Floodplain soils

from Wikipedia, the free encyclopedia

Alluvial soils form class A of the German Soil Systematics and are located in the department of soils influenced by groundwater . They are the dominant soil types in the floodplains of large river systems and are often closely associated with gullies .

Origin and Distribution

Alluvial soils are of Holocene origin and are found in the valleys of flowing waters. Their soil is composed of fluvial sediments and is influenced by a high groundwater level , which can fluctuate considerably depending on the water level of the river. Depending on the river, several meters are possible over the course of a year. Areas not diked are subject to periodic to episodic flooding and are therefore characterized by constant erosion and sedimentation (“floodplain dynamics”). In areas behind the dykes, the relocation of material has stopped, although overflows due to smoke water are possible.

A low level of hydromorphic characteristics (rust spots, anaerobia) is typical for floodplain soils , so that deep floodplain soils look more like terrestrial soils at first glance . For these properties to develop, the soil and the water it contains must contain oxygen. On the one hand, the groundwater level is mostly below 8 dm, so that long aeration phases occur outside of the floods. However, there is also a strong supply of oxygen due to the constant changes in the groundwater level. Since the groundwater is hydraulically connected to the river water, changes in the water level in the river are propagated in the groundwater. If the groundwater conditions are tense, the changes in water level can spread quickly and very far, even into the diked hinterland. The pore space of the soil is thus constantly filled and emptied with water, which at the same time results in the discharge of soil air and an entry of atmospheric oxygen. In the dike area, the river water, which is in exchange with the water in the bank areas, contains oxygen compared to normal groundwater. Depending on the flow dynamics, a mixing zone with oxygen-carrying groundwater can develop. The prerequisite for the strong groundwater fluctuations (and the intensive soil air exchange) is a large water level dynamic in the river. That is the reason why classic floodplain regions are regionally very limited. In Germany this certainly applies to the valleys of the Danube , Rhine and Elbe . The Weser must already be viewed as borderline for broad floodplains. Smaller rivers have too little runoff and water level dynamics to transfer groundwater dynamics over a wide area to their neighboring areas. Gleye therefore dominate in their valleys. The floodplain area can, depending on the size of the river, take a few meters up to 4 km from the river bed.

During sedimentation, the sorting of grain sizes that is typical for river valleys also occurs . The deposited sediments become finer and finer in the direction of the river mouth and with increasing distance to the river, as the flow velocity decreases. Due to the grain size and thickness of the horizons, as well as any organic matter contained in them, one can give precise information about earlier river courses, floods and water levels.

Leveling

The floodplain soils usually show the leveling aAh / aC / aG, whereby the “a” stands for floodplain dynamics (sedimentation layers from floods).

  • aAh - The topsoil horizon (A) is usually humus (h).
  • aC - The starting material (C) is fluvial sediment. The soil type becomes finer and finer as the flow rate decreases. The material itself can e.g. B. be diverse with regard to the lime content, because it depends on the sediments carried over by the region.
  • aG (o) - The horizon (G) influenced by the groundwater begins below 8  dm , often even much deeper. As a rule, it has oxidizing conditions (o).

In the floodplain soils, just like in the gleyas, some areas of the soil are below the groundwater level all year round . However, this G horizon contains oxygen. A reduction horizon (Gr) is therefore missing for floodplain soils! Hydromorphic influences (rust spots) in the A or C horizon are also subordinate or absent.

Soil types

Alluvial soils are divided into five types of soil , the occurrence of which is usually highly dependent on the relief (relief sequence). The individual soil types occur in an idealized manner in certain areas of the river, whereby the flow speed of the water is decisive:

The former Borowina soil type has been added to the Kalkpaternia since 2005. The floodplain soils develop into terrestrial soils after embankment, like a regosol or a pararendzina , since erosion and sedimentation (floodplain dynamics) do not occur. Floodplain soils are generally very closely associated with gleyas. If the G horizon is permanently between 4 and 8 dm, there is a transition to the gley. Despite the common ground (groundwater influence), they can be differentiated, because gleye are not necessarily composed of fluvial sediments, must show hydromorphic features already in the first 4 dm and have a clear reduction horizon (Gr) with a lack of oxygen.

use

The soils of the middle and lower reaches have been enriched with nutrients and beneficial soil material by flooding and are therefore very fertile. That is why they are very popular for agricultural use and, in order to counter the flood as the only significant disadvantage, they are diked. There are very fertile floodplains that date back to historical times, such as the Wetterau , where one of the most productive soils can be found. Here, the bottom value numbers possible to 80 or 90th Because of the high yield expectations, areas in front of the dikes are often plowed despite the risk of flooding. Usually, however, use as (mowing) pasture predominates there. A major problem with the agricultural use of the floodplain soils is the entry of pollutants and fertilizers into the nearby river via the groundwater.

With natural settlement, soft or hard wood meadows are to be expected on alluvial soils . Due to the high fertility of the floodplain sites, the natural vegetation has largely been replaced by arable farming and grassland. In Germany there are only very small and mostly severely disturbed residues. In the last few decades, especially after high flood damage, there has been a rethink with regard to their flood protection function, so that these biotopes are strongly protected and promoted today.

Microplastics

Scientists from the University of Bern have examined floodplain soils in Swiss nature reserves for microplastics and found what they were looking for. Although the sites are in nature reserves, microplastics have been found in 90 percent of the soil. Projections assume that the amount of microplastics that gets into the soil every year with sewage sludge is greater than the amount that ends up in the world's oceans . The researchers estimate that there are around 53 tons of microplastic in the top five centimeters of the floodplain. Even many soils in remote mountain areas are contaminated with microplastic, which suggests an Aeolian transport . New studies suggest that microplastics in the soil can kill earthworms, for example . Since earthworms fulfill important functions in the soil, this could also impair soil fertility .

literature

  • Ad Hoc Working Group Soil: Soil Science Mapping Guide. 5th edition. 2005, ISBN 3-510-95920-5 .
  • E. Mückenhausen: The soil science. 4th edition. 1993, ISBN 3-7690-0511-2 .
  • H. Montenegro, T. Holfelder, B. Wawra: Investigation of the interactions between surface water and groundwater in floodplains. In: Auenreport - Articles from the Elbe River Landscape Biosphere Reserve - Brandenburg. Special volume 1, 1999, pp. 27-40.
  • H. Montenegro, T. Holfelder, B. Wawra: Modeling of the exchange processes between surface water and groundwater in river meadows. In: K. Friese, B. Witter, G. Miehlich, M. Rode (Eds.): Substance budget of floodplain ecosystems. Soils and hydrology, pollutants, ratings. Springer Verlag, Berlin, 2000, pp. 89-98.

Web links

Individual evidence

  1. ↑ Soils in Swiss nature reserves contain considerable amounts of microplastic . In: Media release from the University of Bern . April 27, 2018 ( unibe.ch [accessed on May 2, 2018]). Soils in Swiss nature reserves contain considerable amounts of microplastics ( Memento of the original from April 28, 2018 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / www.unibe.ch
  2. ↑ Michael Scheurer, Moritz Bigalke: Microplastics in Swiss Floodplain Soils . In: Environ. Sci. Technol. 2018, doi : 10.1021 / acs.est.7b06003 .