Historic floods (largest recorded flood)

Historic floods show where water really reached along a river. Czech Decree No. 79/2018 Coll. defines the area flooded by the highest recorded natural flood as the area bounded by a flood line matching the highest historically recorded and documented water level. The river manager shows it in the flood zone proposal where the data allow (Section 7(1)(f) of the decree).

Where the data come from

• The national layer is kept by the T. G. Masaryk Water Research Institute (VÚV TGM) in its DIBAVOD database as the area of the largest recorded natural flood. Each river section shows the flood with the highest recorded water level, the oldest from 1880, the most recent from 2013. Most often it is the spring flood of 2006 and the floods of July 1997, August 2010, June 2013 and August 2002.
• The September 2024 flood is in a separate layer. VÚV TGM mapped it during the flood evaluation from aerial and satellite images, field measurements and records from the public, in two accuracy categories. The first, more accurate one was agreed with the state river basin companies (Povodí), usually on sections where the peak exceeded the 20-year flood. The second is an order of magnitude less accurate.
• Only water that overflowed from the river channel counts, not rainwater caught in hollows.

How it differs from the Q100 flood zone

The Q100 flood zone and other N-year floods are calculations: a hydraulic model shows where water would reach in a flood of a given size. A historic flood extent is a record of a real event. The real flood may have been smaller or larger than the 100-year flood, and the channel and flood defences may have changed since. That is why both are worth looking at.

How to read it

• The mapping is indicative. For binding information VÚV TGM refers to the river manager or the water authority.
• The layer only covers rivers where the extent was documented. If a river is missing, it does not mean there has never been a flood.
• Smaller floods and flash floods from slopes and fields are not included.
• During a viewing, ask neighbours or the municipal office where the water reached. Flood marks on buildings and the municipal flood plan also help.