SPECIFIC HAZARDS AND MITIGATION MESURES 2

Land instabilities

Mechanism of destruction
Landslides
Parameters of severity
Volume of material dislodged (m^{3), area buried or affected, velocity (cm/day), boulder sizes.}
Causes

Gravitational forces imposed on sloping soils exceed the shear strength of soils that hold them in position. High water content makes soil heavier, increasing the load, and decreasing shear strength. With these conditions heavy rainfalls or flooding make landslides more likely to happen. The angle of slope at which soils are stable is a physical property of the soil. Steep cuttings through some types of soils makes them unstable. Triggering of the collapse of unstable soils can be caused by almost any minor event: storms, minor ground tremors or man-made actions. Liquifaction is caused by earthquake vibrations through loose soils, usually with high water content.

Hazard assessment and mapping techniques
Identification of previous landslides or ground failures by geotechnical survey. Identification of probability of triggering events such as earthquakes. Mapping of soil types (surface geology) and slope angles (topographic contouring). Mapping of water tables, hydrology and drainage. Identification of artificial land fill, man-made mounds, garbage pits, slag heaps. Investigation into the probability of triggering events, especially earthquakes.

Potential for reducing hazard
Landslide risk for a slope reduced by shallower slope angles (excavating top layer to cut back slope), increasing drainage (both deep drainage and surface run-off) and engineering works (piling, ground anchors, retaining walls). Shallower angles for embankments and cuttings, terracing slopes and forestation can prevent loss of surface material to depth of root penetration. Debris flows can be directed into specially constructed channels if they are expected. Rockfall protection barriers (trenches, slit dams, vegetation barriers) can protect settlements.
Onset and warning
Most landslides occur gradually at rates of a few centimeters an hour. Sudden failures can occur without warning. Rockfalls are sudden but noisy. Debris flows sudden, but precursory trickles of material may give a few minutes of warning if population is prepared.

Elements most at risk
Settlements built on steep slopes and softer soils or along cliff tops. Settlements built at the base of steep slopes, on alluvial outwash fans or at the mouth of streams emerging from mountain valleys. Roads and other communication lines through mountain areas. Masonry buildings. Buildings with weak foundations. Large structures without monolithic foundations. Buried utilities, brittle pipes.

Main mitigation strategies
Location planning to avoid hazardous areas being used for settlements or as sites for important structures. In come cases relocation may be considered. Reduce hazards where possible. Engineering of structures to withstand or accommodate potential ground movement. Piled foundations to protect against Liquefaction. Monolithic foundations to avoid differential settlements. Flexible buried utilities. Relocation of existing settlements or infrastructure may be considered.
Community participation
Recognizing land instability potential and identifying active landslides. Avoidance of siting houses in hazardous locations. Construction of strong foundations for structures. Compaction of ground locally. Slope stabilization through terracing and forestry. Rockfall barriers (trees and earth banking).
　
　
Strong winds (typhoons, hurricanes, cyclones, tropical storms and tornados)

Mechanism of destruction
Pressure and suction from wind pressure, buffeting for hours at a time. Strong wind loads imposed on a structure may cause it to collapse, particularly after many cycles of load reversals. More common damage is building and non-structural elements (roof sheets, cladding, chimneys) blown loose. Wind-borne debris causes damage and injury. High winds cause stormy seas that can sink ships and pound shorelines. Many storms bring heavy rains. Extreme low air pressure at the center of a tornado is very destructive and houses may explode on contact.

Parameters of severity
Velocity of wind. Wind scales (e.g. Beaufort) gale severity scale. Local hurricane/typhoon scales.

Causes
Winds generated by pressure differences in weather systems. Strongest winds generated in tropics around severe low pressure systems several hundreds of kilometers diameter (cyclones) known as typhoons in the Pacific and as hurricanes in Americas and elsewhere. Extreme low pressure pockets of much narrower diameter generate rapidly twisting winds in tornados.

Hazard assessment and mapping techniques
Meteorological records of wind speeds and direction at weather stations gives probability of high winds in any region. Local factors of topography, vegetation and urbanization may affect microclimate. Past records of cyclone and tornado paths give common patterns of occurrence for damaging wind systems.

Potential for reducing hazard
None. Cloud seeding may dissipate rain content.

Onset and warning
Tornados may strike suddenly but most strong winds build up strength over a number of hours. Low pressure systems and tropical storm development can be detected hours or days before damaging winds affect populations. Satellite tracking can help follow movement of tropical storms and project likely path. The movements of weather systems are however, complex and still difficult to predict with accuracy.

Elements most at risk
Lightweight structures and timber housing. Informal housing sectors and shanty settlements. Roofs and cladding. Loose or poorly attached building elements, sheets and boards. Trees, fences, signs etc. Telegraph poles, pylons and high-level cables. Fishing boats or other maritime industries.

Main mitigation strategies
Engineering of structures to withstand wind forces. Wind load requirements in building codes. Wind safety requirements for non-structural elements. Good construction practices. Micro-climatic siting of key facilities, e.g. in lee of hillsides. Planting of windbreaks, planning of forestry areas upwind of towns. Provision of wind-safety buildings (e.g. strong village halls) for community shelter in vulnerable settlements.
Community participation
Construction of wind-resistant or easily rebuilt houses. Securing fixing of elements that could blow away and cause damage or injury elsewhere, e.g. metal sheeting, fences, signs. Preparedness for storm action. Taking shelter in strong, wind-resistant buildings. Protection measures for boats, building contents or other possessions at risk.
　
Technological hazards
Explosions cause loss of life, injury and destruction of buildings and infrastructure; transportation accidents kill and injure passengers and crew, and may release hazardous and polluting substances; industrial fires can achieve very high temperatures and affect large areas; hazardous substances released into the air or water can travel long distances and cause contamination of air, water supply, land, crops and livestock making areas uninhabitable for humans; wildlife is destroyed, and ecological systems disrupted. Large-scale disasters can threaten the stability of the global ecology.

Parameters of severity
Quantity of hazardous substances released; temperature of fire; extent of explosion destruction; area of contamination of air, sea, groundwater; local intensity of contamination (parts per million, Becquerels/liter for radio-activity).
Causes
Fire; failures of plant safety design; incorrect plant operating procedures; failures of plant components; accidental impact; arson and sabotage; earthquakes.
Hazard assessment and mapping techniques
Inventories and maps of storage locations of toxic/hazardous substances and their characteristics; common transportation routes for dangerous substances; maps of possible zone of contamination and contamination intensity in the event of a release of any given size; traffic corridors and historical accident records for transportation hazard areas;

Potential for reducing hazard
Improved safety standards in plant and equipment design; anticipation of possible hazards in plant design; fail-safe design and operating procedures; dispersal of hazardous materials; legislation; preparedness planning

Onset and warning
Rapid (minutes or hours) or sudden (no warning); industrial plant design should incorporate monitoring and warning systems for fire, component failure and build-up of dangerous conditions; release of pollutants may be slow enough for warning and evacuation of plant operatives and public; explosions can in some cases be anticipated.

Elements most at risk
Industrial plant or vehicle and its employees or crew; passengers or residents of nearby settlements; adjacent buildings; livestock/crops in the vicinity of the plant (up to hundreds of kilometers in the case of large-scale releases of airborne pollutants and radioactive materials); regional water supply and hydrology; fauna and flora.
Main mitigation strategies
Reduce or eliminate hazard by the means listed above; improve fire-resistance by use of fire-resistant materials, building fire barriers, smoke extraction; improving detectors and warning systems; preparedness planning - improve firefighting and pollution dispersal capabilities, and emergency relief and evacuation planning for plant employees and nearby settlements, (crew and passengers in the case of vehicles). Initiate on-site and off-site safety plans, conduct drills in conjunction with local fire departments. Improve capabilities of civil defense and emergency authorities. Limit or reduce storage capacity of dangerous or flamable chemicals.

Community participation
Action to monitor pollution levels, to ensure inspection and enforcement of existing safety standards and to improve safety legislation. Prepare evacuation plans.
　
Drought and desertification
Lack of water affects health of crops, trees, livestock, humans: land becomes subject to erosion and flooding; effects are gradual but if not checked, crops and trees and livestock die, people lose livelihood, are forced to move, and may starve if aid is not provided: then buildings and infrastructure are abandoned and decay and cultural artifacts are lost.
Parameters of severity
Rainfall level, rainfall deficit (mm), period of drought; extent of loss of soil cover, extent of desert climatic zone.
Causes
Drought mainly caused by short-term periodic fluctuations in rainfall level; possibly by long-term climatic changes; desertification caused by loss of vegetation and subsequent land erosion caused by combination of drought, overgrazing and poor land management.
Hazard assessment and mapping techniques
Rainfall map indicating areas of desert and semi-desert climatic conditions; mapping of erosion rates and desertification.
Potential for reducing hazard
Drought is uncontrollable; desertification can be reduced by improved land management practices, forest management, infiltration dams, irrigation and range management (control of land use and animal grazing patterns).
Onset and warning
Slow onset, period of years, many warnings by rainfall levels, river, well and reservoir levels, human and animal health indicators. Onset of severe drought, causes death of livestock, rise in infant mortality, migration.
Elements most at risk
Crops and forests; human and animal health, all economic activities dependent on continuous water supply; entire human settlements if drought is prolonged.

Main mitigation strategies
Water rationing; conserving or replacing failing water supply by watershed management, construction of dams, pipelines or aqueducts; conserving soil and reducing erosion rates by checking dams, levelling, planting, herd management; reducing firewood cutting by improved fuel stoves, introduction of flexible farming and cropping patterns; population control; education and training programs.
Community participation
Construction of check dams, reservoirs, wells, water tanks, planting and afforestation; changing cropping patterns; introducing water conservation policies; changing livestock management practices; development of alternative non-agricultural industries.

Mechanisms of destruction

Mechanism of destruction

destroy structures, roads, pipes and cables either by the ground moving out from beneath them or by burying them. Gradual ground movement causes tilted, unusable buildings. Cracks in the ground split foundations and rupture buried utilities. Sudden slope failures can take the ground out from under settlements and throw them down hillsides. Rockfalls cause destruction from fragmentation of exposed rock faces into boulders that roll down and collide into structures and settlements. Debris flows in softer soils, slurry material, man-made spoil heaps and soils with high water content flow like a liquid, tilling valleys, burying settlements, blocking rivers (possibly causing floods) and blocking roads. Liquefaction of soils on flat land under strong vibrations in earthquakes is the sudden loss of the strength of soils to support structures that stand on it. Soils effectively turn temporarily to liquid allowing structures to sink or fall over.