KM-I-5: Change of flow direction | Umweltbundesamt

KM-I-5: Change of flow direction

At the Baltic Sea, storm surges in particular can lead to a change in the direction of flow in sections of watercourses close to the coast. Due to climate change, the risk of such backwater situations is increasing. At the gauging station Anklam, Mecklenburg-Western Pomerania, the number of days with flow direction changes has increased significantly since 1961. The readings from the other gauging stations do not show any trends, possibly because the time series are still relatively short.

Discharge from coastal watercourses influenced by storm surges

Where sea and rivers meet, the interplay of natural forces creates unique natural spaces. In estuarine areas and coastal watercourses, surf and tides interact with the river currents from further inland. Owing to variable freshwater and saltwater proportions, the dynamic landscapes are characterised by specific ecological properties. Zones of brackish water provide habitats for specialised animal and plant species, that can be used as ‘nurseries’ by fish species considered of economic importance. In many places, estuarine and brackish water areas inform the character of the North German coast.

For example in the North Sea, the tides influence the hydrodynamic scenarios in sections with coastal watercourses: During ebbtide, the river water can flow unhindered into the estuarine area, whereas at high tide this flow is slowed down or the river water is backed up. Especially when strong wind and a rough swell force seawater into estuarine areas and inland waters, it can happen that in a section near the coast, the flow direction of a watercourse is temporarily reversed. In cases of any such backwater events, the gauging stations alongside the river sections concerned record negative discharge values. If the tidal force is rather weak as in the Baltic Sea, storms are always the decisive factor responsible for causing backup in a watercourse. In particular when water levels are high, strong wind can push the seawater far into the inland waters. Climate change favours the occurrence of backwater events: Rising sea levels generally cause higher water levels on the coast. Rising sea levels also cause the baseline level of storm surges to rise thus causing these surges to increase water levels (cf. Indicator KM-I-3).

Apart from the maritime influence of storm surges and rough seas, the backing up of watercourses is linked with specific characteristics of water bodies. Watercourses prone to turning into backwaters are characterised by a (very) weak water discharge and low valley floor gradient (< 0.5 permille, more rarely up to ≤ 2 permille). These conditions apply to the young moraine landscapes of Mecklenburg-Western Pomerania and eastern parts of Schleswig-Holstein shaped by the last ice age. In areas of shallow Baltic Sea coasts, watercourses flow into a slightly brackish coastal water, silted up to various extents. The Peene river, for instance, flows into the Stettiner Haff, a brackish water area separated from the Baltic Sea by a spit or the island of Usedom. The mineral-rich bed of a watercourse prone to causing backwater is usually distinctly below the water level of the Baltic Sea. Owing to natural sedimentation processes, mud is often deposited in the bed of the water body.

When a watercourse is backed up, this can bring about changes in the ecological and hydro-dynamic characteristics in, and pertaining to, sections of the watercourse concerned. For example, during storm surges the river mouth of the Peene into the Baltic Sea is shifted inland thus exposing areas to the influence of the sea which were never before exposed to such influences. The mud deposits can cause lack of oxygen impacting the water quality as well as the ecological structure of the water. Moreover, backwater can also exacerbate the risk of flooding areas alongside the inland watercourse. In such cases dykes would be the primary flood protection tool. However, such dykes are usually dimensioned for flood events which arise in areas further inland.

Four gauging stations were selected for the indicator to show the number of days with flow direction changes for selected watercourses – prone to backing up – flowing into the Baltic Sea⁸¹. It was significant for the gauging station to be selected in a location that was as little as possible influenced by anthropogenic activity. The gauging stations differ in terms of their exposure to the Baltic Sea, the topographic nature of the beds of the watercourses and the gradient of the valley bottom. Two of the gauging stations are located at the Peene river: The gauging station Anklam is located in the Peene’s river mouth where it flows into the Stettiner Haff. The gauging station Demmin is located 58 kilometres further inland thus indicating how far inland the watercourse is backed up. The gauging station Lübeck-Moisling at the Trave river is located in an area that is exposed to both coastal and river flooding. Despite its distance of 20 kilometres from the coast, this gauge reacts to raised water levels in the Baltic Sea and to storm surges. The gauging station Füsing at Füsinger Au is located in the backup area of the river Schlei.

Owing to the influence of individual extreme events, the time series proceed subject to fluctuations. In the hydrological year of 1989 the gauging station Anklam witnessed – for the first time – a flow direction change caused by backwater occurring on more than 100 days. Notably in August of that year, negative discharge values were recorded when a severe storm with hurricane-like gusts resulted in flooding the Baltic Sea coast with waves several metres high causing substantial damage. In the hydrological years of 2019 and 2020, with similar frequency, a backwater situation was recorded at the gauging station Anklam. In the beginning of 2019 two storm surges in quick succession contributed to flow direction changes in the river Peene. In that year the highest value of the time series so far occurred, with flow direction changes recorded on 84 days by the gauging station Demmin. The greatest number of days with flow direction change recorded by the gauging station Lübeck-Moisling was measured in September 2014: At the end of the month, the low-pressure system ‘Gudrun’ impacted the Baltic Sea area. As the wind in the western part of the Baltic Sea gained momentum and shifted north, the water level on the coast reached the monthly maximum value and pushed seawater up the coastal watercourses.

In northern parts of Germany, the summers of 1992, 2003, 2018 and 2019 were characterised by extraordinary heat and low precipitation levels. Consequently, the runoff from watercourses was distinctly reduced. The low discharge volumes of the rivers Peene, Trave and Schlei may have favoured the flow direction changes recorded in those years.

^{81 - Pottgiesser T., Sommerhäuser M. 2008: Begleittext zur Aktualisierung der Steckbriefe der bundesdeutschen Fließgewässertypen (Teil A) und Ergänzung der Steckbriefe der deutschen Fließgewässertypen um typspezifische Referenzbedingungen und Bewertungsverfahren aller Qualitätselemente (Teil B), UBA-Projekt (Förderkennzeichen 36015007) und LAWA-Projekt O 8.06. Steckbrief Typ 23: Rückstau- bzw. brackwasserbeeinflusste Ostseezuflüsse: https://www.flussgebiete.nrw.de/system/files/atoms/files/21_typ23_april2008.pdf}