5.
Hazard to life
5.1
Introduction
5.1.1 This section presents a
summary of the analysis and findings of the Hazard to Life Assessment (HA)
undertaken for the proposed construction and operation for this Project, in accordance
with Section 3.4.5 of the EIA Study Brief (ESB-288/2015).
5.1.2 This HA aims to achieve the
objectives specified in the Section 2.1 of the EIA Study Brief. Technical
requirements in the Study Brief are listed in the following:
3.4.5 Hazard to Human Life
3.4.5.1 The Applicant shall follow the criteria
for evaluating hazard to human life as stated in section 2 of Annex 4 of the
TM.
3.4.5.2 The hazard to human life assessment
shall follow the detailed technical requirements given in Appendix C.
Appendix C – Requirements for Assessment of
Hazard to Human Life
1.
The Applicant shall carry out hazard assessment to evaluate potential
hazard to life during construction and operation stages of the Project due to
Silvermine Bay Water Treatment Works (SMBWTW). The hazard assessment shall
include the following:
(i)
Identify hazardous
scenarios associated with the transport, storage and use of chlorine at SMBWTW
and then determine a set of relevant scenarios to be included in a Quantitative
Risk Assessment (QRA);
(ii)
Execute a QRA of
the set of hazardous scenarios determined in (i), expressing population risks
in both individual and societal terms;
(iii)
Compare individual
and societal risks with the criteria for evaluating hazard to life stipulated
in Annex 4 of the TM; and
(iv)
Identify and assess
practicable and cost-effective risk mitigation measures.
2.
The methodology to be used in the hazard assessment shall be agreed with
the Director and should be consistent with previous studies having similar
issues (e.g. Integration of Siu Ho Wan and Silver Mine Bay Water Treatment
Works)
5.2
Environmental Legislation, Standards and Guidelines
5.2.1 The key legislation and
guidelines that are considered relevant to the Project are as follows:
· Dangerous
Goods Ordinance, Chapter 295; and
· Environmental
Impact Assessment Ordinance (EIAO), Chapter 499.
-
EIAO Technical
Memorandum (EIAO-TM)
-
Hong Kong
Government Risk Guidelines (HKRG), EIAO TM Annex 4
5.3
Study Approach
5.3.1 The overall approach to the
assessment is represented in Figure 5.1.
5.3.2 The major phases in the HA
are:
(i)
Hazard Identification: Identify hazard scenarios associated with the transport, storage and
use of chlorine, and then determine a set of relevant scenarios to be included
in a QRA.
(ii)
Frequency Assessment: Assess the likelihood of occurrence of the identified hazard scenarios.
(iii)
Consequence Assessment: Assess the consequences and impact to the surrounding population.
(iv)
Risk Summation and Assessment: Evaluate the risk level, in terms of individual risk and societal risk.
The risks will be compared with the criteria stipulated in Annex 4 of the
Environmental Impact Assessment Ordinance – Technical Memorandum (EIAO-TM)[1] to determine their
acceptability.
(v)
Identification of Mitigation Measures:
Identify and assess practicable and
cost-effective risk mitigation measures. The risks of mitigated cases will then
be reassessed to determine the level of risk reduction.
5.3.3
When conducting the HA, reference to studies of similar nature or
facility in Hong Kong will be made, so as to maintain consistency with similar
studies in Hong Kong. The studies proposed to be collected and reviewed are:
(i)
An Environmental Impact
Assessment (EIA) Report for “Integration of Siu Ho Wan and Silver Mine Bay
Water Treatment Works” (Register No.: AEIAR-158/2011) (the “Previous Report”)[2] was prepared in 2010 and
approved by EPD in 2011. Hazardous scenario identified and assessed in the Hazard
Assessment of this report will be adopted as a basis for the current HA.
(ii)
In 2009, a feasibility study was carried out for a Potential Housing
Site in Tuen Mun East Area by the Planning Department (PlanD) and Civil
Engineering and Development Department (CEDD)[3]. In that report the Tai Lam
Chung No.2 Chlorination Station (now called Tai Lam Chung Chlorination Store)
was studied in form of a QRA. Since the methodology and modelling approach of
the report is very similar to that used in the Previous Report, that report
will be studied and referenced where applicable.
5.3.4 The overall approach and
methodology of the assessment were discussed in detail in Methodology Paper and
was agreed by Environmental Protection Department.
5.4
Risk Acceptability Criteria
5.4.1 As stipulated in the Annex
4 of the EIAO-TM, the risk guidelines comprise two measures shown as follows:
i. 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.
ii. Societal Risk: it can be presented graphically as in Figure 5.2. The Societal Risk Guideline is expressed in terms of lines plotting the frequency (F) of N or more fatalities in the population from accidents at the facility of concern. In the figure, ALARP means As Low As Reasonably Practicable. Risk in this region should be mitigated to As Low As Reasonably Practicable.
5.5
Cases to be Studied
5.5.1 Four cases were considered
in this study to demonstrate the changes in risk level caused by the proposed
development:
5.5.2 In the calibration case,
all the available parameters in the Previous Report were adopted. Individual
risk contour and societal risk FN curve were generated using the in-house risk
summation software RISKSUM, so as to demonstrate that the simulation approach
is able to produce similar result as the Previous Report.
Baseline without proposed Project (year 2020)
5.5.3 The baseline study assessed
the risk at the operation year (year 2020) without the proposed project, i.e.
without the Lai Chi Yuen Cemetery extension.
Construction Stage (year 2019)
5.5.4 The construction case
assessed the risk of increased construction workers at the proposed site. Year
2019 is the tentative final year of construction of the proposed project.
5.5.5 The operation case assessed
the increase of risk due to the increase of population in Lai Chi Yuen Cemetery
as well as the associated roads due to the operation of the Lai Chi Yuen
Cemetery extension.
5.6
Meteorological Data
5.6.1 The meteorological
conditions affect the consequence of release in particular the wind direction,
speed and stability, which influences the direction and degree of turbulence of
gas dispersion.
5.6.2 For consistency,
meteorological data used in the Previous Report was adopted for calibration
case. It is summarized in Table 5.1 and Table 5.2. Although
the sum of the frequencies is 0.5 in those tables, during actual calculation
the frequencies were automatically normalized to 1 for day time and night time
respectively.
Table 5.1 Statistics
of Frequencies of Different Weather Categories (Day Time - SMBWTW)
|
Sector |
Degree from North |
B |
D |
D |
D |
E |
F |
Total |
|
1 |
0o |
0.0025 |
0.00125 |
0.02574 |
0.0085 |
0 |
0 |
0.03798 |
|
2 |
22.5o |
0.00545 |
0.0019 |
0.01519 |
0.00265 |
0 |
0 |
0.02519 |
|
3 |
45o |
0.0062 |
0.00265 |
0.01854 |
0.01035 |
0 |
0 |
0.03773 |
|
4 |
67.5o |
0.0034 |
0.00215 |
0.04223 |
0.06362 |
0 |
0 |
0.11141 |
|
5 |
90o |
0.00165 |
0.00105 |
0.03264 |
0.04323 |
0 |
0 |
0.07857 |
|
6 |
112.5o |
0.00085 |
0.0007 |
0.00805 |
0.00755 |
0 |
0 |
0.01714 |
|
7 |
135o |
0.0009 |
0.00055 |
0.0039 |
0.003 |
0 |
0 |
0.00835 |
|
8 |
157.5o |
0.00065 |
0.0008 |
0.01364 |
0.0039 |
0 |
0 |
0.01899 |
|
9 |
180o |
0.0007 |
0.00085 |
0.01529 |
0.0025 |
0 |
0 |
0.01934 |
|
10 |
202.5o |
0.0007 |
0.0006 |
0.0071 |
0.002 |
0 |
0 |
0.01040 |
|
11 |
225o |
0.00115 |
0.00115 |
0.007 |
0.00165 |
0 |
0 |
0.01095 |
|
12 |
247.5o |
0.00305 |
0.00155 |
0.01614 |
0.00065 |
0 |
0 |
0.02139 |
|
13 |
270o |
0.0067 |
0.0017 |
0.02064 |
0.00125 |
0 |
0 |
0.03029 |
|
14 |
292.5o |
0.00675 |
0.0009 |
0.01574 |
0.00445 |
0 |
0 |
0.02784 |
|
15 |
315o |
0.0039 |
0.0009 |
0.00875 |
0.0041 |
0 |
0 |
0.01765 |
|
16 |
337.5o |
0.0036 |
0.001 |
0.01619 |
0.006 |
0 |
0 |
0.02679 |
|
Total |
0.04813 |
0.01969 |
0.26679 |
0.16538 |
0 |
0 |
0.5 |
|
|
Wind Speed (m/s) |
15 |
1 |
4 |
7 |
3 |
1 |
|
|
Table 5.2 Statistics of Frequencies of Different Weather Categories (Night Time - SMBWTW)
|
Sector |
Degree
from North |
B |
D |
D |
D |
E |
F |
Total |
|
1 |
0o |
0 |
0.00066 |
0.0099 |
0.00413 |
0.00388 |
0.00434 |
0.02291 |
|
2 |
22.5o |
0 |
0.00087 |
0.01668 |
0.00582 |
0.01056 |
0.0077 |
0.04163 |
|
3 |
45o |
0 |
0.00133 |
0.00949 |
0.00184 |
0.00832 |
0.01031 |
0.03129 |
|
4 |
67.5o |
0 |
0.00179 |
0.01143 |
0.00709 |
0.01 |
0.01454 |
0.04485 |
|
5 |
90o |
0 |
0.00148 |
0.02683 |
0.04357 |
0.01643 |
0.01888 |
0.10719 |
|
6 |
112.5o |
0 |
0.00071 |
0.02112 |
0.01939 |
0.01602 |
0.01454 |
0.07178 |
|
7 |
135o |
0 |
0.00046 |
0.005 |
0.00515 |
0.00403 |
0.00826 |
0.0229 |
|
8 |
157.5o |
0 |
0.00036 |
0.00224 |
0.00204 |
0.00189 |
0.00668 |
0.01321 |
|
9 |
180o |
0 |
0.00056 |
0.00826 |
0.0027 |
0.00679 |
0.00944 |
0.02775 |
|
10 |
202.5o |
0 |
0.00061 |
0.00959 |
0.00173 |
0.00832 |
0.00867 |
0.02892 |
|
11 |
225o |
0 |
0.00041 |
0.00423 |
0.00138 |
0.00413 |
0.00525 |
0.0154 |
|
12 |
247.5o |
0 |
0.00077 |
0.00393 |
0.00112 |
0.00301 |
0.00622 |
0.01505 |
|
13 |
270o |
0 |
0.00107 |
0.00571 |
0.00046 |
0.00138 |
0.00531 |
0.01393 |
|
14 |
292.5o |
0 |
0.00117 |
0.00668 |
0.00082 |
0.00128 |
0.00454 |
0.01449 |
|
15 |
315o |
0 |
0.00061 |
0.00724 |
0.00301 |
0.00173 |
0.00275 |
0.01534 |
|
16 |
337.5o |
0 |
0.00061 |
0.0051 |
0.00281 |
0.00209 |
0.00275 |
0.01336 |
|
Total |
0 |
0.01347 |
0.15345 |
0.10305 |
0.09984 |
0.13019 |
0.5 |
|
|
Wind
Speed (m/s) |
15 |
1 |
4 |
7 |
3 |
1 |
|
|
5.6.3 Other than the “Calibration
Case” (i.e. “baseline”, “construction’ and “operation” cases), the average meteorological data from Peng Chau automatic weather
station from year 2006 to year 2015 was adopted from Hong Kong Observatory.
Peng Chau automatic weather station is the nearest station in Hong Kong
Observatory that has similar altitude to the Mui Wo Area to best represent the
meteorological condition of the area.
5.6.4 In order to maintain
consistency with the Previous Report, the number of wind directions and weather
classes were followed. The average wind speeds and wind directions were classified
as daytime (0700-1900 hrs) and night time (1900-0700 hrs) and are summarized in
the following tables and wind roses (Figure 5.3 and Figure 5.4). As illustrated by the
wind roses, the prevailing wind directions in the area are from east (E)
followed by from north north-west.
Table 5.3 Average Wind Statistics for Daytime (Peng Chau 2006-2015)
|
Sector |
Degree from North |
B |
D |
D |
D |
E |
F |
Total |
|
1 |
0o |
0.01631 |
0.00463 |
0.01573 |
0.01442 |
0.00485 |
0.00633 |
0.06227 |
|
2 |
22.5o |
0.00716 |
0.00217 |
0.00236 |
0.00179 |
0.00088 |
0.00227 |
0.01664 |
|
3 |
45o |
0.00805 |
0.00208 |
0.00501 |
0.00136 |
0.00155 |
0.00198 |
0.02003 |
|
4 |
67.5o |
0.01824 |
0.00246 |
0.01509 |
0.01533 |
0.00306 |
0.00222 |
0.05640 |
|
5 |
90o |
0.08947 |
0.00487 |
0.04634 |
0.11857 |
0.00559 |
0.00489 |
0.26973 |
|
6 |
112.5o |
0.03522 |
0.00320 |
0.00595 |
0.00702 |
0.00179 |
0.00339 |
0.05656 |
|
7 |
135o |
0.04350 |
0.00401 |
0.00599 |
0.00148 |
0.00177 |
0.00435 |
0.06110 |
|
8 |
157.5o |
0.04377 |
0.00456 |
0.00776 |
0.00086 |
0.00205 |
0.00442 |
0.06342 |
|
9 |
180o |
0.05188 |
0.00929 |
0.00941 |
0.00062 |
0.00246 |
0.00707 |
0.08073 |
|
10 |
202.5o |
0.05515 |
0.00795 |
0.00766 |
0.00014 |
0.00196 |
0.00602 |
0.07889 |
|
11 |
225o |
0.02075 |
0.00346 |
0.00489 |
0.00043 |
0.00141 |
0.00399 |
0.03493 |
|
12 |
247.5o |
0.00122 |
0.00081 |
0.00050 |
0.00007 |
0.00029 |
0.00122 |
0.00411 |
|
13 |
270o |
0.00267 |
0.00107 |
0.00050 |
0.00010 |
0.00029 |
0.00186 |
0.00649 |
|
14 |
292.5o |
0.02225 |
0.00131 |
0.00568 |
0.00296 |
0.00153 |
0.00420 |
0.03794 |
|
15 |
315o |
0.02414 |
0.00239 |
0.01382 |
0.01263 |
0.00313 |
0.00544 |
0.06155 |
|
16 |
337.5o |
0.02125 |
0.00363 |
0.02731 |
0.02509 |
0.00614 |
0.00578 |
0.08920 |
|
Total |
0.46103 |
0.05790 |
0.17404 |
0.20288 |
0.03873 |
0.06542 |
1.00000 |
|
|
Wind Speed (m/s) |
15 |
1 |
4 |
7 |
3 |
1 |
|
|
Table 5.4 Average Wind Statistics for Night-time (Peng Chau 2006-2015)
|
Sector |
Degree from North |
B |
D |
D |
D |
E |
F |
Total |
|
1 |
0o |
0.00000 |
0.00007 |
0.02584 |
0.01628 |
0.01495 |
0.02478 |
0.08192 |
|
2 |
22.5o |
0.00000 |
0.00012 |
0.00284 |
0.00125 |
0.00217 |
0.00698 |
0.01336 |
|
3 |
45o |
0.00000 |
0.00010 |
0.00864 |
0.00147 |
0.00578 |
0.00708 |
0.02307 |
|
4 |
67.5o |
0.00000 |
0.00010 |
0.02904 |
0.02018 |
0.00987 |
0.00867 |
0.06786 |
|
5 |
90o |
0.00000 |
0.00012 |
0.09186 |
0.13521 |
0.02290 |
0.01705 |
0.26714 |
|
6 |
112.5o |
0.00000 |
0.00012 |
0.01471 |
0.00660 |
0.00946 |
0.01864 |
0.04953 |
|
7 |
135o |
0.00000 |
0.00005 |
0.01026 |
0.00154 |
0.00867 |
0.02155 |
0.04207 |
|
8 |
157.5o |
0.00000 |
0.00007 |
0.01081 |
0.00065 |
0.01074 |
0.02463 |
0.04691 |
|
9 |
180o |
0.00000 |
0.00010 |
0.01731 |
0.00075 |
0.02032 |
0.04801 |
0.08649 |
|
10 |
202.5o |
0.00000 |
0.00005 |
0.00809 |
0.00034 |
0.01382 |
0.04226 |
0.06456 |
|
11 |
225o |
0.00000 |
0.00007 |
0.00602 |
0.00084 |
0.00778 |
0.02463 |
0.03935 |
|
12 |
247.5o |
0.00000 |
0.00005 |
0.00082 |
0.00010 |
0.00089 |
0.01009 |
0.01194 |
|
13 |
270o |
0.00000 |
0.00005 |
0.00017 |
0.00014 |
0.00031 |
0.01233 |
0.01300 |
|
14 |
292.5o |
0.00000 |
0.00002 |
0.00320 |
0.00205 |
0.00178 |
0.02023 |
0.02728 |
|
15 |
315o |
0.00000 |
0.00012 |
0.01700 |
0.01043 |
0.00848 |
0.02374 |
0.05977 |
|
16 |
337.5o |
0.00000 |
0.00024 |
0.03942 |
0.02601 |
0.01842 |
0.02167 |
0.10576 |
|
Total |
0.00000 |
0.00144 |
0.28604 |
0.22382 |
0.15635 |
0.33235 |
1.00000 |
|
|
Wind Speed
(m/s) |
15 |
1 |
4 |
7 |
3 |
1 |
|
|
5.7
Review of the Current Site
5.7.1 Lai Chi Yuen Cemetery is located
at the south of Mui Wo. It is in the middle of Nam Shan and is at about
+100mPD. Surrounding the site is mostly hilly area. There is only one small
access road connecting the project site to the South Lantau Road. A photo of
the Lai Chi Yuen Cemetery is shown in Figure 5.5. The entrance is at the
top of the slope and the Cemetery span downhill with burial spaces arranged in
levels along the slope.
5.7.2 The proposed project is an
extension of current Cemetery. It is located at the entrance of the Lai Chi
Yuen Cemetery, in form of an elevated platform to accommodate 790 outdoor
niches and ancillary facility including a joss paper burner and planters.
5.7.3 The location of the Lai Chi
Yuen Cemetery and its relative location to SMBWTW is shown in Figure 5.6. The chlorine store within
SMBWTW is separated from the project site for about 700m. The consultation zone
of SMBWTW as stipulated in the Potentially Hazardous Installations (PHI) register
is illustrated in Figure 5.7.
5.7.4 No Dangerous Goods is
stored and will be stored in the Lai Chi Yuen Cemetery site.
5.8
Review of SMBWTW
5.8.1
Chlorine is used in this water treatment works to disinfect the water before
sending out to the potable water network. During operation, two chlorine drums
are moved to the designated loading area, where they are connected to fixed
pipework for chlorination process. One drum is the duty drum and another one is
in standby. When the chlorine in the duty drum is depleted, the standby drum
will automatically kick in to replace the duty drum, by the auto-changeover
system in the pipework.
5.8.2
Chlorine is transported to SMBWTW by truck
in form of 1-tonne chlorine drums. The chlorine in the drum is stored in liquid
form at equilibrium pressure inside the drum which varies with ambient
temperature.
5.8.3
A site visit to SMBWTW was carried out on 23rd Nov. 2015.
WSD has confirmed that there is no change in plant operation parameters
compared to the Previous Report. The design parameters are summarized in the
following table:
Table 5.5 Design Parameters of SMBWTW
|
Parameter |
Value |
|
Design treatment capacity |
159,000 m3/day |
|
Form of chlorine storage |
1-tonne drum |
|
Chlorine inventory |
Max. 17 tonnes |
|
Number of evaporators |
2 |
|
Number of Chlorinators |
3 |
|
Chlorine dosage rate |
2-5ppm |
|
Number of chlorine drums delivered per truck |
6 |
5.8.4 Layout of SMBWTW is shown
in Figure 5.8.
5.8.5
The delivery of chlorine is calculated based on the
design dosage rate of chlorine into the water. It is assumed that the chlorine
delivery is carried out during daytime.
5.8.6
The chlorine store is equipped with chlorine detection
system and “Contain-and-Absorb” system. The chlorine detection system can
detect the presence of chlorine as low as 3 ppm and automatically activate
alarms as well as the “Contain-and-Absorb” system. Once it is activated, the
air inside the chlorine store will be directed to a chlorine scrubber which can
absorb the gaseous chlorine and the treated air will be circulated back to the
chlorine store.
5.8.7
The scrubber is a packed tower with sodium
hydroxide solution as the absorbing media. The sodium hydroxide solution in the
chlorine scrubber is designed to treat 1-tonne of chlorine release.
5.8.8 Unloading of chlorine from
truck to the store is carried out within the store with the roller shutter shut
off.
5.9
Population Assumptions Adopted in this Study
5.9.1 Population data to be
considered include the 2010 population assumption from the past report (for the
calibration case), and population around SMBWTW
which may be affected by the hazardous scenarios of SMBWTW. This would cover
the 1km Consultation Zone (CZ), as well as population outside the CZ which
might be affected by potential chlorine releases. The population area
considered in this study is illustrated in Appendix 5.1.
5.9.2 Estimation of future
population is based on the following sources:
i.
Assumptions made in “Integration of SHW and SMBWTW EIA” report
ii.
Hong Kong Population Census and other population statistics published by
Census and Statistics Department (C&SD)
iii.
Traffic Impact Assessment (TIA) by traffic consultant
iv.
Desktop search conducted by consultants
v.
Site surveys undertaken by the consultants
General Population Growth Rate
5.9.3 The growth rate of
residential population was estimated from the “Projections of Population Distribution 2015-2024” published by
Planning Department and C&SD. Other populations such as those in the
commercial area would follow the assumption in Previous Report with additional
population growth to account for the major civil works in the area (the Mui Wo
Facelift, further discussed in Section 5.9.13).
5.9.4
The population projection of Tertiary Planning Unit (TPU) 962 is
representing the Mui Wo area (Figure
5.9). Projected population by TPU at
units 961 and 962 is listed in the following table:
Table 5.6 Population Projection for TPU 961 and 962
|
Tertiary Planning Units,
TPU(s) |
Projected Population (1) |
||||||
|
2014 |
2015 |
2016 |
2017 |
2018 |
2019 |
2020 |
|
|
961 & 962 |
5 900 |
6 000 |
6 000 |
6 000 |
6 100 |
7 800 |
7 800 |
|
% change
from past year |
N/A |
2% |
0% |
0% |
2% |
28% |
0% |
Note:
(1) Data extracted from Table 15 of the "Projections
of Population Distribution 2015-2024" Report
5.9.5 The general population
trend is about stagnant or a 2% annual increase in some years (year 2015 and
year 2018). The sudden jump in year 2019 corresponds to the new public housing
development in Mui Wo. From the above data, an annual population growth rate of
1.5% is approximated for residential population in the study area.
Major Population Groups within the Study Area
5.9.6
At present, FEHD manages the existing 276
coffin burial spaces and 332 urn burial spaces provided in Lai Chi Yuen
Cemetery. Among which, 149 coffin burial spaces and 224 urn burial spaces are
occupied as in May 2014.
5.9.7
The current project proposes to construct
a platform next to the entrance of Lai Chi Yuen Cemetery to accommodate some
790 niches. Hence, there will be a total of 1398 burial spaces/niches in Lai
Chi Yuen Cemetery upon full development of the cemetery.
5.9.8
In this study, it is conservatively
assumed that the number of visitors to the Cemetery on Festival Day and the
following Sunday equals to the number of burial spaces/niches.
5.9.9
During the site survey carried out by
traffic consultant in year 2014 Ching Ming Festival, there were about 361
visitors visited the cemetery, which corresponded to about 1 visitor per
occupied niche. This ratio was used to project the population pattern of the
Cemetery after the construction of new niches.
5.9.10
During construction stage 50 construction
workers is assumed to be working on site.
5.9.11 Mui Wo town centre
can be briefly divided into two parts. The southern part is the area around the
Mui Wo public pier. The area comprises of pier, bus terminal, carparks,
restaurants and mixed commercial/ residential buildings (Location ID: 21, 14B,
15). The northern part is mainly residential and municipal facilities such as
garden, playground, public swimming pools and sport complexes. There is also a
hotel along the Silver Mine Bay Beach and a number of rental houses scattered
in this area. Compared with the Previous Report, there are a number of
public housing development near Ngan Wan Estate (Location ID: 10 and 4E) which
will increase the population in the area.
5.9.12 The population groups in
the Mui Wo town centre area as listed in the Previous Report were checked individually
through site survey and desktop study.
Mui Wo Facelift (The Improvement Works in Mui Wo)
5.9.13 CEDD has commenced Mui Wo
Improvement Works
which aimed at improving the Mui Wo Town Centre in various aspects. According to CEDD, the Phase 1 and Phase 2 (stage 1) Works are now
in construction stage and will be completed by year 2018. The remaining work
(Phase 2 (stage 2)) is targeted to be commenced after Phase 2 (stage 1), i.e.
in year 2018. However, there is no definite schedule of the work and is
subjected to funding approval.
5.9.14 The major works in Mui Wo
town area involve the following:
·
Phase
1 works:
1.
a 230-metre (m) long and 11.5-m wide segregated pedestrian walkway and
cycle track along the waterfront between Mui Wo Cooked Food Market and River
Silver
2.
a 35-m long and 4.8-m wide footbridge across River Silver
3.
a civic square near Mui Wo old town with associated landscaped area,
recreational and leisure facilities and a performance venue
4.
seven amenity areas in villages at Mui Wo
5.
ancillary works including signage, landscaping, drainage and utilities
works
·
Phase 2 Stage 1 works:
1.
realignment of Mui Wo Ferry Pier Road and extension of existing car park
2.
ancillary works including slope works, signage, landscaping, drainage
and utilities works
·
Remaining
phases include:
1.
Re-provisioning of new cooked food market and covered cycle parking area
2.
improvement to south waterfront promenade
3.
re-provisioning of cargo loading and unloading area
4.
enhancement of Entrance Plaza
5.
improvement/provision of cycle tracks and heritage trails in Mui Wo
5.9.15 The population of the above
new/improved facilities were either obtained from the user departments or
assumed based on the best available information.
5.9.16 Although there is no new
commercial area to be developed under the “Mui Wo Facelift”, a 3% annual
population growth, which is in-line with the increase in transient population,
is assumed for the existing commercial population in the Mui Wo town area.
Facilities for children and elderly
5.9.17
In the “Integration of SHW and SMBWTW EIA” study, there was a Home
for the elderly (Location ID: 26) near Mui Wo River Silver Garden. However
according to the latest government map and the site visit carried
out by consultant on 23 Nov 2015, the home of the elderly has been relocated.
The home of the elderly is confirmed to be located inside Silver Plaza
(Location ID: 26)
5.9.18 Another elderly home
identified within the site is the Silver View Center for the Aged (Location ID:
4C_EH), which is in the Block 5-8 of Silver View Center (Location ID: 4C). The
maximum capacity is 32 residents. Maximum capacity is assumed in this study.
Number of staff is approximated according to the staff requirement as set out
by Social Welfare Department.
5.9.19 There are a number of
kindergartens within the Consultation Zone:
(i)
The Lick Hang Kindergarten
(Leafy Branch) is in Ling Tsui Tau (Location ID: 4G) and in Ma Po Village (as
part of Location ID: 18E).
(ii)
The Little Lamb Education is
next to Lick Hang Kindergarten in Ma Po Village (part of Location ID: 18E).
(iii)
The Mui Wo OWLS School is
in Mui Wo South (Location ID: 14D)
(iv)
The Little Lantau Montessori
Kindergarten is in Silver View Center (Location ID: 4C_K)
(v)
The Kind Hing Trinity International Kindergarten
is near the Mui Wo Ferry Pier car park (Location ID: 14E)
5.9.20 The populations of these
kindergartens were collected through the datasheets from Education Bureau or
from site survey carried out by Consultant.
5.9.21 The only primary school
within the consultation zone is Mui Wo School which is located in Tai Tei Tong.
Another school The Heung Yee Kuk Southern District Secondary School included in
“Integration of SHW and SMBWTW EIA” (Location ID: 15) has been discontinued
since year 2007.
Area affected by Ching Ming
/ Chung Yeung Festival
5.9.22 Mui Wo South (Bus Terminus,
Ferry Pier, restaurants (Location ID: 21) as well as the Commercial Area
(Location ID: 14C) near the pier were expected to have increase in population
due to the Ching Ming / Chung Yeung Festival. The number of visitors per day on
Festival Day and the following Sunday were assumed to be equal to the number of
available niches in the cemetery (as discussed in Section 5.9.27). Each visitor is further assumed to stay in the area (pier and the
commercial area) for a total of 2 hours. By using these assumptions, the
population increase during Festival Day for base case, construction case and
operation case can be deduced respectively. It would be added to the Sunday
population to derive the Festival Day population.
5.9.23 The population may vary
within the day. To account for this, the following time periods would be
adopted.
Table 5.7 Definition of Time Periods
|
ID |
Time Period |
Period |
Weighing per day |
% Distribution per year |
|
TM1 |
Working Day |
Monday to Friday |
12 hours |
33.38% |
|
TM2 |
Saturday Day |
Saturday |
12 hours |
7.14% |
|
TM3 |
Holiday Day |
Sunday and public holidays (except Ching Ming and Chung Yeung) |
12 hours |
8.93% |
|
TM4 |
Night |
Monday to Sunday |
12 hours |
50% |
|
TM5 |
Ching Ming and Chung Yeung (Day) |
4 days - daytime |
12 hours |
0.55% |
Note:
It is assumed that
there are 52 weeks a year, and 17 public holidays in Hong Kong per year which
are all on weekdays
5.9.24 The following shows the
details on how the Time Periods are defined:
TM1 = (52 weeks * 5 weekdays per week – 17 public holidays) / 365 * 0.5
= 0.3338
TM2 = (52 weeks * 1 Saturday per week) / 365 * 0.5 = 0.0714
TM3 = (52 weeks * 1 Sunday per week + 17 public holidays – Ching Ming
& Chung Yeung and following Sundays (4 days)) / 365 * 0.5 = 0.08929
TM4 = 12 hours per 24 hours = 0.5
TM5 = 4 days per year / 365 * 0.5 = 0.00548
5.9.25 Time period 5 (TM5), which
represents the population during Ching Ming and Chung Yeung Festival, as well
as the Sunday following these festivals, is added to this study compared with
the time period assumption in Previous Report.
5.9.26 During TM5 the majority of
the population will have the same population fraction as Sunday (TM3), plus
some additional population (608 for base case, 1392 for operation case, derived
from the number burial places in their respective phase) to the relevant
population group such as the pier and bus terminus (Location ID: 21) and the
commercial area near the pier (Location ID: 14C).
5.9.27 TIA has carried out a
survey in 2014 Ching Ming Festival and summarized that for Lai Chi Yuen
Cemetery the number of visitors on Ching Ming Day was approximately equal to
the number of occupied niches. And the Sunday following the Ching Ming Day
would have visitors approximately equal to 20% of the number of occupied
niches. In this study, Festival Day and the following Sunday were assumed to
attract number of visitors equal to the total number of available niches, which
is a very conservative assumption. As a result, 4 days per year for TM5 is
assumed, as shown in the above calculation.
5.9.28 Each population is assigned
with an indoor fraction which indicates the portion of population which can
receive extra protection by staying indoor. The indoor fraction used in this
study generally followed the assumption of the Previous Report based on the
population type.
5.9.29 In Previous Study, for the
modelling of Siu Ho Wan Water Treatment Works, the road population was modelled
as 95% indoor and “normal indoor population” as indicated in Table 9.8.2 and
Section 9.7.10 of Previous Report. However, considering that the traffic speed
in the current study is relatively low (50km/hr or 30km/hr), it is more
conservative to assume that the population in vehicle as 100% outdoor.
Population associated with the road vehicles as well as pedestrian were modelled
as 100% outdoor, which is consistent with other
similar studies.[4]
5.9.30 There is only one major
transient population within the consultation zone which is the South Lantau
Road. Other roads such as the access road to Lai Chi Yuen Cemetery/Lai Chi Yuen
Tsuen, Ngan Kwong Wan Road etc. were also considered in this study although
they have significantly fewer population.
5.9.31
Road traffic as well as pedestrian population were assumed based
on the findings in Traffic Impact Assessment (TIA) and Annual Traffic Census (ATC)
conducted by Transport Department (TD).
5.9.32 The ATC counting station
used in this assessment is Core Station 5015 (South Lantau Road (from Mui Wo
Ferry Pier to Chi Ma Wan Road)). For roads other than South Lantau Road, the
traffic characteristics (peak hour traffic flow w.r.t. normal hour traffic
flow, distribution of traffic between weekday and weekend, day and night, etc.)
were assumed to also follow South Lantau Road since all other roads receive the
traffic from this road, unless specific data is available in TIA.
5.9.33 The TIA has calculated the
total trip generations by the existing cemetery as well as the proposed
development. The result can be summarized in the following table:
Table 5.8 Peak hour trip Generation (Person/Hour) for Existing Cemetery and Proposed Development in TIA
|
Existing (vacant plus occupied coffin burial or
urn burial) |
Existing plus proposed development |
|
748 |
1713 |
5.9.34 The numbers in the above
table were used to derive the transient population in festival days (TM5). The population
in other time periods was derived using the weekly traffic flow variation graph
correlation in ATC.
5.9.35 The peak hour trip
generation (person/hour) is preferred instead of the number of vehicle per hour
forecast (both data can be found from TIA) because the traffic in Ching
Ming/Chung Yeung peak hour with the Special Bus services has a much higher
occupancy ratio. Direct addition of the vehicle trip generation figure to the
ATC based AADT figure will lead to a relatively not conservative figure.
5.9.36 The TIA also analysed the
annual traffic growth rate. It has assumed that the annual growth rate is 3%
for weekdays (starting from year 2014) and 6% for weekends (the extra 3% growth
rate is due to the public housing development in Mui Wo). These factors were used
to calculate traffic population in year 2019 and year 2020 from the ATC data in
year 2014.
5.9.37 Since occupancy per vehicle
data was not available in both ATC and TIA for South Lantau Road, in this
assessment it is assumed to be similar to the pattern of traffic station 5014
(Route Twisk from Chuen Lung to Cheung Pei Shan Road),
which is a major rural road similar to South Island Road. 2.0 passengers per
vehicle is thus conservatively assumed in order to calculate the road
population from traffic data.
South Lantau Road (Location ID: R1)
5.9.38 South Lantau Road is the
only major road connecting Mui Wo to other part of the Lantau Island. Annual
Traffic Census (ATC) has been conducted on this road by Transport Department
(Core station 5015). The daily variation graph showed that about 80% of traffic
happens in daytime (0700-1900) and 20% of traffic in night time (1900-0700). It
also showed that the normal hour traffic flow is about 80% of peak hour traffic
flow.
5.9.39 The speed of vehicle is
assumed to be 50 km/hr at all times[5].
The access road to Lai Chi Yuen Tsuen (Location ID: R2)
5.9.40 The road is used to
access Lai Chi Yuen Tsuen and surrounding areas. However, during Ching
Ming / Chung Yeung Days many private cars/taxis and other buses make use of the
road to assess Lai Chi Yuen Cemetery. There is also substantial increase in
pedestrian population on the road during Ching Ming / Chung Yeung Day. The TIA
finding is used to calculate the population.
5.9.41 The speed of vehicle is
assumed to be 30 km/hr at all times conservatively, due to the relatively poor
road condition.
The footpaths at the junction of South Lantau Road and access road to Lai Chi Yuen Cemetery (Location ID: P1, P2, P3)
5.9.42
TIA has carried out site survey in year 2014 to count the pedestrian
flow in Ching Ming Day, weekday and Sunday for the
footpath P1, P2 and P3, which are the major footpaths that will be affected by
the proposed project. The result can be summarized in the following table:
Table 5.9 Site Survey for Peak Hour Pedestrian Population (Person/Hour) in Year 2014
|
Road |
Ching Ming Day |
Sunday |
Weekday |
|
South Lantau Road (between the bus stops) |
69 |
49 |
10 |
|
Single track access road |
69 |
49 |
10 |
|
From junction of single track access road to Lai Chi Yuen Cemetery |
459 |
139 |
10 |
5.9.43 The traffic survey shows
that during festival days, the pedestrian population for the section of
footpath from the junction of the access road to Lai Chi Yuen Cemetery (Lantau
Trail Section 12) is significantly increased.
5.9.44 The annual growth rate of
pedestrian accessing P1, P2 and P3 on non-festival days is assumed to be 3%, in
order to be in-line with the general road traffic growth rate (without the
additional effect of public housing in Silvermine Bay as described in Section
5.9.36).
5.9.45 Additional population will
be added for festival days (TM5) by using the percentage increase in niches in
Lai Chi Yuen Cemetery in operation phase.
5.9.46 The moving speed of
pedestrian is conservatively assumed to be 3 km/hr considering the hilly road.
Ngan Kwong Wan Road (Location ID: R3) and Mui Wo Ferry Pier Road (Location ID: R4)
5.9.47 No ATC or TIA data is
available for estimation of traffic in these roads. Based on site survey
carried out by Consultant in Nov 2016, it was observed that all the buses
(Route 1 and 3M) from South Lantau Road would first go to the bus stop at Ngan
Kwong Wan Road before returning to the bus terminus at the pier via Mui Wo
Ferry Pier Road. Based on Consultant’s observation, 50% of the traffic of South
Lantau Road is assumed heading to Ngan Kwong Wan Road, and 50% of traffic is
assumed heading to Mui Wo Ferry Pier Road.
5.9.48 Special speed limit of
30km/hr is imposed on Ngan Kwong Wan Road and it is used in this study for
population calculation.
5.10
Modelling Approach and Calibration
Proposed Risk Modelling Approach
5.10.1 This assessment made use of
the consequence modelling results in the Previous Report as well as the
consultant’s in-house risk summation software, RISKSUM, to assess the level of
risk.
5.10.2 For the toxic chlorine cloud
dispersion upon an accidental release, cloud dispersion contours from
consequence modelling results in the Previous Report will be adopted. The
consequence modelling in Previous
Report was based on the Britter and McQuaid dense gas
dispersion model.
5.10.3 Occurrence frequencies,
population, wind speed and other parameters are then entered into RISKSUM for
risk summation process to obtain the Individual Risk and Societal Risk and will
be presented in Individual Risk Contour Plots and FN Curves respectively.
5.10.4 In order to ensure
compatibility in using different models in evaluating the risk associated with
the SMBWTW, a calibration exercise is necessary.
5.10.5 In Previous Report, a dense gas
dispersion model called Britter and McQuaid was adopted to simulate the extent
of effect of chlorine dispersion. The dispersion results together with
population information were then further processed in consultant’s in-house
risk summation tools, to assess the level of risk.
5.10.6
A model calibration exercise has been carried out using in-house risk
summation software RISKSUM and is attached in the Methodology Paper of this Study.
5.11
Hazard Identification
Review of identified Hazards in SMBWTW
5.11.1 SMBWTW is a typical water
treatment works in Hong Kong equipped with similar chlorination facilities as
other water treatment works except that SMBWTW is relatively small in scale. By
summarizing the hazard identification exercises carried out for the QRA of
other similar water treatment works in Hong Kong[3], the following hazardous
scenarios can be identified:
1. Hazardous scenario inside the chlorination store
i. Spontaneous tank failure
ii. Dropped container
iii.
Spontaneous
failure of larger bore manifold pipework
iv. Spontaneous failure of pigtail
v. Drive away whilst the truck is still unloading
vi. Failure to identify faulty connection (human error)
vii. Failure to tighten connections (human error)
viii. Isolation error (human error)
ix. Failure to identify faulty valve (human error)
x. Fire (internal or external)
xi. Failure of QA procedure allowing water ingress, or presence of NCl3 causing severe corrosion to the drum / pipework, following pressure low alarm failure
xii. Missile domino effect
2. Hazardous scenario happens on road
i. Road accident leading to load shedding
ii. Road accident leading to roll over
iii. Road accident leading to vehicle fire
iv. Road accident leading to truck being crushed in rear
v. Road accident leading to truck being crushing at the side
vi. Spontaneous failure
viii. Missile domino effect
3. Hazardous scenario happens at the store which can lead to direct release of chlorine to atmosphere
i. Air crash
ii. Earthquake
iii. Subsidence/landslides
iv. Dropped container
v. Spontaneous drum failure
vi. Impact while manoeuvring to enter the chlorine store
vii. Road accident leading to truck being crushed in rear
viii. Road accident leading to truck being crushing at the side
ix. Spontaneous truck fire
x. Missile domino effect
5.11.2 Past Incidents regarding
use, storage and transport of chlorine has been extensively reviewed by various
recent EIA studies in Hong Kong for water treatment works. In particular, the
world wide accident database Major Hazard Incident Data Service (MHIDAS), was
independently reviewed in order to update the Hazard Identification conclusions
(Appendix 5.2). However, only a few
relevant chlorine incidents occurred worldwide since the previous review, and
after examination of their nature was concluded that no revision of the
previously identified hazard scenario is necessary.
5.11.3 The primary causes of
worldwide chlorine release incidents identified from previous 8 WTWs
Reassessment Study include:
i. Equipment Failure
ii. Human Error
iii. Corrosion
iv. Fire/Overheating
v. Contamination
vi. Road accident
vii. Marine Accident
viii. Rail accident
5.11.4 Since the design and operation
of SMBWTW is very similar to other Water Treatment Works in Hong Kong having
chlorine disinfection facility, their findings will generally be adopted.
Review of Existing Chlorine Facilities
5.11.5 Site visit to SMBWTW was
performed on 23rd Nov 2015. The operators of SMBWTW confirmed that
the general arrangement of the use, storage and transport of chlorine is
similar to other Water Treatment Works in Hong Kong, and there is no change in
chlorine store design, chlorine inventory, unloading and procedures, etc.
compared to the Previous Report.
Hazard associated with the Construction and Operation of the Project to SMBWTW
5.11.6 Since the project site is
away from the SMBWTW for at least 300m, the construction and operation of the
Project will not create any additional hazard to SMBWTW.
5.12
Frequency Assessment
5.12.1 With the potential hazards
identified, the likelihood of each hazardous scenario is then determined. The
occurrence frequencies were adopted directly from Previous Study, which is
supplemented by statistics from historical data if necessary. For some of the
cases where historical accident data is not fully available or insufficient, a
Poisson distribution can be assumed to derive the future occurrence rate of
such events, by using the number of “no occurrences”. Or in other cases, Fault
Tree Analysis technique can be used to derive the frequency using best
available equipment failure data.
5.12.2 As discussed in Section 5.8.3, there is no change to the operating parameters of SMBWTW since
Previous Study, thus the event frequencies used in the Precious Report are
still valid and can be used in the current study. The Event frequencies from
Table 9.6.3 of the Previous Study were extracted and presented in the table
below.
Table 5.10 Event Frequency adopted in this Study
|
Hole Size |
Isolation |
Confinement |
Event Frequency (per
year) |
|
|
Internal Release |
||||
|
Leak from single 1
tonne drum |
Small |
No |
Vent On |
3.39x10-7 |
|
Large |
No |
Vent On |
1.99x10-7 |
|
|
Rupture |
No |
Vent On |
1.87x10-8 |
|
|
Leak from single 1
tonne drum |
Small |
No |
Vent Off |
7.42x10-6 |
|
Large |
No |
Vent Off |
4.34x10-6 |
|
|
Rupture |
No |
Vent Off |
4.09x10-7 |
|
|
Leak from
pipework, 100% drum level |
Small |
3 min. |
Vent On |
2.53x10-6 |
|
10 min. |
Vent On |
8.54x10-7 |
||
|
No |
Vent On |
3.42x10-8 |
||
|
Leak from
pipework, 100% drum level |
Small |
3 min. |
Vent Off |
5.52x10-5 |
|
10 min. |
Vent Off |
1.87x10-5 |
||
|
No |
Vent Off |
7.46x10-7 |
||
|
Leak from
pipework, 25% drum level |
Small |
3 min. |
Vent On |
5.13x10-6 |
|
10 min. |
Vent On |
1.73x10-6 |
||
|
No |
Vent On |
6.94x10-8 |
||
|
Leak from
pipework, 25% drum level |
Small |
3 min. |
Vent Off |
1.12x10-4 |
|
10 min. |
Vent Off |
3.79x10-5 |
||
|
No |
Vent Off |
1.51x10-6 |
||
|
Leak from 7 drums
in store |
Large |
No |
Vent On |
6.00x10-11 |
|
Leak from 7 drums
in store |
Large |
No |
Vent Off |
1.31x10-9 |
|
Leak from 6 drums
in store |
Large |
No |
Vent On |
6.00x10-11 |
|
Leak from 6 drums
in store |
Large |
No |
Vent Off |
1.31x10-9 |
|
Rupture of second
drum due to missile |
Rupture |
No |
Vent On |
1.87x10-9 |
|
Rupture of second
drum due to missile |
Rupture |
No |
Vent Off |
4.09x10-8 |
|
External Release
at the Store |
||||
|
Leak from single 1
tonne drum on truck or storage within building |
Small |
No |
External |
7.82x10-5 |
|
Large |
No |
External |
2.11x10-5 |
|
|
Rupture |
No |
External |
1.40x10-6 |
|
|
Leak from 7 drums
in store on storage within building |
Large |
No |
External |
5.00x10-7 |
|
Rupture |
No |
External |
6.09x10-8 |
|
|
Leak from 6 drums
in store on storage within building |
Large |
No |
External |
5.00x10-7 |
|
Rupture |
No |
External |
6.09x10-8 |
|
|
Leak from three 1
tonne drums on truck stationary at store |
Large |
No |
External |
1.56x10-7 |
|
Rupture of second
drum due to missile |
Rupture |
No |
External |
1.55x10-7 |
|
External Release
from the Road |
||||
|
Leak from 3
1-tonne drums on road |
Large |
No |
External |
2.65x10-6 |
|
Leak from single 1
tonne drum on road |
Small |
No |
External |
1.60x10-5 |
|
Large |
No |
External |
2.89x10-5 |
|
|
Rupture |
No |
External |
1.68x10-7 |
|
|
Rupture of second
drum due to missile |
Rupture |
No |
External |
1.68x10-8 |
5.13
Consequence Assessment
5.13.1 Chlorine is a greenish
yellow gas in room temperature and pressure, and is considered highly toxic.
Contact by inhalation will cause irritation, damage to respiratory organs or
even death depending on the chlorine concentration and contact time. The
following table summarize the toxic effect of chlorine to human[6]:
Table 5.11 Toxic Effect of Chlorine
|
Concentration (ppm) |
Effects |
|
1000 |
May be fatal with brief exposure |
|
400-300 |
A predicted average lethal concentration
for 50% of active healthy people for 30 minutes |
|
150-100 |
More vulnerable people might suffer
fatality from 5-10 minute exposure |
|
20-10 |
Causes immediate irritation of nose, throat
and eyes with coughing and lachrymation for half to 1 hour exposure |
|
10 |
Causes coughing in less than 1 minutes
exposure |
|
3-6 |
Causes stinging or burning sensation but
can be tolerated without undue ill effect for up to 1 hour |
5.13.2 In order to be consistent
with the Previous Report, the rate of discharge, i.e. the chlorine release rate
from chlorine drum to the atmosphere is adopted from the Previous Report. In
summary, there are mainly 3 types of releases, which are:
i. Internal release in the chlorine store
ii. External release on road during transportation
iii. External release at the store
Internal Releases
5.13.3
For internal releases, the final release rate to the atmosphere
depends greatly on the status of the ventilation system and the Contain and
Absorb system at the time the release occurs:
i.
Contain and Absorb system functions properly (which results in no
off-site impact)
ii.
Ventilation on, scrubber system on, or failure (forced ventilation)
iii.
Ventilation off, scrubber failed and the chlorine escape from the store
from ineffective door and window seals (release due to expansion)
5.13.4 In the event of the normal
ventilator failing to switch off, the emission rate is determined by the normal
ventilation rate. Success or failure of the absorber has minimal effect on the
emission rate if normal ventilation is ongoing. Under the perfect mixing
assumption, the released chlorine will perfectly mix with the air inside the
store, and will be extracted to the atmosphere by the normal ventilation
system.
5.13.5 The release rate to the
atmosphere for internal releases with forced ventilation are calculated using
the following equation[7, 8]:
Where
R is the release rate
in the building
N
is the number of air changes per hour
t
is the time elapsed (in hours)
5.13.6 The above equation is a
solution of the differential equation obtained from first principles
consideration of the mass balance of chlorine in the air in the store, the air
and the chlorine in the store is assumed to be effectively mixed.
Ventilation driven by expansion of release
5.13.7 For cases that the
ventilation is off while scrubber failed to start, the liquid chlorine released
will quickly evaporate into gaseous phase and trapped inside the store. Due to
the increase in gas pressure inside the store, chlorine is assumed to leak and
escape from the store to the atmosphere through the weak points such as window
and door seals. In these cases the mass release rate will be lower compared
with the cases with ventilation on.
5.13.8 In reality, the inventory
of chlorine inside the store varies with time. Following the assumption of “Hazard Assessment for Uprating of Silvermine
Bay Treatment Works to 200Mld: Final Assessment Report”, the chlorine inventory is modelled as following [7]:
Ÿ Full for 33.3% of the time
Ÿ Quarter full for 66.7% of the
time
Ÿ Store full 50% of the time
Ÿ Store at minimum stock level
(i.e. 1 truckload less than full capacity of the store) 50% of the time
The average release rates used in the
consequence calculation, is calculated by analytical integration of the above
time-dependent release rates Q(t) over the release duration (T):
5.13.9 The effect of these
scenarios has been accounted for in the Previous Report as a resultant
effective release rate to the atmosphere. These release rates were adopted
directly in this study. They are summarized in the following table:
Table 5.12 Discharge Rates and Their Duration of Each Internal Release Scenario
|
Consequence |
Hole Size |
Isolation |
Confinement |
Release Rate to the Atmosphere (kg/s) |
Duration (s) |
|
Leak from single 1 tonne drum |
Small |
No |
Vent On |
2.5x10-1 |
3443 |
|
Large |
No |
Vent On |
5.5x10-1 |
1244 |
|
|
Rupture |
No |
Vent On |
7.1x10-1 |
827 |
|
|
Leak from single 1 tonne drum |
Small |
No |
Vent Off |
6.1x10-2 |
3600 |
|
Large |
No |
Vent Off |
4.2x10-1 |
1565 |
|
|
Rupture |
No |
Vent Off |
7.1x10-1 |
827 |
|
|
Leak from pipework, 100% drum level |
Small |
3 min. |
Vent On |
4.3x10-2 |
1006 |
|
10 min. |
Vent On |
1.1x10-1 |
1426 |
||
|
No |
Vent On |
2.5x10-1 |
3443 |
||
|
Leak from pipework, 100% drum level |
Small |
3 min. |
Vent Off |
5.6x10-3 |
3600 |
|
10 min. |
Vent Off |
1.8x10-2 |
3600 |
||
|
No |
Vent Off |
6.1x10-2 |
3600 |
||
|
Leak from pipework, 25% drum level |
Small |
3 min. |
Vent On |
4.3x10-2 |
1006 |
|
10 min. |
Vent On |
1.1x10-1 |
1426 |
||
|
No |
Vent On |
1.2x10-1 |
1488 |
||
|
Leak from pipework, 25% drum level |
Small |
3 min. |
Vent Off |
5.6x10-3 |
3600 |
|
10 min. |
Vent Off |
1.8x10-2 |
3600 |
||
|
No |
Vent Off |
2.0x10-2 |
3600 |
||
|
Leak from 7 drums in store |
Large |
No |
Vent On |
3.3 |
1244 |
|
Leak from 7 drums in store |
Large |
No |
Vent Off |
3.3 |
1244 |
|
Leak from 6 drums in store |
Large |
No |
Vent On |
3.3 |
1244 |
|
Leak from 6 drums in store |
Large |
No |
Vent Off |
3.3 |
1244 |
|
Rupture of second drum due to missile |
Rupture |
No |
Vent On |
7.1x10-1 |
827 |
|
Rupture of second drum due to missile |
Rupture |
No |
Vent Off |
1.4 |
827 |
5.13.10 Hazardous events relate to building
collapse are classified as external releases which can be caused either by an
incident during transportation of the chlorine by truck or by major damage of
building due to earthquake or aircraft. In these scenario the building is
assumed to be unable to contain the chlorine and the initial chlorine discharge
rate will become the effective release rate to the atmosphere.[2]
Table 5.13 Summary of External Releases
|
Consequence |
Hole Size |
Release Rate to the Atmosphere (kg/s) |
Duration (s) |
|
Leak from single 1 tonne drum on truck or storage within building |
small |
0.38 |
2618 |
|
large |
2.4 |
419 |
|
|
rupture |
1000 |
1 |
|
|
Leak from 7 drums in store within building |
large |
17 |
419 |
|
rupture |
7000 |
1 |
|
|
Leak from 6 drums in store within building |
large |
14 |
419 |
|
rupture |
6000 |
1 |
|
|
Leak from three 1-tonne drums on truck stationary at store |
large |
7.2 |
419 |
|
Rupture of second drum due to missile domino effect |
rupture |
2000 |
1 |
Releases during Chlorine Transportation
5.13.11 Loss of containment
scenarios during transportation will lead to a direct release of chlorine to
atmosphere. The following table, referenced from Table C1a of the Previous Report, summarizes the release scenarios and their respective release rates
and mass released which are adopted in the current study:
Table 5.14 Summary of Release Sizes during Chlorine Transportation
|
Hole Size |
Release Rate (kg/s) |
Mass Released |
|
Small Leak (single drum) |
0.38 |
1 tonne |
|
Large Leak (single drum) |
2.4 |
1 tonne |
|
Catastrophic Failure (single drum) |
1 tonne
instantaneous |
1 tonne |
|
Missile Domino Effect causing rupture of second drum |
2 tonnes
instantaneous |
2 tonnes |
|
Large Leak (3 drums on truck) |
7.2 |
3 tonnes |
5.13.12 The chlorine releases on
road cannot be contained or isolated.
Dispersion of Chlorine in Atmosphere
5.13.13
In the Previous Study, Britter and McQuaid Model (The BM Model) for
dense gas dispersion was used. The BM Model is an empirical model which has
been detailed in Workbook written
by Britter and McQuaid in 1988[9].
The aims of developing the co-relation was to enable non-specialists to prepare
estimates of the dispersion of pollutant dense gas emissions. The primary aim
of the model is to enable estimate of:
(i)
Average concentration levels along the plume axis, for continuous
release, and
(ii) Maximum concentration levels along the down-wind cloud path, for instantaneous releases.
5.13.14 In addition, Britter and
McQuaid give rules of thumb to estimate iso-concentration contours.
5.13.15 The BM model consists of a
collection of curves for down-wind maximum concentrations in form of nomograms.
The nomograms were derived from widely-accepted field test data and from wind
tunnel data.[10]
5.13.16 The dispersion results, in
form of LD90, LD50 and LD03 contours were summarized in Table C1a of the
Previous Report. These contours were represented by d/c/s/m parameters which
can be explained in Figure
5.10.
5.13.17 The toxic effect of chlorine
is implicitly expressed in the dispersion modelling in the Previous Report.
Three toxic levels (contours) were used to express the lethality of the
chlorine gas cloud:
Ÿ
LD90 – 90% of healthy people outdoor will die within this contour
Ÿ
LD50 – 50% of healthy people outdoor will die within this contour
Ÿ
LD03 – 3% of healthy people outdoor will die within this contour
5.13.18 The typical ventilation
rate of 0.5 air exchanges per hour and 600 seconds tail time was adopted in the
Previous Report. These assumptions would lead to an approximate 90% reduction
of risk indoor. It is adopted in the current study.
5.13.19
The possibility of persons out of doors escaping to a less exposed location
indoors and thus reducing their dose from the outdoor to the indoor rate was
reviewed. The escape rate is related to the chlorine concentration of the
dispersion cloud. Referring to the Previous Report, it was considered that 80% of persons out
of doors in areas of LD03 contour would escape indoors, 20% of persons in areas
of LD50 would escape indoors and 0% in LD90 contour can escape indoors. As a
result, the percentage of fatality against chlorine concentration can be
summarized in the following table:
Table 5.15 Fatality Rate for Indoor and Outdoor Population Considering Escape Probability
|
Chlorine Cloud
Concentration (LD) |
Fatality for
Outdoor Population |
Fatality for
Indoor Population |
|
90 |
0.9 |
0.09 |
|
50 |
0.41 |
0.05 |
|
3 |
0.0084 |
0.003 |
5.13.20 Since there is no high
speed transportation service such as railway in this Study, all road traffic
population in this study were thus modelled without additional protection
factor, which is in line with Previous Report.
Chlorine Cloud Height
5.13.21 Information on the heights
of chlorine clouds were adopted from the Previous Report Table C1a. It is
assumed that the population above the chlorine cloud is not affected. For
population in high-rise buildings such as Ngan Wan Estate, a protection factor
is assumed based on the chlorine cloud height and the height of the building.
Sensitive Population Consideration
5.13.22 Vulnerable population
factor of 3.3, as used in the Previous Report and in line with data published
by Withers and Lees in 1985[11], is adopted in current
Study. The fatality rate for indoor and outdoor population is summarized in the
following table:
Table 5.16 Fatality Rate for Indoor and Outdoor Population Considering Escape Probability (Vulnerable Population)
|
Chlorine Cloud
Concentration (LD) |
Fatality for
Outdoor Population |
Fatality for
Indoor Population |
|
90 |
1 |
0.297 |
|
50 |
1 |
0.165 |
|
3 |
0.099 |
0.0099 |
5.14
Risk Assessment
5.14.1 By combining the results of
frequency estimation and consequence analysis, risk levels of the assessment
scenarios were characterised in terms of individual risk (presented in
individual risk contours plot) and societal risk (presented in FN curves and Potential
Loss of Life). This integration of risk was processed by the RISKSUM software.
Determination of Risk Acceptability
5.14.2 The level of risk in terms
of Individual Risks and the FN curves determined from the risk summation
process were compared with the criteria stipulated in Annex 4 of the TM to determine their acceptability.
5.14.3 The off-site risk level was
checked whether it is less than 1 x 10-5 / year (i.e. 1 in 100,000
per year).
5.14.4 The determination of
acceptability of societal risk in the form of FN curves is based on the
following criteria:
i. Unacceptable where risks are so high that they should usually be reduced regardless of the cost or else the hazardous activity should not proceed;
ii. ALARP (As Low As Reasonably Practicable) where the risks associated with the hazardous activity should be reduced to a level “as low as reasonably practicable”, in which the priority of measures is established on the basis of practicability and cost to implement versus risk reduction achieved; and
iii. Acceptable where risks are so low that no action is necessary.
5.14.5 The individual risk result
of SMBWTW is illustrated in Figure
5.11. It showed that the region with individual risk higher than 1x10-5
/year (brown area) is slightly outside the boundary of the SMBWTW. The Risk
Guidelines in Hong Kong states that when utilizing risk contours, the estimated
duration of exposure of a person to the PHI should be taken into consideration
to determine the individual risk for comparison with the Risk Guidelines. Since
the brown area outside site boundary only covers hillside and a small section
of road only used by SMBWTW, the individual risk result is compliant with the
criteria stipulated in Annex 4 of EIAO-TM.
5.14.6 The societal risk results
were summarized in the form of F-N curve in Figure 5.12.
5.14.7 Result showed that the F-N
curve for base case, construction case and operation case in this study all lie
within the ALARP region. A comparison of these curves indicated that the
additional risk by the proposed project (either construction or operation
phase) is not significant.
5.14.8 Societal risk can also be
represented in the form of Potential Loss of Life (PLL). It expresses the risk
to the population as a whole and for each scenario and its location. The PLL is
an integrated measure of societal risk obtained by summing the product of each
F-N pair:
5.14.9 The PLLs in all the cases
studied are summarized in the following table:
Table 5.17 Summary of PLLs
|
Case |
Overall
PLL (/year) |
PLL of
Lai Chi Yuen Cemetery (/year) |
% of PLL
to overall PLL |
|
Base Case (Population ID: 28) |
1.443x10-4 |
3.034x10-7 |
0.210% |
|
Construction Case (Population
ID: 28+ 28_C) |
1.426X10-4 |
5.706x10-7 |
0.400% |
|
Operation Case (Population ID:
28 + 28_O) |
1.445X10-4 |
7.699x10-7 |
0.533% |
5.14.10 The result shows that the
PLL contribution of Lai Chi Yuen Cemetery is very minimal. The PLL increase due
to the proposed project is less than 1%, which is also insignificant. It can be
concluded that the risk contribution of the proposed project is not
significant.
5.15
Uncertainty Analysis
5.15.1 This analysis was based on
a number of assumptions and was based on ‘cautious best estimates’:
1. The cemetery population was assumed conservatively. The number of visitors corresponded to the maximum capacity of the cemetery (with extension) was assumed. In fact, the cemetery now is still far from fully occupied. It was also conservatively assumed that the Sunday following the Ching Ming/Chung Yeung Festival will have the same number of visitors as the Festival Day.
2. The traffic flow was assumed to be the peak hour flow for the whole day during Tsing Ming / Chung Yeung Festival
3. Many population is conservatively assumed to a higher value. For example, the future civic square is assumed to have 2000 people (maximum capacity as provided by CEDD) in every Sunday, and 200 people over the night.
5.15.2 These assumptions will
effectively push the resultant FN-curve to the conservative side. It is
considered that the FN curve shown in Figure 5.12 is reasonably conservative
and the curve will stay in the current ALARP region even uncertainty is
considered.
5.16
Risk Mitigation Measures Identification and Analysis
Identification of Risk Mitigation Measures
5.16.1
Since the societal risk of the SMBWTW is within the ALARP region,
Cost-Benefit Analysis (CBA) is required to select and analyse risk mitigation
measures.
5.16.2 Operation and Construction
stage risk mitigation measures of the Lai Chi Yuen Cemetery project are
reviewed and consulted with relevant stakeholders. Measures that are relevant
and applicable to the current project are selected for CBA and presented in Table 5.18.
Table 5.18 Candidate Risk Mitigation Measures
|
Option |
Measures |
Remarks |
|
Operation stage |
||
|
A |
Prepare face masks at every
level of the Cemetery as well as the Extension |
To be assessed in CBA |
|
B |
Setting up temporary shelter at
the cemetery during the Ching Ming and Chung Yeung Festival |
To be assessed in CBA |
|
C |
Enhance communication with WSD
and Police and identify emergency response in case of major chlorine incident |
Recommended to implement as a
good practice |
|
Construction Stage |
||
|
D |
Prepare 50 face masks for
construction workers |
Since the number of face masks
involved will not be significant, this mitigation measure will be implemented
by contractor without the need of CBA assessment |
|
E |
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. |
5.16.3 Assumptions of the relevant
measures for further analysis by CBA are listed in Table 5.19.
Table 5.19 Assumptions for Candidate Mitigation Measures
|
Option |
Detail provisions |
Project Stage |
|
A |
Chemical, Biological and Radiological (CBR) Gas
Mask can be place at every level of the existing cemetery as well as at the
cemetery extension. 2000 of the gas masks should be prepared to cater for the
maximum capacity of the Cemetery with Extension. Such gas masks cabinets can
also be found in some large metro/subway stations in Seoul. Due to the site
condition, it is expected that only 10% of the visitors can have enough time
to access the gas masks and put them on timely and properly before the chlorine
gas cloud arrives the cemetery. |
Operation stage |
|
B |
One or two temporary shelters can be set up
during Ching Ming / Chung Yeung Festival so that in case of chlorine release
some visitors can stay in the shelter and wait for evacuation by Fire
Services Department/ Police. The temporary shelter should be able to sustain
a filtered fresh air supply for a certain period of time. Due to the space
available and the site constraints, 10% of visitors during festival days are
assumed to be able to be protected by the shelter. |
Operation stage |
Analysis of Mitigation Measures
5.16.4 Cost Benefit Analysis (CBA)
is a widely used method for evaluating the cost-effectiveness of mitigation
measures. The method can also be used to demonstrate that all reasonably practicable
measures have been taken to reduce risk. The Implied Cost of Averting a
Fatality (ICAF), which was adopted previously in the Reassessment Study, will
be used in this HA. The ICAF is calculated as follows.
5.16.5 The ICAF can be compared
with the value of life to determine whether the implementation of the
identified mitigation measures is reasonably practicable. In line with other
similar studies in Hong Kong[4, 13], a value of HK$33M will be
used in this HA.
5.16.6 Aversion factor may be
applied to adjust the value of life, usually upward, to reflect the public’s
aversion to high risk. The same methodology, as adopted in the Reassessment
Study of Eight Existing Water Treatment Works in Hong Kong[4], will be used, which is to
determine the factor on a sliding scale from 1 for risk near the lower boundary
of the ALARP region, up to a maximum of 20 for risk near the upper boundary of
the ALARP region. The ICAF can be compared with the value of life (proposed to
be HK$33M in this study) to determine whether the implementation of the
identified mitigation measures is reasonably practicable. 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$660M will be adopted.
5.16.7 In Option A, it is assumed
that 10% of the population will be protected by correctly wearing the Gas Mask.
The PLL reduction (for cemetery population) of this mitigation is from 7.70x10-7/year
to 6.93 x10-7/year.
5.16.8 In Option B, it is assumed
that 10% of the population during Ching Ming / Chung Yeung Festival can be
protected by staying in the temporary shelter. The PLL of the cemetery
population will drop from 7.70x10-7/year to 7.64x10-7/year.
5.16.9 The result of CBA is
summarized in the following table (Table 5.20):
Table 5.20 Candidates for Risk Mitigation Measures to be considered in CBA
|
Option |
Estimated Cost (HK$M) |
PLL Reduction |
Design Life (Years) |
ICAF (HK$M) |
Cost-effective Measures |
|
A |
1 |
7.70x10-8 |
5 |
2.60x106 |
No |
|
B |
5 |
5.89x10-9 |
10 |
8.49x107 |
No |
5.16.10 It can be easily seen that
both Option A and Option B are not cost-effective measures to reduce PLL. It also
agrees with the fact that the total PLL contribution of Lai Chi Yuen Cemetery
is very low compared with the overall PLL of SMBWTW, that only very low cost
mitigation measures can be justified under the ALARP principle.
5.16.11 On the other hand, during
construction stage some recommendations (Option E) are made which can
effectively reduce the risk to construction workers, it is summarized in
Section 5.16.13.
5.16.12 The CBA exercise above has
identified all practicable risk reduction measures and the risk has been
reduced to As Low As Reasonably Practicable.
5.16.13
There are a number of measures recommended during construction stage,
which include:
1. The number of workers on site during construction stage should be kept within the level as assessed in this report.
2. FEHD/ArchSD/the responsible personnel of the construction site should keep effective communication with Police or relevant authorities to ensure a proper evacuation/emergency response in case of a gas release incident. FEHD/ArchSD/the responsible personnel of the construction site should ensure all workers on site to be familiar with the route to escape. Diagram showing the escape routes to a safe place should be posted in the site notice boards and at the entrance/exit of site.
3. Specific 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 SMBWTW should be determined and made known to the construction workers.
4. The construction site officer should establish a communication channel with the SMBWTW operation personnel during construction stage. Upon receiving the notice of an external gas leak at the SMBWTW, the construction site officer should direct the workers to evacuate by following the instructions of Police or relevant authorities as appropriate.
5. Induction Training should be provided to any staff before working on site.
6. The responsible officer of the construction site should ensure all construction staff are familiar with the evacuation routes and /or location of the protective gears (if available).
5.16.14 During operation stage, due
to the nature of the site (many visitors are not familiar with the place and many
visitors may be slow in walking), emergency public announcement and evacuation
etc. are not recommended. FEHD should
keep effective communication with Police or relevant authorities to ensure a
proper emergency response in case of a gas release incident (Option C in Table 5.18).
5.17
Conclusion
5.17.1 Hazard to Life Assessment
has been carried out for the proposed Lai Chi Yuen Cemetery Extension. The aim
of the study was to assess the effect of the proposed Cemetery Extension on the
overall risk level of the nearby Silvermine Bay Water Treatment Works (SMBWTW)
due to the increased population in the area.
5.17.2 After analysing all the
possible events of chlorine release of SMBWTW, the Individual Risk (IR) contours
and the F-N Curves were generated and compared against the requirement as
stipulated in the Annex 4 of EIAO-TM.
5.17.3 It has been concluded that
both the societal risk and individual risk of SMBWTW are compliant to the Annex
4 of EIAO-TM. The increase of population in the proposed Cemetery Extension
will induce a very slight increase in the overall risk of the SMBWTW. Since the
overall societal risk is in the ALARP region, mitigations measures were
proposed and Cost-Benefit Analysis was performed. Other recommended practices
to reduce the overall risk were also proposed. After implementing all the
recommended risk mitigation measures, the overall risk is in compliance with
the ALARP principles and Risk Guidelines (Annex 4 of EIAO-TM).
5.18
Reference
[1] Environmental Impact Assessment Ordinance, Chapter 499.
[5] "Tips on driving on Lantau Closed Roads," ed: Hong Kong Transport Department, Feb 2016.