(introduction...)

The following several pages deal with the particular characteristics of several hazard types and the main mitigation strategies used to reduce their effects.

Floods and water hazards

Mechanism of destruction

Inundation and flow of water with mechanical pressures of rapidly flowing water. Currents of moving or turbulent water can knock down and drown people and animals in relatively shallow depths. Debris carried by the water is also destructive and injurious. Structures are damaged by undermining of foundations and abutments. Mud, oil and other pollutants carried by the water is deposited and ruins crops and building contents. Flooding destroys sewerage systems, pollutes water supplies and may spread disease. Saturation of soils may cause landslides or ground failure.

Parameters of severity

Area flooded (km²), depth or height of flood, velocity of water flow, amount of mud deposited or held in suspension. Duration of inundation. Tsunamis or tidal waves measured in height (meters).

Causes

River flooding results from abnormally high precipitation rates or rapid snow melt in catchment areas, bringing more water into the hydrological system than can be adequately drained within existing river channels. Sedimentation of river beds and deforestation of catchment areas can exacerbate conditions leading to floods. High tides may flood coastal areas, or seas be driven inland by windstorms. Extensive precipitation in urban areas or drainage failures may lead to flooding in towns as hard urban surfaces increase run-off loads. Tsunamis are caused by underwater earthquakes or eruptions. Dam failures or collapse of water retaining walls (sea walls, dikes, levees).

Hazard assessment and mapping techniques

Historical records give first indication of flood return periods and extent. Topographic mapping and height contouring around river systems, together with estimates of capacity of hydrology system and catchment area. Precipitation and snow-melt records to estimate probability of overload. Coastal areas: tidal records, storm frequency, topography and beach section characteristics. Bay, coastal geography and breakwater characteristics.

Potential for reducing hazard

Retaining walls and levees along rivers, sea walls along coasts may keep high water levels out of flood plains. Water regulation (slowing up the rate at which water is discharged from catchment areas) can be achieved through construction of reservoirs, increasing vegetation cover to slow down run-off, and building sluice systems. Dredging deeper river channels and constructing alternative drainage routes (new river channels, pipe systems) may prevent river overload. Storm drains in towns assist drainage rate. Beaches, dune belts, breakwaters also reduce power of tidal surges.

Onset and warning

Flooding may happen gradually, building up depth over several hours, or suddenly with the breach of retaining walls. Heavy prolonged precipitation may warn of coming river flood or urban drainage overload. High tides with high winds may indicate chance of coastal flooding some hours before it occurs. Evacuation may be possible with suitable monitoring and warning system in place. Tsunamis arrive hours or minutes after earthquake.

Elements most at risk

Anything sited in flood plains. Earth buildings or masonry with water-soluble mortar. Buildings with shallow foundations or weak resistance to lateral loads or impact. Basements or underground buildings. Utilities: sewerage, power, water supply. Machinery and electronics including industry and communications equipment. Food stocks. Cultural artifacts. Confined/penned livestock and agriculture. Fishing boats and other maritime industries.

Main mitigation strategies

Land-use control and locations planning to avoid potential flood plain being the site of vulnerable elements. Engineering of structures in floodplain to withstand flood forces and design for elevated floor levels. Seepage-resistance infrastructure.

Community participation

Sedimentation clearance, dike construction. Awareness of flood plain. Houses constructed to be flood resistant (water-resistant materials, strong foundations). Farming practices to be flood-compatible. Awareness of deforestation. Living practices reflect awareness: storage and sleeping areas high off ground. Flood evacuation preparedness, boats and rescue equipment.

Earthquakes

Mechanism of destruction

Vibrational energy transmitted through the earth's surface from depth. Vibration causes damage and collapse of structures, which in turn may kill and injure occupants. Vibration may also cause landslides, liquifaction, rockfalls and other ground failures, damaging settlements in the vicinity. Vibration may also trigger multiple fires, industrial or transportation accidents and may trigger floods through failure of dams and other flood retaining embankments.

Parameters of severity

Magnitude scales (Richter, Seismic Moment) indicate the amount of energy release at the epicenter - the size of an area affected by an earthquake is roughly related to the amount of energy released. Intensity scales (Modified Mercalli, MSK) indicate severity of ground shaking at a location - severity of shaking is also related to magnitude of energy release, distance away from epicenter of the earthquake and local soil conditions.

Causes

Energy release by geophysical adjustments deep in the earth along faults formed in the earth's crust. Tectonic processes of continental drift. Local geomorphology shifts. Volcanic activity.

Hazard assessment and mapping techniques

Past occurrence of earthquakes and accurate logging of their size and effects: tendency for earthquakes to recur in the same areas over the centuries. Identification of seismic fault systems and seismic source regions. In rare cases it may be possible to identify individual causative faults. Quantification of probability of experiencing various strengths of ground motion at a site in terms of return period (average time between events) for an intensity.

Potential for reducing hazard

None.

Onset and warning

Sudden. Not currently possible to predict short-term earthquake occurrence with any accuracy.

Elements most at risk

Dense collections of weak buildings with high occupancy. Non-engineered buildings constructed by the householder: earth, rubble stone and unreinforced masonry buildings. Buildings with heavy roofs. Older structures with little lateral strength, poor quality buildings or buildings with construction defects. Tall buildings from distant earthquakes, and buildings built on loose soils. Structures sited on weak slopes. Infrastructure above ground or buried in deformable soils. Industrial and chemical plants also present secondary risks.

Main mitigation strategies

Engineering of structures to withstand vibration forces. Seismic building codes. Enforcement of compliance with building code requirements and encouragement of higher standards of construction quality. Construction of important public sector buildings to high standards of engineering design. Strengthening of important existing buildings known to be vulnerable. Location planning to reduce urban densities on geological areas known to amplify ground vibrations. Insurance. Seismic zonation and land-use regulations.

Community participation

Construction of earthquake-resistant buildings and desire to live in houses safe from seismic forces. Awareness of earthquake risk. Activities and day-to-day arrangements of building contents carried out bearing in mind possibility of ground shaking. Sources of naked flames, dangerous appliances etc. made stable and safe. Knowledge of what to do in the event of an earthquake occurrence; participation in earthquake drills, practices, public awareness programs. Community action groups for civil protection: fire-fighting and first aid training. Preparation of fire extinguishers, excavation tools and other civil protection equipment. Contingency plans for training family members at the family level.

Volcanic eruption

Mechanism of destruction

Gradual or explosive eruption, ejecting hot ashes, pyroclastic flows, gases and dust. Blast pressures may destroy structures, forests and infrastructure close to the volcano and noxious gases may kill. Hot ash falls for many kilometers around the volcano, burning and burying settlements. Dust may carry for long distances, and fall as a pollutant on other settlements further away. Molten lava is released from the volcanic crater and may flow for many kilometers before solidifying. The heat of lava will burn most things in its path. Snow-capped volcanoes suffer ice-melt causing debris flows and landslides that can bury buildings. A volcanic eruption may alter the regional weather patterns, and destroy local ecology. Volcanoes may also cause ground upheaval during their formation.

Parameters of severity

Volume of material ejected. Explosiveness and duration of eruption, radius of fall-out, depth of ash deposit.

Causes

Ejection of magma from deep in the earth, associated with mantle convection currents. Tectonic processes of continental drift and plate formation.

Hazard assessment and mapping techniques

Identification of active volcanoes. Volcanoes readily identifiable by their topographical and geological characteristics. Activity rates from historical records and geological analysis. Seismic observation can determine whether a volcano is active.

Potential for reducing hazard

Lava flows and debris flows may be channelled, dammed and diverted away from settlements to some extent, by engineering works.

Onset and warning

Eruption may be gradual or explosive. Seismic and geochemical monitoring, tiltmeters, and mudflow detectors may be able to detect build up of pressure over the hours and days preceding eruption. Mud flow detection, geotechnical monitors and tiltmeters are some of the monitoring strategies available. Evacuation of population away from volcano environs is often possible.

Elements most at risk

Anything close to the volcano. Combustible roofs or buildings. Water supplies vulnerable to dust fall-out. Weak buildings may collapse under ash loads. Crops and livestock are at risk.

Main mitigation strategies

Location planning to avoid areas close to volcano slopes being used for important activities. Avoidance of likely lava-flow channels. Promotion of fire-resistant structures. Engineering of structures to withstand additional weight of ash deposit.

Community participation

Awareness of volcano risk. Identification of danger zones. Preparedness for evacuation. Fire-fighting skills. Taking shelter in strong, tire-resistant buildings.

Land instabilities

Mechanism of destruction

Landslides 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.

Parameters of severity

Volume of material dislodged (m³), 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

Mechanism of destruction

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

Mechanisms of destruction

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.