Flash floods

Flash floods are common to most mountain areas and a continuous challenge to water resources management and disaster reduction management in particular. They repre- sent some of the most devastating types of floods due to rapid occurrences, large quan- tities of water, high sediment content and fast moving flood waves. They are a typical feature of steep topography where the land has limited retentive capacity in areas with high variability in rainfall, such as in monsoon regions.

While GLOFs can be considered a special type of flash flood, flash floods in general can originate in different ways and are affected by multiple processes. Perhaps the most common are episodes of intense rainfall, typically in monsoon and rainy seasons. The phenomenon is well known to many mountain communities and populations living below mountain ranges. In many mountain communities people have learned to in- terpret weather signs as warning of flash floods and have to some extent adapted reasonably well to these dangers. How- ever, flash floods often do not follow predictable patterns, and there may not be much warning as the floods vary with the amount of rain that is discharged and can also be caused by rapid melting of snow and ice, something which may be re- lated to climate change in ways not previously experienced in these communities. Flash floods are also caused by failure of man-made and natural dams and can be triggered by land- slides and debris or seismic activity.

Flash floods pose a serious threat to people in mountain regions and have severe downstream effects. In contrast to most GLOFs which occur in sparsely populated high moun- tain regions, a large proportion of flash floods occur in more densely populated middle hill and mid-mountain regions where social, humanitarian and economic impacts are much greater. Numbers are highly uncertain and most likely under- reported, but flash floods in the Himalayas are estimated to cause the loss of at least 5000 people every year (Jianchu et al. 2006). An unknown, but probably much higher number is affected in a variety of negative ways. Floods destroy bridges, roads, buildings, agricultural lands and settlement areas. Flooding

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also increases erosion and the sediment load of the water. They can also lead to unusually high water flows in steep riv- ers and have been known to exceed predictions for maximum flows. For example the Kulekhani hydropower plant in Nepal was designed for a maximum flow of 8000 cubic meters per second. A previous peak flow of 12000 cubic meters was re- jected as a statistical outlier. However, in 1993 a cloudburst led to a flash flood with 15 000 cubic meters per second that severely damaged the plant. Similarly a single flash flood the same year knocked out half of the country’s electricity pro- duction for several months leading to a major economic im- pact to Nepal (NCVST 2009).

Many mountain regions are characterised by limited space that is both suitable and safe for human habitation. In north- ern Pakistan for instance, communities are often located on or close to alluvial fans since almost any other space is unin- habitable due to the ruggedness of the terrain. However, al- luvial fans are frequently the core path of flash floods. Flash floods have significant negative impacts on socio-economic conditions and livelihoods. Flooding destroys productive land, buildings and roads, hence loss of valuable property and work- ing capacity of communities. It also causes human injury and deaths, and typically elderly, children and women are the most vulnerable as in most cases related to natural hazards.

 Figure 11: Overview of natural disasters in Asia between 1975 and 2004. Floods account for a very significant propor- tion of the disasters observed.