Flash Flood

Flash Flood Study Group

The term "Flash Flood" has been defined, in English speaking countries and in international organizations as well, in several ways. National Oceanic and Atmospheric Administration (NOAA) of the United States defines flash flood as a flood of which peak appears within, in general, six hours from the onset of torrential rainfall. The definition of International Center for Integrated Mountain Development (ICIMOD) compares a flash flood with an ordinary flood and defines flash flood as flood characterized by hyper concentration of sediment and drifting wood and, furthermore, quick rise of the flood level in several minutes or a few hours as a result of torrential rainfall or breach of natural dam. A debris flow triggered by the breach of natural dam in Japan and the one triggered by the outburst of either morainal lake or glacier lake falls also on the category of flash flood. The latter case is referred to as glacier lake outburst flood (GLOF) and poses serious threat to the life in Nepal.

GLOF is one of the growing evident concerns in the areas of not only Himalayan but also other high mountain ranges including European Alps since a number of glacier lakes have been developed as glaciers retreat due to recent global warming.

United States Geological Survey (USGS) pointed out that flash flood takes place often even in arid and semiarid regions. In fact, the arid regions of the mid-western states of the United States are notorious for flash flood which causes flood disasters in lower reaches. It is observed that rainfalls due to mostly lightning storm are too heavy to penetrate the soil in short period of time so that trigger flash flood which abruptly fills and runs dry chanels¹⁾.

In the arid areas in the North Africa, the Middle East, the South-West and the Central Asia, it is observed that almost all the precipitations cause hazardous flash flood. On the other hand, however, farmers in the above areas have been making it possible to utilize flash flood water for spate irrigation to irrigate wheat fields on alluvial fans in a controlled manner employing unique technique called "Pachad Irrigation" since times immemorial and have successfully reduced the flood risk in downstream areas. The Pachad irrigation is characterized by multiple earth dams built immediately before the rain in the channel and segmented farmland on a target alluvial fan.

The floodwater dammed by the first earth dam located at the apex of the fan is diverted to the segments located on the same level with the dam. As soon as the segments on the top level are sufficiently irrigated, the first dam is breached in order to allow the second earth dam to divert flood water to the segments on the corresponding level to the second dam.Likewise, flood water is efficiently utilized for irrigation. Flood disaster is therefore successfully prevented as a result by employing specific technique which follows the morphological processes of the fan formation.

The term "Banjir Bandang" which is equivalent to flash flood has been frequently used in Indonesia. It is spoken through the ages and means in origin a kind of flood and associated inundation due to, for example, embankment breach. It is currently used to describe all the large floods which take place abruptly and runs fast.

Department of Public Works of Indonesia defines banjir bandang as, comparing with ordinary flooding, a specific flood of which flood level raising is very quick, generation mechanism has something to do with slope failure, kinetic energy is so large and associated changes in river morphology are so destructive and irreversible. The flood which had taken place in January 2006 in Junbur, East Java is an example.

The survey team which had made a reconnaissance immediately after the event disclosed that the cause of the banjir bandang was the breach of natural dam which had broken out due to massive slope failure²⁾.

As observed in the above, the definitions of flash flood range widely and differ from country to country from a flood of which peak flood comes several hours later to the one of which peak flood comes immediately after slope failure or breach of natural dam.

No specific equivalent in Japanese to flash flood is defined yet, but floods which take place in watershed and contain a lot of sediments and drifting woods have been referred to as variety of terms such as debris flow, sediment flow^* and flashflood. The term flashflood therefore covers very wide ranges of hyper concentrated flows.

Disasters due to flashflood are growing concerns in not only the areas above mentioned but worldwide. FLASH FLOOD 2006, an international conference, organized jointly by World Meteorological Organization (WMO) and NOAA in March 2006 in Costa Rica attracted worldwide attention and was attended by researchers, administrators and practitioners from as many as 20 countries such as, in addition to the United States and Central American countries, Germany and France from the west Europe, Russia and Japan, Bhutan and Philippines from Asia. Fruits of the latest research works and papers on the field application were presented at the conference.

Furthermore, in Krakau, Poland, in October 2008, WMO will organize a workshop on local preparedness and community participation to cope with flashflood in the regions of the Central and East Europe. Flashflood and associated disasters are growing social issues in the regions of the Central and East Europe, too.

Consequences and impacts of disasters due to flashflood are much more serious in the societies and countries in which populations suffer from unemployment because of poor industries. Japan, during the period immediately after WWII, was a good example. A disaster due to flashflood was referred to as a disaster of cadet branch because repatriated refugees and soldiers had been obliged to work and live on the land which had been long abandoned because of high risk of flashflood. The cases of cadet branch disaster are, unfortunately, increasing worldwide.

The term, flashflood, reminds us, in general, of ordinary flood of which discharge pattern is not so high convex in terms of hydrograph and is sometimes used for an ordinary flood in the United States, but it is certain the term means the specific discharge pattern which we can identify in the catchment basin designated as the hazard area due to erosion.

Technologies and know-how we have acquired are therefore applicable to all the aspects of flashflood worldwide. International cooperation for hazardless environment through flashflood sciences must therefore be strengthened.

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The flow which falls on the transitional phase between mass-transportation and suspended flow can be called sediment flow. Sediment flow can be observed in the reach of which gradient is less than 10 degrees, i.e. the sedimentation section on an alluvial cone of which gradient is less than 10 degrees.
(The Japan Society of Erosion Control Engineering (2004), Glossary of Sabo Engineering, Sankai Doh)

References

1)

Kurihara et al., "Characteristics of Flash Flood and Research Needs in the World", Journal of Public Works Research Institute, Vol. 48, No. 10.
pp.60-65, 2006 >>> Civil Engineering Journal

2)

Watanabe et al., "Disasters due to Banjir Bandang in Jember, East Java, Indonesia", Journal of the Japan Society of Sabo Engineering, Vol. 58, No. 6,
pp.31-38, 2006 >>> Journal of the Japan Society of Erosion Control Engineering

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Translated from Journal of Japan Sabo Association, Sabou to Chisui, Vol.179, pp.105-106, 2007