2.2.2. Type Plan—Flood

Floods mean a temporary significant increase in the level of water in rivers or other surface water bodies which causes the water to flood an area outside of the riverbed and may cause damage. Due to its significant amount of broken terrain, the Czech Republic has a very dense hydrographic network spanning a total of about 85 thousand kilometers. It is located in the temperate climate zone with a regular seasonal temperature and precipitation cycle. Aside from these long-term swings, short-term temperature changes are caused by frequent transitions of atmospheric fronts, which separate colder and warmer air and are often accompanied with rainfall.

In general, the long-term amount of rainfall increases at higher altitudes, but the predominant wind direction in mountainous areas is a significant factor as well. The average annual volume of outflow from the territory of the Czech Republic is about 15.1 billion m3, corresponding to a nominal outflow of 6.1 L·s<sup>−</sup>1·km−2. The outflow ratios are, however, not equally distributed. The ratio between the average and maximum flow during a 100-year flood is 1:20 to 1:50 on larger rivers, may reach nearly 1:100 on smaller rivers, and small mountainous rivers may have even larger ratios. In the vast majority of cases, floods in the Czech Republic are decisively influenced by hydrological causes and phenomena that also occur in the Czech Republic. Floods coming from abroad may only be considered on the river of Ohra (flow into the Skalka reservoir), Lužnice (flow into the Tˇrebo ˇn pond system), Dyje (flow into the Vranov reservoir) and Stˇenava (flow from Poland).

The floods caused by long-lasting regional rain (1997, 2002) caused the rise of floods on a large scape on a regional level. They usually occur on all watercourses in the afflicted region and propagate on medium-sized and larger watercourses (e.g., the Berounka, Vltava, Elbe, Morava, Thaya river basins).

Extreme winter and spring floods are caused by the melting of the snow cover, especially in combination with heavy rains. It can affect both mountain regions, where they happen most often at the end of the winter season, when intensive rains fall on the remains of snow cover, then primarily in foothills and middle elevations, but also in lowlands (Znojmo region 2006). In the second aforementioned case, the negative factors for the rise of significant flooding are, in particular: a large amount of snow cover in lower and middle elevations that can thaw over the course of a few short days; rainfall; the temperature of the air exceeding 8 ◦C on average and even persisting in the night on a level of about 4 ◦C and more; freezing soil under the snow cover, which limits the permeation and speeds up the runoff; and the strong convection increasing rainfall on the windward side and the increasing melting of snow.

Melting significant enough for a flood to arise can occur practically from the end of November until April. In a year with an abundance of snow, approximately 5 billion m<sup>3</sup> of water accumulates in snow throughout the entire territory. The height of the snow cover reaches an average of 10–20 cm in the lowlands, 40–60 cm in middle elevations and more than 100 cm in the mountains. The period of the melting of the snow cover is not regular, over the long-term, the maximum average snow cover in lowlands is in the middle of February, in mountains in the second half of March. The risk of flooding from spring thaws has decreased when compared to the period of the first half of the 19th century as a result of warming, the regularity of spring flooding has decreased, though the threat of significant spring flooding remains.

Flooding caused by ice phenomena occur even at relatively lower flow rates in sections of the flow susceptible to the creation of ice sheets and ice jams in sites with lower flow rates or in a narrow section of the riverbed (dangerous ice phenomena can arise on various flows and tend to occur very often on, for example, the rivers Berounka, Otava, Beˇcva, Moravská Sázava, Sázava, Divoká Orlice, etc.).

A critical situation may be announced if the ongoing critical water or flow levels and their impacts threaten the operation of critical infrastructure and where the handling of such situations is beyond the capabilities of individual regions in view of unfavorable weather forecasts. The impacts of large floods may include significant material damage and may have a long-term impact on the affected territory, and the impacts may require the use of all the country's available resources including the possible use of international humanitarian aid even beyond the duration of the flood itself.
