● Hazard maps created before the eruption of Mt St Helens were extremely
useful, as the mudflows generated by these eruptions had very similar
distributions to those produced in earlier times
● Some engineering schemes can be applied to reduce the damage of mass
wasting
● Eg. rock bolts and retaining walls - drainage through weep holes
● Drainage pipes, plants, down drain
● Slope regarding - slope oversteepened by erosion, bulldozering the materials
from the top of the slope to the bottom
● However, such methods are expensive, so eliminating human activities in
areas where slides are likely to occur may be the best way to reduce damage
and loss of life
Landslides
● Slope stabilisation techniques such as retaining walls, slope reinforcement,
or soil nailing
● Drainage systems control water infiltration and prevent saturation of the soil
● Land-use planning to avoid construction on steep slopes or in areas prone to
landslides
Mudslides
● Proper land management practices, including vegetation cover and
reforestation, to stabilise slopes and reduce erosion
● Retention basins or debris dams to trap and control the flow of
sediment-laden water
● Channelization or diversion structures to redirect the flow away from
vulnerable areas
● Stormwater management systems to control excessive runoff and prevent
saturation of the soil
Rockfalls
● Rockfall barriers or catchment fences to intercept and contain falling rocks
● Rock bolting or netting to secure loose rocks and prevent detachment
● Rockfall drapes or mesh to stabilise rock faces and prevent rock falls
● Regular inspections and maintenance of rock slopes to identify potential
hazards
, 9.3 Hazards Resulting from atmospheric disturbances
Large-scale atmospheric tropical disturbances - tropical storms
(cyclones)
● Tropical cyclones are known as hurricanes in the Atlantic, Caribbean and the
north-western Pacific
● They are called cyclones in the Indian Ocean and South Pacific
● Tropical storms bring intense rainfalls and very high winds, which may cause
storm surges and coastal flooding and other hazards such as mudslides
● Tropical storms are characterised by enormous quantities of water, due to
their origins over tropical seas
● High-intensity rainfall (up to 500 mm in 24 h) causes flooding
● Their path is erratic, so preparation for it only gives about 12 hours in
advance, which is an insufficient amount of time for proper evacuating
measures
● Tropical storms develop in low-pressure systems over tropical oceans
● Winds spiral rapidly around a calm, central area, known as the eye
● In a mature tropical storm, pressure may fall as low as 880 mb
● The combination of the low atmospheric pressure and the pressure in the eye
of the tropical storm leads to very strong winds
● Tropical storms move excess heat from low latitudes to higher latitudes
● They normally develop in the westward flowing air, just north of the equator
● They begin as a small-scale tropical depression, which forces the warm air to
rise up (a tropical depression is a low-pressure system that forms over warm
ocean waters near the equator)
● This causes thunderstorms that last for 24 h and may develop into tropical
storms, which have greater wind speeds than 117 km/h
● Only 10% of tropical disturbances become tropical storms
● For tropical storms to form, a number of conditions are needed:
1. Ocean water above 28 C is needed for the proper amount of water to
evaporate, warm water must be about 60 m deep because storms stir
up the ocean, bringing up cold water from below
2. Winds need to be coming together, converging near the surface
3. The air needs to be unstable and keep eddying for it to keep rising
4. Air up to about 5500 m needs to be humid as it is pulled into the
storm, the extra water vapour provides latent heat energy
5. Pre-existing winds should be coming from nearly the same direction
and at the same speeds and altitudes to avoid ripping the storm apart
6. The upper atmosphere high-pressure area helps pump away air
rising in the storms
