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Cover Page

Executive Summary

Introduction

Data Sources

Deaths Due to Natural Hazards

A Building Damage Index

20th Century Building Damage

Alternative Perspectives on Damage

Spatial Variation in Damage

A More Refined View

Discussion

Conclusion

Further Reading

Acknowledgements

Issues in Risk Science
Natural Hazards Risk Assessment: An Australian Perspective - Russell Blong

A Building Damage Index

Natural hazards damage not only includes buildings and infrastructure but also agriculture and other economic activity. Damage other than to buildings is difficult to get a grip on – we have taken the easy path and focused on building damage which is probably the most important aspect of damage, in both economically advantaged and economically disadvantaged societies. However, our emphasis on building damage reflects the difficulty of dealing with other aspects, the woeful lack of reasonable data, and our interest in insurance issues. This implies that our third question above has been modified to: “Which natural hazard in Australia causes the most damage to buildings?”

Even simplifying the question still leaves some tough issues: How do you compare severe damage to a dozen houses to destruction of the local pub? Or hospital? We approached this issue by developing a purpose built damage index that we hope will have wide applicability (Blong, 2003).

The Risk Frontiers Damage Index reduces building damage to House Equivalents (HE); for example, 2 houses half-destroyed is equivalent to 1 house totally destroyed. Buildings other than houses are made equivalent to houses using comparisons of floor areas and per m2 construction costs. Much of the necessary data can be found readily in construction handbooks (e.g. Rawlinsons, 1999).

For example, if we set the cost of building an average Australian house at AUD$800/m², and the cost of building a supermarket at AUD$1,130m², the cost ratio for the supermarket is about 1.4 (setting the house construction cost to 1.0). Then, if the floor area of an “average” Australian house is 180m² and the floor area of the supermarket is 2,000m², setting the replacement cost of the house to 1.0, the Replacement Ratio (RR) for the supermarket is about 16.0 [($1130x2000m²)/($800x180m²)= 15.7]. Thus, the cost of replacing the supermarket is roughly 16 times the cost of replacing a house (RR=16).

A tornado takes out 10 houses, the supermarket, the local pub, and half-destroys 6 more houses. With a cost ratio of 1.9 and a floor area of 1,000m², the RR for the pub = 11. Thus, the tornado damage amounts to 10+16+11+(0.5x6) = 40 House Equivalents.

The Damage Index is concerned only with building damage. As constructed, it ignores damage to motor vehicles, parking areas, swimming pools, gazebos, fences, barbecues and other important elements of Australian life. It also ignores building contents, though all the elements named above (plus aeroplanes, power pylons, gas pipes and fire engines) could be readily turned into House Equivalent values given sufficient time and desire. Obviously, the Index also ignores the social value or utility of the buildings as is evident from the relative replacement ratios for the supermarket and the pub.

In the tornado example above six houses were described as “half-destroyed”. Often, we will want to be more sophisticated than that. Table 2 outlines a scheme relating Damage Classes to Central Damage Values (CDV) and ranges in these values for each class.

Table 2: Central Damage Values (CDV)

Damage Class	Central Damage Value	Range
Light	0.02	0.01-0.05
Moderate	0.10	0.05-0.20
Heavy	0.40	0.20-0.60
Severe	0.75	0.60-0.90
Collapse	1.00	0.90-1.0

Table 2 shows that Heavy Damage implies damage equivalent to about 40% of the replacement value of a building. Thus for our supermarket, Heavy damage implies 40% of the Replacement Ratio (0.4x16) = 6.4 House Equivalents.

For a single building, Damage (HE) = RR x CDV.

Obviously, if we have more specific information about the cost of damage or the replacement value of the supermarket we can vary the CDV and the RR.

The single-word Damage Class descriptors in Table 2 convey only limited information. Table 3 provides more detailed information for tropical cyclone and landslide damage – those familiar with the literature will note my indebtedness to Leicester and Reardon (1976) and to Alexander (1989). Details for tornado, hail, earthquake, bushfire, flood and tsunami are listed in Blong (2003).

Table 3: Damage descriptions for specified Central Damage Values

PERIL

CDV

0.02

Light

0.10

Moderate

0.40

Heavy

0.75

Severe

1.00

Collapse

Tropical cyclone

Negligible – missile damage to cladding or windows

Loss of half roof sheeting

Loss of roof structure + some damage to walls

Loss of all walls

Loss of walls, floor and some support piers on elevated houses

Landslide

Hairline cracks (<0.1mm) in walls or structural members

Minor settlement of foundations

Walls out of perpendicular by several degrees; floors inclined; or heaved; open cracks in walls

Structure grossly distorted; partition walls and brick infill at least partly collapsed; footings lose bearing; service pipes disrupted

Partial/total collapse

We can now express damage as:

Damage (HE) = No of Buildings x RR x CDV

In the tropical cyclone that struck Endsnigh in 1998, 40 houses suffered Moderate wind damage, 60 houses Severe damage, a grandstand (RR=10) totally Collapsed, and a Motel suffered Heavy damage. Thus:

Damage (HE) = [40x1.0x0.1]+[60x1.0x0.75]+[1x10.0x1.0]+[1x7.0x0.4]

= 4+45+10+2.8 = 61.8 House Equivalents

The same tropical cyclone caused landsliding in the suburb of Slippery Slope, destroying 12 houses, while a debris flow entered a single-storey office block (RR=6, CDV=0.3):

Damage (HE) = [12x1.0x1.0]+[1x6.0x0.3] = 13.8 HE

The same tropical cyclone produced flooding in Gurgle, another Endsnigh suburb, with water entering 180 houses (CDV=0.1), floating debris Severely damaging a 1000m2 warehouse (RR=4.2), producing Heavy damage to a suburban police station (RR=2.1) and destroying 5 adjacent small retail outlets (RR=0.5):

Damage (HE) = [180x1.0x0.1]+[1x4.2x0.75]+[1x2.1x0.4]+[5x0.5x1.0]

= 24.5 HE

Thus the total damage produced in Endsnigh by the cyclone is 100.1 HE, cunningly allowing the House Equivalents shown in Table 4 to also serve as percentages. We can note that more than 60% of the total building damage was produced by the cyclonic winds and that nearly 80% of the total damage was to residential buildings.

Table 4: 1998 Endsnigh tropical cyclone damage summary – House Equivalents

	Wind	Landslide	Flood	Total
Residential	49.0	12.0	18.0	79.0
Commercial	2.8	1.8	5.65	10.3
Govt./Public	10	-	0.9	10.9
Total	61.8	13.8	24.5	100.1

Table 4 characterises some of the real benefits of considering building damage in this way. We have a good understanding of the components of damage in the 1998 Endsnigh cyclone. We can compare total damage and the components with earlier cyclones that struck Endsnigh and with the consequences of cyclones that have struck other parts of this great country.

We can also compare the consequences of cyclones with the consequences (for buildings) of other natural perils. Now we have the basis for a reasonably rational natural hazards risk assessment.

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