Water bed

The river bed (sometimes also channel bed ) is the part of a river that limits the flowing water below and to the sides. In hydraulics , the river bed is called a channel . Depending on the size of the flowing water, a distinction is often made between a river bed (for large flowing water, rivers ) and a stream bed (for small flowing water, brooks ).

The river bed consists of the river bed - the bottom of the river - and the bank , depending on the definition up to the top of the embankment or to the mean water line . The water body consists of the water body (the volume of water itself), the water bed (the enclosure of the water from the bed and the bank) and the associated aquifer . The investigation and description of water beds is the task of hydromorphology . Bodies of water are formed in the complex interplay of local climate, geology and geomorphology, vegetation and the influences of human use. The natural waters bed forming processes are erosion ( erosion ), transport and silting (entrained by the flowing water sediment sediment called). Water beds and the like are described. a. through the longitudinal and transverse profile and the lines (running shape). There are interactions with the natural floodplain, the floodplain . However, this is not counted as part of the river bed. The shape of the valley also has no direct connection to the river bed; it was often created under completely different hydraulic conditions in the geological past, while the river bed (without direct human influence) is in balance with the current flow regime.

In the applied context, especially in the case of renaturation, the characteristics of the water bed are described as water structure (see article Water structure quality ).

The term is not common for banks and bottoms of standing water.

Water bed formation

The energy for the formation of the water bed is supplied by the water flowing away following the force of gravity; the force acting on the water bed is called shear stress . The transport force of the water is directly dependent on the flow velocity, which increases with increasing gradient and with increasing water flow . By at flood sharp rise in water levels therefore are responsible mostly for the bed formation in the natural state floods. In addition, the transport force for solids depends heavily on their grain size. Coarse material is carried along as debris on the river bed , while fine material is carried along as suspended matter in the water . The actual transport also depends on how much solids the flowing water is already carrying. Depending on the speed of the flow, solids on the bed and bank are removed (eroded), transported with the flowing water or deposited again; they are sorted according to grain size as the current decreases. The bottom brakes the water depending on its roughness , which means, for example, with the same amount of water, narrow and deep waters flow faster than broad ones (up to an optimal width below which the influence of the banks is decisive). However, a very wide sole does not remain level; Even small disturbances lead to irregularities, which eventually grow into gravel banks and islands. The vegetation also affects the roughness. The following are essential for the natural formation of the river bed: gradient, local climate (determines the amount and distribution of precipitation and thus the amount of water and the runoff regime ), the geology of the catchment area (weathering resistance of the rocks and the resulting grain sizes ).

Run development

branched river bed with diamond-shaped gravel islands: the Lech in Austria

meandering river bed with oxbow lakes on the Yamal Peninsula , Siberia

interwoven bed of the Rakaia River , New Zealand

Depending on the gradient, the interaction of erosion and accumulation of sediments leads to a completely different shape of the river beds.

straight

In very steep waters the transport force is so high that virtually no debris is deposited, the result is a straight course of water following the valley slope (can also occur at lower flow velocities if the solid load is very low, for example in the outflow of lakes)

branched

With a slightly lower gradient, a lot of coarse debris is carried along, which sediments again in areas with a slightly reduced flow velocity. The sediments form sand or gravel banks, which dry out as islands when the water level drops. Within the wide, just delimited flood channel, the water then flows in a system of interconnected channels that flow around a characteristic, diamond-shaped pattern of islands. Branched channels typically occur on slopes between about 4 and 2 percent.

meandering

If the gradient is very slight, curvature erosion predominates on the outside of the bends in the water, while the bed of the water is hardly eroded. As a result, all curves gradually increase by themselves, so that the water flows in loops and loops. Two opposing loops can move towards each other until the bank in between is completely eroded. A meander breakthrough occurs, whereby the old, no longer flowed loops are cut off as oxbow lakes. The typical meander formation only occurs with a gradient of less than 2 percent.

In reality, these ideal-typical forms are linked to one another through mixed forms and transitions. Many of these hybrid forms have been given their own names, examples would be interwoven flow or anastomosing flow .

Running types

In addition to the characterization of water beds as straight, branched or meandering, numerous other typologies have been developed that enable a finer differentiation. For example, the Montgomery and Buffington typology for mountain streams has found widespread use. They first differentiate according to the character of the material into which the water cuts:

• standing rock (bedrock). Bodies of water with a river bed occur in very steep bodies of water whose transport capacity is high in relation to the sediment supply.
• Colluvium . Waters in colluvial slope sediments (which were not deposited by the water itself) are usually small spring streams and upper streams.
• Alluvium . The course in sediment beds deposited by them is typical of all larger bodies of water.

The alluvial waters form a multitude of other morphological types, which are mainly determined by the slope:

• Cascades (cascade). In very steep water beds with embedded boulders with rapid to turbulent discharge, cascade-like flow stretches form.
• Step pool. With a slightly lower gradient, calm flowing stretches or even basin-like widenings that are interspersed in the cascade form at fairly regular intervals behind larger obstacles such as boulders.

Cascade flow route with small river beds: the Teichbach in the Black Forest

• flat bed of water (plane). If the gradient is a little smaller, the water flows turbulently over an almost flat bed of sand, gravel and stones. The rhythmic sequence of cascades with a shooting current and calm widening is missing. A reinforced river bed is typical, in which a freely eroded pavement made of coarse rubble protects the fine material lying underneath from deposits. These are only relocated during very strong floods.
• Fords and pools (alternatively: quick and quiet) (pool riffle): With an even lower slope, below approx. 1.5 percent bed slope, the waters usually do not form straight, but rather winding forms. The river bed forms a rhythmic sequence of overflowing sand or gravel banks (fords), in which, quite regularly with about five to seven times the width of the water, calm widenings, the pools , are inserted. Pools form on the outside of bends or behind obstacles across the water axis like tree trunks ( dead wood ). Sediments are transported from scour to scour during floods.
• Dunes and corrugations (dune-ripple): Especially in slowly flowing waters with a sandy bottom, the sediment forms transport bodies with a wavy surface, which are called corrugations with dimensions in the centimeter to decimeter range, and dunes in the decimeter to meter range. If they migrate upstream at a higher flow velocity, one speaks of anti-dunes. Typically, sediment is rearranged even at low flow velocities. If this also occurs when the water level is low, which is mostly due to human interference, it is called "sand drift". Waters with drifting sand are biologically hostile to settlement.

The material of the water bed

The water bed of alluvial water bodies depends, without human influence, primarily on the flow velocity of the water body. The faster the water flows, the coarser material it can transport. Fine sediments, however, form soil aggregates that increase their stability when eroded compared to a single-grain structure. As a result, material that was deposited at a certain flow rate can possibly only be mobilized again at a higher rate. The transport power of the water also depends on how much sediment it is already carrying. In order to prevent deep erosion and other erosion, many bodies of water are therefore artificially added to bed load (bed load management), especially in water bodies that are artificially depleted of bed load due to other hydraulic engineering interventions (e.g. reservoirs).

The following overview gives a rough overview of which grain sizes can be transported at which flow speeds of the water:

• more than 300 cm / s: blocks, blocks, more than 80 mm in diameter
• 200 to 300 cm / s: stones, more than approx. 60 to 80 mm in diameter
• 150 to 200 cm / s: coarse gravel , 20 to 60 mm in diameter
• 75 to 150 cm / s: medium gravel, 6 to 20 millimeters in diameter
• 50 to 75 cm / s: fine gravel, 2 to 6 millimeters in diameter
• 25 to 50 cm / s: coarse sand , 0.6 to 2 millimeters in diameter
• 17 to 25 cm / s: medium sand, 0.2 to 0.6 mm in diameter
• 10 to 17 cm / s: fine sand, 0.06 to 0.2 mm in diameter

Sediment carried along by the flowing water as debris is transported rolling or hopping on the ground and thus rounded off as rubble at the edges. Even finer material than fine sand, i.e. silt and clay , is usually not carried along as debris, but finely distributed as water turbidity (suspension). It can also be transported at very low flow rates.

Due to this connection, the material from which the water bed is built depends on the flow velocity. This, in turn, depends, in addition to the amount of water, above all on the gradient. As a result, sand -shaped streams form in regions with a low gradient , and gravel-shaped streams in landscapes with a higher gradient . Since the flow speed in the middle of the channel is usually higher than on the banks, where it is also slowed down by the bank vegetation, a large part of the material can be deposited in the bank area in slowly flowing rivers, where it can form natural dams ( dam bank river ). Sometimes the erosion also leads to the fine material being washed away at the top, so that the bed of the river consists of coarse stones that protect the fine sediment below from further erosion (“armored” bed). If this river bed is breached in a disaster-like event, it is known as a bottom breakthrough .

In addition to the inorganic sediment, organic material of plant origin is also involved in the formation of the bed in natural flowing waters. In streams, this is mainly material coming from outside, such as the fall foliage of the annual leaf fall or the dead wood of dead trees on the banks.

There are special features in bodies of water whose beds are eroded into rocks and soils consisting of loose sediments, such as loess ; Such material coming from outside is called allochthonous . In these cases, the sediment is largely determined by the starting material in addition to the flow rate.

swell

• Heinz Patt, Peter Jürging, Wener Kraus: Natural hydraulic engineering . Springer Verlag, 1998. ISBN 3-540-61666-7

Individual evidence

1. ^ Entry in the Lexicon of Geography, Spektrum.de
2. ↑ DIN 4047 part 5: Agricultural hydraulic engineering, terms: development and maintenance of water bodies
3. ↑ DIN 4049 Part 1: Hydrology, Basic Terms.
4. ↑ founded by Luna Leopold & M. Gordon Wolmann: River Channel Patterns: Braided, meandering and straight . US Department of the Interior, Geological Survey Professional Papers 282 B, 1957.
5. ↑ David R. Montgomery & John M. Buffington (1997): Channel-reach morphology in mountain drainage basins . Geological Society of America Bulletin vol. 109, no. 5: 596-611.
6. ↑ Peter A. Bisson, David R. Montgomery, John M. Buffington: Valley Segments, Stream Reaches, and Channel Units . Chapter 2 in F. Richard Hauer and Gary A. Lamberti (editors): Methods in Stream Ecology , Second edition, Academic Press; Elsevier. 23-49. ISBN 978-0-12-332908-0
7. ↑ Brunke, M., Purps, M., Wirtz. C. (2012): Fords and ponds in flowing waters of the lowlands: morphology, habitat function for fish and renaturation measures. Hydrology and Water Management 56 (3): 100–110.
8. ↑ Bed load management. Bavarian State Office for the Environment, 2016
9. ↑ after Wilfried Schönborn, Ute Risse-Buhl: Textbook of Limnology. 2nd edition 2012. Schweizerbart-Verlag, Stuttgart. ISBN 978-3-510-65275-4 . Table 12 on page 89.

Web links

Commons : Stream beds  - collection of pictures, videos and audio files