Untersuchungen zu anthropogenen Beeinträchtigungen der Wasserstände am Pegel Magdeburg-Strombrücke
M. Simon (September 2010)
Investigations of long-term observations on water levels and discharges are especially interesting as a proof of ongoing and anticipated effects of climate change on the discharge conditions of large rivers. Effects of climate change on the discharge conditions will be expected also for the river Elbe, therefore, available observation series of selected Elbe gauge stations had to be analysed. Low water conditions are in the focus of interest here.
For a first analysis, the gauge Magdeburg-Strombrücke which shows the longest observation period at the river Elbe was selected. For this gauge, continuous water level observations are available since April 17, 1727. Long observation periods are also available for the gauges Dresden (since 1776), Wittenberg/Lutherstadt (since 1817), and Barby (since 1841).
The recorded water levels cannot be used for statistical analysis without considering water engineering measures in the close area around the gauge as well as in the area downstream and upstream the gauge, so the author investigated the anthropogenic impacts on water levels at the gauge Magdeburg-Strombrücke. The question whether the constantly decreasing trend of low water levels is explicable by anthropogenic effects had to be analysed particularly.
Main focuses of the analysis were:
• the chronological development of the run of three Elbe channels into the city area of Magdeburg,
• the architectural changes in the close area around the gauge profile Magdeburg-Strombrücke,
• the different weir buildings with their different overfall highs in the Old-Elbe in Magdeburg (weir of Cracau),
• the construction of the Elbe bypass canal east of Magdeburg with the weir of Pretzien and its effects on water levels in the areas Schönebeck and Magdeburg,
• the effects of the recess of the Elbe riverbed as a result of water engineering measures, particularly by watercourse shortenings of the river Elbe by cut-offs and by construction of groins for medial and low water regulations,
• the effects of ice rates on water levels.
As a result, it could be shown that the following main factors were responsible for the development of the water levels at the gauge Magdeburg-Strombrücke:
• The watercourse shortening of the river Elbe by cut-offs downstream Magdeburg, particularly the three connected sinuosity cut-offs closely downstream Magdeburg from 1740 to 1789, caused the enlargement of the riverbed slope which induced an increase in flow velocity. So since 1740, a strong, long lasting, retrograde erosion arose with a recess in the Elbe riverbed with decreasing low water levels upstream the medial water region and overstepping the gauge station Magdeburg-Strombrücke.
• The medial water regulation (1844-1892), both low water regulations (1913-1930 and 1931-1950) as well as the increased regulation (1934 and 1935) with an extensive construction of groins still enforced the riverbed erosion resulting from cut-offs with decreasing low water levels.
• The profile extension of the river Elbe near the Magdeburg citadel (1876/78) in the area around the gauge Magdeburg-Strombrücke caused a decrease of the water level at least for medial and high water levels.
• The different reconstructions of the weir of Cracau in the Old-Elbe in Magdeburg with different heights of the overfall crown played a certain role for the gauge water levels in the area downstream the medial water at the gauge Magdeburg-Strombrücke in the river-Elbe. The blasting of the weir in 1806, until its rebuilding in the years 1816/17, had also effects on the low water level of the river Elbe because greater amounts of water ran off again by the Old-Elbe.
• Since 1875, the Elbe bypass canal with the weir of Pretzien improved the low water levels and the medial water levels in the river Elbe in Magdeburg because the discharge of water into the Old-Elbe lowlands east of Magdeburg was prevented by the closed weir. However, the water level decrease caused by riverbed erosion could not be prevented but only be reduced. In cases of larger floods, the drain of a part of the flood waters (up to 24%) through the opened weir and the Elbe bypass canal caused the decreasing of the water level in Schönebeck (up to 70 cm) and at the gauge Magdeburg-Strombrücke (up to 50 cm).
• Major dike constructions upstream Magdeburg with an essential reduction of flood areas, particularly from the middle of the 19th century on, led to increasing flood peaks during major floods, with a centennial flood of 15 cm. Hence, the effect of the weir of Pretzien was partly offset.
• An essential influence on the water levels can be also caused by important ice rates. In particular, closed ice covers (ice level) as well as agglomerating and drifting floes can cause noticeable water level decreases which could be a danger in case of flood waters.
In summary, one can say that the trend in the decrease of low and medial water levels at the gauge Magdeburg-Strombrücke started in 1740, mainly due to riverbed erosion as a result of the water course shortening (cut-offs) downstream Magdeburg, and due to the construction of groins as a result of the river regulations in the Elbe (since 1844). The high water levels are decisively influenced by the operation of the weir of Pretzien in the river Elbe bypass canal and by ice rates.
