Types of flood

When we talk of floods, it is important to realise that they may be due to quite different causes. This is of great significance in the context of insurance. The three main types are: storm surges, river floods, and flash floods.

Then there are a number of special cases like groundwater flooding, tsunamis, dam-break floods, glacial lake outburst floods, backwater floods (caused by landslides falling into rivers, ice jams, clogged bridges, etc.), debris flows, and rising levels in the sea and lakes.

Storm surges

Storm surges can occur on the coast and along the banks of large lakes. They have extreme loss potential and may cause hundreds of thousands of fatalities. The Bangladesh storm surges with death tolls of 300,000 (1970) and 140,000 (1991) are the best-known of the recent past but not the only ones.

Even in Europe, storm surge events still claimed the lives of thousands of people in the second half of last century (North Sea, 1953: 2,000 dead). Major improvements in sea defences and, in particular, the enhancement of forecasting and early-warning facilities in recent years have led to great storm surge catastrophes becoming less common.

Nevertheless, storm surges still represent an immense loss potential in what is a relatively limited strip of land on the coast. Insurance cover for this hazard is virtually impossible due to the problem of antiselection.

The accelerating rise in sea levels that is certainly to be expected will aggravate the risk of storm surge and coastal erosion all around the globe — and this will be one of the most detrimental effects of global warming.

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River floods

River floods are the result of copious rainfall usually continuing for a period of days over a large area. The ground becomes saturated and cannot cope with any more water so that the rain flows directly into the rivers. Frozen ground has the same effect since it prevents the water from seeping away.

River floods do not occur abruptly but build up gradually - although sometimes in a short time. As a rule, they last from a few days to a few weeks. The affected area may be very extensive if the river valley is flat and broad and the river carries a large volume of water.

In narrow valleys the flooded area is restricted to a relatively thin strip along the river, but the water may be quite deep and flow at high speeds.

This type of flood is problematical in terms of insurance because — as in the case of storm surges — only a relatively small proportion of the entire building stock is actually at risk from flood waters. What is more, these buildings are to be found in areas that are affected frequently and in some cases even regularly.

Defining the exposed areas is extremely difficult and it is almost impossible to determine the probability of loss at any given point. This is particularly true when flood control measures are in place, because, on the one hand, they may be effective even beyond the design value, but may, on the other hand, fail because of unforeseen circumstances even during moderate floods.

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Flash floods

Flash floods can occur almost anywhere, so that nearly everybody is threatened. Sometimes they mark the beginning of a major river flood, but usually they are separate, individual events of only local significance, scattered randomly in space and time.

Flash floods are caused by what are usually short periods of intense rain often occurring over a very small area and typically in conjunction with thunderstorms. The soil is not usually saturated; but as the rainfall intensity exceeds the infiltration rate, the water runs off on the surface and soon gathers in the receiving waters.

This results in a rapidly growing flood wave which can gush downhill and in next to no time forge into areas where it may not even have rained at all. The mechanical forces associated with the high rates of flow are a critical factor as is the erosion potential: both can cause buildings to collapse and increase the size of losses enormously.

The term "flash flood" is also used in connection with cloud bursts on even terrain, when floods occur because the water cannot flow off fast enough due to the lack of a sufficient gradient. It is almost impossible to forecast flash floods, simply because they happen too suddenly.

In most cases it is only a question of minutes, thus more or less ruling out short-term loss reduction measures. Flash floods are also much shorter in duration than river floods. Most of the water has disappeared again after a few hours.

Insurance against flash floods is no problem because the necessary spread of risks over space and time will certainly be achieved. However, a high market penetration depends upon large numbers of people being sufficiently aware of the risk involved with this kind of hazard.

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Mountain streams, debris flows, landslides

Floods in mountain regions — even in low mountain ranges — are special kinds of flash floods with characteristics of their own. As a rule, there is a large proportion of sediment in the flowing water. When the solid content exceeds about 30%, the term used is "debris flow".

In this kind of flow event, the mixture of water and solids shoots downhill at high speed, carrying along with it everything that stands in its way. Debris flows are capable of transporting boulders measuring several cubic metres. They usually occur in a number of surges (or pulses).

They stop abruptly and deposit the solid materials in piles often a few metres high. As a result of the warmer climate, the permafrost areas in the mountains are thawing, which leads to more landslides. This means that there is sufficient non-consolidated material for debris flows to form.

Tsunamis

Just after Christmas 2004, a type of flood that was considered exotic by many captured the attention of the whole world: a tsunami. Tsunamis are gravity waves, generated by large volumes of water being displaced by, for example, large earthquakes, submarine or onshore landslides, or volcanic eruptions.

With a wave height of a few decimetres and a wave length of up to several hundred kilometres, they travel through the open sea at the speed of a passenger jet without losing much of their energy. As they approach the coast, their speed decreases but at the same time their height increases — sometimes dramatically.

Wave crests more than 10 metres high have been observed. These waves may reach run-up heights exceeding 30 m on the shore and destroy everything in their way. Very often the water is seen to retreat from the shore before a tsunami reaches the coastline, giving people a chance to escape if they can read the signs properly.

Unfortunately, few could in December 2004. The most devastating natural catastrophe in many decades not only hit about ten countries directly. It also affected dozens of countries around the world through its impact on the tourist sector. The tsunami event claimed the lives of over 170,000 people, but with more than 100,000 reported missing, the exact number will never be known.

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