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Frequently asked questions (FAQs)

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On this page we collect the most frequently asked questions. If you have any suggestions for expanding this list, please send us an email or give us a call (contact details at the end of the page). The FAQ list is constantly updated.

Floods are part of the natural water cycle. This phenomenon is not caused by human hands, but by nature itself. The cause of floods is (almost) always the same: rain.

In Saxony, around 700 mm (or liters per square meter) of precipitation fall on average each year. Almost 13 billion cubic meters of water fall on the total area of ​​the Free State. That corresponds to a cube with an edge length of thirteen kilometers. In comparison: Lake Constance has a volume of 48 billion cubic meters.

Some of the precipitation evaporates, seeps into the ground or is absorbed by plants. Another part of the water eventually flows into the sea via our streams and rivers. But like on our motorways, this can lead to traffic jams - if it rains too much at once, floods occur.

Every flood is different

Floods are always spoken of in everyday life. Science, however, distinguishes between different types of flood. These depend to a large extent on the duration, strength and extent of the precipitation as well as on the natural conditions on site. Two types of precipitation are of particular importance for the development of floods:

  • Short-term, very intensive and locally limited heavy precipitation (e.g. during thunderstorms): In extreme cases, this type of precipitation can reach 100 liters of water per square meter per hour and trigger flash floods. Floods as a result of heavy rain events are usually only of short duration and occur spatially very limited. As a rule, they result in extensive "wild" runoff in sloping terrain and impoundments in flat terrain that are not necessarily tied to flowing waters. That is why they are difficult to predict. Short and locally limited precipitation does not have a noticeable effect on large rivers such as the Elbe.
  • Long-lasting, large-scale precipitation: Continuous rain has a particular influence on the flooding of large rivers. Like a sponge, the soil can only absorb a limited amount of water. Once this amount is reached, precipitation can no longer seep into the ground and flows off on the surface. This means that more water can enter a river from a large catchment area than the river bed can hold.

Note: Information of the article provided by the initiative Hochwasser.Info.Bayern

Sensible handling of floods demands everyone: the Free State, the districts and municipalities, but also every individual can and must contribute to reducing their individual flood risk. Basically, flood protection is therefore a task for society as a whole. Of course, the Free State in particular plays a central role in providing services of general interest to its citizens.

This is reflected in the area of ​​responsibility: All larger bodies of water are - as so-called first-order bodies of water - within the jurisdiction of the Free State of Saxony. So-called flood protection concepts are drawn up for these bodies of water, which, among other things, describe measures to reduce the flood risk. This includes, for example, classic technical flood protection, such as dikes, mobile protective walls or retention basins. The executive authority for the Free State is the State Reservoir Administration.

Almost all other (especially the smaller) bodies of water are in the area of ​​responsibility of the municipalities (or those directly adjacent to the water body), which are responsible for drawing up and implementing appropriate plans, at least if there is a significant flood risk. This explicitly does not apply to floods as a result of wildly flowing water during heavy rain, although the first steps are currently being taken to take these phenomena into account in flood planning (for example in our RAINMAN project).

In the event of an incident, the most important actors in alerting the population and carrying out countermeasures are initially the municipalities, which have so-called alarm and deployment plans in place. These are based on the alarm level declared by the Lower Water Authority (i.e. the districts or urban districts) (the alarm level is NOT declared by the LHWZ).

But every individual also has a role to play in reducing flood risks in terms of personal provision; For example, by taking precautionary measures such as insurance, drawing up an emergency plan or technical protective measures on your own home, each individual can reduce the harmful effects of floods to a level that is tolerable for them in an emergency.

Therefore, please inform yourself in good time about your personal flood risk and protect yourself against financial damage. Because even extensive state and municipal flood protection has limits. Here is more information about what you can do.

Additional information

Further information on the topics of flood risk management and flood protection can be found in the Saxony water portal:

The word flood PROTECTION is actually misleading, because there can never be one hundred percent protection against every conceivable flood event. "Protection" can even be disadvantageous, namely precisely when, on the more deceptive assumption that protection would be sufficient for all conceivable situations, large values ​​are exposed to a flood risk.

In principle, floods can occur anywhere; be it classically out of the water, or as a result of heavy rain and so-called »wildly flowing water«. So you will not be able to eliminate the risk of flooding, but must deal with it in a suitable manner.

And this is where a - admittedly cumbersome - term comes into play, which means dealing with a flood risk that cannot be eliminated: flood risk management. The aim is to reduce the risk that floods entail, for example in the way that - where technical measures would not work or would be too expensive - the flood is avoided or precautions are taken to minimize the damage in the event of flooding to minimize.

The most important component in this risk management is to take adequate precautions. So, in relation to each individual, to find out about the dangers of flooding on their doorstep (for example by means of maps), as well as to insure themselves and to draw up an emergency plan. In the field of floods, we have what each individual can do to better protect themselves against floods - what can I do? compiled for you.

Basically it has to be said that floods can occur anywhere in the Free State; be it "classic" from the water, or as a result of heavy rain and so-called "wildly flowing water". In short: Floods should always be considered and taken into account.

Maps are an indispensable tool for finding out about flood risks. We have prepared a quick access flood map for you to the most important flood maps.

With regard to building law considerations, reservations, etc., please contact the building authority responsible for you, i.e. usually your municipality.

Levels serve various purposes. In addition to capturing Water levels, for example for the flood warning, they also perform hydrological tasks, such as recording the Flow rate (Amount of water per unit of time). Water levels are usually NOT referred to as gauges; »Level« means the entirety of the measuring devices of a measuring station, including the associated channel section.

At levels are Direct and continuous water levels detected. The The flow rate is determined indirectly via flow velocity measurements, which are carried out regularly.

Water levels are determined using different technologies, for example by means of floats or water pressure sensors, which are connected to a data acquisition and transmission device. The water level measurement always relates to a fixed reference horizon, the zero point of the water level, which can be both below and above the deepest bed in the water level cross-section. The only prerequisite for its determination is that positive water levels always occur in the area of ​​the occurring flows.

To determine the flow rates, proceed as follows: The flow velocities required for this are recorded with special devices, the so-called hydrometric blades, in points regularly distributed over the gauge cross-section like a network. Hydrometric blades have a calibrated propeller, from whose number of revolutions the flow velocity for a certain point in the cross-section through which the flow passes is derived.

The mean flow velocity in the gauge cross section is then determined from the individual flow velocities. In addition, the depth of the water is determined from soundings of the water depth evenly distributed over the width of the water level. The multiplication of both quantities (speed and area) gives the flow.

Flow determinations take place at different time intervals in different flow ranges from the lowest to the highest water level, whereby this is explicitly recorded. This makes it possible to set up a water level-flow relationship, with the help of which flows can also be specified as a continuous time series for the continuously measured water levels.

The flow velocity measurements required to determine the flow rate are carried out at least four times a year, in addition at low water levels and during and after high water levels, and up to weekly in the case of summer weeds or icing in winter.

Additional information

The level data is transmitted to the LHWZ as quarter-hourly mean values ​​every quarter of an hour and is processed internally around two to three minutes after each measurement interval and is sent to the web portal and other information offers such as the Meine level app. The LHWZ web portal is updated every five minutes.

Precipitation data is obtained from the LHWZ from different providers (BfUL, LTV, DWD, etc.) in different temporal resolutions (one minute to one hour). Here, too, the data is processed within a few minutes of being received by the LHWZ. Hourly precipitation values ​​are continuously generated and updated if there are higher than hourly data (with a note such as "current time range cumulative").

In principle, these instructions should be observed with regard to the current data (raw data) available in the LHWZ web portal.

The LHWZ provides its warning products to the relevant flood apps and flood portals, which can and should be used for self-notification. You can find an overview here:

Flood portals and flood apps

  • The central information hub on the subject of floods in Saxony.
  • Cross-border flood portal (LHP) The flood situation in all of Germany and the neighboring countries at a glance.
  • ZÜRS public - Information system on risks from natural events How high is my risk? »Zoning system for flooding, backwater and heavy rain«.
  • My level app Current water levels of around 2,500 levels in Germany with notification when individually configurable levels are reached and when there are flood warnings.
  • "WarnWetter" app Official weather and severe weather warnings from the German Weather Service (DWD) as well as warnings about natural hazards (such as floods) directly via notification on the mobile device.
  • »NINA« app Receive flood warnings via notification, among other things, with the official NINA app from BBK; shows the same messages as the warning apps KATWARN and BIWAPP.
  • »KATWARN« app KATWARN is a warning system that some municipalities use; shows the same messages as NINA.
  • »BIWAPP« app Some municipalities use BIWAPP; shows the same messages as NINA.

RSS is a standardized news format that can be accessed via the Internet. Wherever RSS feeds are offered in the LHWZ web portal, you will find a corresponding symbol at the top right of the website. Among other things, there are RSS feeds for flood warnings and early flood warnings.

Additional information

We have put together some tips for dealing with warnings in RSS format below:

There are different warnings about the natural hazard "flood". Floods can occur very locally (in individual districts, for example), regionally (in smaller streams and rivers) or supraregional (as in the floods in 2013 or 2002).

For the warning of a supraregional flood hazard use the flood warnings from the state flood center, which are based on observed water levels at the flood warning levels and hydrological forecasts.

For the warnings above regional floods the flood early warning of the state flood center is suitable. The early warning provides an approximate estimate of the flood risk to be expected for small catchment areas up to 24 hours in advance.

Local flood can develop very quickly as a result of heavy rain and are very difficult to predict in terms of location and time of occurrence. First of all, use the weather warnings of the German Meteorological Service to warn of such events.

The following figure summarizes the different warning products and their validity:

The LHWZ is currently making predictions of the water flow for around 80 and thus a large part of the flood reporting levels in the Free State. Of these, however, the forecasts are only published regularly for around 30 gauges. Why is that?

Many of the Saxon waters have their origin in the Free State and have rather small catchment areas. The smaller the area for which a forecast is to be made, the greater the uncertainties to be expected. You know that for sure: In summer you cannot say exactly whether heavy rain will occur at a certain point or ten kilometers away.

This makes hydrological forecasts for small catchment areas comparatively uncertain and, above all, unstable in a certain way; It is quite possible that forecasts change, for example when new weather forecasts are available. These changes can be much more blatant for small catchments than would be the case, for example, for larger rivers.

For the reasons mentioned above, the selection of the forecasts displayed in the LHWZ web portal represents a compromise between what is technically feasible and what is technically sensible.

Additional information

In principle, precipitation data from different measurement networks are displayed in the LHWZ web portal. For legal reasons, we are not allowed to pass on data that is not collected by the Free State of Saxony. This applies to precipitation data from the measurement networks of the German Weather Service, the MeteoGroup and the state weather services of Poland and the Czech Republic (IMGW and CHMU). You can see who operates which station using the relevant information here in the LHWZ web portal (see the following figure). Data from the Free State's precipitation measurement network can be downloaded here.

Additional information

The following links provide access to precipitation data:

Contact person for the FAQs:

Petra Walther

Telephone: 0351 8928-4514

Email: [email protected]