Description of natural hazards

Flood means the temporary covering by water of land not normally covered by water. It occurs when a precipitation event is significantly above a normal event in duration and/or intensity, or intensive snowmelt occurs. Both processes can also overlap (European Flood Directive). Floods lead to damage to the building (construction), in cellars and underground garages, but can also directly affect motor vehicles and people. Objects transported in the water (such as trees) can also cause damage.

Debris flows are displacement processes of sediment, water and often also woody debris in torrent catchment areas, representing a considerable hazard potential in populated regions. Together with rapid velocities and high densities, debris flows are characterised by a high destruction potential for infrastructural facilities and buildings (ExtremA, 2019).

In heavy precipitation events, large amounts of precipitation fall within a relatively short period of time. Heavy precipitation can be divided into large-scale and small-scale events. Large-scale heavy precipitation events can occur throughout the year and are triggered by low-pressure areas or damming effects on mountains. Small-scale heavy precipitation is triggered by convection, i.e. precipitation in the form of showers and thunderstorms. The latter mainly occur in the summer half-year (ZAMG, 2020).

Landslides are fractured and/or fracture-free displacements of solid rock (= rock) and/or loose rock (= clays, sands, gravels etc. as well as their mixtures), that are directed downhill under the effect of gravity. They are very complex in their formation. They are rarely triggered by a single cause. They occur when the balance of forces in the slope is unfavourably changed by restraining and driving forces as a result of physical processes (Suda & Rudolf-Miklau, Bauen und Naturgefahren, 2012).

Rockfall is the falling, jumping, sliding or rolling of individual, independent rock fragments (stones and blocks) (ExtremA, 2019). Trigger mechanisms include freeze/thaw cycles, precipitation, earthquakes, progressive weathering, etc. Often, the forest also plays a crucial role in triggering rockfall. The roots and constant leverage movements of trees cause the rocks to loosen. On the other hand, the forest exerts a significant braking effect on rockfalls (collision of stones with trunks). Large stones or boulders can break through house walls and roofs. If stones fall on roads or tracks, this is not only dangerous for travellers, it can also lead to closures and thus considerable detours (Suda & Rudolf-Miklau, Building and natural hazards, 2012).

An avalanche is defined as masses of snow which, when falling rapidly on steep slopes, can cause hazards or damage due to their kinetic energy, the air pressure wave they causes or due to their deposition (ExtremA, 2019). An avalanche can destroy concrete buildings and lead to considerable disruption on traffic routes (barriers, metre-high deposits, etc.) (Suda & Rudolf-Miklau, Bauen und Naturgefahren, 2012).

On heat days the daily maximum temperature exceeds 30 °C. These days have increased considerably on average throughout Austria in recent decades. The increase is most significant in the south-east of Austria. Tropical nights, when the temperature does not drop below 20 °C, are also problematic for human well-being (ZAMG).

Drought is defined by water deficit, which is mainly caused by a lack of precipitation or an excess of evaporation. Drought causes significant restrictions for agriculture, forestry or water supply, and extreme situations can occur if critical values are exceeded. Drought is therefore itself referred to as a natural hazard, but at the same time drought also forms the basis for other natural hazards such as forest fires, crop failures or pest infestations (ExtremA, 2019).

A forest fire is any uncontrolled fire that at least partially affects forest or forest soil, regardless of the fire type (smouldering fire, ground fire, crown fire), the cause, the vegetation type (also grass fire underneath a high forest, fire on clear cut area or in the windbreak belt) or the fire area (for example, also rootstock fire or fire of a single tree caused by lightning). Mixed forms and unclear cases are recorded as forest / wildland fires. https://fireblog.boku.ac.at/ If appropriate suppression measures fail, forest fires quickly develop into wildfires (Suda & Rudolf-Miklau, Building and natural hazards, 2012).

A storm is defined as flow velocity of the air relative to the earth's surface exceeds 75 km/h or 9 Beaufort. Storms occur when there are considerable pressure differences over a relatively short distance, conditions that are particularly prevalent in strongly developed low-pressure systems (ExtremA, 2019).

Hail is precipitation in the form of ice grains at least 5 mm in diameter, which may be clearly or milky, spherically or irregularly shaped. Hail is formed in convective clouds when solid precipitation particles are held in suspension by upwinds or repeatedly lifted into the air, gaining mass by freezing supercooled water droplets (ExtremA, 2019).

In nature, lightning strikes are a spark discharge or a temporary electric arc between clouds or between clouds and the earth. Lightning strikes occur during a thunderstorm as a result of electrostatic charging of the cloud-forming water droplets or raindrops. They can also - depending on the polarity of the electrostatic charge – origin from the earth. Lightning is a equalisation of electric potential within a cloud (cloud lightning) or between the ground and the lower part of the cloud (earth lightning) (Suda & Rudolf-Miklau, Building and natural hazards, 2012).

Snow load is the vertical force exerted by the stationary snow cover due to its own weight. Long-lasting heavy snowfalls, high specific weight of the snow, wind-induced snow transport (formation of cornices) as well as strong water saturation of the snow cover due to rain lead to extreme loads on buildings (roofs) (Suda & Rudolf-Miklau, Bauen und Naturgefahren, 2012). However, high-voltage power lines, solar panels or wind turbines, as well as trees, can also be affected by this natural hazard. A 50-year event has become established as a reference value (ExtremA, 2019).

Ice loads result from the accumulation of icing on various structures. A distinction is made between "precipitation icing" and "fog icing" (ExtremA, 2019).

Late frost is a frost event in spring when many plants are already in full bloom. The frost can lead to the death of young shoot tips. Countermeasures include frost irrigation, smoking, frost candles or winding.

Pest calamities are diseases of forest stocks due to the mass reproduction of pests such as bark beetles or larvae.

Invasive species are alien plants or animals that have undesirable effects on other species, biotic communities or biotopes. They compete with native species for habitat and resources and displace them.