SPECIFIC HAZARDS AND MITIGATION MESURES 1

Floods and water hazardsMechanism 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 (km^{2), 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
　
　
　EarthquakesMechanism 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 eruptionMechanism 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.