Groyne

Called a groyne, as Stack, Hoeft, Kribbe, scoop or in the Alpine space battle, is a built approximately at right angles to the beach during pre-built into a sea or from the banks of a river from wall or mound -like structure that serves the coastal protection or river engineering.

• 2.1 fairway deepening
• 2.2 restoration
• 2.3 Miscellaneous

At sea it can involve rows of piles of wood or concrete to act a steel sheet pile wall or a stone wall, the latter is sometimes lined with two rows of poles. The effect of the groynes that shore -parallel currents are reduced in flow rate to the extent that no sand erosion takes place and instead the entrained sediments are accumulated.

Groynes, which were built in the early 19th century, consisted of driven wooden piles, later they tried it with concrete piles. Also dyke -like stone with bitumen or Betonverguss groynes were created. In the 1950s they developed for exposed coastal sections of heavy concrete tetrapods that are used for the protection of groynes. Recently, it has also been experimenting with plastics, which have not proven successful for different reasons.

The average life span of wooden groynes was formerly about 50 years, until in the 1990s feeding damage occurred by the shipworm. To some extent, since used instead of softwood logs from tropical wood piles, which are not attacked by the shell.

The effect of the breakwaters for coastal protection in some places was less than expected, as they are elsewhere used successfully today. On the west coast of Sylt groynes for example, could hardly stop the sand loss and are now complemented by beach nourishment, which replace regularly direct the lost sediment.

In the traditional land reclamation from the sea of the Wadden Sea groynes are often supplemented by fascines, buhnen similar knee-high faschinierte double peg rows, the increase in the enclosed Lahnungsfeldern by flow reduction sedimentation.

Pictures of sea groynes

Breakwater of concrete piles

Flooded Nordseebuhne in Bournemouth

Groynes on rivers

In river engineering are groynes, used depending on the model to two conflicting purposes. Either they serve the fairway deepening or renaturation. Groynes are generally built on river sections with relatively low flow gradient.

Navigation channel deepening

From both sides in pairs projecting into the river groynes deepening of the shipping channel and the shore protection act. The groynes pairs artificially reduce the cross sectional area of the river bed. Since the amount of water remains the same, increases due to the Venturi effect, the flow rate in the middle of the river. Due to the higher flow rate of erosion and the sediment of the riverbed rises is removed. Thus, the fairway is less, making the outflow cross -section increases again.

Along the shore arise between the groynes still water zones without flow or even with a slight backflow ( Neerströmung ); these are also referred to as groyne fields areas as to an increase sediments, thereby protecting the shore. Modern groynes are built with about 10 ° inclination to the flow. This is in case of flow in the event of a flood, the flow in the middle of the river held and prevented an erosion of bank sediments.

At the groyne heads meet water bodies of different flow rates to each other. They form strudel with its axis vertical, which often generate meter deep potholes in the unpaved ground waters. In the area of the Middle Rhine groyne fields are often used for bathing. Since the body of water to the groyne heads is mostly unpaved, swimmers can drown when they are pulled off of swirls in the scours.

If one side of the river is sufficiently secured against erosion, groynes can there sometimes be omitted, and correspondingly longer unilateral groynes are applied instead of the groynes pairs.

Groynes are usually poured dams, which are protected by pavement or gravel, partly blast furnace slag was used. Groynes must be set regularly serviced, have jurisdiction in Germany, the Waterways and Shipping Offices. Some German inland waterways groynes are marked on the shore by groynes stones, indicating the approximate location of a jetty.

Renaturation

Groynes can also be used in the context of river restoration projects. In this case, they are arranged so that in a straightened watercourse new meanders, scouring and bank erosions occur. The prerequisite is that the stage opposite shore is unpaved. Paradoxically, this decreases by groynes, the flow rate, as the rehabilitated river extended by the river bends, which the slope decreased. Often these groynes are implemented biologically engineering, for example, woody debris or boulders are used as a breakwater.

Others

Simple flow structures of various types are known in particular in the Eastern Alps as a battle.

In North America and Nepal since the 1960s instead of river groyne fields perpendicular chosen baffles, known as Iowa Vanes used.