Volcanoes - Ministry of Civil Defence and Emergency Management

Water supplies
Contamination of open water supplies is
common, even from relatively small ashfalls. Both
turbidity and acidity are the most common problems
affecting water supplies but will usually return to
normal levels within a few hours to days unless
ashfalls are prolonged. Hazardous changes in water
chemistry are rare. However, leachates from ash can
mix with small volumes of water such as roof-fed
water tanks, stock water troughs and shallow water
bodies and cause chemical contamination to levels
above recommended guidelines for drinking water.
Other indirect problems can result from increased
water demand for clean-up operations by communities
affected by ashfall.
Waste
Sewage and stormwater systems are highly
vulnerable to damage from volcanic ashfalls, because
ash blocks pipes, damages pumps and other machinery
and interferes with sewage treatment processes. When
ash falls on impervious surfaces, such as roads, roofs and
other paved areas, it is easily washed into stormwater
systems by rain, or during clean-up operations. It may
also enter the sewage system via illegal connections,
manholes, sediment trap overload or inter-connections
to a stormwater system. Since the grainsize and density
of ash particles decreases with distance from an erupting
volcano, it is at more distant localities where fine ash
enters the system most easily. Very fine ash may remain
in suspension and be transported to sewage treatment
plants depending on pipe size, fluid pressure and
velocity. Where pipes become blocked, local flooding
results. Sewage pumps may also be damaged by ashladen
sewage or they may fail if ash impacts on their
electricity supply system. This may result in backing up
of sewage in urban areas. To remove ash from sewage
and stormwater systems is a time-consuming and costly
exercise.
Sewage Treatment Plants
Ash-laden sewage may enter a treatment plant
overloading solid removal equipment at both the pretreatment
and primary treatment stages. Milliscreens,
mechanical grit/sludge removal mechanisms and other
equipment may become damaged. Ash falling directly
into sedimentation tanks will add to the volume of
material which has to be removed. Low density pumice
and finer pumice shards may float on the surface of
ponds. Ash entering secondary treatment facilities,
such as oxidation ponds or biofilters, will tend to reduce
or halt the oxidation process until the ash settles out
or is removed. Ash may affect the acidity or toxicity
level of effluent to such an extent that bacterial growth
may be damaged or lost. If there is plant failure and/or
deliberate shutdown, untreated sewage may have to
be released into waterways. Costs of repair may also be
extreme. Shutdown and diversion of raw sewage during
and immediately after ashfall may significantly decrease
damage and thus diversion duration and also cost in the
longer term.
Transportation
Transportation networks (eg. road, rail and
air) are extremely vulnerable to volcanic ashfalls,
being subject to widespread disruptions and damage.
Volcanic ash falling on roads is extremely disruptive to
transportation. It reduces visibility on roads and is easily
raised in clouds by passing vehicles. This presents an ongoing
visibility hazard. Wet ash can turn to mud, causing
further problems with vehicle traction. Fine ash causes
clogging of air filters causing engine failure and radiator
blockages resulting in cars overheating. Vehicle brakes
are susceptible to damage and it may also enter the
engine causing wear on moving parts, reducing vehicle
life. To remove ash from roads is a deceptively timeconsuming
and highly costly exercise.
Rail transportation is less vulnerable to volcanic
ash than road, with disruptions mainly caused by
poor visibility and breathing problems for train crews.
Trains will also stir up fallen ash which can affect
residents close to railway tracks. Ash will affect rail
engines in a similar fashion to car engines. Light rain
on fallen ash may also lead to short-circuiting of signal
equipment.
Air transportation is extremely vulnerable to
volcanic ash. Severe impacts can result from aircraftash
encounters, as temperatures reach 3000 o C in
modern jet engines - enough to melt ash. Over 90 ash
encounters have been reported world-wide in the period
1960-1996, with eight aircraft having lost in-flight jet
engine power over that period. Luckily, to date, none
have crashed as a consequence of such encounters.
Drifting volcanic ash can affect large volumes of airspace,
commonly resulting in wide aircraft exclusion
since ash cannot be detected by aircraft radar. This was
the main cause of flight disruptions during the 1995-
1996 Ruapehu eruptions. Extensive night shutdown is
often required as a precaution in times of possible ash
presence. With world-wide air traffic planned to double
over the next decade, and with future aircraft being
bigger and with fewer engines, the vulnerability will
continue to increase. Even minor ashfalls on airports
may shut them down, with damage to both aircraft and
facilities.
Communications
Communications can be severely disrupted
around an erupting volcano. Such disruptions may
result from interference to radio due to atmospheric
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TEPHRA
June 2004