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Australian Government Emergency Management Australia Community Safety: Severe Storms CHARACTERISTICS OF SEVERE STORMS Severe storms are very localised events, not usually affecting wide areas as tropical cyclones and floods do, so their devastating impact is often under-estimated. These storms can occur anywhere in Australia, and do so much more frequently than any other major natural hazard. On average, each year, severe storms are responsible for more damage (as measured by insurance costs) than tropical cyclones, earthquakes, floods or bushfires. Unfortunately, storms also kill people; between five and ten deaths are caused by lightning strikes each year. Deaths also occur when strong winds cause tree limbs to fall; debris to become projectiles; and small boats in open water to capsize. In fact, although many people believe that tornadoes do not occur in Australia, 41 tornado-related deaths have been recorded here. The Bureau of Meteorology is responsible for provision of warnings of dangerous weather to the Australian community, with the aim of minimising injury and damage. The service is provided from Regional Forecasting Centres of the Bureau in State and Territory capitals, and information is transmitted to authorities such as Police, State and Territory Emergency Services, and to radio and television stations. Further information can be obtained from the Bureau of Meteorology web site WHICH STORMS ARE SEVERE STORMS? Severe storms can be divided into two types: severe thunderstorms; and land gales. Severe thunderstorms are more common, and generally more dangerous than land gales, which are described later. SEVERE THUNDERSTORMS They are defined by the Bureau of Meteorology, as those thunderstorms which produce either:
Most thunderstorms do not reach the level of intensity needed to produce these dangerous phenomena, but they all produce lightning, which can cause death and injury. Lightning and Thunder
Hail
Wind Gusts
Flash-Floods
Tornadoes
WHAT CAUSES SEVERE THUNDERSTORMS? Thunderstorms develop when dense cold air overlies warm, moist air, which is less dense. A 'trigger' such as solar heating, a front, or a trough, can then begin the development of a thunderstorm. Strong upward currents of air develop, and heat energy stored in the air and water vapour, is converted into wind and electrical energy. When the atmosphere is especially unstable, and the surrounding wind flow happens to be arranged to provide the most efficient input of energy to a growing cloud, a severe thunderstorm develops, with well-organised, complementary up- and down-draughts. WHERE AND WHEN DO SEVERE THUNDERSTORMS STRIKE? Severe thunderstorms can occur throughout the year, although they are very rare during the dry winter months in the north. Most strike from September to March, when the supply of solar energy is greatest, but severe winter storms linked to cold fronts, are common in the south west of WA. The geographical spread of severe thunderstorms in Australia, is difficult to determine, because of our low population density, and lack of observations over most of the continent. While records of storm impacts show that the most damaging storms have occurred in the populous south east quarter of the continent, analysis of wind, hail and tornado data, suggests that severe thunderstorms are a significant threat throughout the Country. The most damaging individual storms, have hit south eastern Queensland and the central NSW coast. LAND GALES Land gales are simply gale force winds (62 km/h) or stronger, over the land, and usually affect a much wider area, and last much longer than thunderstorms. Gales blow when large differences in atmospheric pressure are concentrated over a small distance. This can happen between 'deep' low pressure systems, and strong highs, or near intense cold fronts. In the southern half of Australia, extreme winds generally occur in winter and spring, and are usually due to land gales. In the tropical north, the strongest winds usually hit in summer and autumn, and are often due to tropical cyclones. PROTECTION AGAINST LIGHTNING STRIKES In statistical terms, lightning poses a greater threat to individuals, than almost any other natural hazard in Australia, accounting for five to ten lives, and well over 100 injuries annually. During thunderstorms the following precautions should be taken: OUTDOORS PROTECTION
INDOOR PROTECTION
FIRST-AID
SEVERE STORMS ACTION GUIDE Trim trees and branches well clear of your home. Check / secure roofing. Clear guttering and downpipes. Fit windows with shutters, or at least metal screens. Clear yard of loose materials that may blow about.
List your emergency phone numbers. Check your home insurance is current and adequate. If you live in a caravan, ask your State or Territory Emergency Service for a 'Protecting Caravans' pamphlet. Listen to local radio for storm 'Advice and Warnings'. Shelter and secure pets and animals. Put outdoor items, garden furniture, toys, etc, inside. Fill bins with water. Shelter vehicles, or cover with tied tarpaulins/blankets. Secure doors and windows, and draw curtains. Put valuables, medications, spare warm clothing in plastic bags with your emergency kit. Disconnect electrical items and outside TV/radio aerials. Stay inside. Shelter well clear of windows and skylights. If the building starts to break up, shelter in the strongest part, (cellar, internal room, hallway, or built-in wardrobe) under a mattress, doona, or a strong table or bench. Listen to your portable radio for storm updates. If outdoors, seek solid enclosed shelter (not a tree). If driving, stop clear of trees, powerlines and streams. Don't use a fixed telephone during a thunderstorm. Listen to local radio for official warnings and advice. Check for damaged windows, walls, or roof. Temporarily cover with plastic sheeting and nailed-on wooden strips. For help, see Emergency Assistance below. If you don't need help, check and / or help your neighbours. Beware of fallen powerlines, damaged buildings and trees, and flooded watercourses. Don't go sight-seeing. If unable to contact emergency services by phone: PROTECTING CARAVANS FROM HIGH WINDS From November to April each year, the Australian coast north from PERTH through DARWIN, and around to northern NEW SOUTH WALES, faces the threat of cyclone, with consequent casualties and destruction of property. Other types of storms, even tornadoes, can strike any part of Australia, at almost any time of the year. No area can be singled out as being safe from destructive winds. These notes are for the guidance of caravan users, in making safe vans and equipment during times of high winds. They apply especially to caravan users, in established caravan parks, but should be followed as closely as possible by those in other situations. If it is impossible or impracticable to provide the recommended ground anchors, lesser protection may be obtained by making anchors of buried logs, steel or concrete. THE DANGER TO CARAVANS Caravans are necessarily of lightweight construction, with generally flat sides, and will be endangered in high winds, unless special precautions are taken to reinforce their structure, to protect them from damage from flying debris, and to prevent them from being tossed off their supports. POSITIONING Placement with a narrow end facing the prevailing wind, will provide some degree of protection for your van. Caution: Proper positioning alone, cannot assure safety. Your van should be tied down securely; the parking brake applied; and the wheels chocked. Natural barriers, such as trees, provide worthwhile windbreaks. Even if your caravan is securely tied down, you should seek other shelter, when warned of approaching high winds. In some areas, flooding is associated with storms, and you should consider this when positioning your van. MAKE SAFE IN STAGES The precautions suggested in these guidelines should be taken in three stages. However, it is important to realise, that even if the suggested precautions are taken, it is not possible to cyclone-proof a caravan, to make it a cyclone shelter. Consequently, broadcast instructions for the public to occupy shelters should be heeded immediately. The first stage is chassis tie-down, which should be done as a matter of normal course, even when temporarily located in high wind areas (principally cyclone areas). This is to save time later. Stage 1 and Stage 2, must both be carried out to make the van safe. One stage without the other, is not sufficient. The second and third stages, which are roof tie-down, and equipment make-safe, should be done when a high wind warning or tropical cyclone warning is issued by the Bureau of Meteorology. STAGE 1 - CHASSIS TIE-DOWN The chassis tie-down is designed to prevent the van from being blown off its supports. Established caravan parks in high wind areas usually provide ground anchor points for chassis tie-down. In these cases, use either short lengths of strong chain and turnbuckles, or sound rope of at least 9.5 mm (3/8 in) diameter, to secure the chassis to the ground anchor points. When these points are not provided, the suggested tie-down system, comprises a nylon rope, anchored at ground level, and attached to the van chassis. Before starting work on the tie-down, ensure that the van supports are sound and stable. The effectiveness of the tie-down will be lost, if the van supports fail during high winds. The steel pin type anchor, is suitable for most installations. The pin should be angled to the vertical, and driven into position, preferably by a jack-hammer with an adaptor on the driving ram. After driving the pin, weld to top of pin, a bar or loop, made from 8 mm m.s rod. Where rock is encountered, pre-drill a 25 mm (1 in) diam. hole 600 mm (24 in) (plus any earth cover) into the rock, and drive a star picket to full depth of hole. Enquiries should be made, and care taken, to avoid damaging underground pipes or cables, or other facilities, when driving pickets into the ground. When completed, the chassis tie-down will not interfere with use of the annex, but in most cases, the annex will need to be removed during the tie-down installation. STAGE 2 - ROOF TIE-DOWN The roof tie-down keeps the van from overturning, or swaying off its chassis. Because of the interference caused to the use of an annex, installation of the roof tie-down, can be left until a high wind or tropical cyclone warning is issued. The first step in roof tie-down, is to remove and roll up the annex for stowage. This should be done immediately a high wind or a tropical cyclone warning is broadcast. The most effective roof tie-down, is a net firmly fixed at ground level, and passing over the van roof, for the full length of the van. This also affords protection against flying debris. The nylon rope should be tensioned as much as possible by hand. The corners of the caravan, and the ropes, should be protected from damage from each other, by the use of metal angle pieces, or some other form of padding. STAGE 3 - EQUIPMENT MAKE-SAFE After a high wind or tropical cyclone warning has been broadcast, and the Stage 1 and Stage 2 tie-down of the van has been completed, as much equipment as possible should be stowed inside the van. This includes the annex, awnings, and other items such as lawnmowers, bicycles, toys, chairs, garbage bins, and the countless small household articles which seem to accumulate. Heavy or bulky items that cannot be stowed in the van, should be laid on their side, tied together with strong rope, and lashed to suitable anchor points. Before leaving the site to seek shelter in a place protected from wind-borne objects:
Folding caravans with canvas sides, would need to be closed down, before the roof tie-down is tightened. If alternative shelter is not available, go to one of the designated high wind or cyclone shelters in your area. ROPE USE IN A CARAVAN TIE-DOWN STAGE 1: Chassis tie-down. All rope should be 8 kn.* (1800 lb) breaking strain (or stronger) nylon rope (3/8 in or 9.5 mm diameter). All rope should span no more than two spans (ie. from ground picket to chassis member, to ground picket). Rope should be tied down loosely with secure knots to ground pickets and chassis members, and then tightened to a 'twanging' tightness with 12 mm. (1/2 in) diameter 300 mm (12 in) long steel rods or timber dowel used as a twitch. Once the rope is tightened, the twitching rod should be tied along the tightened rope with twine, to prevent the twitch unwinding. Do not overtighten. Where rope cannot be passed around a chassis member, provide loops made from 8 mm. dia.m.s. rod, secured to chassis with a minimum of 50 mm. long x 6 mm fillet weld to each leg of loop. This system is recommended by the Darwin Reconstruction Commission, and the Australian Department of Housing & Construction. No guarantee can be given for the structural sufficiency of individual caravans. STAGE 2: Roof tie-down. All rope should be 8 kn.* (1800 lb) breaking strain nylon rope. Tie-down caravan with rope, with knots at each ground picket. Hand tighten ropes only - twitching is not required. Do not overtighten. Tie all caravan roof ropes together, with ropes running horizontally at the junction of the caravan roof and side walls, and knotted to each roof rope. This can be used to tighten roof ropes (if loose) but the primary purpose is to keep the roof ropes in position. * A useful conversion factor when converting from Imperial to metric Breaking Force is 0.1 Ton F = 1 kN (Kilo Newton) and for converting Breaking Force expressed in kN into Breaking Force expressed in kg is 1 kN = 100 kg. ****************************** This Community Support Website is |
