**2.1 Definitions and approaches**

Flash floods (synonym: storm-driven floods) can be defined from various aspects: as hydrometeorological phenomena, natural hazards or geomorphic agents. Inundations can be referred to four basic classes: riverine floods, excess water (from rising groundwater table), coastal floods and flash floods (Lóczy, 2010). Although riverine floods along major rivers remain to be the most severe natural hazard which threaten to inflict serious damage to human life and property, recently the latter classes have also attracted more attention in scientific circles.

Flash Flood Hazards 29

flash flood hazard assessment should be substantially different from that applied for modelling inundations along large rivers, in coastal areas or in valleys and lowlands due to

Flash floods are often associated with other natural hazards. Then their damage is partly due to the fact that they often trigger debris flows, i.e. hyperconcentrated flows, where the proportion of sediment load surpasses that of water discharge (Iverson, 1997). The particle size of the sediment swept at rates up to 20 m s-1 – confined within narrow valleys or erosion gullies – may range from clay to large blocks. The rainfall that produces flash floods often saturates entire hillslopes and subsequently may induce extensive slumps. Through blocking valleys and impounding stream flow, the slumps create suitable conditions for the

Disastrous flash flood events can be cited from almost all continents. The majority of the 'classic', best documented events have been reported from the United States. Every year, riverine, coastal and flash floods are responsible for more fatalities than any other meteorological phenomenon in the Unites States. On the 30-year average, flood-related death toll totals 120 fatalities annually. From 1996 through 2003, 3000 flash flood events were documented in a year on average (Collier, 2007). Although some authors note that remarkable progress in research and warning have been made in the US and in some other countries (e.g. in the UK), flash floods are still among the most dangerous natural phenomena worldwide (Davis, 2001). Detailed documentation is available since the 1970s. In 1972 alone two disastrous floods were recorded in the US: 125 people were killed in Buffalo Creek, West Virginia, as a consequence of the failure of a coal-waste dam (Davies et al., 1972) and 238 people in Rapid City, South Dakota, where 380 mm rain fell within 6 hours (Davis, 2001). One of the best documented flash floods of all time occurred in the Thompson Canyon, Colorado, a small watershed (181 km2) drained by one of the tributaries of the Colorado River. In 1976, 350 mm rain fell in less than six hours, flooding the narrow canyon floor (Caracena et al., 1979) and when the water level rose suddenly and unexpectedly by 6.5 m (Davis, 2001). 145 people were killed, 418 houses destroyed and 138 damaged. Total

A flash flood (well studied and even documented in videos, now on YouTube) took place in England, on the Cornwall Peninsula, in Boscastle, on August 16, 2004 (Golding, 2005). It provided an opportunity for the application of the land surface model called MOSES-PDM (Met Office Surface Exchange Scheme incorporating the Probability Distributed Model) to portray the evolution of soil moisture conditions from meteorological information (radar rainfall and satellite cloud observations). The entire rainfall event lasted for only about seven hours, but was very localized. The total 24-hour cumulative rainfall reached 200.4 mm at one location (Otterham) (Golding, 2005). In places upstream from Boscastle rainfall intensity reached 24 mm within 15 minutes, while in Boscastle 89 mm rain fell in an hour (Golding, 2005). The probability of such a high-intensity rainfall in Boscastle, at least according to the available statistical data, is 1 to 1,300. Antecedent high precipitation also influenced soil moisture and runoff on the higher portions of the catchment. The intense rainfall was followed by a 2-metre rise in the water level of the local Valency Stream, the discharge of which reached 180 m3 s-1, a value of an estimated return time of 400 years

elevated groundwater levels (Czigány et al. 2011b – see also below).

next flood.

**2.2 Flash floods in the world** 

material damage amounted to USD 40 million.

From a hydrometeorological aspect, flash floods are best described as events involving "too much water in too little time" (Grundfest & Ripps, 2000). This means that exceptionally high amounts of rainfall, combined with very efficient and rapid runoff on relatively small catchments, are typical of flash floods. A flash flood immediately follows the inducing storm event. The term 'flash' itself indicates a sudden rapid hydrological response of a usually small catchment, where water levels may rise to their maximum within minutes or a few hours after the onset of the rain event. Flash floods are highly localized in space: they are restricted to basins of a few hundred square kilometres or less. They are also restricted in time: response times not exceeding a few hours or are even less. Therefore, extremely short time is left for warning (Georgakakos, 1987, 2006; Collier, 2007; Carpenter et al. 1999).

It is often emphasized that heavy rainfall is a necessary but not sufficient condition for inducing flash floods. Since the entire physical environment influences their origin, flash floods are proper subjects for physical geographical investigations (Czigány et al., 2008). For instance, soil moisture conditions prior to the rainfall events are major hydrological controls of flash flood generation (Norbiato et al., 2008; Czigány et al., 2010). It is only with knowledge on the topography, soils and human impact on the catchment (steep slopes, drainage density, impermeable surfaces, saturated soils and land use) that the flood/no flood threshold can be established with some precision. Anthropogenic influences are important because some basins respond particularly rapidly to intense rainfall in the wake of disturbances in the natural drainage (stream channelization, deforestation, housing development, fire etc.) (Norbiato et al., 2008). As hydrometeorological phenomena, flash floods are best characterized by their magnitude (total amount and intensity of inducing rainfall), return interval, total runoff and similar parameters.

As geomorphological phenomena flash floods are short-duration events caused by an abrupt rise in the discharge of a river or stream, which may have remarkable geomorphic impacts through erosion and sedimentation (Reid, 2004). Previously, some geomorphologists restricted this concept to the ephemeral streams of arid and semiarid areas (Reid et al., 1994), but now the view is more excepted that the 'flashy' flood hydrographs of subtropical seasonal climates and even of humid temperate regions can also be covered in the flash flood category. There may be, however, significant differences in runoff generation and geomorphic consequences (Bull & Kirkby, 2002). The geomorphic consequences of flash floods are usually judged from the stream flood hydrograph, sediment load transported and sediment accumulation.

Flash floods are naturally not novel phenomena, the frequency of their occurrence, however, shows an increasing tendency. Until some recent disasters, flash floods have not been so intensively studied as conventional large riverine floods. In some particularly affected countries (e.g. in the United States and the United Kingdom), however, their research dates back to the 1970s and 80s (e.g. Grundfest, 1977, 1987; Georgakakos, 1987; Schmittner & Giresse, 1996; Carpentier et al., 1999; Pontrelli et al., 1999).

In the case of a sophisticated hydrological approach, in addition to precipitation, several environmental factors are also to be considered in flash flood modelling as boundary conditions. Soil characteristics (actual moisture content, permeability, ground surface alterations and vertical soil profile) influence runoff production and help define flash flood prone areas. Various catchment characteristics (e.g. size, shape, slope, land cover) also affect runoff and the potential occurrence of flash floods. Consequently, the approach towards

From a hydrometeorological aspect, flash floods are best described as events involving "too much water in too little time" (Grundfest & Ripps, 2000). This means that exceptionally high amounts of rainfall, combined with very efficient and rapid runoff on relatively small catchments, are typical of flash floods. A flash flood immediately follows the inducing storm event. The term 'flash' itself indicates a sudden rapid hydrological response of a usually small catchment, where water levels may rise to their maximum within minutes or a few hours after the onset of the rain event. Flash floods are highly localized in space: they are restricted to basins of a few hundred square kilometres or less. They are also restricted in time: response times not exceeding a few hours or are even less. Therefore, extremely short time is left for warning (Georgakakos, 1987, 2006; Collier, 2007; Carpenter et al. 1999).

It is often emphasized that heavy rainfall is a necessary but not sufficient condition for inducing flash floods. Since the entire physical environment influences their origin, flash floods are proper subjects for physical geographical investigations (Czigány et al., 2008). For instance, soil moisture conditions prior to the rainfall events are major hydrological controls of flash flood generation (Norbiato et al., 2008; Czigány et al., 2010). It is only with knowledge on the topography, soils and human impact on the catchment (steep slopes, drainage density, impermeable surfaces, saturated soils and land use) that the flood/no flood threshold can be established with some precision. Anthropogenic influences are important because some basins respond particularly rapidly to intense rainfall in the wake of disturbances in the natural drainage (stream channelization, deforestation, housing development, fire etc.) (Norbiato et al., 2008). As hydrometeorological phenomena, flash floods are best characterized by their magnitude (total amount and intensity of inducing

As geomorphological phenomena flash floods are short-duration events caused by an abrupt rise in the discharge of a river or stream, which may have remarkable geomorphic impacts through erosion and sedimentation (Reid, 2004). Previously, some geomorphologists restricted this concept to the ephemeral streams of arid and semiarid areas (Reid et al., 1994), but now the view is more excepted that the 'flashy' flood hydrographs of subtropical seasonal climates and even of humid temperate regions can also be covered in the flash flood category. There may be, however, significant differences in runoff generation and geomorphic consequences (Bull & Kirkby, 2002). The geomorphic consequences of flash floods are usually judged from the stream flood hydrograph,

Flash floods are naturally not novel phenomena, the frequency of their occurrence, however, shows an increasing tendency. Until some recent disasters, flash floods have not been so intensively studied as conventional large riverine floods. In some particularly affected countries (e.g. in the United States and the United Kingdom), however, their research dates back to the 1970s and 80s (e.g. Grundfest, 1977, 1987; Georgakakos, 1987; Schmittner &

In the case of a sophisticated hydrological approach, in addition to precipitation, several environmental factors are also to be considered in flash flood modelling as boundary conditions. Soil characteristics (actual moisture content, permeability, ground surface alterations and vertical soil profile) influence runoff production and help define flash flood prone areas. Various catchment characteristics (e.g. size, shape, slope, land cover) also affect runoff and the potential occurrence of flash floods. Consequently, the approach towards

rainfall), return interval, total runoff and similar parameters.

sediment load transported and sediment accumulation.

Giresse, 1996; Carpentier et al., 1999; Pontrelli et al., 1999).

flash flood hazard assessment should be substantially different from that applied for modelling inundations along large rivers, in coastal areas or in valleys and lowlands due to elevated groundwater levels (Czigány et al. 2011b – see also below).

Flash floods are often associated with other natural hazards. Then their damage is partly due to the fact that they often trigger debris flows, i.e. hyperconcentrated flows, where the proportion of sediment load surpasses that of water discharge (Iverson, 1997). The particle size of the sediment swept at rates up to 20 m s-1 – confined within narrow valleys or erosion gullies – may range from clay to large blocks. The rainfall that produces flash floods often saturates entire hillslopes and subsequently may induce extensive slumps. Through blocking valleys and impounding stream flow, the slumps create suitable conditions for the next flood.

#### **2.2 Flash floods in the world**

Disastrous flash flood events can be cited from almost all continents. The majority of the 'classic', best documented events have been reported from the United States. Every year, riverine, coastal and flash floods are responsible for more fatalities than any other meteorological phenomenon in the Unites States. On the 30-year average, flood-related death toll totals 120 fatalities annually. From 1996 through 2003, 3000 flash flood events were documented in a year on average (Collier, 2007). Although some authors note that remarkable progress in research and warning have been made in the US and in some other countries (e.g. in the UK), flash floods are still among the most dangerous natural phenomena worldwide (Davis, 2001). Detailed documentation is available since the 1970s. In 1972 alone two disastrous floods were recorded in the US: 125 people were killed in Buffalo Creek, West Virginia, as a consequence of the failure of a coal-waste dam (Davies et al., 1972) and 238 people in Rapid City, South Dakota, where 380 mm rain fell within 6 hours (Davis, 2001). One of the best documented flash floods of all time occurred in the Thompson Canyon, Colorado, a small watershed (181 km2) drained by one of the tributaries of the Colorado River. In 1976, 350 mm rain fell in less than six hours, flooding the narrow canyon floor (Caracena et al., 1979) and when the water level rose suddenly and unexpectedly by 6.5 m (Davis, 2001). 145 people were killed, 418 houses destroyed and 138 damaged. Total material damage amounted to USD 40 million.

A flash flood (well studied and even documented in videos, now on YouTube) took place in England, on the Cornwall Peninsula, in Boscastle, on August 16, 2004 (Golding, 2005). It provided an opportunity for the application of the land surface model called MOSES-PDM (Met Office Surface Exchange Scheme incorporating the Probability Distributed Model) to portray the evolution of soil moisture conditions from meteorological information (radar rainfall and satellite cloud observations). The entire rainfall event lasted for only about seven hours, but was very localized. The total 24-hour cumulative rainfall reached 200.4 mm at one location (Otterham) (Golding, 2005). In places upstream from Boscastle rainfall intensity reached 24 mm within 15 minutes, while in Boscastle 89 mm rain fell in an hour (Golding, 2005). The probability of such a high-intensity rainfall in Boscastle, at least according to the available statistical data, is 1 to 1,300. Antecedent high precipitation also influenced soil moisture and runoff on the higher portions of the catchment. The intense rainfall was followed by a 2-metre rise in the water level of the local Valency Stream, the discharge of which reached 180 m3 s-1, a value of an estimated return time of 400 years

Flash Flood Hazards 31

disaster also here. The steep slopes of Blue Mountain ridge enhanced and localized convection over the region. In addition to antecedent rainfall, land-use practices (e.g. floodplain

Some examples of flash floods can also be cited from a continent infamous of rather extreme spatial and temporal variations in weather conditions, Australia. The most frequent cause of flash flooding is slow-moving thunderstorms. These systems, related to the El Niño– Southern Oscillaton (ENSO) circulation pattern, can involve strong updrafts of air which suspend huge amounts of rain before releasing a deluge onto the ground (Allan, 1993). Water in creeks, drains and natural watercourses can rise at dangerous rates. On the evening of 26 January 1971, seven people died in Canberra as flash flood waters from a nearby thunderstorm flooded roadways near a drainage channel. It was estimated that around 95 mm of rain fell in one hour during the event. On another occasion, in Sydney on 7 November 1984, 127 mm fell in one hour leading to damage of around AUD 128 million (in July 1996 terms) (Australian Government, Bureau of Meteorology [AG BoM]). In the drier ('outback') regions of Australia flash floods are more common, but – the vulnerability being

Flash floods are increasingly observed in urban areas, where the surface is unable to absorb large amounts of water in a short period. In urban areas the hazard is exacerbated by various – and not exclusively physical – contributing factors and vulnerability is significantly higher – often because of the sudden rise of water levels. For instance, many cities of Latin America show uncontrolled and disorganized urban growth (Stevaux & Latrubesse, 2010) with infrastructure and production systems (railways, roads, plants) concentrated in densely populated valleys. Disasters are produced by a combination of tropical storms inducing flash floods and landslides and urban occupation of the valleys. It is often emphasized that the increased proportion of impervious urban surfaces and the limited drainage capacity are responsible for flooding. A storm on December 14–16, 1999, caused catastrophic landslides and flooding along a 40-km coastal strip north of Caracas, in the coastal state of Vargas, Venezuela with its extremely steep and rugged topography (mountains 2700 m high within about 6–10 km of the coast) (Brandes, 2000). The rivers and streams of this mountainous region drain to the north and emerge from steep canyons onto alluvial fans before emptying into the Caribbean Sea. Damage to communities and infrastructure was so serious because here little flat area is available for development with the exception of the alluvial fans. In Vargas state probably almost 50,000 people were killed, more than 8000 individual residences, and 700 apartment buildings were destroyed or damaged and total economic losses are estimated at USD 1.79 billion (Wieczorek et al., 2001). On average, at least one or two major flash-flood or landslide events per century

have been recorded in this region since Spanish occupation in the 17th century.

Until very recently flood hazard research in Hungary had focused on riverine floods, particularly those along the two major rivers, the Danube and its main tributary, the Tisza (Lóczy & Juhász, 1996; Lóczy, 2010). There are relatively few papers published on flash flood events in Hungary (Gyenizse & Vass, 1998; Fábián et al., 2009). In the mainly lowland and hill environments of Hungary flash floods do not appear a major hazard. Another reason is the lack of appropriate monitoring systems in the flash flood affected catchments (Vass, 1997). In the wake of the events of the first decade in the 21st century, however, flash

encroachment) also increase flood hazards.

lower – their documentation is not so good.

**2.3 Flash floods in Hungary** 

(Bettess, 2005). During the Boscastle flash flood event, 100 residential homes were destroyed and 75 cars were swept to the sea. Due to the efficient assistance of the available rescue teams, no fatalities happened. This rare event resulted from a combination of hydrometeorological factors (Golding, 2005): unusually high rainfall efficiency (relative to the moisture content of the inflowing air) and the exceptionally long stay of intense storms over the same catchment.

For similar reasons, mountain environments in the Mediterranean are also seriously threatened by flash flooding (e.g. Borga et al., 2007). In the Aragonian Pyrenees, the catchment of the Barranco de Áras stream (only 19 km2 in area) was affected by enduring (5 hour) rainfall with 500 mm h-1 peak intensity and an estimated 243 mm total amount on 6 August 1996. In the Biescas camp-site 87 people died because flash flooding of 600 m s-1 discharge was combined with a debris flow transporting 68,000 m3 debris (Gutiérrez et al., 1998). The tragic underestimation of the capacity of check dams by engineers had also contributed to the disaster. In the French Côte d'Azur, in Draguignan (Var *département*) the 10 June 2010 flash flood killed 37 people in the town and its neighbourhood, caused blackouts and cut away the village from the world (Telegraph, 2010). It was triggered by a huge cloudburst (350 mm within 20 hours), unobserved in the area since 1827. (For a more complete overview of flash floods in Europe see Gaume et al., 2009.)

As it has been mentioned, however, arid and semiarid regions are the most favoured environments for flash flood generation (Reid et al., 1994). According to research in Israel (Cohen & Laronne, 2005), for instance, flash floods of arid regions involve both bedload and suspended sediment concentrations much higher than in the perennial rivers of humid environments. In arid or desert regions storms cut arroyos (intermittent gullies with flat floors and vertical walls). Flash flooding in an arroyo can occur in less than a minute, with enough power to wash away sections of pavement, large boulders, cars and even houses. Although the sediment yield of individual events is large, fortunately, flood events rarely occur and mean annual sediment yields remain low in arid environments (Graf, 2002).

The prediction of heavy rainfall and ensuing flash floods is particularly challenging in the tropical belt, especially on islands with intense, localized and mostly convective rainfalls (e.g. Kodama & Barnes, 1977). Devastating events have been reported from various Caribbean islands (Laing, 2004). During an El Niño winter, on 5–6 January 1992, heavy rainfall produced flash floods in Puerto Rico and caused 23 deaths and 88 million U.S. dollars in damage (National Oceanic and Atmospheric Administration, National Weather Service [NOAA NWS] 1992). At a few stations the amount of the rainfall, associated with a quasi-stationary front at the surface and an upper-level trough, was up to 500 mm (Laing, 2004).

High relief often generates heavy rainfall through orographic lift or by creating persistent low-level convergence which induces new convection (Weston & Roy, 1994). Such hydrometeorological and other environmental conditions were associated with another deadly flash flood, too, that occurred in Jamaica on 3–4 January 1998. The northeastern region was affected by heavy rainfall, which induced both flash floods and mudslides which caused five deaths and more than nine million U.S. dollars in damage to property, agriculture, and infrastructure (Laing, 2004). The situation was aggravated by antecedent rainfall from a strong cold front, currents of moist air masses at lower and higher topographic levels. Similarly to the Puerto Rico event, orographic lifting contributed to the

(Bettess, 2005). During the Boscastle flash flood event, 100 residential homes were destroyed and 75 cars were swept to the sea. Due to the efficient assistance of the available rescue teams, no fatalities happened. This rare event resulted from a combination of hydrometeorological factors (Golding, 2005): unusually high rainfall efficiency (relative to the moisture content of the inflowing air) and the exceptionally long stay of intense storms

For similar reasons, mountain environments in the Mediterranean are also seriously threatened by flash flooding (e.g. Borga et al., 2007). In the Aragonian Pyrenees, the catchment of the Barranco de Áras stream (only 19 km2 in area) was affected by enduring (5 hour) rainfall with 500 mm h-1 peak intensity and an estimated 243 mm total amount on 6 August 1996. In the Biescas camp-site 87 people died because flash flooding of 600 m s-1 discharge was combined with a debris flow transporting 68,000 m3 debris (Gutiérrez et al., 1998). The tragic underestimation of the capacity of check dams by engineers had also contributed to the disaster. In the French Côte d'Azur, in Draguignan (Var *département*) the 10 June 2010 flash flood killed 37 people in the town and its neighbourhood, caused blackouts and cut away the village from the world (Telegraph, 2010). It was triggered by a huge cloudburst (350 mm within 20 hours), unobserved in the area since 1827. (For a more

As it has been mentioned, however, arid and semiarid regions are the most favoured environments for flash flood generation (Reid et al., 1994). According to research in Israel (Cohen & Laronne, 2005), for instance, flash floods of arid regions involve both bedload and suspended sediment concentrations much higher than in the perennial rivers of humid environments. In arid or desert regions storms cut arroyos (intermittent gullies with flat floors and vertical walls). Flash flooding in an arroyo can occur in less than a minute, with enough power to wash away sections of pavement, large boulders, cars and even houses. Although the sediment yield of individual events is large, fortunately, flood events rarely occur and mean annual sediment yields remain low in arid environments (Graf, 2002).

The prediction of heavy rainfall and ensuing flash floods is particularly challenging in the tropical belt, especially on islands with intense, localized and mostly convective rainfalls (e.g. Kodama & Barnes, 1977). Devastating events have been reported from various Caribbean islands (Laing, 2004). During an El Niño winter, on 5–6 January 1992, heavy rainfall produced flash floods in Puerto Rico and caused 23 deaths and 88 million U.S. dollars in damage (National Oceanic and Atmospheric Administration, National Weather Service [NOAA NWS] 1992). At a few stations the amount of the rainfall, associated with a quasi-stationary front at the surface and an upper-level trough, was up to 500 mm (Laing,

High relief often generates heavy rainfall through orographic lift or by creating persistent low-level convergence which induces new convection (Weston & Roy, 1994). Such hydrometeorological and other environmental conditions were associated with another deadly flash flood, too, that occurred in Jamaica on 3–4 January 1998. The northeastern region was affected by heavy rainfall, which induced both flash floods and mudslides which caused five deaths and more than nine million U.S. dollars in damage to property, agriculture, and infrastructure (Laing, 2004). The situation was aggravated by antecedent rainfall from a strong cold front, currents of moist air masses at lower and higher topographic levels. Similarly to the Puerto Rico event, orographic lifting contributed to the

complete overview of flash floods in Europe see Gaume et al., 2009.)

over the same catchment.

2004).

disaster also here. The steep slopes of Blue Mountain ridge enhanced and localized convection over the region. In addition to antecedent rainfall, land-use practices (e.g. floodplain encroachment) also increase flood hazards.

Some examples of flash floods can also be cited from a continent infamous of rather extreme spatial and temporal variations in weather conditions, Australia. The most frequent cause of flash flooding is slow-moving thunderstorms. These systems, related to the El Niño– Southern Oscillaton (ENSO) circulation pattern, can involve strong updrafts of air which suspend huge amounts of rain before releasing a deluge onto the ground (Allan, 1993). Water in creeks, drains and natural watercourses can rise at dangerous rates. On the evening of 26 January 1971, seven people died in Canberra as flash flood waters from a nearby thunderstorm flooded roadways near a drainage channel. It was estimated that around 95 mm of rain fell in one hour during the event. On another occasion, in Sydney on 7 November 1984, 127 mm fell in one hour leading to damage of around AUD 128 million (in July 1996 terms) (Australian Government, Bureau of Meteorology [AG BoM]). In the drier ('outback') regions of Australia flash floods are more common, but – the vulnerability being lower – their documentation is not so good.

Flash floods are increasingly observed in urban areas, where the surface is unable to absorb large amounts of water in a short period. In urban areas the hazard is exacerbated by various – and not exclusively physical – contributing factors and vulnerability is significantly higher – often because of the sudden rise of water levels. For instance, many cities of Latin America show uncontrolled and disorganized urban growth (Stevaux & Latrubesse, 2010) with infrastructure and production systems (railways, roads, plants) concentrated in densely populated valleys. Disasters are produced by a combination of tropical storms inducing flash floods and landslides and urban occupation of the valleys. It is often emphasized that the increased proportion of impervious urban surfaces and the limited drainage capacity are responsible for flooding. A storm on December 14–16, 1999, caused catastrophic landslides and flooding along a 40-km coastal strip north of Caracas, in the coastal state of Vargas, Venezuela with its extremely steep and rugged topography (mountains 2700 m high within about 6–10 km of the coast) (Brandes, 2000). The rivers and streams of this mountainous region drain to the north and emerge from steep canyons onto alluvial fans before emptying into the Caribbean Sea. Damage to communities and infrastructure was so serious because here little flat area is available for development with the exception of the alluvial fans. In Vargas state probably almost 50,000 people were killed, more than 8000 individual residences, and 700 apartment buildings were destroyed or damaged and total economic losses are estimated at USD 1.79 billion (Wieczorek et al., 2001). On average, at least one or two major flash-flood or landslide events per century have been recorded in this region since Spanish occupation in the 17th century.

#### **2.3 Flash floods in Hungary**

Until very recently flood hazard research in Hungary had focused on riverine floods, particularly those along the two major rivers, the Danube and its main tributary, the Tisza (Lóczy & Juhász, 1996; Lóczy, 2010). There are relatively few papers published on flash flood events in Hungary (Gyenizse & Vass, 1998; Fábián et al., 2009). In the mainly lowland and hill environments of Hungary flash floods do not appear a major hazard. Another reason is the lack of appropriate monitoring systems in the flash flood affected catchments (Vass, 1997). In the wake of the events of the first decade in the 21st century, however, flash

Flash Flood Hazards 33

Fig. 2. The drainage network of Southern Transdanubia with the catchments studied and an

inserted location map (from the river network database of Hungary)

flood related disasters and their consequences have been appearing more and more frequently in the Hungarian media.

Some recent events that made news took place in the Mátra Mountains (North-Hungary) in 1999 (Koris & Winter, 1999) and again on 18 April 2005 (Horváth, 2005). The rainfall resulted from an atmospheric complex of several convective cells transporting moist air like a conveyor belt against the mountain slopes. Huge boulders of volcanic rock were transported by the local stream (Fig. 1). As an aftermath of the flood, slumps in 500 m length along the undercut bank are regularly generated.

Fig. 1. Deposits of debris flow after the Mátrakeresztes flash flood (by permission of the Nógrád County Disaster Prevention Directorate)

Some of the most disastrous events in Hungary occurred on 15–16 May 2010, when a strong cyclone reached the Carpathian Basin. Hitherto virtually unknown stream names (e.g. Hábi Canal, Bükkösd Stream and Baranya Canal) appeared in the media. The ensuing floods caused significant economic losses in Southern Transdanubia (Southwest-Hungary), a region of mostly dissected hill topography and a dense drainage network (Fig. 2). Daily precipitation amounts, intensities and stream stages broke records and cumulative precipitation locally exceeded 300 mm in the Kapos drainage basin during May and June. In Csikóstőttős village 65 people were evacuated. A one metre high flood swept away a children's camp in Szekszárd, where firemen assisted to evacuate the campers. On 16 June 2010 182 mm of rainfall fell on the village of Iklódbördőce in the Zala Hills (Southwest-Hungary) and caused a mudflow. Estimated by the insurance companies, the May and June events caused ca HUF 100 billion (EUR 360 million) economic losses, at least 3,100 residential homes were damaged and the agricultural damage totals ca HUF 30 billion (EUR 110 million). A summary of water-related damage recorded by insurance companies shows the distribution of insurance events in Southern Transdanubia between 1980 and 2005 (Fig. 3). In the light of the 2010 floods, the number of events presented here seems to be underestimated (property insurance was probably not comprehensive), but the map is informative of the zones of highest flood risk.
