Protective Measures

Protective measures against volcanic hazards

1: ash falls
2: volcanic blasts and pyroclastic flows
3: lahars
4: lava flows

Protection against ash falls

Heavy ash falls tend to cause a sudden and complete blackout, reducing visibility and rendering even powerful lights ineffective. Ash deposits can bury essential equipment such as fire hydrants, making them difficult to find if needed to deal with fires which may be caused by volcanic impacts. This can be overcome by keeping hoses attached to the hydrants under pressure and keeping them above ground. This was successfully carried out under heavy ash fall during the Heimaey (Iceland) eruption of 1973.

Accumulation of heavy ash falls on building roofs and their resultant collapse is the main type of ash fall-related damage. This is particularly true if the ash becomes wet and even heavier. In areas of potentially heavy ash falls, equipment must be readied and people prepared to remove the ash from roofs, before too much of it accumulates. In addition, a survey of the area should be made to determine the thickness of dry and wet ash that roofs will bear without collapse. At Vestmannaeyjar (Iceland), affected by the 1973 Heimaey ash falls, roofs with a slope > 20^o suffered no damage. However, buildings with lower sloped or flat roofs collapsed if the ash was not continually removed. Steep roofs of metal sheeting do not retain ash and are also resistant to ignition by hot rock fragments, hence, these should be used in areas subject to major ash fall hazards.

Indirect effects of ash falls should also be expected and planned for. Toxic components adhering to ash (including fluoride and sulphuric acid) and electrical discharges can often cause serious adverse effects. Water supplies (particularly rainfall collection systems) should be protected (by disconnection) from potentially toxic compounds carried by some ash falls. Sensitive electrical equipment should also be protected from electrical discharges and plans emplaced to deal with fires potentially started by lightning.

Although it may be impossible to protect all people in an area of heavy ash fall, public information programmes can be in place to forecast ash falls and to educate people about how to deal with the ash. Radio broadcasts can advise of shortest evacuation routes or inform about steps to take to protect people and property from the ash fall. Rescue personnel in heavy ash fall areas should be equipped with helmets, face masks, heat-resistant capes and also gas masks if toxic gases are present.

Shovelling or sweeping ash by hand from roofs is the most common way of protecting property against ash fall. On very large buildings with flat, strong roofs small mechanical shovels have been used - in Vestmannaeyjar in 1973, 600 tonnes of ash was removed from the hospital roof. Large amounts of ash cleared from roofs can build up at the sides of buildings and exert pressure on walls; bulldozers are required to smooth or remove the ash and keep streets clear. In very heavy ash falls and poor visibility, power lines can be a danger when the former ground level has been built up by metres of ash. After eruptions ash can prove useful as a material for road, airport or building foundations. Close to volcanoes there is greater potential for the fall of red-hot ash that can start fires. Large fragments can smash through windows and set fire to the interiors of houses; 25 homes were burnt down in the 1973 Heimaey eruption. Further damage was prevented by shielding windows and roofs facing the volcano with metal sheeting. Fuel tanks were protected by fitting wire mesh over ventilation tubes.

Protection against volcanic blasts and pyroclastic flows

In areas subject to these hazards, building destruction will be almost total. Only an underground, reinforced, hermetically sealed, impact-resistant structure will give protection against blasts and pyroclastic flows (e.g. a nuclear bomb shelter). For most countries suffering volcanic hazards such structures are too expensive for private homes or even the state. However, such structures would be appropriate at volcano observatories and for police and other officials that maintain essential services in evacuated areas. Other installations located in high-risk areas (e.g. power stations and communications centres) would require similar protection.

In developing countries where more buildings will be constructed from reinforced concrete in future years, modifications are possible where basement areas could be converted into volcanic eruption shelters in emergencies. Another form of protection could be provided by arranging public buildings to have doors and windows sealable from hot volcanic dust clouds that occur on the fringes of pyroclastic flows. This may help to stop the asphyxiation of people in their undamaged houses that has occurred in many historic eruptions.

Protection against lahars

Small lahars can be diverted by barriers or artificial channels to lead them away from populated areas. Some can also be contained by pre-emptied dam reservoirs. However, in many cases the volume and power of large lahars are impossible to control.

The best way to avoid damage from lahars is to restrict or stop building in areas subject to lahars in the historic and recent geologic record. However, in many cases, ignorance of this has led to settlements in highly hazardous areas close to volcanoes. The only protection possible is evacuation of the area once eruptions have begun and lahars appear likely. An improvement on this would be installation of real-time lahar monitoring stations sufficiently upstream of settled areas to provide adequate warning for evacuation.

Protection against lava flows

The first attempt in recorded history to divert a lava flow was in 1669 in Sicily when lava from Etna was flowing toward the city of Catania. People covered themselves with wet cowhides and opened a breach in the side of the flow with iron bars. This succeeded as lava flowed through the breach and in another direction. However, because it began to flow toward another village, its inhabitants stopped the operation and the breach closed up and the main flow continued toward the city. Several other methods have later been tried to divert lava flows.

Aircraft bombing of low-viscosity basaltic lava flows has been attempted to open new channels or to break up and clog flows threatening valuable property. In 1942 a long lava flow was breached by bombing high on the slopes of Mauna Loa (Hawaii) and its flow front 20 km away stopped moving. However, this corresponded with a decrease in eruption intensity and it was not proved that the bombing was solely responsible for the lava flow stopping. Aircraft bombing is generally not accurate enough and relies on good visibility, which is not normally the case during eruptions, guided missiles may prove more successful. It is doubtful that bombing would have any affect on thick flows and a misdirected bomb may in fact increase the flow in the wrong direction.

In recent history many experimental methods have been attempted to divert lava flows from Mt. Etna in Sicily. Explosive breaching of a flow was experimented with in 1983, using charges installed by hand into shallow bore holes. In 1992 several attempts at slowing a flow from Etna by controlled explosions failed, and also early attempts at constructing concrete barriers met with little success. Eventually the lava was diverted by constricting its flow in a lava tube with concrete blocks.

There have been many other attempts at diverting lava flows with barriers. In 1881 a barrier was constructed to slow a lava flow heading from Mauna Loa to Hilo, but the eruption stopped before the barrier was completed. In northern Iceland where lava is very free flowing (and hence likely to be diverted by a barrier rather than push it over) two barriers have been built to protect a village and factory in the Krafla area. In addition, a ridge has been levelled to direct lava away from the settlement. However, since this was completed no eruptions have occurred to test the effectiveness of the work. Barriers built in 1983 using bulldozers and trucks were successfully used to divert a lava flow from Etna away from a hotel and recreational area.

Another method tried to slow lava flows was to spray water onto their fronts to cool them. This was found to work for a short while on a small lava flow in Hawaii in 1960 that was sprayed by two fire hoses. In 1973 a larger flow from Heimaey (Iceland) was heading toward the town of Vestmannaeyjar and its harbour. After several pumps were assembled, around 100 L of water per second was sprayed onto the 500 m wide advancing flow front. After a short while the lava slowed down and piled up to about 20 m high. However, it continued to flow on either side of the sprayed area at the same speed. Pumping capacity was increased to 400 L/s and with addition of a special pumping ship up to 1200 L/s. In the 150 days that this operation continued 6.2 million m³ of sea water were sprayed onto the lava. Holes drilled into unsprayed parts of the lava after the eruption indicated a temperature of 500-700^oC at 5-8 m deep, whilst on sprayed areas this was reached only at 12-16 m deep. Whether or not the operation was worthwhile was debated. However, although the spraying cost around $1.6 million, if without it the port area would have been inundated, then it paid for itself many times over.