= Probit value
C = Chlorine Concentration (mg/l)
T = exposure time (minutes)
8.6.9
The
effective outdoor probability of fatality was applied in current assessment. Table 8‑9 presented the probability adopted in this study.
Table 8‑9 Effective Outdoors Probability of
Fatality
|
Nominal
Outdoor Fatality probability (for a person remaining outdoor)
|
%
of population attempting escape
|
Effective
outdoor fatality probability (taking into account the probability of escape)
|
|
90%
|
0%
|
90%
|
|
50%
|
80%
|
31%
|
|
3%
|
80%
|
0.7%
|
Sensitive Population Consideration
8.6.13
The
results of the wind tunnel tests with a wind speed of 2m/s
adopted in current study are summarised in Table 8‑10. Cloud dispersion contours for LD levels 90, 50
and 3 are adopted. These results will be scaled for risk modelling using RISKSUM.
Table 8‑10 Wind Tunnel Simulations for Tuen Mun WTW
|
Test No.
|
Release Location
|
Release Type
|
Release Rate or Quantity
|
Wind Direction (degrees)
|
|
1
|
Southeast
corner of Chlorine Store
|
Instantaneous
|
500kg
|
360,
30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330
|
|
2
|
Access
Road
|
Instantaneous
|
50kg
|
360,
60, 120, 180, 240, 300
|
|
3
|
Access
Road
|
Continuous
|
5kg/s
|
260,
60, 120, 180, 240, 300
|
8.6.14
To
model various chlorine dispersion scenarios, scaling factors were derived to
modify the cloud contours to estimate the consequences in different scenarios.
8.6.15
Rotational
transformation is adopted for “wind smoothing” purpose. Cloud dispersion
contours are re-generated every 10°
in wind direction using the closest wind direction contour.
8.6.16
Based
on the wind tunnel data, factors are applied to scale up/down wind tunnel data
for various release quantities and wind speed. However, effect of stability
class on the cloud geometry is considered not significant. Horizontal scaling factor follows ½
power law for release mass and a simple scaling factor 0.7 is applied for wind
speed 5ms-1. Overall scaling factors of various scenarios are given
in Table 8‑11 below.
Table 8‑11 Scaling Factors for Release
Events
|
Scenario
Reference
|
Release
Case
|
Horizontal
scaling factor (1/2 power law)
|
Wind
speed factor (D2)
|
Wind
speed factor (D5)
|
Overall
scaling factor (D2)
|
Overall
scaling factor (D5)
|
|
RU50MML
|
5 kg/s
continuous release
|
1
|
1
|
0.7
|
1
|
0.7
|
|
RU50SRU
|
50 kg
instantaneous
|
1
|
1
|
0.7
|
1
|
0.7
|
|
EU50MRU1
|
150 kg
instantaneous
|
1.7
|
1
|
0.7
|
1.7
|
1.19
|
|
EU50MRU2
|
1400kg instantaneous
|
1.7
|
1
|
0.7
|
1.7
|
1.19
|
|
EU50MRUH
|
1400kg instantaneous
|
1.7
|
1
|
0.7
|
1.7
|
1.19
|
8.6.17
These
scaled contours are served as the consequence input for the RISKSUM.
General
8.7.2
All
hazardous scenarios identified are modelled using RISKSUM.
8.7.3
With
the event frequencies, scaled LD contours and the population distributions, the
individual risk contours is produced and presented in Figure 8.5.
8.7.4
For
the 10-5 per year individual risk contour, it can be seen that it
extends beyond the site boundary to the immediate south west of the chlorine
store. However, this area is an unpopulated cut slope adjacent to the site and
therefore risk to actual offsite individual complies with the Risk Guidelines.
8.7.5
The
societal risk results for the Year 2009, with and without TMR widening works,
are presented in Figure 8.6 in a form of FN curves for comparison with
the Government Risk Guidelines.
8.7.6
From
Figure 8.6, it is demonstrated that there is an insignificant increase
in risk to nearby populations as well as construction workers at Tsing Tin
Interchange. The FN curves are located in the low ALARP region of the criteria.
8.7.7
The societal risk to the construction site workers
at TMR work site lies in the acceptable region as indicated in Figure 8.6.
Therefore, societal risk to construction workers is considered acceptable.
8.7.8
The
societal risk results for Year 2024 are presented in Figure 8.7 in a
form of FN curve for comparison with the Government Risk Guidelines. The FN curves of the Year 2024 case are
located in the low ALARP region of the criteria.
8.7.9
The
FN curve for the TMR population at Tsing Tin Interchange after road widening is
plotted in Figure 8.7 and lies in the acceptable region. Therefore, the societal risk
to transient TMR population is considered acceptable.
8.7.10
Risks
in terms of Individual Risk (IR) and Societal Risk are determined using RISKSUM. The IR at the TMR
widening work site is determined less than 1E-8 per year that is far below the
acceptance IR criteria (1E-5 per year).
8.7.11
The
FN curves demonstrated that there is no substantial increase in societal risk
during construction and the increase in traffic in TMR for Year 2009 and Year
2024 respectively.
General
8.8.1
The
societal risks of the TMWTW, identified in Section 8.7 and plotted in the FN
Curves, have been shown to lie in the low ALARP region of the Risk Guideline.
Therefore, mitigation measures have been identified and analysed.
8.8.2
The
mitigation measures identification exercise included a review on the candidate
mitigation measures proposed in the EIA Study Report of Route 8 (Formerly Route
16).
8.8.3
The
following sub-sections presented the candidate risk mitigation measures which
are relevant to current study and the results of the Cost-Benefit-Analysis
(CBA) of these measures.
Review of Candidate Risk
Mitigation Measures of Route 8 Study
8.8.4
Operation
and Construction stage risk mitigation measures of the Route 8 project are
reviewed with WSD and HyD. Measures, that are relevant and applicable to the
current project, are selected for CBA and presented in Table 8‑12
Table 8‑12 Candidate
Risk Mitigation Measures
|
Reference
|
Measures
|
|
|
Operation Stage
|
|
A
|
Provision of Road Enclosure
|
To be assessed in CBA.
|
|
Construction
Stage
|
|
B
|
Suspension of construction work during chlorine deliveries
|
To be assessed in CBA.
|
|
C
|
Enhance emergency response arrangements, e.g. provision of
visual and audible alarms, training etc.
|
Provision of a means of
alerting construction workers in case of chlorine release is considered an
effective mitigation measure. Therefore, it is recommended to implement as a
good practice.
|
8.8.5
Assumptions
of the relevant measures for further analysis by CBA are listed in Table 8‑13.
Table
8‑13 Assumptions of Mitigation
Measures for CBA
Options
|
Details
provisions
|
Project
Stage
|
|
A
|
Provision of 500m of Road Enclosure at Tsing Tin Interchange of TMR
(section of TMR with LD30 Contour reach)
|
Operation
|
|
B
|
Suspension of
construction work during chlorine deliveries (4 hours works suspension and 125 truck
delivery per year)
|
Construction
|
Analysis of
Mitigation Measures
8.8.6
In
this study, the cost effectiveness is assessed by CBA using calculation of the
Implied Cost of Averting Fatality (ICAF) for each mitigation measures
identified. The ICAF is calculated using the equation as follows by taking into
account the reduction in potential loss of life (PLL) using calculation
|
ICAF =
|
Cost
of Mitigation Measure
|
|
(Reduction
in PLL Value x Design Life of Mitigation Measure)
|
8.8.8
Aversion factor of 20
(Maximum Aversion Factor for risks at the upper region of the Risk Guidelines)
is proposed to adjust the Value of Life to reflect people’s aversion to high
risk. This is a conservative factor adopted even though the FN curves located
at the low ALARP region. With this factor applied, the adjusted Value of Life
of HK$660M will be adopted.
8.8.9
The
costs of implementation of these two options have been estimated and are
presented as follows.
8.8.10
For
Option A, it is conservatively assumed that the cost of construction of the
enclosure to be HK$150,000 per metre. This cost was previously adopted in the
Route 8 study. Therefore, a total cost of HK$75M is estimated to construct the
500m enclosure in TMR at Tsing Tin Interchange.
8.8.11
With
the enclosure, the population associated with the chlorine-affected area
assumed to be relocated to the portals of the enclosure (outside the cloud
affect zone). Hence the reduction of the PLL would be 4.74E-7.
8.8.12
For
a design life of the mitigation measures assumed to be 50 years, the ICAF for
this option would be HK$ 3,165,111M.
8.8.13
For
Option B, it is assumed that 4 hours works suspension is required due to
delivery of chlorine to TMWTW and on average HK$100 per hour for each worker.
In the 2 years construction period, total of 250 delivery of chlorine is
estimated (125 truck delivery per year adopted as a conservative estimation).
Therefore, a cost of HK$ 2M
is required. Therefore, the ICAF for option B is estimated to be HK$19,481,408M.
8.8.14
The
finding of the CBA is summarised in Table 8-14.
Table 8‑14 Results of CBA
Options
|
Estimated
Cost (HK$M)
|
PLL
Reduction
|
Design
Life (Years)
|
ICAF
(HK$M)
|
Cost-effective
Measures
|
|
A
|
75
|
4.74E-7
|
50
|
3,165,111
|
No (>HK$660M)
|
|
B
|
2
|
5.13E-8
|
2
|
19,481,408
|
No
(>HK$660M)
|
8.8.15
From Table 8-14, it is demonstrated that both
mitigation measures are not cost-effective and not reasonably practicable to
implement.
8.8.16
To
evaluate the justifiable expenditure on risk mitigation measures at this risk
level, Maximum Justifiable Expenditure (MJE) are determined as follows.
|
MJE =
|
Decrease
in PLL Value (per year) x Value of Life (HK$) x operating life time of Widen
TMR (years) x aversion factor
|
8.8.17
For
construction stage (Year 2009), the PLL at TMR worksite is found to be 5.13E-8
(per year). Therefore, the MJE for Year 2009, given below, is calculated to
HK$68.
|
MJE (Construction) =
|
5.13E-8
x 33E6 x 2 x 20
|
= HK$68
|
8.8.18
The
PLL at TMR is found to be 3.89E-7 (per year) and 4.74E-7 (per year) for Year
2009 and Year 2024 respectively. Therefore, the MJE for Year 2024, given below,
is calculated to be HK$2805.
|
MJE (Operation) =
|
(4.74E-7-3.89E-7)
x 33E6 x 50 x 20
|
= HK$2805
|
8.8.19
From
the MJE, expenditure of HK$2805 or below is justified to spend to reduce the
level of risk due to the project.
8.8.20
In
view that both options A and B assessed are considered not cost effective to
implement,
only the recommended good practice measure (Option “C”) as mentioned in Table 8‑12 are recommended. Details of option “C” are presented
in subsequent section 8.9.
8.9.1
The
assessment evaluated the risk associated with the TMWTW to the nearby
population including the increase of traffic in TMR in Year 2009 and Year 2024.
8.9.2
There
is no substantial increase of risk to construction workers, nearby population
and transient population due to widening of the TMR during construction stage
and operation stage.
8.9.3
Two
Candidate mitigation measures were identified and analysed using CBA. Both
options assessed are not cost-effective and reasonably practicable
to implement. However,
the following measures are provided for HyD to implement during construction
stage of the Widening works.
a)
The
number of workers on site during construction stage should be kept as the
level as assessed in this report.
b)
Emergency
evacuation procedures should be formulated and HyD should ensure that all
workers on site should be familiar with these procedures as well as the route
to escape in case of gas release incident occur. Relevant Departments, such as
WSD and FSD, should be consulted during the development of Emergency
procedures. Diagram showing the escape routes to a safe place should be posted
in the site notice boards and at the entrance/exit of site.
c)
The
emergency procedures should specify means of providing a rapid and direct
warning (e.g. Siren and Flashing Light) to construction workers in the event of
chlorine gas release in the TMWTW.
d)
The
construction site officer of HyD should establish a communication channel with
the TMWTW operation personnel during construction stage. In case of any
hazardous incidents in the treatment works, operation personnel of TMWTW should
advise the site officer to evacuate the construction workers.
8.9.4
Induction
Training should be provided to any staff before working on site at the Tsing
Tin Interchange work site.
8.9.5
Periodic
drills, with the participation of WSD, should be coordinated and conducted to
ensure all construction staffs are familiar with the evacuation procedures.
Upon completion of the drills, a review on every step taken should be conducted
to identify area of improvement.
8.10
Conclusion
8.10.2
Hazardous
scenarios associated with the chlorine storage and on-site transport of
chlorine in the TMWTW and the respective occurrence frequencies have been
identified and confirmed by reviewing of historical hazardous incident database
to ensure hazards and its respective occurrence frequencies are acceptable for
use.
8.10.3
A
QRA expressing population risks in both individual and societal terms has been
conducted to evaluate the risk to population in the vicinity of the TMWTW. This
assessment considered the future growth of nearby population and the increase
of traffic flow in future due to the opening of the HK-SWC and DBL.
8.10.4
From
the results of the QRA, individual risk at the construction site is found to be less than 1E-08 per
year which is acceptable in comparison with the criteria (1E-05 per year) in
the Risk Guidelines. With respect to societal risk, the FN curves indicated
that no significant change in the risk level during construction stage (Year
2009) and Operation Stage (Year 2024) of the project when compared with the
baselines FN curve.
8.10.5
Furthermore,
no construction hazard was identified that would affect the storage,
use or transport of chlorine in the TMWTW.
8.10.6
Mitigation
measures have been identified and demonstrated by CBA that selected measures
are not cost-effective to implement.
8.10.7
Recommended
“good practice” measures for the construction period are given in order to
ensure that the construction workers in the project site would understand the
evacuation procedures when working in the consultation zone of the TMWTW.
8.10.8
In
conclusion, the risks during construction and operation of the TMR project are
considered to be reduced to as low as reasonably practicable with the
implementation of the “Good Practice” measures during construction period.