9.4 Sustainable management in hazardous environments

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

Category F0: Light damage (<117 km/h)

Category F3: Severe damage (254–332 km/h)

Category F5: Incredible damage (419–512 km/h)

Description

Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged Roofs and some walls torn off wellconstructed houses; trains overturned; most trees in forests uprooted; heavy cars lifted off ground and thrown Strong-frame houses lifted off foundations and swept away; automobile-sized missiles fly through the air in excess of 100m; 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

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 200–410mm of rainfall each month Post-landslide restoration works

Revised

involving conventional civil designs are costly and sometimes not practical at remote sites. Because of these constraints and the low risk to lives and property, bioengineering was the option taken for erosion control, slope stabilisation and vegetation establishment. Two study plots were chosen and one control plot. Initial work involved soil nailing, using 300 live stakes of angsana tree branches and 200 cut stems of ubi kayu. Subsequently, major ground works involved the installation of geostructures. Tall saplings of Toona sinensis, a fast growing tree species, were then planted at the toe of the slope for long-term stability.

Live stakes and cut stems

l At the end of 6 months, the live stakes became living trees. l A high percentage of angsana stakes (93%) sprouted shoots and roots after a month, and 75% of ubi kayu stems sprouted leaves within a week. l Vegetation cover on slopes helped reduce soil erosion because shoots lowered the intensity of raindrops falling on the exposed soil. l Furthermore, roots functioned like mini soil nails to increase the shear strength of surface soils. l Thus, live stakes were effective in stabilising unstable slopes, and their use in bioengineering should be promoted in the wet tropics.

Slope stability

l Without the erosion control measures, there was aggressive soil erosion during heavy downpours, which caused scouring of the steep slope below the tarred road and resulted in an overhang of the road shoulder.

Trapped sediments and vegetation establishment

l After one year about 75% of one study site was covered by vegetation, while 90% of the second plot was revegetated. l There was no more incidence of landslide at these two plots. l However, at the control plot, there was further soil erosion, which resulted in further undercutting of the slope face. l At the control plot, after 1 year, only seven plant species were present. These were weeds. l The poor vegetation cover is probably due to unstable soil conditions caused by frequent soil erosion and minor landslides.

Evaluation

l The geo-structures were installed at a cost of US$3078, which was cheaper than restoration works using conventional civil structures such as rock gabions, which would cost about US$20,000. l As the site is fairly remote, higher transportation and labour costs would have contributed to the higher cost of constructing rock gabions at this site. l On the other hand, the geo-materials, which were abundantly available locally, were relatively cheap to make or purchase, and this contributed to the low project cost. The geo-structures were non-polluting, required minimal post-installation maintenance, were visually attractive and could support greater biodiversity within the restored habitats. l The geo-materials used in the project, such as coir rolls and straw wattles, biodegrade after about a year and become organic fertilisers for the newly established vegetation. l After 18 months, the restored cut slopes were almost covered by vegetation, and there was no further incident of landslides.

Bioengineering is the use of vegetation in engineering, for example the selection of suitable plant species for the recolonisation of areas following landslides. Geo-materials refer to the use of naturally occurring materials such as vegetation in engineering. Geo-structures are structures constructed from geo-materials such as bamboo bundles (fachines), coir rolls and straw wattles.

19 Evaluate the success of one management scheme in a hazardous environment that you have studied.

Answer on p.219

Exam-style questions

1 (a) Outline the range of hazards associated with volcanic eruptions. [10] (b) How does the impact of hurricanes vary with levels of economic development? [15] 2 (a) Outline the natural and human reasons why mass movements occur. [10] (b) Describe ways in which it is possible to limit mass movements. [15]

Exam ready

Typical mistake

Some students forget to comment about the control study – to evaluate the success of a research project there needs to be a contrast between the experimental and control studies.