Address-based Natural Hazard Risk Ratings focus on the peril – earthquake, flood, hail, bushfire, etc. – rather than the probability or severity of any resulting property damage. In interpreting these risk ratings, it should be noted that building damage is often non-linearly related to peril magnitude or intensity and also depends upon structural considerations including building age in some cases.
All perils are ranked on 5-point scales with 5(1) referring the most (least) severe.
Bushfire and flood ratings refer to individual street addresses while hail storms, earthquakes and tropical cyclones refer to larger scales - Census Collection Districts, Postcodes or coastal zones - that include the specified address. Address location in this data refers to the centroid of the land parcel; note that the actual location of the asset within the property may be different and this may be important for flood and bushfire. Risk ratings cover both residential and commercial addresses. Elevation refers to the height above mean sea level, and its accuracy depends on the resolution of the best available Digital Terrain Model; this will vary between sites, ranging from 5m to 90m.
This document is not a
substitute for a site inspection. Rather it should be treated as a means to
raising awareness of possible natural perils and enabling better value to
be obtained from such an inspection.
Hazard
Scales
Bushfire
Analysis by Risk Frontiers
of major historical bushfires show distance from the bushland fringe to be
the single-most important factor determining the probability of building
destruction given an extreme fire 1 in
adjacent bushlands. A five-point scale is applied to addresses in
bushfire-prone areas:
Very High (5)
|
distances
less than 100 m from extensive bushland
|
High (4)
|
distance
between 100 – 200 m from extensive bushland
|
Medium (3)
|
distance
between 200 – 400 m from extensive bushland
|
Low (2)
|
distance
between 400 and 700 m from extensive bushland
|
Negligible (1)
|
distance
larger than 700 m from extensive bushland
|
Areas with a high
potential for bushfire are largely confined to the southeast and southwest
of the continent and Tasmania 2. Areas north
of the Tropic of Capricorn do experience bush or grassfires but to date
have not involved significant property losses. Accordingly we do not
categorise bushfire risk in these areas.
Site elevation and aspect can influence bushfire risk and these are also included in the database.
Note that the existence of corridors of trees between extensive areas of bushland may aggravate risk beyond the risk rating provided despite the distance between the building and the bush. On the other hand, specific site factors or owner preparedness may mitigate this risk.
Mainstream Riverine
Flood
The probability of experiencing a flood of a particular magnitude is generally expressed as an average recurrence interval (ARI) in years. The ARI is the average interval between events affecting the property. It is an average taken over a very long period so a 20-year ARI flood does not occur regularly every 20 years. A flood in a particular catchment with an ARI of 100 years is often used to define a severe flood for planning purposes.
Two flood risk measures are provided: (1) the ARI (in years) of inundation of the address at ground level, and (2) the water depth during a flood with an ARI of 100 years. The first metric represents the probability of flooding at the property and it is this that dictates the Overall Risk. The second provides a surrogate indication of the potential loss from flooding as water depth is a key determinate of damage. Both measures refer to the centroid of the land parcel. The actual location of buildings with respect to the centroid may increase or decrease risk to property.
ARI is categorised into 5
levels and the Overall Risk (right hand column) is based on this table:
Very
High (5)
|
ARI
less than 20 years
|
High
(4)
|
ARI
between 20-50 years
|
Medium
(3)
|
ARI
between 50-100 years
|
Low
(2)
|
ARI
above 100 years
|
Negligible
(1)
|
ARI
above the Probable Maximum Flood
|
The Probable Maximum Flood is the most severe flood considered possible by hydrologists and is generally given an ARI of 10,000 years.
The ratings are for riverine flooding only and do not indicate the risk of flash flooding or storm water overflow. The risk ratings incorporate the effect of levees and it is assumed that the levees operate as designed.
Ratings are available for 1.3 million of the most flood prone addresses in Australia.
For information about coastal flooding refer to the Tropical Cyclone section.
Earthquake
Australia is located entirely within a tectonic plate and experiences low to moderate levels of seismic activity compared with say Japan, New Zealand and California. Nonetheless destructive earthquakes have occurred as witnessed by the 1989 Newcastle earthquake (ML = 5.6), which resulted in some $1.1 billion (Year 2003 dollars) in insured losses. Poor building stock, in particular, older unreinforced, masonry construction can exacerbate losses from even modest levels of ground shaking. Newer engineered structures or smaller wood-framed buildings are much less susceptible.
Building codes prescribe how much horizontal force building should be able to withstand during an earthquake. Peak Ground Acceleration (PGA) is an estimate of the maximum horizontal acceleration experienced by a solid mass at the soil surface in an earthquake. The 5–point peril scale adopted here is based on PGA from the latest Risk Frontiers’ QuakeAUS catastrophe loss model, rather than from the Global Seismic Hazard Assessment Program (GSHAP) 3 used in the previous risk rating database Versions 2.0 and 2.1. This scale gives resultant peak ground accelerations, with soil conditions already considered and modeled at each site, for a 10% chance of exceedance in 50 years (equivalent to a 475 ARI).
Very
High (5)
|
PGA
larger than 4.0 g
|
High
(4)
|
PGA
between 2.4-4.0 g
|
Medium
(3)
|
PGA
between 0.8-2.4 g
|
Low
(2)
|
PGA
between 0.2-0.8 g
|
Negligible
(1)
|
PGA
less than 0.2 m/s2 g
|
The actual ground shaking intensity felt at ground level can be strongly modulated by the response of the soils and weathered material overlying basal rocks. This tendency to amplify ground motions is given by a five-point ground zonation 4. The simplified version is as follows:
Zone
5
|
Unconsolidated
and swampy soils
|
Zone
4
|
Variable
alluvial, estuarine and wind-blown deposits, including sands, organic
materials and unconsolidated clays.
|
Zone
3
|
Thicker
soils and sediments of older river terraces and valley fills, well-drained
coastal and inland sand dunes.
|
Zone
2
|
Competent
bedrocks but subsoils may be plastic or have high shrink-swell potential
leading to cracking of structures.
|
Zone
1
|
Shallow
soils on competent bedrock.
|
The Overall Rating (right-hand column) is based on the resultant PGA from Risk Frontiers’ QuakeAUS catastrophe loss model.
Hail
Hailstorms can cause substantial damage to property. The April 1999 hailstorm in Sydney caused $1.7 billion (2003 dollars) in insured losses, making it Australia’s most costly natural disaster. Most areas on the mainland south of the Tropic of Capricorn are at risk from hail. Damaging hail is rare in the tropics and in Tasmania. Hail risk is a function of the intensity and frequency of hail, where intensity is represented by hail stone size and frequency is the number of storms per unit area per year 5.
The 5-tier risk scale is
based on the product of maximum-recorded hail stone size and the normalized
annual frequency for key locations around the country.
Very
High (5)
|
For
regions that experience very frequent hailstorms and damage.
|
High
(4)
|
For
regions that experience relatively frequent hailstorms and that have a
history of damaging hail .
|
Medium
(3)
|
For
regions where damaging hail is possible.
|
Low
(2)
|
For
regions that rarely experience damaging hail at all.
|
Negligible
(1)
|
For
regions that have never experienced damaging hail.
|
Tropical Cyclone
Tropical cyclone wind risk (flood and storm surge not included) in Australia is analysed on a broad scale with risk varying across Australia for regions north of latitude 30°S. A five-tier scale is used to categorise the risk geographically, based on Risk Frontiers’ tropical cyclone wind model CyclAUS. An annual probability of occurrence at a 1% level is applied to categorise risk levels for tropical cyclone prone coastal areas.
Very
High (5)
|
Areas with tropical cyclone windspeed greater than 45m/s
|
High
(4)
|
Windspeeds between 37 - 45m/s
|
Medium
(3)
|
Windspeeds between 28 – 37m/s
|
Low
(2)
|
Windspeeds between 17 – 28m/s
|
Negligible
(1)
|
Windspeeds less than 17m/s or locations south of 30°S
|
The windspeed thresholds used in the five-tier scale above are locally adjusted (i.e. adjusted for terrain and topography effects) 3-second gust speeds at a height of 10m.
To determine the windspeed thresholds, windspeeds were calculated at the centroids of a 10km × 10km grid extending over the region of Australia north of 30oS. This method ensured an even distribution of windspeed values across Australia. Statistical distributions were fitted to the windspeed values and the thresholds calculated from the addition/subtraction of the standard deviation from the mean of the distribution.
Proximity to shorelines refers to the shortest distance between an address and shorelines of any coastal waters (e.g., rivers, lakes, lagoons and estuaries) that are directly connecting to open ocean. For addresses located within 25km of the coast, a spatial resolution of 250m was used in the analysis, and for addresses located beyond 25km, the spatial resolution was 500m. The accuracy of this calculation is within 1km (upper integer values are provided).
An indication of storm surge risk can be gained by considering the elevation and the distance from the shoreline together.
Coverage
The coverage of the
underlying database is national although Risk Frontiers does not claim that
all attributes are currently available for all addresses.
Perils
|
Coverage for all G-NAF addresses
(~11 millions)
|
Bushfire
|
100%
|
Mainstream riverine
flood
|
29 urban areas of eastern Australia
|
Earthquake
|
100% for peak ground acceleration
>99.9% for soil zonation
|
Hail
|
100%
|
Tropical cyclone
|
100%
|
Distance to shoreline
|
100%
|
Elevation
and aspect |
100%
|
While Access Macquarie Limited, through Risk Frontiers - the Natural Hazards Research Centre, has used the most current and extensive database of addresses in Australia, we cannot guarantee that Risk Ratings exist for every address in a company portfolio. This could be due to the fact that the address is not contained in our database or the address details do not match any contained in our database due to misspellings or incorrect data entry.
Disclaimer
The purpose of the Natural Hazard Risk Rating Database is to provide an overview of the natural hazards affecting an individual address or geographical area in which a property is located. The risk ratings are not a substitute for an on-site inspection. They are not intended to be, and should not be taken to be, a rating or assessment of the desirability or market value of the property or its features. In the preparation of the database, Risk Frontiers has relied on data provided by third parties and while all reasonable care has been used to review the data for reasonableness it is not possible to verify the accuracy and completeness of that data.
Access Macquarie Limited, Macquarie University and MapData Sciences their officers, employees, agents or
contractors and the authors are not liable for any loss (including legal
costs and expenses), injury or damage including death, economic loss and
consequential loss or liability incurred or suffered by any recipient of
this report or their successors in title or any other party acting or
purporting to act in reliance on the contents of this report.
References
1Chen, K and
McAneney, K.J. 2004. Quantifying bushfire penetration into urban areas in
Australia . Geophysical Research Letters. 31, L12212,
doi:10.1029/2004GL020244.
2Blong, R,
Sinai, D, and C. Packham. 2000. Natural Perils in Australia and New Zealand . Swiss Re
Australia Ltd.
3The Global Seismic
Hazard Assessment Program (GSHAP), 1999. http://www.seismo.ethz.ch/GSHAP/
4 Greig
Fester. 1997. Earthquake PML: Household Buildings Sydney II. Greig
Fester ( Australia ) Pty Ltd.
5Leigh, R.
and Kuhnel, I. 2001. Hailstorm loss modeling and risk assessment in the
Sydney region, Australia . Natural Hazards, 24:171-185.
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