9 Hazardous environments
global frequency, and are particularly prone at times when cool, dry air from the Rockies overlies warm, moist ‘Gulf’ air.
Tornado damage
About 1000 tornadoes hit the USA each year. On average, tornadoes kill about 60 people per year – mostly resulting from flying or falling (crushing) debris. There are three damaging factors at work: l The winds are often so strong that objects in the tornado’s path are simply removed or very severely damaged. l Strong rotational movement tends to twist objects from their fixings, and powerful uplift can carry some debris upwards into the cloud. l The very low atmospheric pressure near the vortex centre is a major source of damage. When a tornado approaches a building, external pressure is rapidly reduced, and unless there is a nearly simultaneous and equivalent decrease in internal pressure, the walls and roof may explode outwards in the process of equalising the pressure differences. The Fujita scale relates the degree of damage to the intensity of the wind (Table 9.5). It should be used with caution as it does not take into account differences in building structure and materials. A new enhanced F-scale, introduced in 2006, classifies damage F0–F5 according to different types of buildings/materials. Table 9.5 Fujita tornado damage scale Category
Description
Category F0: Light damage (<117 km/h)
Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged
Category F3: Severe damage (254–332 km/h)
Roofs and some walls torn off wellconstructed houses; trains overturned; most trees in forests uprooted; heavy cars lifted off ground and thrown
Category F5: Incredible damage (419–512 km/h)
Strong-frame houses lifted off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m; trees debarked
Now test yourself 18 To what extent is it possible to manage the risk of tornado damage?
Answer on p.219
Tested
Managing tornados
As yet there is no effective way of managing tornadoes. The best advice is to stay indoors and, if possible, underground. There is no proof that cloud seeding can or cannot change tornado potential in a thunderstorm.
9.4 Sustainable management in hazardous environments The use of geo-materials for erosion and sediment control, Fraser’s Hill, Pahang, Malaysia
Revised
In Malaysia, research on bioengineering has involved studies on plant selection for the re-vegetation of cut slopes along highways. Bioengineering designs have great potential and application in Malaysia because in deforested upland sites, landslides are common, particularly during the wetter months between November and January. Fraser’s Hill is an area of lower montane forest and receives 200–410 mm of rainfall each month Post-landslide restoration works
Paper 2 Advanced Physical Geography Options
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