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Technical Assistance Consultant’s Report
This consultant’s report does not necessarily reflect the views of ADB or the Government concerned, and ADB and the Government cannot be held liable for its contents. (For project preparatory technical assistance: All the views expressed herein may not be incorporated into the proposed project’s design.
Project Number: 50028-001 May 2021
Regional: Pacific Disaster Resilience Program
Multi Hazard Disaster Risk Assessment, Tongatapu Interim Vulnerability Data Report
Prepared by Arup Australia Pty. Ltd. (Arup), Australia Sydney, Australia
For Asian Development Bank
Asian Development Bank
Multi-Hazard Disaster Risk
Assessment, Tongatapu
Interim Vulnerability Data Report
MHDRA-REP-005
Final 02 | 09 April 2021
This report takes into account the particular
instructions and requirements of our client.
It is not intended for and should not be relied
upon by any third party and no responsibility
is undertaken to any third party.
Job number 276779-00
Arup Australia Pty Ltd ABN 76 625 912 665
Arup
Level 5
151 Clarence Street
Sydney NSW 2000
Australia
www.arup.com
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Document Verification
Job title Multi-Hazard Disaster Risk Assessment, Tongatapu Job number
276779-00
Document title Interim Vulnerability Data Report File reference
Document ref MHDRA-REP-005
Revision Date Filename MHDRA-REP-005.docx
Draft 1 Feb 23, 2021
Description First draft
Prepared by Checked by Approved by
Name MA, PR, JD, KB KB, IA, ER TM
Signature
Final Mar 25, 2021
Filename MHDRA-REP-005.docx Description Updated to reflect comments provided by ADB on the first draft
Prepared by Checked by Approved by
Name PR, MA, KB KB TM
Signature
Final – Revision 2
Apr 9, 2021
Filename MHDRA-REP-005.docx Description Updated to address further comments provided by the ADB
Prepared by Checked by Approved by
Name Caitlin Jay Tim Mote Tim Mote
Signature
Issue Document Verification with Document
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Contents Page
1 Introduction 6
2 Asset Descriptions 6
2.1 Buildings 6
2.2 Roads 9
2.3 Power infrastructure 9
2.4 Water infrastructure 14
3 Methodology 16
4 Flood Vulnerability 18
4.1 Buildings 18
4.2 Roads 32
4.3 Power infrastructure 34
4.4 Water infrastructure 41
5 Tsunami Vulnerability 43
5.1 Buildings 43
5.2 Roads 45
5.3 Power infrastructure 47
5.4 Water infrastructure 53
6 Seismic Vulnerability 56
6.1 Buildings 56
6.2 Roads 60
6.3 Power infrastructure 62
6.4 Water infrastructure 69
7 Wind Vulnerability 74
7.1 Buildings 74
7.2 Power infrastructure 78
7.3 Water infrastructure 83
8 Conclusion 84
9 References 85
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Tables
Table 1: Summary of assets and hazards included in vulnerability study
Table 2: Components included and excluded in vulnerability models for each of the flood building archetypes.
Table 3: Replacement cost (estimated average) for building replacement.
Table 4: Replacement cost (estimated average) for contents.
Table 5: Replacement cost for building flood archetypes (where SS denotes single story and MS denotes multi-story buildings).
Table 6: Vulnerability summary for Residential building archetypes.
Table 7: Vulnerability summary for General building archetypes.
Table 8: Vulnerability summary for Industrial building archetype.
Table 9: Vulnerability summary for Out building archetype.
Table 10: Vulnerability summary for Industrial building archetype.
Table 11: Vulnerability summary for roads.
Table 12: Vulnerability summary for Transformers.
Table 13: Vulnerability summary for Pole-Mounted Equipment.
Table 14: Vulnerability summary for Power Station.
Table 15: Vulnerability summary for Solar Farm PV Panels
Table 16: Vulnerability summary for solar farm auxiliary equipment.
Table 17: Vulnerability summary for pumps and wells.
Table 18: Vulnerability summary for the Building Structure
Table 19: Vulnerability summary for roads
Table 20: Vulnerability summary for Utility Poles
Table 21: Vulnerability summary for pole-mounted equipment.
Table 22: Vulnerability summary for power station.
Table 23: Vulnerability summary for solar farm.
Table 24: Vulnerability summary for typical pump system.
Table 25: Vulnerability summary for tank.
Table 26: Mapping of structural typology to equivalent seismic damage function
Table 27: Vulnerability summary for Buildings for seismic shaking
Table 28: Vulnerability summary for roads.
Table 29: Vulnerability summary for utility poles and power lines.
Table 30: Vulnerability summary for transformer.
Table 31: Vulnerability summary for power station.
Table 32: Vulnerability summary for solar panels
Table 33: Vulnerability summary for solar farm auxiliary equipment
Table 34: Vulnerability summary for elevated water tank.
Table 35: Vulnerability summary for at-grade water tank.
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Table 36: Vulnerability summary for buried pipelines.
Table 37: Vulnerability summary for Pumps and Wells
Table 38: Mapping of Structural Typology to PCRAFI tropical cyclone damage functions
Table 39: Vulnerability summary for Buildings
Table 40: Vulnerability summary for utility poles
Table 41: Vulnerability summary for diesel tanks
Table 42: Vulnerability summary for solar panel systems
Figures
Figure 1: Timber frame building in Tonga
Figure 2: Masonry building in Tonga
Figure 3: Steel building in Tonga
Figure 4: Reinforced concrete frame building (may include masonry infill) in Tonga
Figure 5: Photos of roads in Tongatapu
Figure 6: Utility poles with pole-mounted equipment and power lines visible.
Figure 7: Ground-mounted transformers.
Figure 8: Diesel generators inside Popua power station building.
Figure 9: Diesel generators from another view inside Popua power station building.
Figure 10: Pumps and piping for cooling water from the lagoon.
Figure 11: The control room at the power station.
Figure 12: PV panels at Popua solar farm
Figure 13: Auxiliary electrical equipment located at Popua solar farm
Figure 14: One of the wind turbines at the wind farm. There is a pin at the base of the structure that allows the entire wind turbine to be lowered to the ground.
Figure 15: Auxiliary equipment located outside.
Figure 16: Building housing equipment such as control panels etc.
Figure 17: Equipment housed inside the wind farm building.
Figure 18: A pumphouse
Figure 19: An access point for the boreholes
Figure 20: Elevated water tanks
Figure 21: At grade water tanks
Figure 22: Comparison of calculated replacement costs and total insurable value.
Figure 23: Fragility curves for interior building partitions.
Figure 24: General (Timber) building archetype vulnerability curve for the 10th, 50th, and 90th percentile losses.
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Figure 25: Comparison of Nofal & van der Lindt’s Small Multi-Unit Commercial Building archetype (black and blue lines), Hazus commercial archetype, and the General building archetypes (Timber as solid light green line, Masonry/Other as dotted dark green line).
Figure 26: Residential building archetypes for flood.
Figure 27: General building archetypes for flood.
Figure 28: Specialized building archetypes for flood.
Figure 29: Out Building archetypes for flood.
Figure 30: Specialized building archetypes for flood.
Figure 31: Road vulnerability for flood.
Figure 32: Ground-mounted transformer flood vulnerability.
Figure 33: Pole-mounted equipment fragility for flood.
Figure 34: Power station vulnerability to flood.
Figure 35: Solar farm PV panel fragility.
Figure 36: Vulnerability for solar farm auxiliary equipment.
Figure 37: Vulnerability for pump and well for flood.
Figure 38: Fragility functions for the Building Structure (FEMA, 2017)
Figure 39: Road fragility curves (Horspool & Fraser, 2016)
Figure 40: Utility Pole fragility for tsunami.
Figure 41: Utility pole fragility for tsunami, taken to apply to pole-mounted equipment. (Horspool & Fraser, 2016)
Figure 42: Power station fragility for tsunami (Horspool & Fraser, 2016)
Figure 43: Solar farm fragility function (Horspool & Fraser, 2016)
Figure 44: Pump system fragility curves for tsunami.
Figure 45: Tank fragility curve for tsunami.
Figure 46: PCRAFI damage function for reinforced masonry where the blue curves represent the baseline performance and the red lines represent the retrofitted performance.
Figure 47: GLOSI vulnerability curve for reinforced masonry building
Figure 48: Vulnerability curves for different building typologies subject to varying earthquake intensity (Bazzurro, 2015)
Figure 49: Road vulnerability for seismic from Hazus. (FEMA, 2020)
Figure 50: Utility pole and power line vulnerability for seismic.
Figure 51: Transformer vulnerability for seismic.
Figure 52: Power station vulnerability for seismic.
Figure 53: Fragility curves of solar panel support frames (Miyamoto International Inc., 2019)
Figure 54: Fragility curves for anchored components in small scale power plants (FEMA, 2020)
Figure 55: Vulnerability for elevated water tank in seismic. (American Lifelines Alliance, 2001)
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Figure 56: Vulnerability for at-grade water tank for seismic (American Lifelines Alliance, 2001).
Figure 57: Vulnerability of buried pipelines for seismic.
Figure 58: Vulnerability of pumps and wells for seismic (FEMA, 2020)
Figure 59: Example of vulnerability curve produced by VAWS – Timber building
Figure 60: PCRAFI vulnerability curve for timber buildings
Figure 61: Vulnerability curves for buildings in wind (Bazzurro, 2015)
Figure 62: Fragility curves for utility poles in wind (Quanta Technology, 2008) (unit for wind speed has been converted)
Figure 63: Steel storage tank buckling fragility at fill (O) levels (Olivar, Mayorga, Giraldo, Sánchez-Silva, & Salzano, 2018)
Figure 64: Solar panel fragility curves in wind (adapted (Goodman, 2015))
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1 Introduction
This report sets out the findings of the vulnerability assessment which has been undertaken as part of the Asian Development Bank (ADB)’s Multi-Hazard Disaster Risk Assessment (MHDRA) for Tongatapu, Tonga.
The purpose of the vulnerability assessment has been to:
• Establish estimates for asset damage as a result of the hazards for a number of building typologies and road segments representative of the building stock and road network; and
• Establish estimates for asset damage as a result of the hazards for power assets (substations, power stations, High voltage (HV)/Low voltage (LV) poles, distribution lines, transmission lines, wind turbines, and solar farms) and water assets (storage tanks, pipelines, pump stations and wells).
This report is structured as follows: Section 2 describes the assets included in scope and summarizes the assets that were studied for each hazard; Section 3 describes our approach to the vulnerability study; Sections 4 through 7 describe the vulnerability information for each asset for flood, tsunami, seismic, and wind respectively; and Section 8 summarizes the overall study.
2 Asset Descriptions
This section outlines the buildings, road, water, and power infrastructure assets that were included in the vulnerability study. The information that was known and used to inform the vulnerability study is detailed in the following sections. For a more detailed breakdown of the assets on Tongatapu, refer to the Exposure Data Development Report (MHDRA-REP-003). (Arup, 2021).
2.1 Buildings
All 28,176 buildings on Tongatapu were in scope for the vulnerability assessment. Details on the features collected and a breakdown of the building stock are provided in the Exposure Data Development Report (MHDRA-REP-003). In conjunction with the features collected in the database, information gathered during the field survey, workshops with local engineers, surveys completed by subconsultants and government ministries (Ministry of Health), and the Tongan Building Code (The Kingdom of Tonga, 2001) (The Kingdom of Tonga, 2007) were used to inform the vulnerability study for the building stock.
National building codes for Tonga have existed since 2001, and are based on multiple other National Building Codes (The Kingdom of Tonga, 2001) and include building design requirements for seismic and wind events. This code has been updated in 2007, although wind and seismic design requirements had been kept the same in the 2007 building code.
For wind events, the 2001 Tongan Building Code references design of buildings to the Australian Standards and New Zealand Standards with local parameters specified such as design wind speeds to reflect the local context. Site wind loads in the Tongan Building Code are stated to be 70 m/s (The Kingdom of Tonga, 2001). A review of these codes completed
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by Arup recommended an update to local design wind speeds (The Kingdom of Tonga, 2018). Recommendations to wind speeds for structural design also pointed to dependence on the Importance Level of a building causing design wind speeds to vary, in comparison to the constant level set out in previous Tongan Building Codes. Buildings at Importance Level 2 (domestic housing) are recommended to have design wind speeds of 66m/s, whereas those of Importance Level 3 and 4 (critical emergency response infrastructure) should be designed for 70m/s and 76m/s, respectively.
For seismic design in the Tongan Building Code, users are directed to the California Building Code (with a Zone/Hazard Factor specified as Z = 0.4) (The Kingdom of Tonga, 2001). Previous work completed by Arup had found that since the issuance of these building codes, the latest research findings indicate that seismicity levels in Tonga are significantly greater that previously understood. The recommended seismic design basis has therefore increased (to Z = 0.7 from Z = 0.4), suggesting that existing buildings may not be as robust in a seismic event as would be required to reflect the seismicity of the region.
The vulnerability of the buildings on Tongatapu depends on a number of factors: the requirements of the local building codes and regulations, how well these codes reflect the latest understanding of hazard levels, and compliance to building code requirements. This vulnerability study, through in-country observations of the construction practices and quality of local materials has attempted to account for all of these factors. These factors may result in some buildings from the surveyed stock performing better or even more poorly than anticipated in certain hazards, however, every effort has been made so that results of this study reflect the performance levels of a large majority of the buildings in Tonga.
For the vulnerability study, the building stock was represented by a number of typologies. The typologies used to represent the building stock have been adapted such that they best represent the building stock and associated vulnerability of that building stock relative to each specific hazard. The typologies adopted vary for the different hazards. This is documented in Sections 4-7. Figure 1 through Figure 4 show some images of different types of buildings on Tongatapu.
Figure 1: Timber frame building in Tonga
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Figure 2: Masonry building in Tonga
Figure 3: Steel building in Tonga
Figure 4: Reinforced concrete frame building (may include masonry infill) in Tonga
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2.2 Roads
The road network on Tongatapu includes over 1,000 km of roads, classified as either paved, unpaved, or informal tracks. There are no bridges on the island. For more detail on the data collected refer to the Exposure Development Report (MHDRA-REP-003).
Local pavement design standards were not available, but based on general construction practice in the Pacific, empirical methods from Australia and New Zealand have been adopted in Tonga. In addition to the data collected during the field surveys, a draft Tonga State of the Roads report was reviewed (Government of Tonga, 2015) as well as some pavement condition assessments for other regions of Tonga.
Figure 5 provides some examples of the roads in Tongatapu.
Figure 5: Photos of roads in Tongatapu
2.3 Power infrastructure
The power infrastructure included the following: the power distribution network, the power station, three solar farms and the wind farm. The decommissioned pilot wind turbine on the southern end of Tongatapu has not been included.
The power distribution network includes overhead and below ground power lines, utility poles, at-grade and pole-mounted transformers, and other pole-mounted equipment (switches, capacitors, fuses). Relevant details are summarized below and examples are shown in Figure 6 and Figure 7.
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• Utility poles:
o The typical utility pole on Tongatapu is made of pine or other softwood and is anchored about 1.5-2 m into the ground with a concrete foundation.
o Island-wide upgrades to the distribution system were in progress in 2020. According to TPL, all poles outside of Nuku’alofa were rehabilitated or replaced as of October 2020 and plans to upgrade the poles in Nuku’alofa were underway.
o HV poles are larger than LV poles. HV poles are approx. 11 m tall on average and approximately 350-450mm in diameter. LV poles are approx. 9 m tall on average and approximately 200-300mm in diameter.
• Overhead lines:
o Aerial bundled cables (ABC) distribute power across the island, which improved upon the previous system which had separate mounted conductors.
o At the connection point from poles to residences, the power lines have been undergrounded.
• Below ground lines: Copper wire of 16mm size buried at a depth of 600mm.
• Ground-mounted transformers: Typically, these are oil-filled and located atop concrete plinths (<0.5m high).
• Pole-mounted equipment:
o Components include switches, capacitors, and fuses, which are all critical to the functioning of the power distribution system.
o Components were observed mounted approximately at 2/3 the height of the utility pole.
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Figure 6: Utility poles with pole-mounted equipment and power lines visible.
Figure 7: Ground-mounted transformers.
The Popua Power Station includes eight diesel generators, a 260,000 L diesel fuel tank with day tanks, a building that houses the control room and three switchboard rooms, pumps that pull cooling water from the lagoon and a series of outdoor transformers.
In addition to information gathered during the field surveys, a workshop was held with TPL and further information was gathered via email correspondence. Relevant details are summarized below and photographs of various pieces of equipment are shown in Figure 8 through Figure 11.
• The building foundation is located 1.5 m above ground level.
• Generator batteries are raised approximately 100 mm above the slab.
• The site was built upon reclaimed land.
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Figure 8: Diesel generators inside Popua power station building.
Figure 9: Diesel generators from another view inside Popua power station building.
Figure 10: Pumps and piping for cooling water from the lagoon.
Figure 11: The control room at the power station.
The solar farms include the photovoltaic (PV) panels and supporting structure, and associated auxiliary equipment, such as inverters, junction boxes, and cabling. Relevant details include:
• PV panels sit on elevated foundations approximately 0.5 m above the ground. They are further elevated by their supporting steel frames by another 1-2 m, depending on the solar farm. The panels have junction boxes located below the panels that are vulnerable to inundation. No information about the IP rating of the panels or associated equipment was available.
• Auxiliary equipment: much of the equipment, including inverters, batteries, and transformers, is elevated and housed in rugged containers as shown in Figure 13.
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Figure 12: PV panels at Popua solar farm
Figure 13: Auxiliary electrical equipment located at Popua solar farm
The wind farms include the wind turbines themselves and auxiliary equipment, such as batteries, control panels, inverters, and feeder and interconnection panels. Relevant details include:
• Wind turbines:
o The turbines are manufactured by Vergnet and the tower by Progressive Energy Corporation.
o There is a pin at the based on the tower than enables the entire structure to be lowered and secured to the ground in advance high wind events.
o The structure was designed for seismic demands; however, the specific seismic rating is not known.
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• Auxiliary equipment: The building on site is elevated approximately 1 m from the ground and much of the outdoor equipment is also raised by approximately 150-250 mm on concrete plinths.
Figure 14: One of the wind turbines at the wind farm. There is a pin at the base of the structure that allows the entire wind turbine
to be lowered to the ground.
Figure 15: Auxiliary equipment located outside.
Figure 16: Building housing equipment such as control panels etc.
Figure 17: Equipment housed inside the wind farm building.
In addition to data gathered during the field surveys, information about the above assets were primarily provided by Tonga Power Limited (TPL) through the documentation and data provided (refer to Exposure Data Development Report MHDRA-REP-003), an online workshop, and through follow-up questions by email.
2.4 Water infrastructure
The water infrastructure includes the following: water tanks (at grade and elevated), buried pipelines, hydrants, pumps, and wells. Figure 18 through Figure 21 show some examples of the water infrastructure.
• At-grade storage tank: The capacity of these concrete tanks is approximately 22 m3. They serve the Nuku’alofa region.
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• Elevated storage tank: These tanks are typically found in towns across Tongatapu. Standard material is plastic (replacement of concrete tanks is in progress) located on a stand approximately 9 m tall.
• Hydrant: These are located in Nuku’alofa. No information was provided specific to the network.
• Pumps and wells: The submersible pumps typically have 3.3 kW capacity, though it varies by town. The pump and well are located inside a small pumphouse.
• Buried pipelines: Typical pipes were constructed of PVC, but ductile iron and asbestos cement pipes can also be found on Tongatapu.
In addition to the data captured during the field surveys, information about the water infrastructure was provided by Tonga Water Board (TWB) and the Ministry of Lands and Natural Resources (MLNR) through datasets, workshops, and follow up emails.
TWB provided the following information about design of their non-building structures: typically designed to 57 m/s wind speeds and peak ground accelerations of 0.3 g.
Figure 18: A pumphouse
Figure 19: An access point for the boreholes
Figure 20: Elevated water tanks
Figure 21: At grade water tanks
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3 Methodology
The approach used in the vulnerability study was both staged and different for each hazard type (as outlined in the Inception Report Methodology). The first stage assessed existing literature and local in-country knowledge for applicability to each hazard and asset/building combination. Where no existing literature was fit for use, a second stage was employed that involved modelling the hazard and building/asset vulnerability combination.
In line with this, the general approach for the methodology of the vulnerability study across both stages was as follows:
• Information gathering: building upon the exposure database, specific details pertinent to the vulnerability study were gathered through discussions with the Government of Tonga and desktop research. Some assets or components were deemed rugged based on research or Arup’s experience, that is not considered vulnerable to a particular hazard. These asset and hazard combinations were then excluded from review.
• Review of previous events: damage reports from Tropical Cyclone Gita and Harold were reviewed to understand past impacts.
• Literature review for roads, water, and power assets for all hazards: a literature review was undertaken to identify existing fragility or vulnerability curves for each asset and hazard pair. Where they existed, they were vetted for their appropriateness for the Tongan context based on the data and assumptions used in their development. Curves were either:
o Directly adopted if they were representative of the assets in Tongatapu,
o Modified by shifting linearly to account for reduced or increased capacity where further information about the asset was available, or
o Deemed unfit and discarded where they were not representative and could not be modified. In this case, new fragilities were developed based on engineering judgement.
As outlined in the Inception Report and Variation Requests, fragility or vulnerability functions for road, power, and water assets for all hazards were going to be adopted/adapted/developed from existing literature. Where gaps in existing literature was encountered, these have been identified and raised with the ADB as a limitation to this approach.
• Literature review for buildings for tsunami, wind, and seismic: a literature review informed the vulnerability assessment for buildings in tsunami. Fragility functions for the building structure were adapted from literature for the Tongan context.
For building vulnerability under seismic and wind hazards, the study leveraged previous work in the region by directly adopting the PCRAFI vulnerability functions (Bazzurro, 2015).
• Vulnerability modelling for buildings for inundation for flood (both coastal and
pluvial): building models for nine building typologies were developed to represent the building stock in Tongatapu. The building models modelled specific components, each with their own fragility functions, and many simulations were run for varying flood depths to develop building level vulnerability curves based on probabilistic
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modelling. For more detail see Section 4.1. These models were developed to better represent the vulnerability of the building stock on Tongatapu and was made possible due to the data collected during the field surveys. This was considered to be the most appropriate approach because in contrast to seismic and wind, no region-specific vulnerability information was available for flood.
The information provided in this vulnerability report includes a combination of fragility functions (that show the probability of being in various damage states based on hazard intensity) and vulnerability functions (that show the loss or total damage ratio as a function of hazard intensity), depending on the data source.
The table below summarizes the assets and hazards that were included in the vulnerability study. The colours indicate the origin of the selected vulnerability or fragility curves and black indicates no information was found. Where boxes are halved, this indicates that more than one method contributed to the asset-hazard combination as each asset class represents a number of individual assets (e.g. within wind farm the turbines were analysed separately to the auxiliary equipment).
Table 1: Summary of assets and hazards included in vulnerability study
Assets Inundation Tsunami Seismic Wind
Buildings
Roads
Po
wer
Power distribution
Power station
Wind farm
Solar farms
Wa
ter
Elevated water tanks
At grade water tanks
Buried pipelines
Pumps and wells
Developed by Arup based on modelling
Developed by Arup based on engineering judgement and first principles approach
Directly adopted from literature
Adapted from literature based on some modification
No information available
Out of scope or deemed rugged and not vulnerable to hazard
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4 Flood Vulnerability
This section outlines the vulnerability study findings for buildings, roads, power and water infrastructure for flood. These curves will be applied to both pluvial and coastal flooding.
4.1 Buildings
Building vulnerability curves were developed using a component-based approach developed by Arup. The vulnerability curves provide the ratio of losses relative to the total cost to replace a building and its contents. Costs associated with building structure replacement conform to modern construction practice in Tonga. The losses sum across the various damage states of the components, which cover scenarios of minor repairs to total replacement and the costs of materials and labour to return the building to the pre-flood condition. This section describes the methodology used to develop the building vulnerability for flood.
The United States Federal Emergency Management Agency (FEMA) flood risk estimation tool, Hazus, documents the many ways that floods damage buildings. The building structure itself is generally expected to survive a flood, whereas the contents and non-structural components (e.g. partitions) are expected to sustain the most damage during flood. (FEMA, 2009)
This evaluation is consistent with Arup reconnaissance experience. During site visits in Houston, TX after Hurricane Harvey in 2017, extensive damage was observed to residential, commercial, and other buildings due to severe flooding. In depths at or less than 1m, most contents were removed from buildings and time was required to dry out and clean the building. In most cases, the partitions and walls were opened to expose the timber framing, which was vulnerable to water damage and mould. The structural integrity was not compromised for buildings observed during Arup’s reconnaissance.
The building vulnerability curves developed for Tongatapu account for the physical damage to a building and its contents, including the costs to clean, clear debris, and repair partially damaged components. Only the physical damage to buildings was considered. The costs of downtime are not included.
After building losses exceed a certain percent of total replacement cost, the building would typically be considered a total loss. For the assessment of losses during seismic events, this value is typically assumed to be 40 percent (Applied Technology Council, 2012). For flood, there is no consensus amongst experts at what this value should be, but from guidance with SMEs and review of the National Flood Insurance Program (NFIP) in the US, fifty percent is a reasonable assumption. Therefore, if the total losses for flood exceed 50% of the total building replacement cost, the building will be considered a complete loss, which equates to 110% of building replacement cost, the extra 10% accounts for the demolition and removal of the unrecoverable structure.
Modelling damage based on depth is appropriate for floods with low velocity, like those which resulted from the hazard modelling of flood scenarios on Tongatapu. Buildings would be more susceptible to structural failure in high-velocity events (Middelmann-Fernandes, 2010), which were not observed in Arup’s hazard models due to the relatively flat topography of Tongatapu. Therefore, the building vulnerability models were developed based on the relationship between depth and damage, and not velocity and damage.
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Flood Building Model Development
Nine building archetypes were developed to model the vulnerability of the buildings across Tongatapu. The process of developing these archetypes, also referred to as typologies, are described below.
Because the losses due to flood are driven by contents and non-structural losses and not structural losses, the building stock on Tongatapu was divided according to their Usage types and not their Construction types. For example, a Timber structure may support anything from a residence to a restaurant or storage shed, all of which would have drastically different contents.
The typology development was driven by the exposure assessment. Typical building areas were calculated for each Usage type to draw similarities across different Usage categories. The purpose of this exercise was to consolidate dozens of building Usage types into 5-10 building typologies from which archetypes could be created. Usage was grouped into four main categories: Residential, General, Specialized, and Out Buildings. The fifth grouping, Hospital Buildings, was developed to accommodate the unique equipment present on the hospital campus.
• Residential buildings cover the majority of the building stock on Tongatapu. From the exposure assessment, about 75% of all buildings were single story residential, and over half of those were timber structures. The remaining were predominantly masonry. All homes contain finishes, partitions, electrical, plumbing, and personal valuables that do not contribute significantly to the overall building value.
• General buildings encompass the greatest number of usage types, amounting to 15% of the buildings on the island. The following buildings are covered by this archetype: Commercial, Education, Government, Public, Emergency, Religious, and Health. Collectively these buildings stand out from the other typologies by containing more robust electrical and mechanical systems (e.g. back-up power, chillers) than Residential buildings and less expensive, specialized equipment (e.g. heavy machinery) compared to the Specialized building archetype.
• Specialized buildings account for approximately 1% of buildings found on Tongatapu. The Usage types associated with these buildings cover Infrastructure, Industrial, and Agricultural buildings. These buildings share the presence of specialized equipment that contributes to a significant portion of the building value.
• Out Buildings, which includes storage sheds and outhouses associated with Commercial and Residential structures, comprise approximately 7% of the building portfolio on Tongatapu. These simply constructed buildings do not contain finishes or partitions like the other archetypes and contain fewer structural elements (i.e. significantly smaller in size) than a residential building.
• Hospital buildings account for a small percentage of the buildings found on Tongatapu, but they represent an important asset in the Health sector. The significant value of the critical medical equipment warranted a differentiation between Hospital buildings and the Specialized building archetype.
After developing these five groupings, further division was made between timber and non-timber archetypes. The wood framing of timber structures is susceptible to rot and severe
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mould in ways that masonry or steel structures are not. After this exercise, a total of nine building archetypes were identified for modelling in the flood vulnerability assessment.
An additional split was considered to analyse single and multi-story buildings separately. Because multi-story buildings comprise less than 7% of building stock on Tongatapu, specific models were not developed for each archetypal category (i.e. Residential, General, Specialized, and Out Buildings) based on the number of stories. Expected flood depths across Tongatapu exceed the first story height of 1% or fewer buildings for only a couple scenarios. Because the majority of multi-story buildings are residential, the multi-story residential vulnerability curve has been provided herein, in which the first-floor components and contents were duplicated to the second story to model the building. For all other multi-story buildings other than residential, the replacement cost for the building will be calculated based on the number of stories and the single story vulnerability curve will be applied. This will lead to conservative estimates however as outlined above this on impacts a very small number of cases and will have minimal impact on the results.
The relative value of each building typology, according to the replacement costs and total insurable value (TIV), was provided by the engineers Kramer Ausenco and determined using the average building area from the Exposure dataset. The replacement costs were estimated with (“SS Replacement Cost”) and without content losses (“SS Replacement Cost + Contents”), where “SS” refers to single story costs. The abbreviations “T” and “M/O” refer to Timber and Masonry/Other archetypes, respectively.
Figure 22: Comparison of calculated replacement costs and total insurable value.
The buildings were populated with components based on workshops, contents surveys, and site visits completed by our partners in Tonga. Each building archetype was assigned an area based on the average building area for each category using the exposure data. In Table 2 the inclusion of components can be compared across the building typologies.
$0
$200,000
$400,000
$600,000
$800,000
$1,000,000
$1,200,000
Resi (T) Resi (M/O) Gen (T) Gen (M/O) Spec (T) Spec (M/O) Out (T) Out (M/O)
Co
st o
r V
alu
e ($
T)
Building Values
SS Replacement Cost SS Replacement Cost + Contents TIV by Usage
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Table 2: Components included and excluded in vulnerability models for each of the flood building archetypes.
Building
Archetype Residential General Specialized Hospital Out
Building
Building
Usage
Residential Commercial,
Education, Public,
Government,
Religious, Emergency,
Health
Industrial,
Agricultural,
Infrastructure
Hospital Out
Building
Structural System
Yes Yes Yes Yes Yes
Electrical System
Yes Yes Yes Yes No
Finished floor Yes Yes Yes Yes No
Appliances Yes Yes Yes Yes No
Partitions Yes Yes Yes Yes No
Chiller No Yes Yes Yes No
Generator No Yes Yes Yes No
Sensitive electrical equipment
No Yes No Yes No
Heavy machinery
No No Yes No No
Plumbing fixtures
Yes Yes Yes Yes No
Cabinetry Yes Yes No Yes No
Contents Yes Yes Yes Yes Yes
Flood Building Model Methodology
To develop the vulnerability curve for each of the nine building archetypes, each representative model was populated with the components described in the previous section. To quantify the individual component vulnerabilities to flood hazard, component-level fragility curves were developed or adopted from literature, capturing the probability that a component reaches various damage states (ranging from minor requiring repairs, to major requiring full replacement) based on different magnitudes of a hazard, in this case meters of flood depth. This state-of-the-art approach allows for implementing the Monte Carlo modelling method to sample points along a fragility curve to generate a damage probability distribution, ultimately used to calculate risk. The probabilistic approach helps address aleatory and epistemic uncertainties that are inherent for any type of risk modelling.
Arup developed this component-based approach for flood risk analysis based on methodology developed for seismic risk analysis, adopted from FEMA P-58 (Applied Technology Council, 2012) published almost ten years ago and enhanced with the Arup Resilience-based Earthquake Design Initiative (REDi) methodology (Arup, 2013). In the past several years, Arup has adapted this seismic component-level approach to flood risk analysis. Other cutting-edge researchers are also adapting this type of approach for detailed modelling for flood (Nofal & van de Lindt, 2020).
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Fragility curves were developed for this project using research of existing literature, site visit data collected by our Tongan partners, workshops with engineers at Kramer Ausenco, interviews with subject matter experts across Arup, and other guides (e.g. codes, specifications, and standards). Each component type has a unique fragility curve for every damage state, which relate flood depths to the probable damage states. An example has been provided below for interior partitions, where DS1 involves aesthetic damage only (cleaning and drying required), DS2 refers to partial failure of the partitions (replacing the bottom drywall panel), and DS3 represents complete failure of the partitions (fully replacing them from floor to ceiling).
Figure 23: Fragility curves for interior building partitions.
Each building model, populated with components according to its size and archetype, was subjected to incremental flood depths up to twenty meters. For each depth, one thousand Monte Carlo simulations were run, sampling the fragilities of each component so that component-level damage results were produced for each of the 1,000 realizations.
From the damage results, the units which needed to be repaired and replaced were obtained for each simulation. This information was used for the financial loss calculation. Consequence functions were developed that link each damage state defined in the fragility curves to specific levels of financial loss based on the repair and replacement costs of different components, provided by Kramer Ausenco. Then, based on the level of damage, the total financial loss is calculated for each realization.
This process produced a distribution of losses for each flood depth. The losses were divided by the total replacement cost of each building archetype, calculated based on the area of the building. This produced a normalized vulnerability curve, which allows for application to buildings of varying areas and replacement costs across Tongatapu during the Risk Assessment.
The median value of the 1000 realizations per flood depth is presented in this report, but, as shown in Figure 24 below, other percentiles were evaluated to understand the distribution of losses. The distribution will inform the uncertainty applied to these vulnerability curves during the Risk Assessment.
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S>
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s)
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Full-height Partition
DS1
DS2
DS3
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Figure 24: General (Timber) building archetype vulnerability curve for the 10th, 50th, and 90th percentile losses.
This methodology was followed for all nine building archetypes developed for Tongatapu. The resulting vulnerability curves were validated through comparison of curves from literature, including Hazus (FEMA, 2009) and Nofal & van der Lindt (Nofal & van de Lindt, 2020). The shape of the curves followed similar patterns, with variation in how steep the initial slope was depending on the building typology and the lowest-lying components modelled. An example for the General building has been provided in Figure 25 below. Nearly all of the building vulnerability curves for flood developed by the Arup team approach 70%-100% loss after several meters of flooding, which is comparable to the vulnerability curves seen in Nofal & van der Lindt’s results (which all reach total loss) and those presented by Hazus, which typically do not reach 100% loss. Overall, the similarities observed increased the team’s confidence in the vulnerability assessment of buildings in flood hazard.
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5%
10%
15%
20%
25%
30%
35%
40%
0 1 2 3 4 5 6
% R
epla
cem
ent
Co
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General (Timber) Building Archetype Losses
p10 p50 p90
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Figure 25: Comparison of Nofal & van der Lindt’s Small Multi-Unit Commercial Building archetype (black and blue lines), Hazus commercial archetype, and the General building archetypes (Timber as solid light green line, Masonry/Other as dotted dark green line).
Some differences between the vulnerability curves published here and those discussed in literature were expected, in part because the component costs were determined specifically for the Tongan context. These costs of contents and non-structural components may contribute different proportions to the total replacement cost, driving the shape of the curve as they get damaged. A summary of the estimated average cost of building typologies and contents are provided in Table 3 and Table 4. This is based on replacement cost data provided by Kramer. When combined with content costs over a whole building, the estimated average cost of replacement for building flood archetypes are as listed in Table 5.
Table 3: Replacement cost (estimated average) for building replacement.
Building Typology Building Costs (T$ per m2)
Single Story Multi-Story
Timber
Residential $2,500 $3,500
General $3,400 $4,400
Specialized $3,200 $4,000
Average $3,000 $3,800
Reinforced Masonry
Residential $2,200 $2,900
General $2,900 $3,800
Specialized $2,600 $3,500
Average $2,800 $3,200
Steel
All $2,700 $3,150
Reinforced Concrete
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$3,100 $3,700
Reinforced Masonry & Concrete
Frame
All $3,200 $3,400
Open (e.g. gas station, typically
steel)
All $2,800 NA
Traditional
All $600 NA
Table 4: Replacement cost (estimated average) for contents.
Building Usage Content Costs (T$ per building)
Residential - Poor Quality $2,200
Residential - Good Quality $9,000
School $18,000
Government $21,000
Public $12,000
Religious $12,000
Commercial $18,000
Agricultural $16,000
Industrial $18,000
Infrastructure $21,000
Health $110,000
Hospital $1,500,000
Emergency $50,000
Out Building $13,500
Table 5: Replacement cost for building flood archetypes (where SS denotes single story and MS denotes multi-story buildings).
Building Archetype Building + Content Costs (T$ per building)
SS Residential (Timber) $321,500
SS Residential (Masonry/Other) $284,000
MS Residential (Timber) $884,000
MS Residential (Masonry/Other) $734,000
General (Timber) $544,400
General (Masonry/Other) $469,400
Specialized (Timber) $1,138,300
Specialized (Masonry/Other) $928,300
Hospital $2,550,000
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Out Building (Timber) $67,900
Out Building (Masonry/Other) $61,400
For more detail on each archetype, please see the following section for one-page summaries of the nine typologies developed for the flood vulnerability assessment.
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Residential Building | Flood
Description
Four Residential Building archetypes were
developed to model approximately 75% of the
Tongatapu building portfolio – two for timber
construction and two for masonry or other
construction types, with one single story curve
and one multi-story curve each. Through
contents surveys, site visits, and cost
workshops, the models were populated with
components specific to Tongan residences.
Damage and failure modes
All Residential archetypes shared the same
interior non-structural components and
contents, developed using input from Tongan
consultants. Damage to the partitions and
electrical system is the largest contributor to
non-structural loss, which gets damaged in
under 1m of flooding. This requires expensive
repairs, before even reaching further damage
states. Contents in residences comprise a small percentage of total building replacement value. So, at
low flood depths, the building itself would be unharmed even though contents may need replacement.
Because timber framing requires costly repairs or replacement in flood compared to masonry or other,
less water-vulnerable construction, the max loss of the Residential (Timber) typologies exceeded the
max loss of the Residential (Masonry/Other) typologies, as seen in Figure 26 above. For further
discussion on the impacts of floodwater on timber framing, please see Section 4.1.
The multi-story building curves replicated the first-floor components and contents onto the second
floor, modelling a two-story residential structure and then the absolute losses were calculated for
flood depths up to 6 meters. To get the total replacement cost, the contents value was added to the cost
of the building structure to generate the total replacement cost. The multi-story buildings cost more to
replace than single story buildings per square meter, so even though the contents were doubled the
percent loss does not double. For example, the max loss as a percentage of replacement costs during 6
m of inundation, approaches 50-65% for multi-story buildings compared to 70-85% for single-story
residential structures. For any typology, as discussed in Section 4.1, the building will be considered a
total loss once losses exceed 50% of the total replacement cost.
Vulnerability curves
As described in Section 4.1, the residential vulnerability curve was developed from a component-level model. The inputs to the components, such as their vulnerability and costs, were developed from a literature review and workshops with Tongan engineers. The curves presented here represent the median loss from thousands of Monte Carlo simulations.
Figure 26: Residential building archetypes for flood.
Type of curve: Vulnerability function
Origin: Arup – developed from component-level building model
Source(s) / reference: Arup
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Damage presented as percent of total replacement cost for a given depth
Table 6: Vulnerability summary for Residential building archetypes.
0%
10%
20%
30%
40%
50%
60%
70%
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90%
100%
0 1 2 3 4 5 6
% R
epla
cem
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Co
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Depth (m)
Residential Building
Residential SS (Timber)
Residential SS (Masonry/Other)
Residential MS (Timber)
Residential MS (Masonry/Other)
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General Building | Flood
Description
Two General Building archetypes were
developed to model approximately 15% of the
Tongatapu building portfolio – one for timber
construction and the second for masonry or
other construction types. This archetype covers
buildings of a variety of usage types (e.g.
Commercial and Government), but these
buildings share characteristics, such as similar
costs of construction and similar vulnerability
of contents (i.e. fewer household appliances,
more computer equipment and supplies).
Damage and failure modes
Unlike residential buildings, the typologies
covered by the General archetype, including
commercial and government buildings, contain
more robust electrical and mechanical systems
critical to their functioning. The building
models were populated with electrical
equipment, chillers, and generators according to the site surveys done by Tongan consultants and
engineers.
Similar to the Residential archetypes, the damage to walls, partitions, and the electrical system drove
the cost to the General archetypes. Water-sensitive electrical and mechanical equipment on the ground
floor also contributed significantly to the cost at depths less than one meter. The General building is
expected to incur damages exceeding 50% of its replacement cost by one meter of flooding.
Vulnerability curves
As described in Section 4.1, the general building vulnerability curve was developed from a component-level model. The inputs to the components, such as their vulnerability and costs, were developed from a literature review and workshops with Tongan engineers. The curves presented here represent the median loss from thousands of Monte Carlo simulations.
Figure 27: General building archetypes for flood.
Type of curve: Vulnerability function
Origin: Arup – developed from component-level building model
Source(s) / reference: Arup
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Damage presented as percent of total replacement cost for a given depth
Table 7: Vulnerability summary for General building archetypes.
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100%
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General Building
General (Timber)
General (Masonry/Other)
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Specialized Building | Flood
Description
Two Specialized Building archetypes were
developed to model approximately 1% of the
Tongatapu building portfolio – one for timber
construction and the second for masonry or
other construction types. This archetype covers
buildings of several Usage types: Industrial,
Agricultural, and Infrastructure. These
buildings were grouped over their similarities
of having expensive, specialized equipment in
their roster of contents.
Damage and failure modes
Industrial buildings, which cover a range of
usage types, including car mechanic shops and
agricultural production facilities, contain
specialized mechanical and electrical
equipment which differentiate these buildings
from residential or commercial buildings. This
equipment contributes to a significant
percentage of the losses. As seen in the
vulnerability curve, the steep incline at less than 1.5 meters of flood depth corresponds to the damage
of these contents of considerable value, in addition to the typical components modelled in the other
building archetypes (e.g. electrical outlets, partitions). The development of the component-level
fragilities for this building archetype drew on research and experience with sensitive equipment
through Arup’s work with mission-critical infrastructure.
Many other components overlapped with the other building types, including the electrical system and
partitions, which contribute to significant portions of the total replacement cost. The Timber archetype
deviates from the Masonry/Other archetype due to the framing experiencing water damage at depths
<1m, whereas this type of damage was assumed to not impact masonry walls which are more robust
in flood.
Vulnerability curves
As described in the Section 4.1, the
specialized vulnerability curve was
developed from a component-level
model. The inputs to the
components, such as their
vulnerability and costs, were
developed from a literature review
and workshops with Tongan
engineers. The curves presented
here represent the median loss from
thousands of Monte Carlo
simulations.
Figure 28: Specialized building archetypes for flood.
Type of curve: Vulnerability function
Origin: Arup – developed from component-level building models
Source(s) / reference: Arup
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Damage presented as percent of total replacement cost for a given depth
Table 8: Vulnerability summary for Industrial building archetype.
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60%
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100%
0 1 2 3 4 5 6
% R
epla
cem
ent
Co
stDepth (m)
Specialized Building
Specialized (Timber)
Specialized (Masonry/Other)
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Out Building | Flood
Description
Two Out Building archetypes were developed
to model approximately 7% of the Tongatapu
building portfolio – one for timber construction
and the second for masonry or other
construction types. This archetype covers out
buildings associated with all usage types,
because they are typically used in similar ways
– for storage or as an outhouse/bathroom.
Damage and failure modes
The Out Buildings have an initial jump when
costs are incurred for cleaning and drying of the
building (<3% of total replacement cost). When
the contents are damaged, they account for
approximately 5% of total building replacement
cost, which creates an increase after 0.5m of
flooding, more easily seen in the
Masonry/Other archetype. The cost of the
Timber archetype is driven by the damage to
the framing, causing the overall vulnerability to increase when the framing sees significant damage
around 0.5m and 3m. The Masonry/Other archetype incurs the cost of wall damage when the flood
depth exceeds the first story at around three meters. At this depth, all modelled components have been
overtopped by floodwater, but the total cost amounts to only 40% of the estimated replacement cost.
This difference may be attributed to the components not explicitly modelled as part of the Out
Building, but that may be included in the estimated cost of a small timber or masonry structure:
electrical system, ceiling, roof, and slab.
Vulnerability curves
As described in the Section 4.1, the specialized vulnerability curve was developed from a component-level model. The inputs to the components, such as their vulnerability and costs, were developed from a literature review and workshops with Tongan engineers. The curves presented here represent the median loss from thousands of Monte Carlo simulations.
Figure 29: Out Building archetypes for flood.
Type of curve: Vulnerability function
Origin: Arup – developed from component-level building models
Source(s) / reference: Arup
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Damage presented as percent of total replacement cost for a given depth
Table 9: Vulnerability summary for Out building archetype.
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Building Archetypes
Out (Timber) Out (Masonry/Other)
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Hospital Building | Flood
Description
Health buildings and Emergency buildings
have been included in the General building
archetypes.
The Hospital buildings were modeled separate
due to the expensiveness of the equipment in
proportion to the building value.
Damage and failure modes
Hospital buildings contain specialized medical
equipment, including x-ray machine, CT scan
machine, ultrasound machine, and
defibrillators, many of which are located on the
ground floor. These components are vulnerable
to flood inundation, as submersion can damage
sensitive electrical or mechanical
subcomponents. The value of these contents
exceeds one million Tonga pa’anga which
contributes a significant percentage to the total
building replacement cost, as estimated by its construction type and usage as a specialized building.
A significant portion of damage, reaching approximately 30% loss, occurs at depths less than 1.5
meters, which would impact the expensive contents and trigger repairs of the partitions which
contribute greatly to overall loss. Similarly, the jump to approximately 70% between 3-4.5 meters
corresponds with damage to the second-story contents, assumed to contribute to half of the total value
of the expensive hospital equipment.
Vulnerability curves
As described in the methodology of
Section 4.1, the hospital
vulnerability curve was developed
from a component-level model.
The inputs to the components, such
as their vulnerability and costs,
were developed from a literature
review and workshops with Tongan
engineers. The curves presented
here represent the median losses
from thousands of Monte
Carlo simulations.
Figure 30: Specialized building archetypes for flood.
Type of curve: Vulnerability function
Origin: Arup – developed from component-level building models
Source(s) / reference: Arup
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Damage presented as percent of total replacement cost for a given depth
Table 10: Vulnerability summary for Industrial building archetype.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 1 2 3 4 5 6%
Rep
lace
men
t C
ost
Depth (m)
Hospital Building
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4.2 Roads
This section outlines the vulnerability curves to be used for roads in Tongatapu subject to flood.
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Roads | Flood
Description
The roads asset includes both paved and unpaved roads on Tongatapu. Discussions with Arup subject matter experts indicated that flood was likely to impact both paved and unpaved roads in a similar manner.
Damage and failure modes
Flood waters erode the sub-base of both paved and unpaved roads by carrying away fine sediments critical to their integrity. This damage occurs at very low levels of flooding, therefore some damage to the foundation would be expected at <0.5m flooding. Structural damage from flooding may also include damage to the pavement, scour, and erosion of the roadway or embankment (Kilgore, Herrmann, Thomas Jr., & and Thompson, 2016). According to the TC Gita Post-Disaster Needs Assessment from 2018, which aligns with description in the Tonga State of the Roads report draft from 2015, “in some areas the condition of roads has deteriorated significantly due to insufficient emphasis on maintenance” which implies that roads have experienced significant degradation and are vulnerable to wash-out and severe scour, especially from coastal wave action (Najera Catalan, 2018) (Government of Tonga, 2015). Though debris was not explicitly considered, it is known that the inundation of residences and other structures leads to debris which must cleared from roads before full service can be returned and repairs made (MEIDECC National Emergency Management Office, 2020).
Vulnerability curves
Most literature discusses the damage to roads caused by flood in qualitative terms or focuses on network disruption. Because no consensus exists around the flood depths that cause damage to roads, other than roads being vulnerable to water at any depth, the economic estimates provided by the USACE analysis of roads in Louisiana, US were used as the basis for the vulnerability presented here, which has been adapted from its original tabular format in US dollars to a lognormal function representing percent of total replacement. In Tonga, methods from Austroads are typically adopted, which was developed from US road design standards. Because of the similarities between codes, these vulnerability curves were adopted for Tonga without modification, however, this does not explicitly incorporate reports of road quality. The information available about road quality was not sufficient to meaningfully adjust the curves above.
Figure 31: Road vulnerability for flood.
Type of curve: Vulnerability function
Origin: Adapted from USACE economic analysis of roads susceptible to cyclone
Source(s) / reference: USACE (U.S. Army Corps of Engineers, 2009)
Basis: US
Intensity measure: Inundation depth
Description of damage
states:
Damage presented as percent of total replacement cost for a given depth
Table 11: Vulnerability summary for roads.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 1 2 3 4 5 6
% R
epla
cem
ent
Co
st
Depth (m)
Road (Paved and Unpaved)
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4.3 Power infrastructure
This section outlines the vulnerability curves to be used for power infrastructure in Tongatapu subject to flood.
Utility poles and power lines were not considered for flood damage, but critical electrical equipment attached to poles was – please see “Pole Mounted Equipment” for additional detail. Several damage mechanisms could impact utility poles and power lines in flood, including debris impact, floatation, and drag on poles and lines. Because flood velocity was not considered in the hazard analysis for pluvial or coastal scenarios, it was assumed that the moving floodwaters of a tsunami would cause significantly more damage to utility poles. For significant damage to occur, the floodwaters must reach the sensitive electrical equipment attached to utility poles. The power lines themselves were considered rugged to inundation, as they are weatherproofed against water intrusion through the cable housing.
Wind turbines were considered rugged to flood damage, but the vulnerabilities of the auxiliary equipment at the wind farms were modelled. See the following section.
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Transformers | Flood
Description
Ground-mounted transformers, though typically weatherproofed to some degree, remain vulnerable to flood inundation.
Damage and failure modes
Individual components of the transformer are susceptible to water damage, like the coils in oil-filled transformers as seen in Tonga. The first damage state corresponds to water entering the transformer and damaging individual subcomponents, which would require replacement. The second damage state leads to total replacement of the transformer and occurs once water has almost completely inundated the interior. These damage states assume that the transformer does not have an IP rating against submersion. The curves presented in Figure 32 should be taken from the base of the transformer. They do not account for the plinth on which exterior ground-mounted transformers are typically found, as observed during site visit data collection. Elevations will be taken into account during the risk analysis process, as damage would not be expected until floodwaters exceed the plinth height.
Vulnerability curves
The two damage states for transformers were developed at Arup from interviews with electrical engineers and other subject matter experts with flood reconnaissance experience. Review of transformer specifications and the photographs from Tongatapu site visits rounded out the assessment of transformer vulnerability and the development of the fragility functions presented
in Figure 32.
Figure 32: Ground-mounted transformer flood vulnerability.
Type of curve: Fragility function
Origin: Arup – engineering judgement, SME input, specification review, and project experience
Source(s) / reference: Arup
Basis: US, International
Intensity measure: Inundation depth
Description of damage
states:
DS1 – Water enters & damages individual components. DS2 – Most components submerged.
Table 12: Vulnerability summary for Transformers.
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4 5 6
P(D
S>
=d
s)
Depth (m)
Ground-Mounted Transformers
DS1 DS2
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Pole-Mounted Equipment | Flood
Description
Pole-mounted equipment, such as capacitors and transformers, are rugged to rainfall but susceptible to damage in inundation.
Damage and failure modes
When floodwaters reach the sensitive electrical equipment mounted on a utility pole, the interior components become inundated and damaged. The pole-mounted equipment was assumed to be designed to an adequate water-resistant standard that can withstand rainfall but not submersion, similar to how ground-mounted transformers are designed. From photographs taken on site visits to Tongatapu, the transformers and other mounted equipment appeared at heights of approximately 6-7m. If flood reached this depth, the pole-mounted equipment would experience significant damage, likely requiring replacement.
Vulnerability curves
The fragility curve developed for the Pole-Mounted Equipment was based on the elevation of the equipment on the pole, determined to be between six and seven meters, depending on the type of utility pole (HV and LV). A generalized fragility was developed using the assumption that once inundated, the equipment would need to be replaced.
Figure 33: Pole-mounted equipment fragility for flood.
Type of curve: Fragility function
Origin: Arup – engineering judgement, SME input, specification review, and project experience
Source(s) / reference: Arup
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Water damages critical sub-components, replacement required
Table 13: Vulnerability summary for Pole-Mounted Equipment.
0.00
0.20
0.40
0.60
0.80
1.00
0 1 2 3 4 5 6 7 8 9 10
P(D
S>
=d
s)
Depth (m)
Pole-Mounted Equipment
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Power Station | Flood
Description
The power station building contains eight diesel generators, three switchboard rooms, and other equipment vulnerable to flood. Also on site are exterior transformers, water pumps, and a large diesel storage tank.
Damage and failure modes
The building of the power station sits elevated on a foundation 1.5 meters above grade. If the water depth exceeds the finished floor elevation, significant damage would be expected to occur to the electrical equipment inside the power station building, including control panels, switchgears, and generators (Miyamoto International Inc., 2019). Some of the exterior equipment, including pumps in the lagoon and the diesel tank, were not modelled individually for flood and assumed to be rugged to inundation or detailed study is required. Exterior electrical equipment, like transformers, would experience damage in significant flood events, but the cost of replacing these components is small when compared to the cost of replacing the entire power station.
Vulnerability curves
Adapted from Hazus vulnerability for Small Power Plants, which applies to generation facilities of less than 100MW and assumes that electrical switch gear is located approximately 1m above grade. These assumptions, though not perfectly aligned with the power station on Tonga, provide a basis of power generation facility vulnerability to flood. As the curve represents both the indoor and outdoor equipment, the curve has not been adjusted to account for the 1.5m above grade foundation. The tabular values were plotted and a lognormal curve fitted to them to produce the vulnerability function presented here.
Figure 34: Power station vulnerability to flood.
Type of curve: Vulnerability function
Origin: Adapted from Hazus (FEMA, 2009)
Source(s) / reference: Arup
Basis: US
Intensity measure: Inundation depth
Description of damage
states:
Damage as a percent of total replacement cost
Table 14: Vulnerability summary for Power Station.
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4 5 6
% D
aa
ma
ge
Depth (m)
Power Station
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Solar Farm – PV Panels | Flood
Description
Photovoltaic panels, junction boxes and transformers were assessed together as part of the solar panel system at the three solar farms on Tongatapu and were evaluated separate from the other auxiliary equipment on site for vulnerability to flood.
Damage and failure modes
The junction boxes associated with solar panels were deemed the most vulnerable subcomponent to damage. From data collected during site visits to Popua Power Station and Maama Mai Solar Facility, the junction boxes were determined to be 0.5-1.5m above the ground. When water inundates these junction boxes, it may damage the electrical components inside and require repairs or replacement. The solar PV panels were assumed protected against adverse weather conditions (i.e. rainfall), but there was not enough information available to determine that the design standard or IP rating would prevent damage from complete submersion. Therefore, the panels were vulnerable to if floodwaters overtopped them. The solar panel structures and foundation were considered rugged to flood inundation. Scour can be an issue but is not a concern for these panels based on their concrete foundations (Miyamoto International Inc., 2019). The cost of damage to junction boxes would be significantly smaller than the cost of replacing the entire solar panel. The transformers on site were considered part of the solar panel system, whereas the inverters and batteries were considered separate, as auxiliary equipment (see next page).
Vulnerability curves
The vulnerability curve for PV panels was developed based on evaluation of site visit photos and discussion with electrical engineers familiar with solar farm design. Complete damage of the junction boxes was assumed to occur between 0.5m and 1.5m, based upon photographs taken at solar facilities on Tongatapu. At depths greater than 1.5m, the likelihood of damage to junction boxes exceeds 90%, and complete inundation of the solar panels was estimated to reach 50% likelihood at two meters.
Figure 35: Solar farm PV panel fragility.
Type of curve: Fragility function
Origin: Arup – engineering judgement, SMEs, Tongatapu site visit data collection
Source(s) / reference: Arup
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
DS 1: Junction box inundated; requires replacement DS 2: PV panel completely submerged; requires replacement
Table 15: Vulnerability summary for Solar Farm PV Panels
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 1 2 3 4 5 6
P(D
S>
=d
s)
Depth (m)
Solar PV Panel
DS1 D2
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Solar Farm – Auxiliary Equipment | Flood
Description
The auxiliary equipment located at solar farms, including inverters, transformers, and batteries, are typically located outside, housed in metal containers, or in a building on-site.
Damage and failure modes
The auxiliary equipment at the solar farm located outside is vulnerable to damage from flooding. Transformers, inverters, and any electrical equipment located inside small metal containers would be damaged if submerged. As most equipment observed from site visit data collection was located atop concrete plinths, the flood depth must exceed the height of the plinth before the fragility curves presented in Figure 36 or Figure 32 would apply. The equipment inside a building present at the solar farm would be susceptible to water damage if the depths exceed the finished floor elevation of the building. Once water enters the building, electrical equipment resting on the ground, like switchgears, would be susceptible to damage.
Vulnerability curves
The fragility curves presented in
Figure 36 was developed by Arup for electrical equipment providing mission-critical services. Based on the review of inverters and transformers housed in similar metal containers as those observed in the US, the fragility functions were taken to apply to the exterior auxiliary equipment present at the solar farms. This fragility excludes the ground-mounted transformers observed from site visit data collection, as the vulnerability for the transformers provided earlier in this section will be applied for those assets. For equipment housed in an auxiliary building present on site, the flood vulnerability curve for Specialized buildings would apply (See Section 4.1).
Figure 36: Vulnerability for solar farm auxiliary equipment.
Type of curve: Fragility function
Origin: Arup – engineering judgement, SMEs, Tongatapu site visit data collection
Source(s) / reference: Arup
Basis: US, Tonga
Intensity measure: Inundation depth
Description of damage
states:
DS 1: Water reaches wiring; replace wiring DS 2: Water reaches critical subcomponents; replace component
Table 16: Vulnerability summary for solar farm auxiliary equipment.
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
P(D
S>
=d
s | d
epth
)
Depth (m)
Solar Farm Auxiliary Equipment
DS1 DS2
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Wind Farm – Auxiliary Equipment | Flood
Description
At the wind farm, the auxiliary equipment includes several ground-mounted transformers outside and a building that houses additional critical electrical equipment, including control panels, switchboards, and batteries.
Damage and failure modes
The exterior transformers present at the wind farm are vulnerable to damage due to inundation. The transformers, though designed for adverse weather conditions, were assumed vulnerable to submersion, which would require the replacement of subcomponents or the entire transformer depending on the depth. The equipment inside the building would be expected to sustain flood damage when the water exceeds the finished floor elevation of the building, which is raised approximately one meter above the ground. Because the equipment rests directly on the floor, albeit in cabinets, it would be vulnerable to damage in even low depths of flooding that reach the interior of the building.
Vulnerability curves
Two vulnerability curves will used to model the auxiliary equipment at the wind farm – one for the exterior ground-mounted transformers and one for the components in the on-site building. The fragility curves for transformer performance in flood were presented earlier in this section under Transformers | Flood. The vulnerability curve for the building and its water-sensitive contents is represented by the Specialized building archetype for Masonry/Other construction, as the structure has a steel frame and metal cladding.
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4.4 Water infrastructure
This section outlines the vulnerability curves to be used for water infrastructure in Tongatapu subject to flood.
The water distribution system, comprised of buried pipelines, was considered rugged to flood hazards. Per FEMA, “no damage [is] expected from submergence” to buried pipelines (FEMA, 2009). Horspool & Fraser also discuss buried pipes in their review of lifelines performance in tsunami, describing how they “performed well” during previous events (Horspool & Fraser, 2016). Since flood events have a less abrasive impact on buried pipes (low velocity compared to tsunamis), these components are considered rugged to flooding. Similarly, both elevated and at-grade water tanks were assumed rugged to flood. This is based on the assumption that the foundations of elevated water tanks are not damaged and that for at-grade tanks, the tank water level exceeds the flood depth and therefore the tank will not float. Contamination of the boreholes due to any type of flooding has the potential to have significant impacts, though this is difficult to quantify.
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Pumps and Wells | Flood
Description
Each well was assumed to contain a submersible electric pump and to be covered by a small timber pumphouse. A transformer was also located on site.
Damage and failure modes
If water exceeds the elevation of the pumphouse concrete foundation, it would be expected to enter the well through the floor openings or vents. This makes the well susceptible to inundation and contamination, requiring extensive clean-up before the water would be safe to drink. (FEMA, 2009) (Arrighi, Tarani, Vicario, & Castelli, 2017) The submersible pump was considered rugged to inundation, as it was designed for submersion in water. The pipework also present at the site was similarly assumed rugged. The transformer on site is vulnerable to water damage in a flood event if the depth reaches sensitive interior components. More information about transformer damage due to inundation can be found in the Transformers | Flood section.
Vulnerability curves
The damage function presented here represents damage to the well, primarily through non-permanent contamination as assumed by Hazus. In its original form, the function assumes that the critical equipment and the well or vent openings are located approximately 1m above the ground. From site visit data collection, the typical pumphouse was raised approximately 20-30cm on its foundation. The Hazus damage function was shifted accordingly. Damage to the on-site transformer is represented by the fragility curves in Figure 32 for ground-mounted transformers in flood.
Figure 37: Vulnerability for pump and well for flood.
Type of curve: Fragility function
Origin: Adapted from Hazus
Source(s) / reference: Arup, FEMA (FEMA, 2009)
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Percent of complete damage
Table 17: Vulnerability summary for pumps and wells.
0%
20%
40%
60%
80%
100%
0.0 1.0 2.0 3.0
% D
amag
e
Depth (m)
Pump and Well Vulnerability
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5 Tsunami Vulnerability
This section outlines the vulnerability study findings for tsunami. In general, there are a lot of similarities with the previous section that focused on flood inundation, however tsunamis often have higher energy flows compared to coastal or pluvial flood. This can result in increased structural damage and damage resulting from debris. Damage modes from pre-tsunami earthquakes were not considered for the tsunami vulnerability assessment.
The fragility and vulnerability functions identified for buildings, roads, water and power infrastructure are detailed in the following sections.
5.1 Buildings
The damage to buildings due to tsunami will be captured through a combination of inundation vulnerability curves outlined in Section 4.1 and the fragilities outlined here. The inundation-based vulnerability curves will cover non-structural and contents components and the momentum flux-based fragility curves will be applied to the building’s structural components. They have not been combined as it is a multivariate fragility function.
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Buildings | Tsunami
Description
Buildings subject to tsunami will be modelled using the flood vulnerability curves in Section 4.1 for contents and non-structural components. The structural components will be modelled separately and are discussed here for all typologies.
Damage and failure modes
Structural damage to buildings due to tsunamis occurs due to the forces from tsunami flow and impact from debris. Water depth, water velocity and the presence of debris (FEMA, 2017) all contribute to the structural damage however, the most common hazard parameter in the literature used to quantify vulnerability are depth-based formulations, which neglect tsunami velocities. The selected fragilities from FEMA (FEMA, 2017) use momentum flux which considers both inundation depth and velocity (momentum flux = depth x velocity2), as a measure of the tsunami hazard so is seen as a realistic measure of damage (Charvet, Macabuag, & Rossetto, 2017).
For resistance of a structure to a tsunami, the lateral capacity of a structure is important. It is assumed that complete damage occurs when lateral forces exceed the lateral strength of the structure. The lateral capacity of the structure under Tsunami flow can be estimated based on the expected seismic capacity of the building (seismic loads also being lateral loads).
Vulnerability curves
The FEMA Hazus Tsunami Model (FEMA, 2017) was selected as it accounted for depth as well as velocity; included fragilities for 36 building types covering the range of building types within the Tongatapu building portfolio and was suitable for the level of building information available from the building surveys completed for the project. The Hazus fragilities (FEMA, 2017) also differentiate based on the stringency of the building Seismic Design Level likely to be in use (Pre-code, Low code, Moderate code or High code). Tongan building codes in the early 2000s refer to the California Building Code for seismic design. Prior to this, building manuals were used to set out guidelines. Based on the local context of building code introduction, building code thresholds, code compliance and material quality, buildings in Tonga are assumed to be ‘Low code’. For buildings built since the mid-2000s that meet building code seismic requirements, a ‘Moderate code’ level could be assumed, giving reinforced masonry, reinforced concrete and steel building typologies significantly better performance. However, insufficient detail is available on the age of construction of the buildings in the exposure assessment to apply this and therefore this study assumes the entire building stock to be ‘Low code’.
Figure 38: Fragility functions for the Building Structure (FEMA, 2017)
Type of curve: Fragility function
Origin: Adopted without modification
Source(s) / reference: FEMA Hazus (FEMA, 2017)
Basis: Analytical models
Intensity measure: Momentum flux
Description of damage
states:
Probability of collapse
Table 18: Vulnerability summary for the Building Structure
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5.2 Roads
This section outlines the vulnerability curves to be used for roads in Tongatapu subject to tsunami.
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Roads | Tsunami
Description
The roads asset includes both paved and unpaved roads on Tongatapu.
Damage and failure modes
In the same way that damage is described for flood in Section 4.2, damage to roads from tsunami may include scouring and debris damage. Scouring occurs as water carries material away from the road base or surface and is related to both depth and flow velocity. Roads with bases of engineered soil are less susceptible (but not immune) to scour. Scour is exacerbated by waves and therefore of particular concern along coastal areas. In addition, roads that are adjacent to drainage channels/rivers/culverts, or elevated on ridges are also more vulnerable (Horspool & Fraser, 2016), though this level of detail has not been assigned to the road network and therefore not included in the assignment of vulnerability information in this study. Debris damage is more likely during tsunami compared to other flood related events due to the increased wave velocities. Descriptions of debris as a function of flood depth can be found in the literature (Horspool & Fraser, 2016) though it is difficult to incorporate in a quantitative manner. It has been incorporated to the extent that debris damage was captured in the data used to develop the fragility curves shown in Figure 39.
For flood related damage, water may enter the road base via all directions (i.e. from above, the edges or from below in the case of ground water). The seal on paved roads prevent water from entering from the top (e.g. direct rainfall) but water may still undermine the based from underneath and from the sides (e.g. overland flow erodes/scours road edges). Therefore, no differentiation has been made between expected performance of paved and unpaved roads.
Vulnerability curves
The fragility functions from Horspool & Fraser were developed based on data from Japan and assumed applicable to the New Zealand context. Local pavement design standards were not available for Tonga but based on consultation with an Arup Australia pavement engineer we have assumed that empirical methods from Australia have been adopted, which are similar to New Zealand and therefore deemed applicable to Tongatapu.
Figure 39: Road fragility curves (Horspool & Fraser, 2016)
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) / reference: Horspool & Fraser (Horspool & Fraser, 2016)
Basis: Japan, empirical data
Intensity measure: Inundation depth
Description of damage
states:
DS 1: Minor damage
DS 2: Damage to one lane DS 3: Damage to all lanes
Table 19: Vulnerability summary for roads
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5.3 Power infrastructure
This section details the vulnerability and fragility curves identified for power infrastructure subject to tsunami.
Transformers will use the same fragility functions as flood to represent damage states from tsunami. Other literature (Horspool & Fraser, 2016) published fragilities that were less conservative for general exterior electrical equipment at substations than the fragility curves derived specifically for transformers in flood from Arup’s prior work with mission-critical structures.
The wind farm, located in the northeast along an elevated area overlooking the ocean, is exposed to significant tsunami hazard. Analysis for turbine performance in tsunami events exists in literature for on-shore and off-shore turbines. However, these turbines differ significantly in their structural typology compared to those in Tongatapu. Turbines in literature (Zhou, Yan, Ma, & Wong, 2014) (Larsen, Arboll, Kristoffersen, Carstensen, & Fuhrman, 2018) (McGovern, et al., 2019), if they include on-shore structures, have a monopile anchored or pad foundation system whereas those on Tongatapu have a hinge at the base with stayed cables to fix the turbine to the ground. The tower component of the turbine is also typically constructed out of a cylindrical structure, rather than a frame. This difference would impact the scour mechanisms expected at the base during a tsunami. There may be additional risk incurred by debris impacting the stayed cables which secure the turbine in place, which would not be an issue in standard wind turbine types found in literature. Finally, the size and mass are distributed differently in the turbines found on Tongatapu, which may change the likely performance under lateral loads generated during tsunami events. No applicable vulnerability functions were found in literature.
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Utility Poles and Power Lines | Tsunami
Description
The power distribution system is primarily
comprised of utility poles and power lines.
Damage and failure modes
Tsunamis cause damage to distribution systems
in several ways. The fast-moving water can
scour the base of the utility poles and
undermine the foundation. This damage can
cause the pole to lean or topple.
Debris strikes also pose a significant hazard to
utility poles and power lines. If the water level
reaches the height of the power lines, floating
debris can sever cables from the utility poles.
This type of damage is difficult to predict and
model, but the empirical data that informed the
development of these fragilities included this
type of damage. (Horspool & Fraser, 2016)
Hydrodynamic forces of the tsunami may exceed the strength of the wooden utility poles or pull
overhead powerlines from their connection to the poles. Toppled utility poles and severed overhead
lines were observed after the 2015 Chile Tsunami in Coquimbo, where water levels reached
approximately three meters (Horspool & Fraser, 2016).
Vulnerability curves
The fragility functions developed
by Horspool & Fraser (Horspool &
Fraser, 2016) were adopted without
modification for wood utility poles.
The utility poles on Tongatapu are
typically constructed of pine, and
the two damage states presented by
Horspool & Fraser. were assumed
to apply, as the authors developed
the functions based on expert
judgment and the review of post-
tsunami damage reports from
Samoa, the Andaman Islands,
Chile, and Japan. Where utility poles are
damaged, the connected lines will also be
considered damaged.
Figure 40: Utility Pole fragility for tsunami.
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) / reference: Horspool & Fraser (Horspool & Fraser, 2016)
Basis:
Post-tsunami damage reports from Samoa, Andaman Islands, Chile, Japan; expert judgment
Intensity measure: Inundation depth
Description of damage
states:
DS 1: Moderate damage to pole, leaning or needs repair
DS 2: Complete damage to pole, washed away
Table 20: Vulnerability summary for Utility Poles
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Pole Mounted Equipment | Tsunami
Description
The pole-mounted equipment covers an array
of assets, including transformers, capacitors,
and fuses.
Damage and failure modes
According to Tang et al. (Tang, et al., 2006) (as
cited in (Horspool & Fraser, 2016)), after the
2004 Indian Ocean Tsunami, many poles
remained standing, but the pole-mounted
transformers were damaged from the saltwater
inundation. Other electrical equipment, like
capacitors and fuses, would be similarly
vulnerable to tsunami flooding.
Hydrodynamic forces that undermine the
structural integrity of utility poles would impact
the pole-mounted equipment. If a utility pole
topples, the pole-mounted equipment could be damaged in the fall or by the floodwaters on the
ground. In this case, once the poles are damaged, a complete replacement of any mounted equipment
is expected.
Vulnerability curves
The fragility functions developed
by Horspool & Fraser (Horspool &
Fraser, 2016) were adopted without
modification for wood utility poles.
The pole-mounted equipment was
taken to be damaged if the utility
pole enters either damage state, in
which the pole requires repair (DS
1) or replacement (DS 2). This will
be coupled with a check of the
fragility shown in Figure 33 for
Pole-Mounted Equipment | Flood if
the pole itself is undamaged. For
more background on the utility
pole fragility functions, please see
Utility Poles and Power Lines |
Tsunami. Horspool & Fraser developed the functions based on expert judgment and the review of
post-tsunami damage reports from Samoa, the Andaman Islands, Chile, and Japan (Horspool &
Fraser, 2016).
Figure 41: Utility pole fragility for tsunami, taken to apply to pole-mounted equipment. (Horspool & Fraser, 2016)
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) / reference: Horspool & Fraser (Horspool & Fraser, 2016)
Basis:
Post-tsunami damage reports from Samoa, Andaman Islands, Chile, Japan; expert judgment
Intensity measure: Inundation depth
Description of damage
states:
DS 1: Moderate damage to pole, leaning or needs repair
DS 2: Complete damage to pole, washed away
Table 21: Vulnerability summary for pole-mounted equipment.
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Power Station | Tsunami
Description
The power station and its associated equipment
are vulnerable to tsunami forces and
inundation.
Damage and failure modes
The inundation and saltwater intrusion can
damage sensitive electrical equipment on the
site, like the switchboards in the powerhouse
and components of the diesel generators.
Because the power station building sits 1.5m
above the ground on its concrete foundation,
significant damage to interior equipment would
not be expected until the tsunami exceeds this
height.
At flow depths less than two meters, damage to
exterior equipment would be expected. On-site
transformers and other electrical equipment
would be susceptible to saltwater intrusion and
require repairs if not full replacement, reflected in the damage states of the provided fragility
functions.
Vulnerability curves
Presented in Horspool & Fraser
(Horspool & Fraser, 2016),
substation fragilities are provided
for indoor and outdoor
components. The damage
description of substation matches
the power station description at
similar depths; for example, under
flow depths of 0.5-2m the
probability of damage is “Medium-
High” for both substations and
power stations with matching
description “saltwater
contamination to electrical
components & structures, debris
and sediment cover, …washout of some
outdoor components.” The three fragilities for
indoor components will be appropriately shifted to represent the raised foundation present at Popua
Power Station.
Figure 42: Power station fragility for tsunami (Horspool & Fraser, 2016)
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) / reference: Horspool & Fraser (Horspool & Fraser, 2016)
Basis:
Post-tsunami damage reports from Samoa, Andaman Islands, Chile, Japan; expert judgment
Intensity measure: Inundation depth
Description of damage
states:
DS 1: Minor water damage; repairs required
DS 2: Moderate damage; replacement of some components DS 3: Complete damage; components washed away
Table 22: Vulnerability summary for power station.
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Solar Farm | Tsunami
Description
Each solar farm contains photovoltaic (PV)
panels and auxiliary equipment, such as
inverters and transformers. They have been
combined in this section.
Damage and failure modes
The solar panels sit on elevated foundations,
approximately half of a meter above the
ground. Their supporting structure raises the
panels an additional 1-2m, depending on the
solar farm on Tongatapu. The inundation-
vulnerable equipment includes the junction
boxes, inverters, and other auxiliary equipment,
which would likely need to be replaced if
flooded with saltwater. For further information about the vulnerability of solar farm equipment in
flood, please see the Section 4.3.
Tsunamis also cause significant damage due to hydrodynamic forces and debris strikes, to both of
which the solar panels and auxiliary equipment are vulnerable. The solar panels are bolted to their
foundations, but at significant flow depths these anchorages could fail, similar to how buildings and
other infrastructure have become stripped from their foundations in previous events (Horspool &
Fraser, 2016). Scour also poses a risk to the concrete foundation (Yeh, et al., 2004) (Bricker, Franics,
& Nakayama, 2012). Generators and other equipment similar in size to inverters seen on Tongatapu
would also be susceptible to debris impact and washout at tsunami flow depths of several meters.
Vulnerability curves
The exterior component curves
presented in Horspool & Fraser
(2016) for the equipment at power
stations was reviewed and applied
to the equipment observed at the
solar farms on Tongatapu. Under
less than half a meter of flooding,
any transformers or other ground-
mounted equipment may be
damaged. At ranges up to two
meters, the junction boxes of the
PV panels may be damaged and
require replacement. At depths of
several meters or greater, the
damage to the solar farms would be
considered extensive to complete. These
evaluations align with the fragility functions
developed by Horspool & Fraser (2016), so the curves were taken to apply to the solar farms on
Tongatapu.
Figure 43: Solar farm fragility function (Horspool & Fraser, 2016)
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) / reference: Horspool and Fraser (Horspool & Fraser, 2016)
Basis:
Post-tsunami damage reports from Samoa, Andaman Islands, Chile, Japan; expert judgment
Intensity measure: Inundation depth
Description of damage
states:
DS 1: Minor water damage; repairs required
DS 2: Moderate damage; replacement of some components DS 3: Complete damage; components washed away
Table 23: Vulnerability summary for solar farm.
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4 5 6
P(D
S>
ds
| d
epth
)
Depth (m)
Solar Farm Fragility
GNS -ExteriorDS1
GNS -ExteriorDS2
GNS -ExteriorDS3
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Wind Farm – Auxiliary Equipment | Tsunami
Description
Outdoor auxiliary equipment on wind farms consist of similar components to those found in solar farms and have been evaluated for vulnerability to tsunami.
Damage and failure modes
Auxiliary equipment (such as transformers, batteries, control panels, inverters, and feeder and
interconnection panels) on wind farms, like solar farms, are vulnerable to tsunami hydrodynamic
forces and inundation. The exterior transformers will be damaged due to inundation and saltwater
intrusion, requiring repairs or total replacement.
Most auxiliary equipment is housed within a building structure. The structure itself, and therefore its
fragility is similar to the steel frame building typology (Section 5.1). Wind farm building structures
that house equipment can be mapped directly to this building typology. Upon collapse of the building
structure, internal equipment is assumed to be damaged to the point of replacement as well. This may
be due to debris impact or buckling/overturning from tsunami forces. The damage due to inundation
will be represented by the Specialized building vulnerability developed for flood inundation.
Vulnerability curves
The indoor auxiliary equipment is covered by curves attributed to the tsunami fragility/vulnerability
of a steel frame building typology (see Section 5.1) and the flood vulnerability of a Specialized
building.
The outdoor auxiliary equipment performs similarly to the transformer fragility presented for flood
and is therefore used, since transformers comprise the main vulnerable outdoor assets observed at the
wind farm (aside from the turbines themselves).
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5.4 Water infrastructure
This section details the vulnerability and fragility curves identified for water infrastructure subject to tsunami.
The water distribution system, comprised of buried pipelines, was considered rugged to tsunami hazards. Similar to performance under flood hazard, damage from inundation is not expected even during a tsunami event (FEMA, 2009). Horspool & Fraser discuss buried pipes in their review of lifelines performance in tsunami, stating that ”in general buried pipes for potable water supply performed well” and, if damage was recorded in a post-tsunami survey, it was typically from the prior earthquake event (i.e. liquefaction) (Horspool & Fraser, 2016). Damage modes from pre-tsunami earthquakes were not considered for the tsunami vulnerability assessment.
Elevated tanks are likely vulnerable to damage caused by tsunami. While there is information about ground mounted tanks (see At Grade Tanks | Tsunami), nothing was found in the literature that represents the elevated water tanks seen on Tongatapu. Impact from debris is likely to be one of the main drivers of damage which is difficult to capture from an analytical approach.
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Pumps and Wells | Tsunami
Description
The submersible pumps inside wells across
Tongatapu are typically covered with a small
pumphouse and many have a transformer
located on site.
Damage and failure modes
Three mechanisms contribute to the damage of
wells, the pumphouses, and on-site
transformers. The submersible pumps
themselves were considered robust to
inundation, though saltwater may lead to
accelerated degradation.
Saltwater intrusion poses a significant threat to
exposed wells. Noted by Horspool & Fraser, “if a well was located within the inundation zone, the
well became contaminated.” (Horspool & Fraser, 2016). According to TWB and MLNR, this would
incur costs to flush the storage tanks or chlorination.
The transformer, typically located within a few meters of the pumphouse, is vulnerable to saltwater
intrusion and inundation damage to critical interior components. If exposed to significant depths, it
may need to be replaced.
At several meters of flow depth, the pumphouse itself may be susceptible to collapse or washout. The
small timber structures sit on a concrete foundation, but, from a review of site visit photos, do not
appear to have robust lateral systems that would aid in resisting tsunami forces.
Vulnerability curves
This fragility curves took into
account several assets on site: the
well, the transformer, and the
pumphouse. Each fragility was
developed based on the data
collected from workshops with
TWB and MLNR, examination of
site visit photographs, review of
relevant literature, and expert
judgment.
Figure 44: Pump system fragility curves for tsunami.
Type of curve: Fragility functions
Origin:
Arup – developed from analysis of assets, literature review, workshops, and expert judgment
Source(s) / reference: Arup SMEs, Horspool et al (Horspool & Fraser, 2016)
Basis: Tonga
Intensity measure: Inundation depth
Description of damage
states:
Damage presented as percent of total replacement cost
Table 24: Vulnerability summary for typical pump system.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 1 2 3 4 5 6
Pro
ba
bil
ity
of
da
ma
ge
Depth (m)
Pump Vulnerability
Transformer
SaltwaterIntrusion
BuildingCollapse
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At Grade Tanks | Tsunami
Description
Concrete water tanks that serve Nuku’alofa are
located at-grade, as opposed to elevated tanks
that support towns across Tongatapu.
Damage and failure modes
Tsunamis can damage storage tanks through
several mechanisms, including floatation.
During previous tsunami events, tanks have
been observed to float in a few meters of flow
depth; this includes tanks made of concrete,
plastic, or steel. Tanks with low water levels
are more susceptible to floatation (Hatayama,
2015) (Horspool & Fraser, 2016). An instance
was recorded post-tsunami in Samoa where a
half-filled tank had moved 170m (EEFIT,
2009). This behaviour is not unique to tsunami inundation, as buoyant forces would act on tanks
during flooding, but the high flow depth hazard for tsunami would more likely lead this damage state
than in flood on Tongatapu.
Scour may undermine the concrete foundation that supports the tanks on Tongatapu (Yeh, et al.,
2004). Additionally, debris strikes may damage the concrete walls of the tanks or potentially dislodge
attached pipes. Any of these damage states may lead to a loss of water or an interruption of service.
Vulnerability curves
A generic tank fragility was developed by Hatayama after analysis of fuel storage tank performance in Japan after the 2011 Tohoku-Oki Tsunami, which results in 100% damage at 9m of flow depth. The smaller tanks (<500 cubic meter capacity) were most damaged, with material specifications not provided (Hatayama, 2015). The capacity of the at-grade storage tanks on Tongatapu is approximately 22 cubic meters, which puts it in the category of tanks most susceptible to damage according to Hatayama. The damage curve was adopted without modification
Figure 45: Tank fragility curve for tsunami.
Type of curve: Fragility function
Origin: Adopted without modification
Source(s) / reference: Hatayama (Hatayama, 2015)
Basis: Japan
Intensity measure: Inundation depth
Description of damage
states: Complete damage
Table 25: Vulnerability summary for tank.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1 2 3 4 5 6
P(D
S>
=d
s)
Depth (m)
Tank Vulnerability
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6 Seismic Vulnerability
This section outlines the vulnerability study findings for seismic for buildings, roads, and power and water infrastructure.
6.1 Buildings
Building upon previous work in the region, the vulnerability assessment for earthquakes utilizes the damage functions developed by AIR for the 2011 PCRAFI study [1]. These damage functions were provided specifically for Federated States of Micronesia, Marshall Islands, Samoa, Tonga, and Vanuatu.
There are seismic damage functions for 14 distinct structural typologies, fewer than those provided for wind, but there were individual functions for single-story and multi-story buildings.
The 14 structural typologies within the exposure data base were mapped to an equivalent PCRAFI typology that had an associated PCRAFI seismic fragility curve as shown in Table 26 below.
Table 26: Mapping of structural typology to equivalent seismic damage function
Structural Typology Equivalent PCRAFI Typology
Timber frame Timber SS
Concrete moment frame RC_Masonry SS
Reinforced masonry RC_Masonry SS
Steel frame Other SS
Concrete frame with masonry infill RC_Masonry SS
Multi-story - Reinforced masonry with timber frame on top RC_Masonry SS
Multi-story - Timber frame on both levels Timber SS
Low level masonry wall with timber frame Timber SS
Open walled structure - wooden pole Open SS
Multi-story - Concrete frame with timber frame on top RC_Masonry SS
Unreinforced masonry URM
Open walled structure - non wooden pole Open SS
Traditional Traditional SS
Multi-story - Timber frame on concrete piers RC_Masonry SS
Assumptions and limitations
Several assumptions needed to be made to account for buildings with no typology:
• As there was no typology for steel buildings, this was assumed to be classified as the ‘Other’ typology as according to the study, this is for mixed or more advanced typology types.
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• There is a large difference in the seismic performance of unreinforced and reinforced masonry structures. Given there was no unreinforced masonry fragility curve, a modification to the reinforced masonry curve was applied. This was done by mapping a log-normal distribution to the existing curve and reducing the Median and Beta parameters to 30% and 50% of the RM equivalent values respectively. This reduction was based on comparing URM and RM performance for FEMA, GLOSI and Hazus defined typologies and damage functions.
• For multi-story buildings with mixed typologies, it was assumed that the first-floor typology would be critical in terms of seismic performance, and as such this was used to map to an equivalent fragility curve.
Review of PCRAFI Damage functions
The seismic damage functions obtained from the PCRAFI report were compared to fragility curves from other sources. The curves represent a total percent of damage a building will undergo for a particular level of hazard intensity. This proportion of damage was developed by assessing the individual cost of replacement and repair for individual components of the building and comparing the ratio of this cost to the overall cost of replacement for the entire building (AIR Worldwide, 2011).
The PCRAFI damage functions were compared to the fragility and vulnerability curves provided in the GLOSI (World Bank, 2021) and Hazus libraries (FEMA, 2009). Hazus provides the fragility curves for different damage states for similar building typologies (i.e. partial failure, total collapse etc.). The fragility curves for total building collapse showed reasonable agreement with the PCRAFI curves, with similar peak ground acceleration (PGA) values for a 100% damage ratio.
GLOSI provides vulnerability curves for reinforced and unreinforced masonry buildings and concrete moment frame buildings. Typically, the GLOSI curves showed a lower level of damage for an equivalent peak ground acceleration (PGA), as shown in the figures below for a reinforced masonry building, where the retrofitted curves showed better agreement. This could be attributed to the fact that the GLOSI curves are for buildings in different countries, with slightly differing typologies.
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Figure 46: PCRAFI damage function for reinforced masonry where the blue curves represent the baseline performance and the red lines represent the retrofitted performance.
Figure 47: GLOSI vulnerability curve for reinforced masonry building
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Buildings | Seismic
Description
This section covers all buildings on Tongatapu.
Damage and failure modes
Buildings are vulnerable to damages both
structural and non-structural in nature from
seismic displacements and accelerations that
exceed values used for design.
Large displacements from earthquake shaking
create large amounts of stress and strain within
building materials that can result in failure. The
amount of displacement that results in failure
will vary depending on component type and
material, as different system types vary in their
ability to dissipate shaking. For example,
unreinforced masonry has little ability to carry
displacement forces and will typically break
apart at lower levels of shaking. For tall buildings, large displacements can result in large overturning
forces that place additional large forces on superstructure elements as well as foundation elements.
Similarly, accelerations that are larger than values used for component design can create large forces
that result in damage as well. Typically, non-structural components (e.g. piping or equipment) are
sensitive to these accelerations. Acceleration induced damage can be tied to either the peak ground
acceleration (PGA) of the earthquake or the peak floor acceleration (PFA), which is a function of the
PGA.
Vulnerability curves
The vulnerability curves shown in
Figure 48 have been directly
adopted for all typologies (except
masonry) from the PCRAFI study
(Bazzurro, 2015) as they were
specifically developed for Tonga
and neighbouring countries. A
review of these functions is
outlined in Section 6.1
There was no unreinforced
masonry fragility curve so a
modification to the reinforced
masonry curve was applied because
there is a large difference in the
seismic performance of unreinforced and reinforced masonry structures. This was done by reducing
the Median and Beta parameters of the existing function to 30% and 50% of the RM equivalent values
respectively based on a comparison of URM and RM performance for FEMA (FEMA, 2020) and the
World Bank’s Global Program for Safer Schools (World Bank, 2021) defined typologies and damage.
Figure 48: Vulnerability curves for different building typologies subject to varying earthquake intensity (Bazzurro, 2015)
Type of curve: Vulnerability
Origin: Adopted from literature with minor modification
Source(s)/reference: PCRAFI study, Bazzurro (Bazzurro, 2015)
Basis: Federated States of Micronesia, Marshall Islands, Samoa, Tonga, and Vanuatu
Intensity Measure: Wind speed
Description of damage
states:
Damage presented as percent of total replacement cost for a given shaking intensity
Table 27: Vulnerability summary for Buildings for seismic shaking
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6.2 Roads
This section outlines the vulnerability curves to be used for roads in Tongatapu subject to seismic shaking. Bridges are often the most vulnerable component of the transportation network, however there are no bridges on Tongatapu so the focus of this section is on paved and unpaved roads.
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Roads | Seismic
Description
Paved and unpaved roads are vulnerable to
seismic hazards that cause permanent ground
deformation.
Damage and failure modes
On Tongatapu, permanent ground deformation
(PGD) is expected to result from liquefaction of
soft soils along the coast. For further discussion
of liquefaction hazard, see the Seismic Hazard
Report (MHDRA-REP-002 [Seismic]).
Minor damage results from settlement of the
road under several centimetres of PGD. As
permanent deformation increases, the road is
susceptible to cracking and further settlement that would disrupt the flow of traffic and require
significant repairs or replacement of the road. In previous earthquake events, significant damage has
resulted from poorly compacted base material (Miyamoto International Inc., 2019) (FEMA, 2020).
Vulnerability curves
The fragility curves for urban roads
from Hazus were adapted without
modification to model road
performance on Tongatapu. In this
case “urban roads” refer to local,
typically two-lane paved roads in
the US (FEMA, 2020). This
description, as opposed to the
description for highways, applied
more appropriately to roads on
Tongatapu based on photographs
and surveys from Tonga State of
the Roads Report and workshops with residents
(TSCP Roads Advisor, 2015). In Tonga, methods
from Austroads are typically adopted, which was developed from US road design standards. Because
of the similarities between codes, these vulnerability curves were adopted for Tonga without
modification, however, this does not explicitly incorporate reports of road quality. The information
available about road quality was not sufficient to meaningfully adjust the curves above.
Figure 49: Road vulnerability for seismic from Hazus. (FEMA, 2020)
Type of curve: Fragility functions
Origin: Adopted from literature without modification
Source(s) / reference: US FEMA (FEMA, 2020)
Basis: US
Intensity measure: Permanent ground deformation
Description of damage
states:
DS 1: Slight settlement DS 2: Moderate settlement or offset of ground DS 3: Extensive/Complete; major settlement of the ground
Table 28: Vulnerability summary for roads.
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6.3 Power infrastructure
This section outlines the vulnerability curves to be used for power infrastructure in Tongatapu subject to seismic shaking.
No appropriate fragility curves were found for wind turbines. The fragility curves for wind turbine performance in seismic events in existing literature are generally available for off-shore and on-shore turbines. However, these turbines are significantly different in construction to those in Tongatapu. Turbines in literature (Myers, Gupta, Ramirez, & Chioccarelli, 2012) (Miyamoto International Inc., 2019) have an anchored or pad foundation system whereas those on Tongatapu have a hinge at the base with stayed cables to fix the turbine to the ground. The tower component of the turbine is also typically constructed out of a cylindrical structure, rather than a frame. Additionally, the size and mass distribution are significantly different and change the likely performance in seismic events.
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Power Distribution System | Seismic
Description
The power distribution system on Tongatapu is
comprised of wooden utility poles for high
voltage (HV) and low voltage (LV) overhead
power lines, pole-mounted equipment like
transformers and capacitors, and a few sections
of underground power lines.
Damage and failure modes
Ground accelerations create forces on the utility
poles, which are anchored into the ground. Pole
mounted equipment may fall to the ground,
particularly if bolted to the cross arms
(Eidinger J. , 2017).
Overhead circuits can be damaged by inertial shaking or in many cases due to “snap loads” when
there is insufficient slack in the lines. Conductor burns and entanglement can also occur due to wire
slapping. Where buildings collapse, this can also lead to damage to the distribution network, known as
“pull down” damage (Eidinger J. , 2017).
Previous events have severely disrupted power distribution networks in other countries (Giovanni, et
al., 2011). Though only PGA is considered here, liquefaction impacted some utility poles in New
Zealand during the February 22nd, 2011 Christchurch earthquake (Giovanni, et al., 2011).
Vulnerability curves
The fragility curves developed by
the engineers who authored Hazus
have been adopted without
modification. These functions
apply to the entire distribution
system, as opposed to the
individual utility pole and power
line components; they encompass
poles, cables, in-line components
(e.g. transformers), and utility-
owned equipment at customer sites.
(FEMA, 2020). They will be
applied to Tongatapu on a regional
basis, taking into account any
differential hazard experienced
on the island.
Figure 50: Utility pole and power line vulnerability for seismic.
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) / reference: US FEMA (FEMA, 2020)
Basis: US
Intensity measure: Permanent ground deformation
Description of damage
states:
DS 1: Slight; 4% of circuits have failed DS 2: Moderate; 12% of circuits have failed DS 3: Extensive; 50% of circuits have failed DS 4: Complete; 80% of circuits have failed
Table 29: Vulnerability summary for utility poles and power lines.
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Transformers | Seismic
Description
Ground-mounted transformers across
Tongatapu are vulnerable to seismic shaking.
Damage and failure modes
During an earthquake, intense shaking may
crack critical subcomponents or dislodge a
ground-mounted transformer from its
foundation. From photographs taken on site
visits to Tongatapu, the transformers appear
anchored to their concrete foundations. Strong
accelerations could lead to shearing of the
anchorage bolts or other failure modes that
dislodge the transformer. Leakages of oil or
from the radiator would require significant
repairs or replacement of the entire transformer
(FEMA, 2020) (Miyamoto International Inc., 2019).
Vulnerability curves
The fragility curve developed by
the engineers who authored Hazus
has been adopted without
modification. This function applies
to the transformers present at small
substations, which corresponds to
34.5kV to 150kV (FEMA, 2020).
The assumption that the equipment
is anchored underlies the fragility.
The photographs from site visits presented the
typical transformers as bolted into concrete
foundations.
Figure 51: Transformer vulnerability for seismic.
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) / reference: US FEMA (FEMA, 2020)
Basis: US
Intensity measure: Peak ground acceleration
Description of damage
states:
Damage corresponds to complete replacement
Table 30: Vulnerability summary for transformer.
0.00
0.20
0.40
0.60
0.80
1.00
0 1 2 3
P(D
S>
ds
| P
GA
)
PGA (g)
Transformer Fragility
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Power Station | Seismic
Description
The power station consists of interior
equipment, like control panels and diesel
generators, and exterior equipment, like the fuel
tank and transformers.
Damage and failure modes
Earthquake shaking can cause severe damage to
the power station and its infrastructure. The
anchorage of any equipment can be
compromised during an earthquake, causing
sliding or toppling of components, including
the generators, transformers, and switchgears.
Subcomponents of the equipment may become
damaged in the shaking, leaving the electrical
systems inoperable. The power station depends
on their functionality, so significant downtime would be expected as a result of damage to the assets
present at the site (Almufti & Wilford, 2013).
Hazus accounts for a variety of failure modes in its description of the four damage states presented in
the fragility curves in Figure 52. Slight damage includes light damage to the generators and the
building itself. Additional damage to instrument panels and racks from chattering and to pumps from
severe shaking correspond to Moderate and Extensive damage states. Severe damage to the building
also triggers higher damage states, with Complete damage corresponding to the building being
structurally compromised (FEMA, 2020).
Vulnerability curves
Power generation facilities in
Hazus account for equipment
such as diesel generators and
electrical panels as well as the
building in which they are
housed. The fragility functions
presented here correspond to a
small generation facility
(<100MW) which applies to
Tongatapu’s 14MW facility. The
components were considered
anchored for seismic design, per
the site visit data collection and
workshops with TPL.
Figure 52: Power station vulnerability for seismic.
Type of curve: Fragility functions
Origin: Adopted without modification
Source(s) /
reference: US FEMA (FEMA, 2020)
Basis: US
Intensity
measure: Peak ground acceleration
Description of
damage states:
DS 1: Slight; light damage to generators, turbine tripping, building in Slight damage state DS 2: Moderate; chattering of instrument panels and racks, building in Moderate damage state DS 3: Extensive; considerable damage to pumps, building in Extensive damage state DS 4: Complete; extensive damage to vessels and valves, building in Complete damage state
Table 31: Vulnerability summary for power station.
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Solar Farm – PV Panels | Seismic
Description
Solar farm photovoltaic (PV) panel structures
were evaluated for fragility to earthquakes.
Damage and failure modes
Evidence exists of a large majority of PV
panels being operational after seismic events in
multiple locations (Fthenakis, 2013). Therefore,
the PV panels themselves are considered
rugged to seismic events. However, the support
structure is vulnerable to damage through a loss
of stability in the frame or through failure in the
connections. Fragility curves therefore focus on
the performance of the steel support frame, and
depends on the level of seismic design.
Vulnerability curves
Fragility curves for multiple
damage states of a Solar Panel
frames are adapted from literature
that uses global data (Miyamoto
International Inc., 2019). If the
frames are designed to a high level
of seismic code compliance, then a
slight improvement is expected in
the performance. However, the
fragility curves highlighted here
give a conservative estimate as the
specific design basis was not
available.
Figure 53: Fragility curves of solar panel support frames (Miyamoto International Inc., 2019)
Type of curve: Fragility curves
Origin: Adopted without modification
Source(s)/reference: Miyamoto International Inc (Miyamoto International Inc., 2019)
Basis: USA
Intensity measure: Peak ground acceleration
Description of damage
states:
DS1: minor, operational
DS2: moderate, operational without repair but some damage
DS3: extensive, operational with repair and considerable damage
DS4: Complete, extensive damage and not repairable
Table 32: Vulnerability summary for solar panels
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Solar Farm – Auxiliary Equipment | Seismic
Description
Typically solar farms include equipment such
as diesel generators, turbines, racks and panels,
boilers, pressure vessels, and the building in
which these are all housed. They have been
considered in the vulnerability study for
seismic events.
Damage and failure modes
The building used to house solar farm
equipment is assessed using
fragility/vulnerability curves in Section 6.1. In
seismic events, however, the buildings and any
housed equipment are impacted independently
of each other and are therefore are assessed separately.
Damage to the equipment can be broadly separated into four categories of varying severity (FEMA,
2020). Slight damage is defined by light damage to equipment, or trips in equipment. Moderate
damage is defined by chattering of panels and racks, and considerable damage to vessels. Extensive
damage is defined by considerable damage to motor driven pumps or large vertical pumps. Lastly,
complete damage is described by extensive damage to vessels beyond repair and extensive damage to
large motor operated valves. The probability of exceeding a damage state of non-building components
of the solar farms (housed equipment and external equipment) are captured in fragility curves shown
in Figure 54.
Vulnerability curves
The fragility curves are adopted
from Hazus for a range of damage
states varying in severity,
dependent on peak ground
acceleration as a measure of
ground-motion intensity, assuming
plant sizes less than 100MW. These
curves are based on probabilistic
combinations of various
subcomponents and their
relationships to each other within a
solar farm (including generators,
racks and panels, boiler and
pressure vessels, etc.) (FEMA,
2020).
Figure 54: Fragility curves for anchored components in small scale power plants (FEMA, 2020)
Type of curve: Fragility curves
Origin: Adopted without modification
Source(s)/reference: FEMA (FEMA, 2020)
Basis: USA
Intensity measure: Peak ground acceleration
Description of damage
states:
Slight: movement of vulnerable equipment, damaged piping/ducts Moderate: higher level of movement, pipe leaks Extensive: stretched bolts at anchored connections, unanchored or spring isolated equipment slides/falls Complete: pipes/ducts fall, equipment is damaged and not operable
Table 33: Vulnerability summary for solar farm auxiliary equipment
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Wind Farm – Auxiliary Equipment | Seismic
Description
Auxiliary equipment in wind farms on Tongatapu were evaluated for seismic events. The equipment
on wind farms are similar in nature to those found in solar farms.
Damage and failure modes
Equipment and machinery found on a wind farm is largely similar to those found on solar farms, and
therefore, the fragility data is similar to equipment on solar farms.
On Tongatapu, the building in which wind farm equipment is housed is a steel frame structure, and is
therefore mapped to a steel frame typology from the building seismic performance
fragility/vulnerability curves shown in Section 6.1.
Vulnerability curves
Refer ‘Solar Farm – Auxiliary Equipment | Seismic’ for further detail of equipment
vulnerability.
Refer ‘6.1 Buildings’ for further detail of building structure vulnerability in seismic
conditions.
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6.4 Water infrastructure
This section outlines the vulnerability curves to be used for water infrastructure in Tongatapu subject to seismic shaking.
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Elevated Water Tanks | Seismic
Description
Elevated plastic water tanks typically rest on 9-
meter tall stands.
Damage and failure modes
Elevated tanks can be damaged in a number of
ways during an earthquake, the most severe of
which is collapse of the tank and its stand.
Inertial overloads lead to tank failure in
collapse, exacerbated when tanks are close to
full (Eidinger J. M., 2012). This damage state is
represented by the fragility curve presented in
Figure 55.
Other damage states can also impact elevated
tanks. Minor damage can be caused from water
sloshing against the tank roof, and cracks or
wrinkles (found in concrete and steel tanks, respectively) that can lead to some loss of the contents.
Major damage includes the buckling of the thin tank wall, as seen in elephant foot buckling typically
seen in steel tanks. Failure of the tank leads to a complete loss of contents, similar to the complete loss
that would be expected from the failure of the tank stand (FEMA, 2020) (American Lifelines
Alliance, 2001).
Vulnerability curves
The fragility developed by the
American Lifelines Alliance (ALA)
(American Lifelines Alliance,
2001) for elevated tanks was
adopted without modification for
this asset. To adopt this curve, the
tank was assumed to be full, which
is a conservative estimate. The
curve corresponds to a damage
state of Collapse, which was
assumed to apply to the elevated tanks
on Tongatapu as this damage state
would not be dependent on the tank material, which differs between ALA (steel tank) and Tonga
(plastic tank). The Non-Seismic curve was adopted because there was insufficient information to
assume that all tanks had been designed to or beyond code design and to analyse the performance of
elevated plastic tanks in shaking. The PGA to which the tank stands were designed was reported to be
0.3g from correspondence with Tonga Water Board, which corresponds to low probability of damage,
supporting the decision to adopt the fragility curve directly from ALA.
Figure 55: Vulnerability for elevated water tank in seismic. (American Lifelines Alliance, 2001)
Type of curve: Fragility curve
Origin: Adopted without modification
Source(s)/reference: ALA (American Lifelines Alliance, 2001)
Basis: USA
Intensity measure: Peak ground acceleration
Description of damage
states:
Collapse of the tank and structure
Table 34: Vulnerability summary for elevated water tank.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.5 1.0 1.5 2.0
P(C
oll
ap
se)
PGA (g)
Elevated Water Tank
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At Grade Water Tanks | Seismic
Description
Several at-grade concrete water tanks supply
water to Nuku’alofa.
Damage and failure modes
Several failure modes could result from the
shaking of at-grade concrete tanks. Cracking
can occur in the walls of concrete tanks,
considered moderate damage according to
Hazus. In severe cases, the walls can fail in
shear or collapse, losing all of its contents
(FEMA, 2020).
Anchored concrete tanks may also slide in
shaking greater than 1g, as well as experience
the overstressing of steel rebar hoops in the
walls. Uplifting and the crushing of concrete
contribute the most significant damage of these
mechanisms (American Lifelines Alliance,
2001). Failure from residual, permanent ground
deformation also poses a significant risk to concrete tanks, but the tanks serving Nuku’alofa reside
outside of the liquefaction zones, so this damage state was not taken into account.
Vulnerability curves
The American Lifelines Alliance
examined the performance of
concrete water tanks and developed
the fragility curve presented in
Figure 56 which has been adopted
without modification. This curve
represents the crushing of concrete,
which occurs at a median PGA of
1.3g, for anchored concrete tanks.
From review of on-site data collection,
the concrete tanks appear solidly
anchored to their foundations.
Figure 56: Vulnerability for at-grade water tank for seismic (American Lifelines Alliance, 2001).
Type of curve: Fragility curve
Origin: Adopted without modification
Source(s)/reference: ALA (American Lifelines Alliance, 2001)
Basis: USA
Intensity measure: Peak ground acceleration
Description of damage
states:
Crushing of concrete walls
Table 35: Vulnerability summary for at-grade water tank.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 1.0 2.0
P(d
amag
e)PGA (g)
Tank Fragilities - At Grade
At-GradeConcrete,Anchored(ALA) - CrushConcrete
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Buried Pipelines | Seismic
Description
Buried pipelines comprise the water
distribution network across Tongatapu.
Damage and failure modes
The failure modes that impact buried pipelines
are driven by permanent ground deformation
(PGD), such as landslides, liquefaction,
settlement, and fault crossings. On Tongatapu,
liquefaction is expected to contribute most
significantly to PGD, which displaces the
pipelines from their original position, causing
failure of the pipe at weak points such as joints.
These breakages leak water until their locations
can be identified and crews sent out to repair
the pipe. (American Lifelines Alliance, 2001)
(Eidinger J. M., 2012) (FEMA, 2020)
The extent of damage depends on the material of the pipe, whether it is considered ductile (e.g. steel
or ductile iron) or brittle (e.g. cast iron or asbestos cement). (American Lifelines Alliance, 2001) The
pipes present on Tonga range from ductile to brittle materials, with most being either asbestos cement
or PVC pipes in the Nuku’alofa area and PVC pipe in other towns across Tongatapu.
Vulnerability curves
The vulnerability for buried
pipelines was adopted from an
equation developed by the
American Lifelines Alliance (ALA)
to model the performance of water
pipelines in earthquakes, based on
empirical observations driven
primarily by liquefaction. As
liquefaction is expected to
contribute most significantly
to PGD on Tongatapu in a
seismic event, the underlying
function was taken to apply to the Tongan context and no modifications were applied for the function
used. Significant uncertainty exists around the curve presented here, as described by the authors of the
ALA report, due to the variability in pipe material, performance, and hazard as part of the empirical
data used to derive the curve (American Lifelines Alliance, 2001). This uncertainty will be
incorporated during the risk assessment.
Figure 57: Vulnerability of buried pipelines for seismic.
Type of curve: Fragility curve
Origin: Adopted without modification
Source(s)/reference: ALA (American Lifelines Alliance, 2001)
Basis: USA
Intensity measure: Permanent ground deformation
Description of damage
states:
Pipeline repairs required per kilometre
Table 36: Vulnerability summary for buried pipelines.
0
2
4
6
8
10
12
14
0 20 40 60 80 100
Rep
air
Ra
te (
rep
air
s/k
m)
PGD (cm)
Buried Pipelines
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Pumps and Well | Seismic
Description
Each well contains a pump to move water into
the distribution system for servicing TWB and
MLNR customers.
Damage and failure modes
Damage to the pump and well system disrupts
the water supply. The disruption can occur due
to the malfunction of the pump and motor due
to loss of power or backup power, which
corresponds to Slight damage in the fragility
curves presented in Figure 58. It is estimated
that this corresponds to up to three days of
disruption, and when Moderate damage states
are reached it could be about a week of disruption. This damage encompasses loss of power and
considerable damage to electrical and mechanical components or to the pumphouse building (FEMA,
2020).
In more severe instances, the building may be extensively damaged during the shaking and fail or the
vertical shaft may be non-functional because of significant distortion. If the building poses a risk to
life safety, then repairs to the pump system may be delayed until the building has been deemed safe to
enter, corresponding to the Complete damage state presented by Hazus (FEMA, 2020).
Vulnerability curves
The well fragility was adopted without modification from Hazus because the well system modelled contains the assets observed at the bore holes in Tongatapu. These assets include the shaft to access the aquifer, the pump, and the building that houses the set-up. Hazus assumes that the equipment is anchored, which was an assumption adopted for the Tongan context so that the fragility in Figure 58 applies. TWB confirmed that their equipment was designed for seismic
shaking.
Figure 58: Vulnerability of pumps and wells for seismic (FEMA, 2020)
Type of curve: Fragility curves
Origin: Adopted without modification
Source(s)/reference: Hazus (FEMA, 2020)
Basis: USA
Intensity measure: Peak ground acceleration
Description of damage
states:
DS 1: Slight DS 2: Moderate DS 3: Extensive DS 4: Complete
Table 37: Vulnerability summary for Pumps and Wells
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7 Wind Vulnerability
This section outlines the vulnerability study findings for wind for buildings, roads, and power and water infrastructure. In general, wind damage as described in this section is limited to damage directly caused by wind and does not account for subsequent damage due to rainfall, unless stated otherwise.
7.1 Buildings
Building upon previous work in the region, the vulnerability assessment for tropical cyclones utilizes the damage functions developed by AIR for the 2011 PCRAFI study (AIR Worldwide, 2011). These damage functions were provided specifically for Federated States of Micronesia, Marshall Islands, Samoa, Tonga, and Vanuatu.
The damage functions available in the published reports from the PCRAFI study are based only on structural typology, and do not differentiate between single and multistory buildings, apart from the damage function for timber buildings (Bazzurro, 2015). The 14 structural typologies within the Tongatapu exposure database were mapped to an equivalent PCRAFI typology that had an associated damage function as shown in Table 38 below.
Table 38: Mapping of Structural Typology to PCRAFI tropical cyclone damage functions
Structural Typology Equivalent PCRAFI Typology
Timber frame Timber SS/MS
Concrete moment frame Concrete
Reinforced masonry Masonry
Steel frame Steel
Concrete frame with masonry infill Masonry
Multi-story - Reinforced masonry with timber frame on top Timber MS
Multi-story - Timber frame on both levels Timber MS
Low level masonry wall with timber frame Timber SS/MS
Open walled structure - wooden pole Open
Multi-story - Concrete frame with timber frame on top Timber MS
Unreinforced masonry Masonry
Open walled structure - non wooden pole Open
Traditional Traditional
Multi-story - Timber frame on concrete piers Timber SS/MS
Assumptions and limitations
Due to the limited number of typologies with available curves, and knowledge of the functions themselves, a number of assumptions were made:
• For multi-story mixed typologies, the top story was taken to be the critical structure for tropical cyclone hazards, so this was used for the mapping to a particular damage
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function. E.g. “Multi-story concrete frame with timber frame on top” was mapped to Timber Multi-story.
• Some curves for retrofitted buildings are given. No retrofitting was assumed for any of the buildings
• No curve was given for unreinforced masonry. Unreinforced masonry buildings were assumed to use the same curve as reinforced masonry buildings for wind events as the failure mechanisms are likely to be similar (roof connection failures etc.)
Review of PCRAFI Cyclone Wind Damage functions
The cyclone wind damage functions obtained from the PCRAFI report were compared to damage functions and fragility curves from other sources for wind. The PCRAFI functions are damage functions showing the total percent of damage a building will undergo for a particular level of hazard intensity. This proportion of damage was developed by assessing the individual cost of replacement and repair for individual components of the building and comparing the ratio of this cost to the overall cost of replacement for the entire building. Other references use fragility functions (i.e. relating hazard intensity to a description of physical damage) making a direct comparison more difficult.
The tropical cyclone damage functions were compared to those developed by Geosciences Australia (GA). The key difference between the two sets of curves, is that the GA curves show a sharp increase from a damage index of 0.5 to 0.9, which would be as expected as this is representative of key structural elements failing with increasing wind speed. This is not apparent in the PCRAFI curves. A comparison of the two are shown below in Figure 59 and Figure 60.
Figure 59: Example of vulnerability curve produced by VAWS – Timber building
Figure 60: PCRAFI vulnerability curve for timber buildings
The GA curves show reasonable agreement with the PCRAFI curves when comparing the wind speed at which total failure occurs, where 100% damage occurs at around 70 m/s from the GA curve produced with their simulation software, VAWS (Wehner, Holmes, & Ginger, 2017); where the PCRAFI curve shows failure typically around 65-80m/s (140-180 mph) depending on the typology. Currently, VAWS also allows the consideration of debris and water ingress, however this has a significant effect on the total damage replacement cost of the building, and it is unconfirmed whether this is accounted for in the PCRAFI curves.
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Accounting for debris and water ingress results in total failure occurring at a much lower wind speed than the PCRAFI curve.
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Buildings | Wind
Description
This section covers all buildings on Tongatapu.
Damage and failure modes
Buildings are vulnerable to damages both structural and non-structural in nature from high wind pressures resulting from high wind velocities and impact from flying windborne debris.
High wind velocities create large internal and external pressures on building envelope components (such as cladding or appurtenances) and on structural components that are attached to these elements. When these pressure loads exceed the design capacity of each element, failure will occur.
Wind loads can have both very large horizontal and vertical components that can create envelope failures such as roofing materials separating from the structure. In addition to this, exterior equipment can be vulnerable to cladding or small component separation as well as global overturning if total horizontal and vertical pressures overcome anchorage or weight of the equipment itself.
Structural elements may experience very high forces from these pressures that result in failure. As these forces can be cyclic in nature, failure may be dominated by modes not always considered in design, such as reverse buckling.
Finally, all components may be vulnerable to impact from flying debris, however, it is very difficult to quantify the exact impact forces from surrounding debris and different elements’ capacity to resist this impact.
Vulnerability curves
The vulnerability curves shown in
Figure 61 have been directly
adopted from the PCRAFI study
(Bazzurro, 2015) as they were
specifically developed for Tonga
and neighbouring countries. A
review of these functions is
outlined in Section 7.1.
Figure 61: Vulnerability curves for buildings in wind (Bazzurro, 2015)
Type of curve: Vulnerability
Origin: Adopted from literature
Source(s)/reference: PCRAFI study, Bazzurro (Bazzurro, 2015)
Basis: Federated States of Micronesia, Marshall Islands, Samoa, Tonga, and Vanuatu
Intensity Measure: Wind speed
Description of damage
states:
Damage presented as percent of total replacement cost for a given wind speed
Table 39: Vulnerability summary for Buildings
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7.2 Power infrastructure
This section outlines the vulnerability curves to be used for power infrastructure in Tongatapu subject to wind.
Based on Arup’s experience, the following power assets were considered rugged and not subject to damage by wind:
From discussions with TPL, the wind turbines are lowered and secured to the ground during high wind events. As high wind events generally have ample warning time to take this action, they have been deemed rugged and not subject to damage from wind. Surface-mounted transformers were also considered rugged. These are typically very heavy pieces of equipment with a low centre of gravity, which gives them a high capacity to resist overturning. Transformers will likely have seismic anchorage, which further increases their resistance to overturning. Transformers are also fully encased in a metal shell and therefore have no small or weak components vulnerable to extreme wind pressures. Damages to conductors connected to the transformer, should they be above grade, will be captured by the vulnerability curves for power distribution and not for transformers.
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Power Distribution System | Wind
Description
Power distribution system described here
includes three components – utility poles, pole
mounted equipment and power lines.
Damage and failure modes
Power lines are the most vulnerable power
distribution component in strong winds and
damage can be caused by fallen trees or debris,
and may in-turn pull adjacent utility poles
further damaging attached lines. TPL
confirmed previous damage to power lines
from fallen trees during previous high wind
events (e.g. TC Harold in 2019). However,
these elements of the distributions system have
a lower monetary cost of repair compared to damaged utility poles.
Although, failure in power distribution through damaged power lines may cause much disruption,
repair/replacement of damage to utility poles themselves are a significantly larger monetary cost. For
this reason, fragility curves have been provided for utility pole failure even though disruption due to
power line failure is acknowledged. Damage of utility poles may occur through foundation failure,
overturning, pole flexure or debris impact.
Existing studies show little impact (approx. 3%) of strong winds on the fragility of the distribution
system if pole-mounted equipment (such as transformers) is present on the utility poles (Salman,
2016) therefore the utility pole fragility will be used to capture this damage as well.
Vulnerability curves
Fragility curve presented here for
power pole failure was generated
using survey data from multiple
hurricane events in the US,
converted from sustained wind
speeds to 3s gust wind speeds.
Resulting fragility curves match
curves in existing literature
(Salman, 2016; Teoh, Alipour, &
Cancelli, 2019). Tongan design
codes suggest design to gust wind
speeds of 155mph (The Kingdom of Tonga,
2007) with concrete foundations between 1.5-
2m. The power pole fragility has been presented as representative for the power distribution system in
Tonga because similar design standards are followed in the US, where foundation depths range 1.4-
1.8m for 9-11m tall poles. (Metra Engineering Department, 2007)
Figure 62: Fragility curves for utility poles in wind (Quanta Technology, 2008) (unit for wind speed has been converted)
Type of curve: Fragility function
Origin: Adopted without modification
Source(s)/reference: Quanta (Quanta Technology, 2008)
Basis: USA
Intensity Measure: Wind speed
Description of damage
states:
Pole failure (based on post-disaster data and therefore may include overturning, flexure and debris impact)
Table 40: Vulnerability summary for utility poles
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Power Station | Wind
Description
Power stations on Tongatapu contain a number
of components including pumps, diesel tanks,
generators, control rooms, etc. and have been
evaluated for wind at a component level.
Damage and failure modes
Low lying components such as pumps are
generally not damaged by wind, except for
debris impact which is difficult to assess due to
site specific conditions. Contents such as
generators, control equipment, etc. are housed
within a building structure and therefore the
damage to these components is governed by the
fragility of the building structure. If the
building is damaged, then it is assumed that the
debris impact and wind speeds are significant
enough to cause damage leading to replacement of equipment.
The components of a power station that is not housed and most vulnerable to winds are the steel diesel
tanks These diesel tanks can be damaged by buckling or overturning in wind, or by debris impact. The
buckling of these tanks depends on how full these tanks are, as any stored liquid resists wind
pressures that cause buckling. For significant damage to occur to the tanks via debris impact prior to
buckling, debris masses would have to be large (>150kg) (Olivar, Mayorga, Giraldo, Sánchez-Silva,
& Salzano, 2018). The overarching damage mode is therefore expected to be buckling caused by
wind.
Vulnerability curves
The above fragility curves were
extracted from existing literature
for fuel tanks, similar in
construction to diesel tanks, and
consider overturning, wall buckling
and debris impact. Fragility curves
are generated through a
probabilistic Monte Carlo
simulation study. While the
location of the modelled tanks in
the source literature are in the Gulf
of Mexico, the wind hazard is assumed to driven
by similar mechanisms to that experienced on
Tongatapu. The building and contents will be captured using the building wind vulnerability curve.
Figure 63: Steel storage tank buckling fragility at fill (O) levels (Olivar, Mayorga, Giraldo, Sánchez-Silva, & Salzano, 2018)
Type of curve: Fragility curves
Origin: Adapted from literature
Source(s)/reference: Olivar et al. (Olivar, Mayorga, Giraldo, Sánchez-Silva, & Salzano, 2018)
Basis: Gulf of Mexico
Intensity Measure: Wind speed
Description of damage
states/curves:
Probability of shell buckling for different fill levels in a tank.
Table 41: Vulnerability summary for diesel tanks
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Solar Farm – PV Panels and Auxiliary Equipment | Wind
Description
Solar panels and their supporting structure are
generally designed to wind loads. Additionally,
most auxiliary equipment in the solar farm is
either housed or considered rugged in wind.
Damage and failure modes
Solar panels are a key component of a solar
farm. Damage may occur to the Photovoltaic
(PV) panels themselves from debris impact, or
damage may occur at the support structure.
Evidence exists of a large majority of PV
panels being operational after cyclonic events
in multiple global locations (Fthenakis, 2013).
Therefore, the critical fragility of a PV system is reliant on the frame structure.
The steel frame is a structure designed to local structural codes in Tonga and therefore the expected
damage state is yielding of the structural frame after local design levels of wind speeds are exceeded.
Any equipment that is housed will be subject to the vulnerability of the building itself (refer to the
Section 7.1). Outdoor auxiliary equipment such as inverters and transformers, are considered rugged
to wind. While debris impact damage is a likely failure mode, establishing reliable fragility curves are
complex and impacts are generally reduced by fencing or casing around equipment, as seen on sites in
Tongatapu.
Vulnerability curves
Fragility curves for PV panel and
structural steel frames have been
adapted from literature (Goodman,
2015) and modified to Tongatapu
structural performance levels.
These fragility curves are adapted
from a study for roof-mounted PV
systems, although literature shows
that the variation of pressures
between ground-mounted and roof-
mounted systems are minimal (Watson,
2018). Furthermore, a linear transformation of the original fragility curves has been completed to
reflect the more stringent design level of the structural frame to withstand 155mph winds.
Figure 64: Solar panel fragility curves in wind (adapted (Goodman, 2015))
Type of curve: Fragility curves
Origin: Adapted from literature
Source(s)/reference: Goodman (Goodman, 2015)
Basis: USA
Intensity Measure: Wind speed
Description of damage
states:
Failure from frame yield
Table 42: Vulnerability summary for solar panel systems
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Wind Farm – Turbines and Auxiliary Equipment | Wind
Description
While, Tongan wind turbines are designed to be lowered in strong winds, outdoor auxiliary equipment
on wind farms consist of similar components to those found in solar farms and have been evaluated
for vulnerability to wind.
Damage and failure modes
Wind turbines on Tongatapu are known to be lowered around a pin on the bottom of the guyed mast
turbines in strong wind events. Therefore, no damage is expected in wind events. Auxiliary equipment
(such as batteries, control panels, inverters, and feeder and interconnection panels) on wind farms,
like solar farms, are assumed rugged to strong wind.
However, all other equipment is housed within a building structure. The structure itself, and therefore
its fragility is similar to the steel frame building typology (Section 7.1). Wind farm building structures
that house equipment can be mapped directly to this building typology. Upon collapse of the building
structure, internal equipment is assumed to be damaged to the point of replacement as well. This may
be due to debris impact or buckling/overturning caused by wind.
Vulnerability curves
The wind turbine is lowered during high wind events; therefore its vulnerability has not been
assessed.
The auxiliary equipment is covered by curves attributed to wind fragility/vulnerability of a steel frame
building typology (see Section 7.1).
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7.3 Water infrastructure
This section outlines the vulnerability curves to be used for water infrastructure in Tongatapu subject to wind.
Based on Arup’s experience, the majority of the water assets were considered rugged and not subject to damage by wind. At grade concrete tanks are of considerable size and not at risk of overturning. Buried pipelines are located underground along with the pumps and wells
and therefore not subject to wind damage.
The only asset that is considered vulnerable to high winds are the elevated water tanks. Evidence of damage to elevated water tanks of much larger size was found to have occurred after Hurricane Katrina in Buras Louisiana (NOAA, 2005) and Tonga after storm events in 2018 and 2019 (The Kingdom of Tonga, 2019) (The Government of Tonga, 2018). No appropriate fragility or vulnerability curves for elevated water tanks subjected to high winds were found in the literature. It is possible to develop fragility curves through simplified models and calculations, however, this is beyond the scope of this study.
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8 Conclusion
The vulnerability study covered buildings, roads, power and water infrastructure on Tongatapu for coastal and pluvial flood, tsunami, wind and seismic hazards.
In general, there is good coverage of all asset and hazard combinations. The level of direct applicability does vary across all combinations, in some cases vulnerability and fragility curves were developed specifically for use in Tonga either by Arup (e.g. buildings for inundation) or in previous studies (e.g. buildings for wind and seismic). In other cases, vulnerability information was adopted from literature either directly where deemed appropriate, or with some modification with information about the assets in Tonga.
A few gaps in vulnerability have been identified that will prevent inclusion in the risk assessment. These include:
• Elevated water tanks and wind + tsunami
• Wind turbines and seismic + tsunami
The findings in literature for these asset and hazard combinations have been detailed in the relevant sections, however none of the existing fragilities appropriately captured their design and anticipated performance in Tongatapu. Vulnerability analysis through modelling was out of scope for these assets, though it may be possible to develop fragility functions based on analytical models if more information is provided or appropriate assumptions can be agreed upon. The results of the Exposure Assessment with cost information will form the basis of the understanding risk for these asset and hazards.
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