• Maximum probable magnitude, which relates to the quantity of energy released by seismic motion.
• Intensity, measured by the Mercalli scale, which takes into account the effects felt by people, the damage to buildings, and the changes to the terrain.
• Likelihood of occurrence.
• Background—seismic events in the past as well as currently active faults. (The seismic history of the area is a key source of data.)
• Quality and types of soil and potential for liquefaction.
• Conditions of groundwater, level and variations over time.

It is important to be aware of potentially unstable areas: soil that is liquefiable or oversaturated, that might be displaced by a seismic event, and so on. The greatest danger is associated with fracture areas, seismic faults, and the former epicenters of destructive earthquakes.

Seismic events may lead to underground instabilities, the terrain caving in, landslides, rock slides or mudflows.  They can also render oversaturated soil too soft, leading to its collapse and damaging system components in the affected area.

The types of damage wrought by earthquakes on water and sanitation systems include the following (Slide 37):

• Total or partial destruction of the collection, treatment, storage and distribution structure.  Slide 38 shows the damage to a treatment plant, which in addition to losing all its panels suffered severe damage to the structure itself.
• Rupture of the pipes and damage to the joints, leading to a drop in the water supply and alteration of its quality (Slide 39).
• Variations in the volume of surface or groundwater.  Changes in the location where water comes out of springs.

Specific damage, as shown in Slide 40, can render the entire system unusable if components vital to its operation are affected.

Hurricanes

Hurricanes, which occur in tropical cyclone basins, originate in the North Atlantic and can affect the Pacific Ocean, the Caribbean, and the Gulf of Mexico.  They are characterized by sustained wind speeds, cyclonic waves, alterations in sea level, heavy precipitation, and various effects inland (Slide 41).  Information on historic events is useful to determine the specific nature of this hazard in any given location.

The strong winds associated with hurricanes are more likely to damage surface works; risk increases in direct proportion to the height of the works and the surface exposed to the wind, and depends mainly on whether the works were designed and built to resist such winds.

Hurricanes can cause different types of damage to water and sanitation systems (Slide 42):

• Partial or total damage to facilities or other structures due to the force of the wind or rains (Slide 43).  Some of the damage to structures may be difficult to anticipate, as in the example shown on (Slide 44).
• The rupture or misalignment of pipes in exposed mountain areas due to flash floods and landslides.
• Damage to superficial components such as catchment facilities (Slide 45) or electrical equipment damaged by flooding (Slide 46).
• Contamination of the water in tanks and pipelines.
• Rupture and failure of components due to subsidence of the terrain as a result of flooding in oversaturated areas.

Floods

Floods are natural phenomena that may be caused by excessive rainfall, hurricanes, abnormal rises in sea level, the thawing of ice and snow, or a combination of the above.

It is important to be aware of the factors that modify runoff behavior in a watershed. Some are climatic: variations in rainfall patterns, intersection areas, evaporation and transpiration.  Others are physiographic: characteristics of the basin such as geological conditions, topography, the course of riverbeds, absorption capacity, type of soil, and land use (Slide 47).

Historical statistics (precipitation levels, river levels, etc.) are a key input for the design of water systems.  Special attention must be paid to recurrence periods and variations in the water level over the years and decades.

Flood-prone areas such as floodplains are the ones most at risk.  When siting system components, the nature of the terrain, including adjacent areas, must be taken into account.

Flood damage can take many forms: the wrenching force of flash floods, the impact of floating debris, landslides in oversaturated areas, rockslides, and so on.  The amount of damage depends on the levels reached by the water, the violence and speed of its flow, and the geographical area covered.

The following are some of the forms in which floods may damage water and sanitation systems (Slide 48):

• Total or partial destruction of collection structures on rivers or brooks.
• Silting up of components.
• Depletion of catchments due to the diversion of river courses.
• Rupture of exposed pipes in the path of rivers or brooks.
• Rupture of pipes in coastal areas and adjacent to river banks due to storm surges.
• Contamination of catchment water.
• Damage to pumping equipment and electrical equipment in general.

Broadly speaking, both too much water and too little can be a problem for water supply and sewerage systems.  In the case of floods, water and sanitation system components are most vulnerable when located where water collects or in the path of flash floods. (Slide 49)

Some water-supply system components themselves may increase the vulnerability of the systems and that of the population, for instance when a dam or reservoir breaks (Slide 50), ruptures occur in high-pressure pipes, or drinking water is supplied to settlements located in unstable terrain without the necessary drainage, so that runoff saturates the soil causing landslides and other mishaps.

During floods, sanitation systems, particularly combined sewers, may become obstructed and fail (Slide 51).  Sewerage obstructions and leaks put water-supply systems at risk from fecal and other contamination (Slide 52), particularly when water-distribution and sewage networks follow roughly the same layout and are thus in close proximity.

It should be expected that different areas, or of different extension, will become prone to flooding at different times, depending on precipitation and recurrence patterns (Slide 53). When waterworks are designed, it is vital that historical variations in precipitation levels or river overflows be taken into account.

Landslides

This phenomenon may be caused by earthquakes, intense rains, volcanic eruptions, even human activities such as those that lead to deforestation.  Regardless of the cause, it occurs in isolated fashion in specific places, hence the need to identify those points in the system that might be affected.

In order to forecast landslides, it is essential to know the geology of the region, particularly steep slopes, ravines, drainage and filtration catchment areas, the topography and stability of the soil, areas with concentrated fissures and places where liquefaction has taken place due to earthquakes or precipitation. (Slide 54).

Vulnerability of water and sanitation systems to landslides is high, particularly in areas where collection facilities are located in mountainous areas and pipes must descend down mountain slopes to reach the areas serviced.  In such areas, landslides may cause the following types of damage (Slide 55):

• Total or partial destruction of vital system components, particularly collection and conduction facilities, located on or near the path of landslides in unstable terrain with steep slopes.
• Water contamination in surface catchment areas in mountainous regions.