Strategies against flood losses

The influences of mankind — in the form of impervious surfaces, river training works, and anthropogenic climate change — can trigger and exacerbate floods. Not every case of flood necessarily leads to flooding, however.

And even if flooding does occur, the losses can still be kept within reasonable bounds and do not always have to be major losses. This presupposes a suitable prevention strategy which embraces all aspects of floods, from their origins to the avoidance of loss potentials.

Little effect will be gained from concentrating on specific aspects like ecological and technical measures (e.g. river restoration and dykes) or organisational and financial mechanisms (e.g. alarm plans and insurance).

Monocausal explanations of the increase in flood catastrophes are repeatedly put forward, citing as the sole trigger impervious surfaces, for example, or river regulation, or the disappearance of natural flood retention areas, or climate change.

Such one-sided allegations do not bear critical examination, however. A much more differentiated view of the causes is required.

Prevention of floods

A flood occurs when there is significantly more water in a river, in a lake, on the ground, or below the surface than normal. Floods are a part of the natural water cycle; but mankind has ways of intervening in this cycle.

The interventions include influencing the climate (resulting in more frequent and more intense precipitation), changing the infiltration capacity of the soil (impervious surfaces, soil compacted by agriculture), discharging water into rivers and lakes (drainage ditches, sewers), and directing it towards the sea (e.g. river regulation, removal of flood retention areas).

Retaining more water must have top priority whenever possible. One thing must be noted, however: extreme floods in large catchments are not attributable to surfaces which have been made impervious by human activity; this has a relatively minor impact.

Likewise, local flood detention, river restoration, and dyke relocations can only reduce extreme flood peaks to a limited degree. We must consider the huge masses of water involved in extreme events on large rivers.

For example, the destructive section of the Mosel flood wave in the 1993 Christmas floods had a flow rate of over 2,000 m³/s and a volume of 840 million m³; a basin the size of Lake Constance with a water depth of 1.56 m would have been required to retain it. Nevertheless, even if some measures have only a limited effect, they must be encouraged and implemented.

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Prevention of flooding

Flooding occurs when the soil, a lake, or a river is unable to take up any more water. The water then stands or flows into areas that are usually dry. Flooding can be influenced by technological measures such as retaining the water at specially designated places (retaining basins, polders, reservoirs), or directing the flood waters by means of dykes within a predetermined area, possibly by means of flood channels. All these measures are based on what is called a design flood, i.e. a relatively high flood value used as the basis for designing protection measures.

Prevention of losses

Losses occur when people and their possessions are affected by flood waters. In such cases, damp, dirt, mechanical forces, and erosion play a major role. The precautions that can be taken are warding off the water or extricating oneself and one's valuables from its effects.

Solutions also include revising land use regulations (prohibiting residential areas in flood-prone districts), adopting permanent and temporary structural measures (building elevated structures, waterproofing cellars and buildings), modifying the management of values (avoiding installations or objects of great value or susceptible to water in lower parts of buildings), and taking appropriate action in the event of an impending flood (e.g. clearing out threatened parts of buildings).

Prevention of risk

The risk is derived from the combination of flood occurrence probability and ensuing costs. At any given place, the risk is nil either if there is no possibility of a flood occurring there or if there are no values there (or both). It can be minimised by suitable measures designed to prevent floods, flooding, and losses.

Nevertheless, there will always be a residual risk; and that is where insurance, for example, comes in. Insurance makes the uncertainty of future financial strains calculable. In return for a premium, the policyholder can either buy complete freedom from that uncertainty or (by paying a lower premium) limit the loss to a certain deductible.

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The partnership for risk reduction

Risk and loss minimisation call for an integrated course of action. At the same time, the flood risk must be carried on several shoulders: the state, the people affected, and the insurance industry. Only when all three partners cooperate with each other in a fine-tuned relationship in the spirit of a risk partnership is disaster prevention really effective.

Reducing the underlying risk for society as a whole is primarily the job of the state. It provides access to observation and early-warning systems, builds dykes, deploys flood retention areas, and by enacting statutory provisions determines the framework for the use of exposed areas.

Those affected are also obliged to make their own contribution to loss prevention by building in an appropriate manner, controlling the exposure of their values (e.g. not converting the basement), being prepared for emergencies (e.g. writing a checklist), and being ready to take action as soon as disaster strikes.

Finally, insurance companies should be on hand, their main task being to compensate financial losses that would have a substantial impact on insureds or even constitute their ruin. This means that although insurers are not social institutions (in the sense of charities), they are indispensable institutions within the social system.

Social institutions redistribute the burden borne by individuals among the entire community of insureds, which is ideally composed in such a way that they all have a chance of being affected — even at different degrees of probability. Furthermore, they perform educational and public relations services, e.g. by publishing brochures in which they draw attention to hazards and explain ways of dealing with them.

Antiselection important

Antiselection plays an important role in the insurance of flood losses. The only people that show any interest in buying insurance are those that live near rivers or on the coast and are often affected by floods.

Owners of property located away from large bodies of water, on the other hand, feel safe from floods, and therefore reject insurance protection. Consequently, the insured community is relatively small and is also comprised of customers that are exposed to a large risk.

There is no danger of antiselection in the case of flash floods because they can happen more or less anywhere. Nevertheless, the general risk of flash floods must first be made clear to everyone, i.e. any subjective misconceptions of the risk must be corrected.

In the discussion of flood control measures, the different types of flood are usually all lumped together. No distinction is made between relatively common floods (e.g. with a return period of five to ten years), major floods (e.g. 100-year events), and catastrophic floods, which only occur on average every few centuries.

This approach is fundamentally wrong and results in conflicting stances and solutions. A distinction must be made between frequent and very rare floods and between small and large catchments, because the measures called for in each case are quite different.

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