Flood protection in Dresden
Dresden lies on the Elbe and several bodies of water that arise in the Eastern Ore Mountains. Due to the proximity of Dresden to the mountains, in which a lot of water can rain down or be stored in large quantities as snow, flood protection plays an important role historically and currently in Dresden . As clear protective measures in urban development, areas near the banks were left free of buildings along the Elbe with the Elbe meadows , embankments were removed and two flood channels were created. Both the historic city center and numerous historic village centers along the Elbe are elevated and are therefore protected from most floods. In particular through theHowever, the Elbe floods in 2002 caused great damage in Dresden. After decades without severe flooding, this reawakened general awareness of the threat to the city.
Location and rivers
Elbe
Dresden is located in the Dresden Elbe valley , a partially narrowed breakthrough valley , largely a flat rift valley . Upstream, the Elbe leaves its Czech catchment area through the narrow and steep valley in the Elbe Sandstone Mountains. There it drains via tributaries such as the Vltava , Orlice (Adler / Orlitz), Jizera (Iser) and Eger the Giant Mountains , the Central Bohemian Uplands and the Ore Mountains as well as the Bohemian Forest and the Bavarian Forest . Down the river from Dresden there is the breakthrough through the Spaar Mountains and the Meißner vineyards. Only behind it does the flat and slow middle course of the Elbe begin.
The Elbe flows through the city in several shallow, but also narrow curves ( meanders ) that narrow towards the west. Shortly before the city center there is a 4 km long curve that bends from north-west to south-west, at the beginning of which the course of the river moves up to a few meters on the northern slope of the Elbe . In the city center, the Elbe bends to the northwest, only to turn again strongly to the south at the so-called Pieschener Winkel. Later, after two more sharp bends, she leaves the city in a north-westerly direction. The entire length of the river in Dresden is around 30 kilometers.
The Elbe valley offered sufficient space in the urban development of Dresden around the river. For a partial flooding (compare Elbe flood canal near Magdeburg) the inner city was on the one hand insufficient , on the other hand the increase in the hinterland of the bank areas was too strong.
Weißeritz
The Weißeritz is an outflow of the Osterzgebirge and arises not far from Dresden through the confluence of the Wilder Weißeritz and Roter Weißeritz in Freital . The total length of the (combined) Weißeritz is 12 kilometers. Both tributaries have about the same size catchment areas, which add up to 323.9 km². They arise at an altitude of 823 m and 787 m respectively, about 30 kilometers as the crow flies southeast of Dresden. The longest tributary of the Weißeritz is the Wilde Weißeritz with a length of 49 kilometers.
The (united) Weißeritz originally flowed into the Elbe not far from Dresden city center and separated the western suburb of Friedrichstadt from the city center. It was moved to the west in 1893, starting in the Plauen district and running through Cotta. The relocation actually took place for reasons of flood protection, since the new course lies in a flood basin of the river. The relocation was not reversible due to the construction of railway systems on the former river bed.
More waters
The Lockwitzbach , which drains an area of 80 km² and whose source is around 500 meters above sea level, also rises in the Ore Mountains . It flows into the Elbe between Kleinzschachwitz and Laubegast .
The Geberbach and the Kaitzbach , which run largely underground in the Dresden city area, come from central locations in the Ore Mountains . The Prießnitz flows into the Elbe from the north and drains the flatter location of the West Lusatian hills and mountains in the northeast of the city.
Flood risk
Dresden is at risk of flooding from two directions. On the one hand, strong floods of the Elbe threaten low-lying parts of the city; on the other hand, the backwaters of water body class I (according to the Saxon water regime ), i.e. above all the Weißeritz and the Lockwitzbach, can also flood higher-lying parts of the city. In addition, there may be local danger from further backwaters of water body class II.
Damage to structures is not only caused by flooding, but also by phenomena accompanying floods such as the increase and displacement of groundwater. Floods of the Elbe trigger very long-lasting changes in the groundwater.
Elbe
Flood
The Elbe in Dresden has an average water level of 200 cm. The high water alarm levels were set from levels of 400 (until July 2012: 350), 500, 600 and 700 cm. This water level is measured at the Augustus Bridge. The flow rate in Dresden is around 350 m³ per second at a level of 200 cm.
Water levels between four and five meters have almost no consequences for the city. If the Elbe exceeds five meters, roads and paths close to the Elbe are flooded by the Elbe and have to be closed. In Dresden city center, this is the Terrassenufer , on which the berths of the White Fleet are located.
Areas near the Elbe are at risk from an increase in groundwater between six and seven meters. As a rule, property protection on individual structures begins at such heights.
Water levels above seven meters endanger the first districts such as Gohlis in the west of the city and Laubegast and Pillnitz in the east. The 3200 meter long dike from Niederwartha , past the village center of Gohlis to Stetzsch , is designed up to a water height of about 7.40 m (Dresden gauge), as the area serves as a flood polders from this height .
Above eight meters, the situation in large parts of the city worsens by leaps and bounds because the Elbe then not only fills the old arms of the Elbe, but flows through them completely, and the flood area is no longer sufficient in many places. The old Elbarm in the southeast of Dresden surrounds the districts of Laubegast and Kleinzschachwitz and extends to the higher districts of Leuben , Dobritz and Seidnitz . The Elbe then also fills the mouths of the backwaters. In this case, the mouth of the Lockwitzbach shifts a few kilometers to the southern edge of the Elbar between Niedersedlitz and Kleinzschachwitz. West of the city center, the Elbe then flows through two almost parallel courses with the two flood channels, which are connected by the main stream halfway. Due to the centrifugal force of the flowing water, the flood channels relieve the tight curve at Pieschen (Pieschener Winkel), which would otherwise lead to massive embankments.
The flood in 2002 had a maximum level of 9.40 meters and a flow rate of more than 4,500 m³ per second. The extreme water level over nine meters also endangered the Semperoper and Frauenkirche . The Elbe flooded an area of around 24.8 km² in the urban area.
Frequency and origin
The Elbe is a rain-snow type of flowing water. In general, the water masses that pass Dresden arise on the courses of the Elbe or Vltava in the Czech Republic and to a lesser extent in Germany. Floods between four and five meters are very common in Dresden, especially after the snow has melted. In particular, due to accelerated thaws (strong temperature jump and rain - therefore rain-snow type ), these then decrease in frequency up to seven meters. The strong thaw flood in spring 2006 was the first spring flood in 52 years that the seven-meter mark in Dresden was able to pass. The strongest winter flood also occurred in March 1845. The low mountain ranges in eastern Germany and especially in the Czech Republic have a problematic effect, as they can experience very intense winters with permanent frost and snowfall due to their continental location. If the general weather conditions change late, the snow in the mountains can then, usually in mid to late March, melt heavily.
The strength of a flood in a thaw depends not only on the weather, but also on the nature of the deposited snow . If, for example, it is saturated with water due to a previous thaw, but is frozen again, it can melt much faster in the event of heavier rain. The depth of penetration of the ground frost or the absorption capacity of the ground also play a role during the melting of the snow. The deposited snow masses are therefore measured especially in the last winter months and are used as water equivalent in hydrological observations and forecasts.
Summer floods after heavy rainfall are very rare on the Elbe. The direction of flow of moist air masses also plays an important role, since not all low mountain ranges have the same ridge orientation on which they rain down. Flood triggering are especially Vb weather conditions from the south , which cause heavy incline rain in the Ore Mountains, Bohemian Central Mountains and Giant Mountains. The flood in 2002 is specified with a return interval as such an event with 100 to 200 years for Dresden.
Floods like the one in August 2002 or June 2013 are not caused by heavy rainfall alone, but only by a certain sequence of defrosting or raining in the Czech Republic, as the areas in the catchment area are far apart and the tidal waves of the Eger, Elbe and Vltava rivers meet must overlay.
For a number of years there has been discussion and research into the extent to which global warming causes extreme weather conditions, i.e. heavy rain in summer or high snow cover and late thawing. It cannot therefore be ruled out that the frequency of Elbe floods has increased or increased. The effects of the Děčín barrage planned in the Czech Republic not far from the German border are also controversial .
Registered high tides
In Dresden, at least extreme flood events have been registered for centuries. The floods in the meteorological summer quarter are shown in bold below. Water levels depend largely on the runoff. This relationship is changed by the flow velocity and the flow profile.
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Groundwater
Dresden is largely located in the Elbe Valley , which was created as a short rift valley in the tectonic connection with the elevation of the Erzgebirge . The Elbe valley basin is shaped by the groundwater-retaining rock layers made of clay , sandstone and plan (layered lime-sand-clay rock). Above it, the Elbe deposited layers of gravel and sometimes sand.
Close to the shore, the groundwater levels in Dresden closely follow the high tides of the Elbe. At Dresden Castle , the peak levels in the groundwater appear about two to three days after passing through a flood peak. In places that are not affected by infiltration or drainage of the flood, there is a less pronounced jump in the groundwater depth and a longer time lag.
After the flood in 2002, the groundwater levels in Dresden remained widespread (and in contrast to the Elbe) permanently above mean water. In 2002, the previous local floods and precipitation also affected the state of the groundwater during the Elbe floods. Where Weißeritz and Lockwitzbach had caused additional seepage through flooding a few days earlier, the rise in groundwater could hardly be dampened. Precipitation also influences the damping behavior of the groundwater. Spring floods have less of an impact on the groundwater, as experience has shown that significantly smaller water masses thaw in Dresden than in the mountainous areas and affect the behavior of the groundwater. The flood in August 2002 triggered a continuous rise in groundwater in places that are further away from the Elbe but still in the bottom of the Elbe valley, which was not yet completed in March of the following year.
Weißeritz
Danger
In addition to the original and new estuary areas, the Weißeritz also endangers other parts of the city in which this was not to be expected. In 2002 she left her bed in Löbtau at the point where she branched off from her original course. This happened at a maximum flow rate of 450 m³ per second, measured approximately at the entrance to the city area, which corresponds to a higher flow rate than the Elbe in Dresden flows through at normal conditions. On the one hand, the Weißeritz ran north and flooded Friedrichstadt and the Wilsdruffer suburbs . In an easterly direction, it filled the suburb of the lake up to the Great Garden and above all the main train station . From this area, in which there were originally several lakes, there was no outflow towards the Elbe. The original ridge within the Elbe valley, which separated the Weißeritz valley from the Kaitzbach valley, was broken through the construction of the railway line into the Weißeritz valley. In 2002 the Weißeritz flooded an area of 5.67 km² in Dresden.
In Dresden, gauges are only measured for the Weißeritz near its mouth, which is more used to assess the amount of water it is fed into the Elbe. If the Weißeritz can be kept in its specified course, floods will have little consequences for the city. Weißeritz floods run quickly and are more difficult to predict than those of the Elbe.
Frequency and origin
As in 2002, flooding in the Weißeritz can be caused by heavy rain or thaw in the Eastern Ore Mountains. In recent years, snow depths over 1.50 meters have been recorded several times in the upper valleys . In the case of continuous rain and warm air masses, a risk from rapid defrosting can arise. The trigger for the floods in 2002 was heavy rainfall of more than 300 liters per square meter in 24 hours in the catchment area of the two rivers.
An event comparable to the summer flood of 2002 occurred much weaker in 1897, the last flood on the Weißeritz in 1958. The State Office for Environment, Agriculture and Geology estimates the recurrence interval for such events at 500 years. Here, too, it is currently being investigated whether such events are increasing. The frequency of the initial meteorological situation of heavy continuous rain was estimated to be less than once in 100 years. The Vb weather situation caused the Oder flood in 1997 and in 2005 also triggered the flood in the northern foothills and central Alps . However, such extreme precipitation (312 mm in 24 hours in Zinnwald-Georgenfeld ) as in 2002 in the Eastern Ore Mountains is still regarded as rare.
In Dresden, the Weißeritz can generally discharge 220 to 420 m³ of water per second without overflowing. There it is expected to overflow as a result of floods every 20 to 50 years. At bottlenecks, however, the capacity in the river bed is only 75 m³ per second and is therefore considerably smaller.
More waters
The Lockwitzbach in the southeast of Dresden flooded Lockwitz and parts of Niedersedlitz , Kleinzschachwitz , Leuben and Laubegast in 2002 . It flooded a system of relief ditches and parts of a river Elbe and had a flow rate of more than 45 m³ per second at the entrance to the Dresden city area. Such events have an accumulation of about 200 years; In 1958 and 1995 the last weaker floods occurred. The river bed of the Lockwitzbach in Dresden holds 25 to 40, at bottlenecks only 15 m³ per second. It is therefore estimated that, on average, an overflow occurs every 20 to 50 years. The Lockwitzbach flooded an area of 2.313 km² in Dresden through the Niedersedlitzer flood ditch and other relief ditches.
The Prießnitz also endangers Dresden on the edge of the Outer Neustadt .
Smaller streams such as the Weidigtbach flowing in the west of Dresden , which was heavily built-up in GDR times, have now been exposed again in many places or even designed to be natural and have been supplemented with smaller retention basins in order to be able to absorb a larger volume during heavy rain and floods.
Endangering the infrastructure
Important components of Dresden's infrastructure are not located in the historical, higher-lying core of the city, but in the upstream suburbs, which today also largely belong to the inner city. Above all, railway systems and public transport hubs are located in a semicircle in Seevorstadt , Wilsdruffer Vorstadt and Friedrichstadt .
All these parts of the city are threatened by flooding from the Weißeritz, but so are the supporting elements in the infrastructure. In 2002, tram, rail and road traffic in the southern old town collapsed in the first 24 hours of the flood disaster. Particularly affected points and traffic hubs are Postplatz , Könneritzstrasse at Dresden Mitte station and Wiener Platz / Hauptbahnhof . Transport hubs that can then take on central tasks are Pirnaischer Platz and Dresden-Neustadt train station .
The city's hospitals, some of which had to be evacuated, such as in Friedrichstadt, were particularly affected . In Dresden, not multi-storey car parks but underground garages were built in numerous places , which completely fill with water when flooded. There is also a road tunnel on Wiener Platz , which was flooded in 2002 together with the adjacent underground car park at the main train station.
The Elbe floods put a strain on the infrastructure, especially when bridges are closed. Apart from the closures of the terrace bank and between Blasewitz and Laubegast, weaker floods have little consequences for the tram and tram network. As a result of strong Elbe floods, however, groundwater damages the substructure of roads.
Endangerment of residential areas
Many districts with different structures and population densities lie along the Elbe ( see map above ). The districts further up the river are populated with 300 to 4,500 inhabitants per square kilometer. In the vicinity of the city center, the density increases to up to 8,600 inhabitants per square kilometer. A total of around 155,000 people live in all parts of the city directly on the Elbe.
Mainly in the southeast are the districts of Zschieren (left Elbe ), Kleinzschachwitz (l), Pillnitz (right Elbe), Wachwitz (r), Laubegast (l) and Tolkewitz (l) with village origins and currently still looser development and settlement. Almost all of the historic village centers in these districts are so high that they were not flooded during the 2002 floods. The southern parts of the city are particularly endangered by enclosure, which makes the supply of drinking water, electricity and food difficult and sometimes impossible. As a rule, therefore, when the enclosing Elbar is flooded, these quarters are evacuated. Later built-up areas in Laubegast and Kleinzschachwitz, both of which are among the best residential areas in the city, are now also significantly lower, partly directly on the shallow silted oxbow lakes of the Elbe. They are threatened by groundwater and surface water. In particular, north-western parts of Laubegast are marked as a floodplain . Through the old Elbarm, parts of Gruna are also flooded in the event of a 100-year flood.
The districts of Blasewitz , Striesen , Johannstadt and Pirnaische Vorstadt are located further towards the city center , some of them also directly on the left bank of the Elbe. Even with strong floods, there are only very few places in these parts of the city (for example in Blasewitz), but then widespread damage is caused by high groundwater.
Below Dresden's inner city, Pieschen (right-Elbe) in particular is at risk from flooding because there, on a sharp curve in the Elbe, strong floods cannot be warded off by fixed dykes. In the event of a 100-year flood, it is assumed at this point that large parts of the Pieschen, Trachenberge and Mickten could be flooded if the Elbe overflows.
Shortly before leaving the Dresden city area, the Elbe passes the Gohlis district , which belongs to the Cossebaude village . Gohlis is one of the first areas on the Elbe in Germany to be protected by a dike (here a pond dike for the controlled flooding of a polder). This is designed for water heights up to about 7.40 meters Elbe level and even withstood the 7.49 meter high water in 2006 . A detailed background description of the nature of protection can be found in the chapter on the history of flood protection .
Its tributaries (as well as the infrastructure) pose a greater risk to residential areas in many respects than the Elbe itself. The rivers are raging during flooding and carry a lot of material and debris with them. The Weißeritz reaches the Dresden city area at 159 meters above sea level and then falls about 55 meters to the point where it flows into the Elbe. As a result, it floods the coastal areas of the Coschütz and Plauen districts with high flow velocities. At the present time, in such situations, it is only possible to evacuate people from their houses with helicopters, especially in the narrow Plauen area . Sometimes there was total destruction of buildings. In 2002 the floods of the backwaters also claimed human lives in Dresden. The focus of improving flood protection in this area is primarily on extending the warning times .
The floods in Friedrichstadt and Wilsdruffer Vorstadt in the west of the city center caused by the Weißeritz hardly differ in their flow behavior from floods caused by the Elbe. But even in these parts of the city, the flow speed was still sufficient, especially on long streets without great flow resistance, to undermine streets and railway tracks.
History of flood protection
As far back as the year 1216, the year Dresden was first mentioned as a city in a document, records of the Elbe floods are available. In Dresden, with few exceptions, village centers on the Elbe were either artificially raised or only laid out in higher elevations on surrounding mountains and slopes.
The Elbe flood in March 1845 can be regarded as an important event for flood protection in Dresden . Like many other cities on the Elbe, it affected Dresden devastatingly, mainly because it was a spring flood with ice drifts. This flood provided information about the runoff behavior and the areas hit by floods. Such a flood was mapped for the first time.
A few years later, the city of Dresden began to grow in the early days . The decisions on urban development made in these years are considered to have shaped the course up to the present day.
In 1865 the width of the Elbe and its banks in Dresden was determined. As a result, the river was deepened in order to increase the flow capacity and improve navigability. The definition of the Elbe meadows went hand in hand with the removal of building land and buildings and was operated under two important aspects: On the one hand, the preservation of the visual relationship between the Brühlscher terrace and Elbe castles and the Waldschlösschen should preserve the cultural value; on the other hand, a flood like Passed the city as harmlessly as possible in 1845. For the Elbe meadows above the mouth of the Prießnitz, building clearance was set.
In 1869, the Saxon Ministry of Finance determined the areas suitable for building. The flood-free riverside street - the Käthe-Kollwitz-Ufer - was created as a building framework between Blasewitz and the city center . In the 1870s the suburbs and suburbs grew enormously, especially Johannstadt . At the turn of the century, this consisted of closed buildings. Blasewitz went on as a suburb with a wealthy population in individual buildings. The proximity to the Elbe no longer stood for less valuable and endangered building land as it was in the villages, but for particularly expensive land and as a guarantee for visual connections. The construction of the three Elbe castles on the northern slope of the Elbe in the middle of the 19th century particularly favored the assessment of the building areas on the opposite bank in Blasewitz and Striesen. Without regulation of the building area, the development would in all probability have channeled the Elbe .
After another flood in 1890, the flood channels were created as flood relief beneath the city center. While the flood channel through the Ostragehege affected an undeveloped area, which was also considered very susceptible to flooding, a lengthy dispute broke out with the landowners about the flood channels between Mickten and Kaditz. The flood channel in the Ostragehege was created in 1904 together with the slaughterhouse and Alberthafen . Starting directly at the Marienbrücke , it relieves the Elbe and prevents it from building up in the city center by slowing down in the following curves. In autumn 1918, construction of the Kaditz flood channel began , which was preceded by expropriation processes.
In 1904 work began on renovating the old Augustus Bridge. The new bridge was wider and therefore more suitable for road traffic and, thanks to its wider arches, made shipping easier. This was accompanied by a reduction in storage space and the risk of ice wedging.
The polder system around Gohlis and Stetzsch in the west of Dresden emerged at the beginning of the 20th century. Gohlis is one of the villages that is low so it was flooded in 1845. When expanding the dykes in the west of Dresden, Gohlis himself rejected a dyke that could protect it from a hundred-year flood. The regulated flooding of built-up areas through the polders during high tides was preferred to the risk of a destructive dike breach. The area is still an important relief area for Radebeul on the opposite side of the Elbe. The spring flood in 2006 showed that the polders can withstand a permanent water level of up to 7.40 meters at the Dresden gauge. When this height is exceeded, the Gohliser Flur begins to flood. Furthermore, the pond dike Gohlis is supposed to protect against the threat of drift ice during the frequent winter floods, as the ice floes cause enormous damage to houses in strong currents. Even in the event of flooding, the dike is intended to guide the main flow of the river past the built-up areas.
The old Elbarme in the east of Dresden and on the areas of Heidenau and Pirna were not diked. As it turned out, these areas represent an important retention area for Dresden's inner city, as they are the first areas after the breakthrough valleys have passed, in which flood peaks can become blunted.
Flood protection
Flood rooms
Dresden has a large flood area for the Elbe. The Elbe meadows run through the entire city and even in 2002 offered protection to residential areas that are in the immediate vicinity of the Elbe (for example Striesen , Johannstadt and Blasewitz ). This meadow landscape is interrupted by groves and hedges and is between a few meters on the Elbe slope and a few hundred meters wide. In some cases, as is the case with Laubegast, buildings outside the city center extend to the shore.
In addition to the Elbwiesen there are two flood channels (in Dresden flood channels called), the left and rechtselbisch meander break through, so the river in case of floods and straighten. The left Elbe flood channel was created within the meadow landscape of the Ostragehege in connection with the construction of the Alberthafen . This flood channel washes around a heaped circular mountain on which the new cattle yard and slaughterhouse, built according to plans by Hans Erlwein , is located. The slaughterhouse bridge was built over the flood channel when the slaughterhouse was set up. Meanwhile, the fair is also located on this hill . For a long time, however, the flood channel was not free of construction because there was a flow obstacle due to the ice rink located there. In the course of a replacement building after the floods of 2002, the hall was demolished, which improved the drainage behavior at the narrow point between the old town and the new town. It should be flown through from an Elbe level of 6.20 meters at the Augustus Bridge.
The right-Elbe flood channel between Mickten and Kaditz , about four kilometers west of the city center, was created between 1918 and 1922. At this point there was already an eroding oxbow lake of the Elbe, which was created during floods (compare meander erosion ). The flood channel means that the Übigau district presents itself as an island during floods. Compared to the first, this flood channel is deeper, but also narrower. It should be flown through from an Elbe level of about 5.50 meters at the Augustus Bridge.
In the southeast, an old Elbarm also serves as a flood area. However, this has not been kept free of buildings in recent years and is only reached by the Elbe at very high levels. It encloses the districts of Zschieren , Kleinzschachwitz and Laubegast . It is connected to the Elbe both between Kleinzschachwitz and Laubegast and between Laubegast and Tolkewitz. It is partially flooded before it can carry water. Since the Lockwitzbach flows into the Elbe between Laubegast and Kleinzschachwitz, water from the Elbe first runs back into the Elbe via this short arm. This eastern part is only used for mining (gravel extraction) and agriculture. The nearby districts, including Sporbitz , are elevated. The connecting roads from Kleinzschachwitz and Laubegast to the outside are flooded during the highest floods and the districts are cut off.
The Elbarm, in which water masses from about seven meters in width spread out, can flatten flood peaks. But it also shifts the edge of the floodplain to the vicinity of districts in the hinterland such as Leuben , Dobritz or Seidnitz . Danger for these parts of the city arises if the oxbow lake is traversed over its full width. However, it is also effective as a retention space before the flow is complete.
A map showing the spread of the flood of March 18, 1845 shows that the corridors of Johannstadt and von Striesen were flooded far beyond the Elbe meadows. At that time this area was used for agriculture or was forested. Until the turn of the century, this flood area was built in the course of the expansion of Johannstadt, Striesen and Blasewitz. The area between Mickten , Trachau and Pieschen was also flooded. This area, which is located on an old Elbarm, was later built on, but was defended in 2002 by a sandbag wall.
Dike systems
In Dresden there are only a small number of dikes , such as in the middle and lower reaches of the Elbe. In the west, Gohlis and parts of Cossebaude are protected by a dyke system that offers protection up to about 7.40 meters high tide. This is not a fully developed protective dike, but a polder system that is intended to provide relief through retention in the event of high floods. In particular, the high risk from eight meters of the Elbe level is due to the fact that there are no dikes protecting the districts, especially in the east near Laubegast and Pillnitz. In these parts of the city, however, the old village centers are located at higher altitudes. The only newer development is between the historic cores and thus deeper. The first dike on the German course of the Elbe is located in Übigau on the stretch between the Flügelwegbrücke and the A 4 bridge .
Often, however, the Elbe meadows and flood channels are closed off by a very flat dike, usually the embankment of a road or a path. One example of this is the Käthe-Kollwitz-Ufer street, which connects Johannstadt with Blasewitz as a riverside street.
The inner city can be protected from floods with a Dresden water level of up to 9.24 m by a flexible protective wall system, see section “Expansion in the 21st century” .
Flood retention
In the last century in particular, systems were created in the surrounding area to catch, hold back and regulate floods from the Erzgebirge runoff . There are no barrage or retention systems for the Elbe in the German upper reaches . In the Czech Republic, there are many reservoirs and stages (→ Vltava cascade ), especially on the Vltava and its tributaries , such as the 68-kilometer-long Orlík Dam and the Lipno Reservoir . The Vltava and Elbe have numerous barrages in their Czech courses, which more or less serve as flood retention.
The floods of the backwaters of the Ore Mountains can be held back in several systems. For the Weißeritz there is the Malter dam (Rote Weißeritz), the Klingenberg dam and the Lehnmühle dam (Wilde Weißeritz). After the flood in 2002, the flood storage space was increased again. These systems not only serve to protect Dresden, but also to protect surrounding communities and cities such as Freital .
There is a retention basin for the Lockwitzbach in the upper reaches of Reinhardtsgrimma . For the Geberbach there is a dam south of Kauscha , some of which is also in Dresden.
Areas near the Strehlen district were kept vacant for the Kaitzbach . These areas were expanded in 1999 to form the Hugo-Bürkner-Park retention basin with a capacity of 11,000 m³. The storage volume turned out to be too small during the floods in 2002 and so the left basin was deepened by two meters at the beginning of 2006. The system now has a capacity of 20,000 m³ and was completely dammed shortly after the expansion when the backwaters flooded during the floods of the Elbe in March 2006. In addition to Strehlen, the facility also protects the southern suburb and especially the Great Garden . Further retention basins in the catchment area of the Kaitzbach are being planned.
Property protection
Individual buildings must be separately protected against flooding, such as the congress center and the Heinz Steyer stadium in the west of the city center . The stadium would already fill up at about seven meters high tide, while the congress center directly on the bank is protected by a wall up to eight meters.
Significantly more buildings have to be secured against uplift due to the rising groundwater level. This is necessary for new buildings that have a pressure-resistant cellar. Buoyancy is prevented by pools of water that are filled in the building. In 2002, for example, this procedure had to be used in the construction of the St. Benno grammar school in the Pirnaische Vorstadt .
As before, numerous streets and buildings are protected with sandbag walls . This is the case between the synagogue (which is elevated) and the Brühlscher Terrasse to defend the Bärenzwinger , near the Yenidze and during extremely strong floods on the Neustädter Ufer .
Responsibilities and missions
Various institutions, offices and administrations are involved in flood protection. Some of these are constantly active or are only convened in the event of a disaster .
Warning and water management
The flood center of the State Office for Environment, Agriculture and Geology is responsible for controlling and monitoring the river level . This flood center serves as a nationwide warning of floods. It issues official warnings that are decisive for the actions of the municipalities and districts in the flood areas. Via the siren warning system of the city of Dresden, spoken flood warnings can be transmitted to the population on a site-specific basis. The dikes along the Elbe are looked after by the state dam administration of Saxony . Not the environmental office of the city of Dresden, but the dam administration decides on the expansion of dikes in the city.
commitment
The state dam administration is responsible for controlling the dams and retention basins. In the event of floods, backwater space is created in the dams in coordination with the flood center and, as far as possible, the flood peaks are influenced. In the course of a flood, the flood center also provides information about the storage space that is still available. Flood storage space can be built up at short notice. In order to expand this storage space at short notice, as is often required, a bottom outlet would have to be installed. However, this is not realistic within the scope of the advance warning times or would overload the flow capacity of the waters below the dams themselves and lead to overflow.
As a municipality, the city of Dresden is responsible for the right and duty to issue disaster pre-alarms and alarms. In Saxony, which is an emergency alert by § 47 of the Saxon Law on fire safety, emergency services and civil protection triggered. With this, the city calls the technical relief organization and the armed forces for administrative assistance . It can also order evacuations in the event of this alarm. In the event of extreme events on the tributaries, in particular on the Weißeritz, evacuations can sometimes only be carried out with the help of the Search and Rescue units of the Federal Republic. These are operational within a few minutes, regardless of the disaster alarm. In 2002, the Bundeswehr's MedEvac aircraft had to be used to evacuate the hospitals . Most of the patients were transported to surrounding hospitals and clinics.
The city generally triggers a disaster pre-alarm if there is a risk that the Elbe will reach a level of seven meters. When this highest flood alarm level is reached, the disaster alarm is triggered for the districts on the Elbe. In the event of floods in the backwaters, the city triggers a disaster alarm for almost the entire city area or does not limit the alarm at all.
Logistics and transport
Dresden Airport is an important base for supplying Dresden, but also the entire conurbation in the Upper Elbe Valley . It is located at a height of 230 meters above zero northwest of the city center and is therefore flood-proof. From this airport, hospital patients can be evacuated, but heavy equipment, such as swimming tanks , can also be flown in.
The federal motorway 4 , which crosses the Elbe in the west of the city, can remain open during floods, which means that the city can be reached from all directions.
documentation
The task of documentation, in particular of the floodplain and the runoff behavior, is carried out by the Environment Agency in Dresden. Both in 2002 and during the flood in 2006, extensive aerial photographs were taken and evaluated with the help of the Air Force . In addition, the city calls on residents, for example on the Internet, to specify the extent of the flood at the peak. In 2002 and 2006 maps were so extensively edited and finished in great detail.
The validity of information about flood plains and especially about runoff behavior in extreme events is quite limited. Floodplains are changing enormously, especially as a result of new development. One example of this is the main train station, the lower terminus of which did not exist at the last event before 2002. Buildings also affect the relationship between flow rate and water level. Documentation plays an important role, especially in the case of floods every ten to twenty years. On the basis of such flood maps and empirical values, decisions can be made more easily as to where floods can be effectively averted by mobile walls or sandbag walls. On the basis of documented findings, it can also be decided which areas and objects can be defended at the predicted water levels.
Construction law is also affected by the designation of floodplain areas. In areas that were flooded in 2002, building permits are currently no longer issued, while between 1990 and 2002 in particular in some places, for example at Laubegast, structures were still built in the silted up arm of the Elbe.
Expansion in the 21st century
Since the floods in 2002, the city has been planning to protect large areas from surface water in the event of very high floods. A system of protective walls made of steel planks is planned, similar to that used in Prague for a long time . There, this measure protects the historic city center in particular. In this regard, there is an acute risk in the inner city of Dresden only for the deeper buildings on Neumarkt and Theaterplatz ; Damage to the Zwinger and the Semper Opera in particular can occur. Since 2011, these areas, including the western suburb of Friedrichstadt , have been secured against floods of up to 9.24 meters using fixed and mobile walls.
The water level of 9.24 meters (16 cm below the high of August 17, 2002) was set in 2004 as the new high for the HQ100 event of the Elbe in Dresden, with a peak of 8.76 m on June 6 In 2013, the devastating floods remained well below this mark eleven years later. Both during floods on the Elbe and during events on second-class waters, however, water levels can also occur that are more extreme than those in 2002.
After the floods in spring 2006, which threatened Gohlis in particular, it was decided to add a protective dike to the polder system there . Gohlis would also be protected from floods over 7.40 meters. The flat dykes on the Elbe, which were originally only intended to prevent soil erosion during floods, will be preserved. The polder area (and with it the flood area) will in all likelihood be smaller due to the protective dike.
The city of Dresden is calling for another retention basin to be built on the course of the Lockwitzbach between Kreischa and Dresden and has given the state dam administration a high priority for this expansion of flood protection. The city also calls for a further gauge to be installed at the level of the Lockwitztal Bridge on the city limits, as the Kreischa gauge in February 2006 did not indicate any danger for the districts along the Lockwitzbach, but the risk arose locally due to the displacement of ice during moderate floods.
Since the walling of Laubegast on the Elbe side proposed by the Environment Agency as a result of the 2002 flood was rejected by some residents of the district, a "participation process" was initiated in the district to find a solution. This was so slow that Laubegast was still defenseless when the Elbe flooded in 2013 and was therefore flooded to a considerable extent, similar to 2002.
See also
- Viennese regulation of the Danube
- Elbe flood canal and Pretziener weir
- Flood (protection) in Würzburg
Web links
General
- City of Dresden: flood
- Themed city map of Dresden: display of public flood prevention measures
Based on concrete floods
- Event analysis of floods in August 2002 in the rivers of the Eastern Ore Mountains. (PDF; 3.94 MB) Saxon State Office for Environment and Geology, accessed on December 15, 2016 .
- Documentation of the flood of August 2002 in the catchment area of the Elbe. (PDF; 4.11 MB) International Commission for the Protection of the Elbe , accessed on December 15, 2016 .
- Influence of the August 2002 flood on the groundwater. (PDF; 2.80 MB) Saxon State Office for Environment and Geology, accessed on December 15, 2016 .
- Flood in August 2002 in the rivers of the Eastern Ore Mountains. Event analysis: Management report. (PDF; 895 kB) Saxon State Office for Environment and Geology, accessed on December 15, 2016 .
- Hans-Ulrich Sieber: Effects of the extreme floods of August 2002 on the safety of the dams, flood retention basins and water reservoirs. German Dam Committee / State Dam Administration of the Free State of Saxony, accessed on December 15, 2016 .
- Map of the Elbe stream within the Kingdom of Saxony: with details of the flood area reached by the flood of March 31, 1845. In: Deutsche Fotothek - Karteforum. Retrieved December 15, 2016 .
Individual evidence
- ↑ Flood alert level 1 for the Elbe. In: dresden.de. State capital Dresden, June 26, 2013, accessed on December 15, 2016 .
- ↑ a b c Environmental Report 2001–2004: Facts on the Environment. (PDF; 4.88 MB) State Capital Dresden, p. 26, Table 5.1 , accessed on December 15, 2016 .
- ↑ Recurrence intervals 2002 floods: Documentation of the floods from August 2002 in the catchment area of the Elbe; P. 26
- ↑ Martin Schmidt: "Floods and flood protection in Germany before 1850", Harzwasserwerke GmbH Hildesheim, Oldenburg 2000
- ^ Environment Saxony: Dresden / Elbe gauge ( memento from May 26, 2009 in the Internet Archive ), June 6, 2013
- ↑ a b Return intervals 2002 floods: Event analysis floods 2002 in the Eastern Ore Mountains rivers; P. 53
- ↑ a b Overflow and capacity of the river beds: Event analysis of the floods in 2002 in the rivers of the Eastern Ore Mountains; P. 67
- ↑ Weidigtbach comes back to daylight in Gorbitz. State capital Dresden, May 22, 2015, accessed on February 16, 2018 (press release).
- ↑ a b Comparison of the floodplain in the "Atlas of Flood Hazard"
- ↑ Landeshauptstadt Dresden, Umweltamt (Ed.): Flood prevention for Dresden - Hugo-Bürkner-Park: A park stops the Kaitzbach when it gets "wild" . Dresden 2007 ( PDF; 2.1 MB ( memento from June 25, 2012 in the Internet Archive )).
- ^ Information sheet on the siren warning system in Dresden. (PDF; 134 kB) State capital Dresden, accessed on December 15, 2016 .
- ↑ Such a map function will be added to the interactive city map
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↑ Dresden Neumarkt is safe from floods. In: Saxon newspaper. March 27, 2007, accessed December 15, 2016 . Trial operation for the new flood protection gate: Weißeritzstraße closed from March 26th to 28th. In: dresden.de. State capital Dresden, March 25, 2011, accessed on August 15, 2015 .
- ↑ Floodplains of the Elbe for a 100-year flood event. (PDF; 35 kB) State capital Dresden, accessed on December 15, 2016 .
- ^ Report on the spring flood 2006 , "Conclusions for flood protection in Dresden" p. 20
- ↑ Living with the river - Laubegast participation process. In: dresden.de. State capital Dresden, accessed on December 15, 2016 .
