Characteristics of the 14 April 1999 Sydney hailstorm based on ground observations, weather radar, insurance data and emergency calls

International audienceHailstorms occur frequently in metropolitan Sydney, in the eastern Australian State of New South Wales, which is especially vulnerable due to its building exposure and geographical location. Hailstorms challenge disaster response agencies and pose a great risk for insurance companies. This study focuses on the Sydney hailstorm of 14 April 1999 ? Australia's most expensive insured natural disaster, with supporting information from two other storms. Comparisons are drawn between observed hailstone sizes, radar-derived reflectivity and damage data in the form of insurance claims and emergency calls. The "emergency response intensity" (defined by the number of emergency calls as a proportion of the total number of dwellings in a Census Collection District) is a useful new measure of the storm intensity or severity experienced. The area defined by a radar reflectivity ?55 dBZ appears to be a good approximation of the damage swath on ground. A preferred area for hail damage is located to the left side of storm paths and corresponds well with larger hailstone sizes. Merging hail cells appear to cause a substantially higher emergency response intensity, which also corresponds well to maximum hailstone sizes. A damage threshold could be identified for hailstone sizes around 2.5 cm (1 cm), based on the emergency response intensity (insurance claims). Emergency response intensity and claims costs both correlate positively with hailstone sizes. Higher claim costs also occurred in areas that experienced higher emergency response intensities

Event trees and epistemic uncertainty in long‐term volcanic hazard assessment of Rift Volcanoes: the example of Aluto (Central Ethiopia)

Aluto is a peralkaline rhyolitic caldera located in a highly populated area in central Ethiopia. Its postcaldera eruptive activity has mainly consisted of self‐similar, pumice‐cone‐building eruptions of varying size and vent location. These eruptions are explosive, generating hazardous phenomena that could impact proximal to distal areas from the vent. Volcanic hazard assessments in Ethiopia and the East African Rift are still limited in number. In this study, we develop an event tree model for Aluto volcano. The event tree is doubly useful: It facilitates the design of a conceptual model for the volcano and provides a framework to quantify volcanic hazard. We combine volcanological data from past and recent research at Aluto, and from a tool to objectively derive analog volcanoes (VOLCANS), to parameterize the event tree, including estimates of the substantial epistemic uncertainty. Results indicate that the probability of a silicic eruption in the next 50 years is highly uncertain, ranging from 2% to 35%. This epistemic uncertainty has a critical influence on event‐tree estimates for other volcanic events, like the probability of occurrence of pyroclastic density currents (PDCs) in the next 50 years. The 90% credible interval for the latter is 5–16%, considering only the epistemic uncertainty in conditional eruption size and PDC occurrence, but 2–23% when adding the epistemic uncertainty in the probability of eruption in 50 years. Despite some anticipated challenges, we envisage that our event tree could be translated to other rift volcanoes, making it an important tool to quantify volcanic hazard in Ethiopia and elsewhere

Communicating Information on Eruptions and Their Impacts from the Earliest Times Until the Late Twentieth Century

Volcanoes hold a fascination for human beings and, before they were recorded by literate observers, eruptions were portrayed in art, were recalled in legends and became incorporated into religious practices: being viewed as agents of punishment, bounty or intimidation depending upon their state of activity and the culture involved. In the Middle East the earliest depiction of an eruption is a wall painting dating from the Neolithic at Çatal Hüyük and the earliest record dates from the third millennium BCE. Knowledge of volcanoes increased over time. In some parts of the world knowledge of eruptions was passed down by oral transmission, but as far as written records were concerned, in the first century CE only 9 volcanoes in the Mediterranean region were recognised, together with Mount Cameroon in West Africa. In the next 1000 years the list grew by 17, some 14 of these volcanoes being in Japan. The first recorded eruptions in Indonesia occurred in 1000 and 1006, and volcanoes in newly settled Iceland increased the number to just 48 in 1380 CE. After this the list continued to increase, with important regions such as New Zealand and Hawaii only being added in the past 200 years. Only from 1900 did the rate of growth decline significantly (Simkin et al. 1981: 23; Simkin, 1993 Siebert et al. 2011; Simkin, 1993), but it is sobering to recall that in the twentieth century major eruptions have occurred from volcanoes that were considered inactive or extinct examples including: Mount Lamington - Papua New Guinea, 1951; Mount Arenal - Costa Rica, 1968 and Nyos - Cameroon, 1986. Although there are instances where the human impact of historical eruptions have been compiled - with examples including the 1883 eruption of Krakatau (Simkin and Fiske (1983) and 1943 -1952 eruption of Parícutin (Luhr and Simkin, 1993) - these are exceptions and there remains a significant gap in knowledge about both the short and long-term effects on societies of major eruptions which occurred before the 1980s. Following a broad review the chapter provides a discussion of the ways in which information has been collected, compiled and disseminated from the earliest times until the 1980s in two case study areas: the Azores Islands (Portugal) and southern Italy. In Italy information on eruptions stretches back to prehistoric times and has become progressively better known over more than 2,000 years of written history, yet even here there remain significant gaps in the record even for events that took place between 1900 and 1990. In contrast, located in the middle of the Atlantic, the Azores have been isolated for much of their history and illustrate the difficulties involved in using indigenous sources to compile, not only assessments of impact, but also at a more basic level a complete list of historical events with accurate dates

Settling-driven gravitational instabilities associated with volcanic clouds: new insights from experimental investigations

Downward propagating instabilities are often observed at the bottom of volcanic plumes and clouds. These instabilities generate fingers that enhance the sedimentation of fine ash. Despite their potential influence on tephra dispersal and deposition, their dynamics is not entirely understood, undermining the accuracy of volcanic ash transport and dispersal models. Here, we present new laboratory experiments that investigate the effects of particle size, composition and concentration on finger generation and dynamics. The experimental set-up consists of a Plexiglas tank equipped with a removable plastic sheet that separates two different layers. The lower layer is a solution of water and sugar, initially denser than the upper layer, which consists of water and particles. Particles in the experiments include glass beads as well as andesitic, rhyolitic and basaltic volcanic ash. During the experiments, we removed the horizontal plastic sheet separating the two fluids. Particles were illuminated with a laser and filmed with a HD camera; particle image velocimetry (PIV) is used to analyse finger dynamics. Results show that both the number and the downward advance speed of fingers increase with particle concentration in the upper layer, while finger speed increases with particle size but is independent of particle composition. An increase in particle concentration and turbulence is estimated to take place inside the fingers, which could promote aggregation in subaerial fallout events. Finally, finger number, finger speed and particle concentration were observed to decrease with time after the formation of fingers. A similar pattern could occur in volcanic clouds when the mass supply from the eruptive vent is reduced. Observed evolution of the experiments through time also indicates that there must be a threshold of fine ash concentration and mass eruption rate below which fingers do not form; this is also confirmed by field observations.Published395V. Dinamica dei processi eruttivi e post-eruttiviJCR Journa

Earthquakes, Volcanoes and God: Comparative Perspectives from Christianity and Islam

This paper asserts that both Christian and Islamic traditions of faith affect the ways in which people both try to make sense of, and respond to, disasters. This contention is supported by the results of empirical research, which demonstrates that differing Islamic and Christian perspectives on human suffering caused by disasters are neither as diverse, nor are they so intractable, as is commonly supposed. Today pastoral convergence between the two traditions may also be discerned, together with a general acceptance of the policies of both State agencies and Non-Governmental Organisations (NGOs) which are concerned with hazard relief and the propagation of policies of disaster risk reduction (DRR). Indeed some important disaster relief NGOs have emerged from Islamic and Christian faith communities and are supported by charitable donations

Preservation of thin tephras

INTRODUCTION Numerous observations attest to the rapid erosion from hillslopes of recently emplaced tephras. A survey of the literature on Soufrière (St Vincent, 1902), Rabaul (Papua New Guinea, 1937), Paricutin (Mexico, 1943-1945), Irazu (Costa Rica, 1963-64), Usu (Japan, 1977) and Mt St Helens (USA, 1980) together with occasional comments about other tephra-producing eruptions suggest the following conclusions: 1. Deep rills and gullies are quickly cut in fresh tephra especially where the development of an impermeable surface crust increases runoff volumes (Cilento, 1937, p47-8; Huggins, 1902, p20; Waldron, 1967, p11; Higashi et al., 1978; Kadomura et al., 1978; Lowdermilk and Bailey, 1946, p286; Collins et al., 1983). 2. Rates of rill and gully erosion are amongst the highest recorded anywhere while sediment concentrations in mudflows or secondary lahars may be as much as 65% by weight (Ollier and Brown, 1971; Waldron, 1967; Higashi et al., 1978). 3. Erosion of tephra is frequently proportional to slope steepness with extensive redeposition on valley floors. Rill erosion and shallow landsliding of tephra are important processes on steeper slopes. Topographic position is also important in determining how much tephra remains at a site (Anderson and Flett, 1903, p437; Segerstrom, 1950; 1960; Collins et al., 1983; Lehre et al., 1983). 4. The amount of vegetation remaining on tephra-mantled slopes influences erosion rates (Collins et al., 1983). Tephra erosion facilitates recovery of surviving vegetation (Lawrence and Ripple, 2000) and vegetation regrowth influences the retention of tephra (Segerstrom, 1950; 1960). 5. As much as one third to one half of the tephra may be removed from the slopes within one year or less of emplacement (Anderson and Flett, 1903, p453; Waldron, 1967, p11), though more detailed studies at Mt St Helens suggest only 11% of tephra was removed in the first year (Collins et al., 1983) and that erosion rates declined dramatically with time (Collins and Dunne, 1986). 6. The decline in erosion rate is not produced by revegetation but by increased infiltration capacity, decreased erodibility of the tephra exposed and the development of a stable rill network (Collins and Dunne, 1986). 7. In the long term, stability of the underlying substrate is an important influence on erosional removal of the tephra mantle (Blong and Pain, 1978). The examples on which the above conclusions are based do not always specify the thickness of the tephra mantle but observations were generally made close to the volcanoes where the tephra was at least 300 mm deep, and sometimes considerably deeper. On the other hand, erosional reworking and/or survival of thin (i.e., 10-300 mm) seems to have not been reported in any detail; although emplacement of thin tephras is usually less destructive of the vegetation cover it is not clear whether erosion of thin tephras is similarly rapid or whether preservation is ensured. Most studies of thin tephras relate to their use as chronostratigraphic marker beds and are commonly based on tephras preserved in lakes and/or swamp deposits. By and large, preservation of thin tephras in other situations is poorly documented. Nonetheless, thin tephras are of considerable value in geomorphic, geologic and archaeologic investigations as they form obvious marker horizons and cover large areas. The present contributions sets out observations on the preservation of thin tephras at four sites: near Mt Hagen and in the Western Finisterre Ranges, Papua New Guinea, on the slopes of Mt Rainier, Washington, USA, and on Kodiak Island, Alaska, USA. Each of the four studies was of only limited duration and detail but, collectively, the results provide considerable data on erosion and survival of thin tephras and the factors that influence their preservation