Flood

Flood is the state of a waterbody where the water level or discharge reaches or exceeds a certain threshold value.

What causes flood?

The interaction of the shape of the area, saturation of the soil, vegetation, available bedload, rate of snow coverage as well as technical intervention in water courses (e.g. dam, retention basin and so on) has a determining influence on the likeliness of flood in a catchment area. Extreme rainfall events, which are either long lasting or of high intensity can trigger floods. A strong snowmelt also can contribute to flood.

Floods act differently, depending on the channel system. In torrent systems intense bed load transport and deposition dominate. Here, one often finds debris flow, bank erosion and overbank sedimentation. In watercourses in the valley bottom, however, flooding become rife, but bank and bed erosion are also possible which leads to undercutting. Besides, bed aggradations might occur aswell.

Regarding the possible consequences, flood hazard in Switzerland is divided into the following categories:

• Debris flow
• Flooding
• bank erosion

Flood means huge masses of water and often high velocities. With its huge force it can sweep away people and goods and it can destroy buildings. Bank erosion and degradation can undermine foundations of buildings. The bed load dragged along by the water often causes heavy damage of arable land, buildings and infrastructure. Since mid-nineteenth century, a lot is being done for flood prevention and protection in Switzerland. Nowadays, securing the upkeep of existing protective structures has top priority. In addition to that damage potential should be reduced by land-use planning. This means that endangered areas are not developed and that the freedom for watercourses are preserved or created. Should those measures not be enough, active measures are taken - such as channel improvement, retention basins or peak flood bypass – which reduce hazard potential.