Active flood zone

The active zone of a flood zone is defined by the water authority at the proposal of the river manager in the built-up area, in development areas under the zoning plan and elsewhere as needed, according to how dangerous the flood flows are (Section 66 of the Czech Water Act). It is set by a measure of general nature, usually together with the flood zone.

How the active zone is defined

Under Section 6 of Decree No. 79/2018 Coll., the active zone covers the river channel, areas of high flood hazard and areas of medium hazard where, during a Q5, Q20 or Q100 flood, the water depth is at least 1.5 m, the flow velocity at least 1.5 m/s or their product at least 0.75 m²/s. For small streams in simple valleys, the 20-year flood extent may be used in justified cases.

What applies in the active zone

• Under Section 67 of the Water Act, structures may not be permitted or built here. Exceptions are water works on the river and flood defences, structures for water intake and for draining waste water and rainwater, and essential transport and utility infrastructure.
• Maintenance and alterations of existing buildings are allowed if they do not worsen the flow of water.
• Fences, hedges and similar obstacles, storing material that can be washed away, and mining or earthworks that worsen the flow are banned.
• Camps, campsites and other temporary accommodation are banned too, except tent camps that were already held at the place before the zone was defined.

What it means for a buyer

A plot in the active zone is practically unusable for a new building, and not even a garden fence may be put up. For the rest of the flood zone outside the active zone the water authority sets restrictive conditions. The exact boundary is in the measure of general nature and in the HEIS map of the T. G. Masaryk Water Research Institute; the water authority or the building authority can explain what it means for a specific project.