Risk is a condition proper to human beings singe the dawn of Human History. The ignorance of how nature works and the initial fear to extreme phenomenons, that has turned recently into a loss of respect for the environment, have made the awaiting for a catastrophe a common element in our daily life. The possibility of suffering a natural disaster is assumed as another element influencing the development of societies in any given historical moment. Humans live in risk on the face of planet Earth, and historically this risk has had a close relationship with water. Among the Pre-Socratic philosophers, Thales of Miletus stated that water was the principal substance, the motor of nature. The presence of water currents was one of the basic conditions to establish human settlements in a territory and for the development of economic activities. And this has been common throughout all periods of history, including the present. With the support of scientific and technical advances, humans have had a continuos battle for domesticating water, for adapting it to our needs, a continuous search for regularity in its natural function that not alway happens. Controlling floods and droughts has always been -and still is- one of the main tasks for the development of societies.
Without water, the action of humans over a geographical space are very difficult; the lack of water causes supply shortages and, in extreme cases, which are still common in some regions of the Earth, famines and death. The excess of water is also a risk for human beings. An extraordinary flood wave and it subsequent overflow causes important damages in the areas affected, that ultimately can also lead to the destruction of human lives. Draughts and floods, pluviometrical and hydrologic extremes are the two natural disasters with more social-economic impact in all of the Earth.
Very frequently, floods affect very ample regions in both developed and developing countries, causing a rupture in the normal running of societies, and in occasions can also turn into catastrophes. Water becomes an impetuous force that has no respect for the labour of man, who normally has not respected the flooding territory when installing their homes or nearby activities. There are few territories in the surface of the planet that are not affected by water floods. This is regardless of the fact that certain fluvial courses register more or less anthropization, and thus have an increased risk for human beings. In general, any territory articulated around an anthropized fluvial space, is in risk of being flooded. But the level of risk will depend on the level of respect that humans have shown towards the natural function of that particular fluvial space, especially in maintaining the drainage and overflow areas when flooding occurs.
There are areas in the surface of the planet which are particularly vulnerable to river, and other fluvial systems, floods. These are territories densely populated where rivers have historically been the basic element for deciding the development of urban locations and economic activities. Also, the areas where traditionally water courses are small but with spasmodic functioning (streams, ravines, gullies), meaning that normally the flow rate is low or non-existent for most of the year, generating a false sense of security and, thus, fostering settlements too near, or even within the natural flow.