Hazard evaluation

HAZARD EVALUATION Also see glossary entry: "HAZARD ASSESSMENT"

To perform risk calculations we need to know the probability of the occurrence of a hazard of a certain level of severity, within a specific period of time, in a given area. The level of severity of natural hazards can be quantified in terms of the magnitude of the occurrence as a whole (event parameter) or in terms of the effect the occurrence would have at a particular location (site parameter). Some of the ways in which the severity of different types of hazard are quantified, using both event and site parameters, are shown in table 3.

Table 3
Hazard evaluation

Natural hazard	Event parameters	Site parameters
Flood	Area flooded (km2) Volume of water (m3)	Depth of floodwater (metres)
Earthquake	Energy release (magnitude)	Intensity of ground shaking (modified mercalli/MSK intensity) Peak ground acceleration (%)
Volcano	Eruption size and duration	Potential to be affected by: ash coverage (m); lava; dust fallout; debris flow
Strong Winds	Windspeed velocity (km/h) Area	Windspeed velocity (km/h)
Landslide	Volume of material dislodged	Potential for ground failure; ground displacement (metres)
Tsunami	Height of wave crest	Depth of floodwater (metres)
Drought	Area affected (km2)	Rainfall deficit (mm)

Like risk, hazard occurrence may be expressed in terms of average expected rate of occurrence of the specified type of event, or on a probabilistic basis. In either case annual recurrence rates are usually used. The inverse of an annual recurrence rate is a return period. Examples of hazard defined in terms of the their occurrence parameters are:

"There is an annual probability of 0.08 of an earthquake with a Magnitude exceeding 7.0 in Eastern Turkey"

This is effectively the same thing as saying

"The average return period for an earthquake of M ³ 7.0 in Eastern Turkey is 12.5 years"

or

"There is a probability of 25% that an earthquake with a Richter magnitude exceeding 7.0 will occur in Eastern Turkey within the next 25 years"

Examples of earthquake hazard expressed in terms of its site characteristics are:

"an annual probability of 0.04 (or 4%) of an earthquake of Intensity VI in the town of Noto" ^I3 (or expected return period of 25 years for the same event - an equivalent definition),

or

"an annual probability of 0.20 (or 20%) of a Peak Ground Acceleration exceeding 0.15% g in the Centre of Mexico City." ¹⁴

The hazard expressed in this way is of course only a partial definition of the hazard, related to events of a particular size range. The definition of the hazard for all possible size ranges cannot be done by a single statement of the type given above, but can be presented graphically, as a relationship between the annual probability and the size of the event, as shown in the examples of hazard maps in figures 8 and 9.

Estimating the occurrence of rare events

The more extreme and severe an event is, the rarer it is.

Natural hazards are extreme cases of normal events; a hurricane is an extreme wind, a destructive earthquake is a large version of the energy released by geological processes that are occurring everyday, a flood is the result of extreme precipitation or storm, or tidal conditions. Extreme meteorological, hydrological or geophysical events pose threats to the human-made environment and to individuals. By definition, extreme events are rare. The more extreme and severe an event is, the rarer it is.

Extreme occurrences of natural hazards are difficult to predict. They occur irregularly - there are very few clearly identifiable patterns of occurrence of natural hazards (although studies are beginning to show that some longer term patterns may be discernible) - and in the short term they appear almost random. Because they happen rarely, there is not a large number of extreme cases in the databases, and statistical forecasting based on past occurrences is unreliable. A volcano that has erupted only once in the past century may erupt once every thousand years or it may have an average eruption rate of once every twenty years and its recent quiescence just happens to be an unusually long gap in its eruption frequency. Estimating the likelihood of another eruption in the near future would need much more than a hundred years statistics on its eruptions. It may be possible to build up a much longer record of how often the volcano has erupted by carefully searching historical records back through previous centuries. It may also be possible for geologists to analyze old lava flows and try to date the eruption frequency from that.

Figure 8 - Hazard Map 1. FLOOD HAZARD IN THE METRO MANILA REGION, PHILIPPINES¹⁵

Similar corroborative evidence on hazard occurrence can sometimes be found for floods (siltation, deposits and high-water marks) and earthquakes (geological evidence of past fault movements) but for most areas where hazards are likely, the main evidence for hazard probability has to come from human records of their occurrence.

Figure 9 - Hazard Map 2. VOLCANIC HAZARDS AT GUNUNG KELUT IN JAVA¹⁶

Research studies have examined how likely extreme cases of natural phenomena are to occur. It has been found that the number of large events of a flood or an earthquake or extreme cases of other natural hazards has some relation to the number of smaller events that occur and thus that the number of small events that occur much more frequently can be used to predict the likelihood of the rarer, more severe ones. Statistical theories on the distribution of extreme values of things, derived from long-term observations indicate that the severity of the event (or logarithm of this severity) may be assumed to be inversely proportional to the logarithm of its frequency of occurrence. The f:N curves of the number of fatalities in different events shown in figure 4 on page 20 illustrate this principle.

Standard methods of plotting the data from smaller, more frequent events can be used to estimate the probability of occurrence of extreme ones. The hazard map in figure 8 shows the distribution of annual flood flows in a particular river plotted in such a way as to offer an estimate of the expected 100 year flood. Usually some corroborative evidence from other geological and historical sources can further justify such projections.

Hazard mapping

Hazard recurrence probability varies from place to place, and one of the most important ways of understanding the risk faced by any community or region is to use the available data to plot hazard maps. According to the type of hazard, various types of hazard maps may be useful. A map of expected maximum site parameter over a specified time period, as shown for an earthquake hazard in map 1 of figure 6, may be useful for some purposes. For other purposes the probability of the occurrence of an event exceeding a certain magnitude may be more useful, as in the following examples.

Flood hazard is often mapped so that the maximum extent of floods with different return periods are superimposed on each other. The hazard map in figure 8 shows the flood hazard for Manila plotted to show the areas inundated by the expected annual flood and the larger areas expected to be inundated by floods with average return periods of 10 years, 25 years and 100 years.

Volcanic hazards are less easily quantified, but areas at greatest risk can easily be identified. The hazard map in figure 9 identifies three areas of Gunung Kelat in Java with increased hazard severity. The area closest to the summit is permanently prohibited; a larger first danger area of about 20 km diameter is identified as being subject to pyroclastic (air-borne volcanic debris) and lahars (lava flows) and liable to be evacuated during eruptions, while parts of the lower slopes which are the presumed paths for lava and mudflows, are identified as a second danger area.

The scale of mapping appropriate for hazard maps depends both on the use and the amount of data available. Knowledge of the spatial distribution of some hazards, such as earthquakes, landslides and floods has reached the level at which variations in risk within a small community can be mapped. Such micro-zoning maps have an important role in land-use planning. Micro-zoning maps can be based on a single event of a single hazard, multiple events of a single hazard, or they can attempt to combine the impact of several different hazards.

The areas of probable occurrence of other hazards, particularly meteorological hazards such as drought and high winds, can only be indicated on maps of much larger areas - for example areas of the world most prone to drought and desertification or tropical storm paths. These maps although not very detailed, nevertheless have an important role in warning development planners of large scale trends and may be useful to UNDP Resident Representatives to identify the hazards to expect and prepare for.

Q. The discussion above points out that there are some hazards which may be mapped in rather fine detail (micro-zoning maps) while others can only be mapped as general trends over much greater areas. In the list given below, identify those hazards which might be presented in the micro-zoning format and those that would be better presented covering a greater area with less detail. (Note: some of the hazards listed are presentable in both formats.)

A.
Hazard type	Micro-zoning map	General trend map
Earthquake	_____________	_____________
Tsunami	_____________	_____________
Volcanic eruption	_____________	_____________
Landslide	_____________	_____________
Tropical storms	_____________	_____________
Floods Drought Population displacements	_____________ _____________ _____________	_____________ _____________ _____________
(caused by war or other hazards)

ANSWER
Hazard	Micro-zoning	General trend
earthquake	x	x
tsunami	x
volcanic eruption	x	x
landslide	x
tropical storms		x
floods	x
drought		x
population
displacements	?	?
(caused by war or other hazards)