Technical measures in the form of dams or walls would be incommensurately high, inconvenient, expensive and often unfeasible. The construction of mobile protection such as the one ready for instance in Prague is time-consuming and torrential floods do not give us this time. Elements of such mobile protection systems are not usable on mountain streams due to how small they are and they can thus be burst by trees and other heavy objects floating in the fast current.
If torrential rain has similar intensity as intensities seen in numerous cases in western Europe and in our country in 2009 and 2010 with discharges reaching multiples of the 100-year discharge, it is practically impossible to protect property along the shores against such floods. Flood control systems cannot be built against such discharges and such character of floods, which carry trees and other floating objects. The same applies to watercourse crossings, bridges, etc.
The often fatal consequences of torrential rain floods may be mitigated by removing barriers that would intensify the flood from places where the flood discharge is envisaged to pass through, namely low footbridges and bridges, piping systems that have low capacity, narrowing of the watercourse, etc.
If people do not want to undergo the risk of floods, they should not build or buy their properties in the vicinity of watercourses. Information about the scope of extreme floods may be obtained from flood hazard maps concerning larger watercourses, which are processed by all EU member states; in the Czech Republic a 500-year repetition flood scope is available. A lot of important information regarding protection against floods can be found on www.povis.cz.
However, it would be desirable that we seek a way how citizens may be warned about when torrential rain is coming. They could thus have a possibility to take at least basic measures to save their own lives and the lives of their family members at least slightly in advance and if possible, they could try to protect some of their movable property. However, the current warning methods are directed at fairly large areas (regions, districts) and it is not possible to implement more radical measures on their whole territory such as for instance evacuation. We therefore have to look for a warning system that would be more targeted area-wise (up to the level of individual municipalities) and one that would come approximately 20 to 30 minutes ahead to allow for evacuation in the very least. However, this path is extremely difficult. Although we know the individual elements of the system (continuous assessment of saturation and condition of the area, precipitation measured by stations and derived from radars, precipitation outflow models that calculate discharge, warning systems that warn citizens such as siren communication. However, the phenomenon that we are facing is extremely dynamic, changeable and potentially extensive. Since torrential rain with local consequences may be expected anywhere on the territory of the Czech Republic, the whole territory would need to be equipped with a fairly dense measurement grid with a reliable data transfer in real time and extreme conditions, an ongoing evaluation of precipitation and discharges and a link to the siren. Such systems would also require the deployment of supercomputers to make sure a forecast arrives in the respective place in time. At present it would be possible to prepare such warning system only for the territory of a region. European countries do not have such systems for larger areas yet. However, these systems are being experimentally developed and should be supported. For instance, research led by prof. Daniel Sempere-Torres from Barcelona is promising.
Authors: Ing. Petr Březina, Ing. Karel Drbal, Ph. D. (T. G. Masaryk Water Research Institute)
Published on: 2/8/2021