50028-001: Pacific Disaster Resilience Program

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

MHDRA-REP-005 | Final | 09 April 2021 | Arup North America Ltd

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Asian Development Bank Multi-Hazard Disaster Risk Assessment, TongatapuInterim Vulnerability Data Report

MHDRA-REP-005 | Final | 09 April 2021 | Arup Australia Pty Ltd


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



5 Tsunami Vulnerability 43

5.1 Buildings 43

5.2 Roads 45



6 Seismic Vulnerability 56

6.1 Buildings 56

6.2 Roads 60



7 Wind Vulnerability 74

7.1 Buildings 74



8 Conclusion 84

9 References 85




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 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 29: Vulnerability summary for utility poles and power lines.

Table 30: Vulnerability summary for transformer.


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.




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.




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 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)




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))




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




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




Figure 2: Masonry building in Tonga

Figure 3: Steel building in Tonga

Figure 4: Reinforced concrete frame building (may include masonry infill) in Tonga




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.




• 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.




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.




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.




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.




• 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.




• 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




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




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




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.




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




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




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).




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.

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 2.5 3.0

P(D

S>

=d

s)

Depth (m)

Full-height Partition

DS1

DS2

DS3




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.

0%

5%

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15%

20%

25%

30%

35%

40%

0 1 2 3 4 5 6

% R

epla

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ent

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st

Depth (m)

General (Timber) Building Archetype Losses

p10 p50 p90




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




$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




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.




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%

80%

90%

100%

0 1 2 3 4 5 6

% R

epla

cem

ent

Co

st

Depth (m)

Residential Building

Residential SS (Timber)

Residential SS (Masonry/Other)

Residential MS (Timber)

Residential MS (Masonry/Other)




General Building | Flood

Description

Two General Building archetypes were


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).


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.


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.


Origin: Arup – developed from component-level building model


Basis: Tonga



states:


Table 7: Vulnerability summary for General building archetypes.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6

% R

epla

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ent

Co

st

Depth (m)

General Building

General (Timber)

General (Masonry/Other)




Specialized Building | Flood

Description

Two Specialized Building archetypes were


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.


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.


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.



Origin: Arup – developed from component-level building models


Basis: Tonga



states:



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6

% R

epla

cem

ent

Co

stDepth (m)

Specialized Building

Specialized (Timber)

Specialized (Masonry/Other)




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.


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.


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.




Basis: Tonga



states:


Table 9: Vulnerability summary for Out building archetype.

0%

10%

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30%

40%

50%

60%

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0 1 2 3 4 5 6

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Depth (m)

Building Archetypes

Out (Timber) Out (Masonry/Other)




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.


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.


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.





Basis: Tonga



states:



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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




4.2 Roads

This section outlines the vulnerability curves to be used for roads in Tongatapu subject to flood.




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.


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).


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.


Origin: Adapted from USACE economic analysis of roads susceptible to cyclone

Source(s) / reference: USACE (U.S. Army Corps of Engineers, 2009)

Basis: US



states:



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0 1 2 3 4 5 6

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Depth (m)

Road (Paved and Unpaved)





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.




Transformers | Flood

Description

Ground-mounted transformers, though typically weatherproofed to some degree, remain vulnerable to flood inundation.


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.


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


Basis: US, International



states:

DS1 – Water enters & damages individual components. DS2 – Most components submerged.

Table 12: Vulnerability summary for Transformers.

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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




Pole-Mounted Equipment | Flood

Description

Pole-mounted equipment, such as capacitors and transformers, are rugged to rainfall but susceptible to damage in inundation.


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.


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.


Origin: Arup – engineering judgement, SME input, specification review, and project experience


Basis: Tonga



states:

Water damages critical sub-components, replacement required

Table 13: Vulnerability summary for Pole-Mounted Equipment.

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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




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.


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.


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.


Origin: Adapted from Hazus (FEMA, 2009)


Basis: US



states:

Damage as a percent of total replacement cost

Table 14: Vulnerability summary for Power Station.

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0.4

0.6

0.8

1.0

0 1 2 3 4 5 6

% D

aa

ma

ge

Depth (m)

Power Station




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.


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).


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.


Origin: Arup – engineering judgement, SMEs, Tongatapu site visit data collection


Basis: Tonga



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




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.


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.


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.


Origin: Arup – engineering judgement, SMEs, Tongatapu site visit data collection


Basis: US, Tonga



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




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.


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.


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.





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.




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.


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.


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.


Origin: Adapted from Hazus

Source(s) / reference: Arup, FEMA (FEMA, 2009)

Basis: Tonga



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




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.




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.


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).


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)


Origin: Adopted without modification

Source(s) / reference: FEMA Hazus (FEMA, 2017)

Basis: Analytical models

Intensity measure: Momentum flux


states:

Probability of collapse

Table 18: Vulnerability summary for the Building Structure




5.2 Roads

This section outlines the vulnerability curves to be used for roads in Tongatapu subject to tsunami.




Roads | Tsunami

Description

The roads asset includes both paved and unpaved roads on Tongatapu.


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.


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


Source(s) / reference: Horspool & Fraser (Horspool & Fraser, 2016)

Basis: Japan, empirical data



states:

DS 1: Minor damage

DS 2: Damage to one lane DS 3: Damage to all lanes

Table 19: Vulnerability summary for roads





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.




Utility Poles and Power Lines | Tsunami

Description

The power distribution system is primarily

comprised of utility poles and power lines.


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).


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.




Basis:

Post-tsunami damage reports from Samoa, Andaman Islands, Chile, Japan; expert judgment



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




Pole Mounted Equipment | Tsunami

Description

The pole-mounted equipment covers an array

of assets, including transformers, capacitors,

and fuses.


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.


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)




Basis:




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.




Power Station | Tsunami

Description

The power station and its associated equipment

are vulnerable to tsunami forces and

inundation.


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.


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)




Basis:




states:

DS 1: Minor water damage; repairs required

DS 2: Moderate damage; replacement of some components DS 3: Complete damage; components washed away





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.


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.


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)



Source(s) / reference: Horspool and Fraser (Horspool & Fraser, 2016)

Basis:




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




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.


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.


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).





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.




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.


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.


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.


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



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




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.


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.


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.



Source(s) / reference: Hatayama (Hatayama, 2015)

Basis: Japan



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




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.




• 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.




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




Buildings | Seismic

Description

This section covers all buildings on Tongatapu.


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.


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


states:

Damage presented as percent of total replacement cost for a given shaking intensity

Table 27: Vulnerability summary for Buildings for seismic shaking




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.




Roads | Seismic

Description

Paved and unpaved roads are vulnerable to

seismic hazards that cause permanent ground

deformation.


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).


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)


Origin: Adopted from literature without modification

Source(s) / reference: US FEMA (FEMA, 2020)

Basis: US

Intensity measure: Permanent ground deformation


states:

DS 1: Slight settlement DS 2: Moderate settlement or offset of ground DS 3: Extensive/Complete; major settlement of the ground






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.




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.


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).


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.




Basis: US



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.




Transformers | Seismic

Description

Ground-mounted transformers across

Tongatapu are vulnerable to seismic shaking.


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).


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.




Basis: US

Intensity measure: Peak ground acceleration


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




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.


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).


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.



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





Solar Farm – PV Panels | Seismic

Description

Solar farm photovoltaic (PV) panel structures

were evaluated for fragility to earthquakes.


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.


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


Source(s)/reference: Miyamoto International Inc (Miyamoto International Inc., 2019)

Basis: USA



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




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.


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.


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)



Source(s)/reference: FEMA (FEMA, 2020)

Basis: USA



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




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.


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.


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.





This section outlines the vulnerability curves to be used for water infrastructure in Tongatapu subject to seismic shaking.




Elevated Water Tanks | Seismic

Description

Elevated plastic water tanks typically rest on 9-

meter tall stands.


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).


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


Source(s)/reference: ALA (American Lifelines Alliance, 2001)

Basis: USA



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




At Grade Water Tanks | Seismic

Description

Several at-grade concrete water tanks supply

water to Nuku’alofa.


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.


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).




Basis: USA



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




Buried Pipelines | Seismic

Description

Buried pipelines comprise the water

distribution network across Tongatapu.


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.


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.




Basis: USA



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




Pumps and Well | Seismic

Description

Each well contains a pump to move water into

the distribution system for servicing TWB and

MLNR customers.


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).


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)



Source(s)/reference: Hazus (FEMA, 2020)

Basis: USA



states:

DS 1: Slight DS 2: Moderate DS 3: Extensive DS 4: Complete

Table 37: Vulnerability summary for Pumps and Wells




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




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.




Accounting for debris and water ingress results in total failure occurring at a much lower wind speed than the PCRAFI curve.




Buildings | Wind

Description

This section covers all buildings on Tongatapu.


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.


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



states:

Damage presented as percent of total replacement cost for a given wind speed

Table 39: Vulnerability summary for Buildings





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.




Power Distribution System | Wind

Description

Power distribution system described here

includes three components – utility poles, pole

mounted equipment and power lines.


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.


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)



Source(s)/reference: Quanta (Quanta Technology, 2008)

Basis: USA



states:

Pole failure (based on post-disaster data and therefore may include overturning, flexure and debris impact)

Table 40: Vulnerability summary for utility poles




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.


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.


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)


Origin: Adapted from literature

Source(s)/reference: Olivar et al. (Olivar, Mayorga, Giraldo, Sánchez-Silva, & Salzano, 2018)

Basis: Gulf of Mexico



states/curves:

Probability of shell buckling for different fill levels in a tank.

Table 41: Vulnerability summary for diesel tanks




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.


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.


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))


Origin: Adapted from literature

Source(s)/reference: Goodman (Goodman, 2015)

Basis: USA



states:

Failure from frame yield

Table 42: Vulnerability summary for solar panel systems




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.


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.


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).





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.




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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Documents

50028-001: Pacific Disaster Resilience Program