Flash flood (critical points)

A flash flood occurs during short but very intense rain. Water has no time to soak in, runs down a slope or field and often carries mud and sediment with it. It can hit a house far from any river, which is why maps of flood zones and N-year flood extents do not cover it.

Critical points

The T. G. Masaryk Water Research Institute (VÚV TGM) uses its methodology to identify critical points - places where a path of concentrated surface runoff enters the built-up area of a municipality. Contributing areas of roughly 0.3-10 km² are considered, and the selection depends mainly on their slope and the share of arable land: the methodology recommends an average slope of at least 3.5% and at least 40% arable land in the catchment. The current methodological guide from 2026 was prepared by VÚV TGM together with the Czech Hydrometeorological Institute (ČHMÚ) and approved by the Ministry of the Environment.

How to read it

A critical point is an indicator of hazard, not a map of exactly where water will go. It says that concentrated runoff from fields and slopes above the village may enter the buildings at this place. How much damage a particular house may suffer depends on the terrain, the buildings and measures in the catchment.

What to look at during a viewing

• Does the house lie below a slope or a field? Where does water flow after rain, and where do ditches and culverts lead?
• Are there banks, grass strips or interception ditches above the buildings? They relate to soil erosion risk.
• Has the village had mud in its streets in recent years? The municipal flood plan or the municipal office can help.
• The slope and shape of the terrain above the house are visible in the digital terrain model, the use of nearby land in the LPIS land parcel register.