Ripple marks

Ripple and ripple marks, waveform, bottom shape, Sohlform, transport body, dune and subaquatic dune are of a flowing medium caused ( on earth air and water) wave-like surface forms at the interface of this medium to a sandy sediment. Ripple and cross-bedding produced by them are among the sedimentary structures. The ripple pattern formation there are two different forms depending on the origin: flow ripple and Oszillationsrippel. In the literature there are a number of different classifications for the product object. Neither the German nor in the English language, a nomenclature prevailed, although that is likely to Ashley (1990 ) are among the most widely used.

Flow ripple

A first minimal increase occurs an obstacle in the surface of the sand ground, can be deposited on the entrained by the flow of sand. The greater the obstacle, the more grains are deposited on (positive feedback). The result is asymmetric flow ripple with a flat windward and leeward side of a steep.

Aquatic flow ripple produced in sand when the flow rate is just above the critical speed. Flow ripples are asymmetrical, with a lower slope on the flow- facing windward side and a steeper slope on the lee side facing away from the flow. The amount of ripple is on average 3-5 cm, their wavelength at 4-60 cm. This ripple arise, one needs a minimum water depth of approximately 3 times the ripple height. Since the ripple formation is determined by the processes in the boundary layer capable of the maximum depth for ripple formation at the top is not limited. The maximum sand grain diameter for the formation of ripples is 0.6-0.7 mm. For larger grain diameters caused major ripples and giant ripple. Ripples and large ripples are not stationary, but migrate in the direction of the flow.

Is the critical flow velocity for the movement of sand grains is reached, the grains begin to move and agglomerate in small clusters. This few grains to form thick irregularities on the sediment surface, affecting the flow in the boundary layer. About the irregularities that form small hills that Stromlininen are closer together and the flow velocity increases. Sediment grains can thus rolling or jumping the windward side of the small hill to be transported up and accumulate on top. If too many grains accumulated, the situation is unstable and the grains slide down the leeward slope and are deposited there. These thin layers of grain are called Leeblätter (English foresets ) and form from the natural angle of repose of about 30-35 °. By repeating this process is able to form on position, and separated by intermediate layers laminated finer sediment the precipitate from the suspension. It gradually creates a ripple. At the top of the hill ( the ripple ), the flow splits. A part of the flow will continue to flow across the sediment surface. The other part is on the leeward side or irregular vortex impinges on the sediment surface, where increased turbulence and erosion forming the troughs between the ripples. A portion of the eroded particles can be transported through the return flow vortex at the base of the Leehanges and deposited there as a thin layer. The other part is taken either in the fluid or transported back over the windward slope of the ripple. If a part of the grains deposited on the windward side, then the so-called Luvblatt forms.

Oszillationsrippel

This also wave or tidal ripple mentioned structures arise when the flow oscillates over the sand, that is, alternately coming from different, opposite ideally directions. The ideal situation is for example given on a flat beach, on the back, the water runs in the opposite sense to the direction from which came the antransportierende shaft. However, any surface acoustic wave generated on the ground reciprocation. The result of this Pendelbewegungungen are ripple marks with a symmetrical cross-section.

According to their size Rippel be divided into three groups:

• Rippel, usually 3-5 cm high and with a wavelength of 4-60 cm
• Large ripple, between 6 cm and 1.5 m high and with a wavelength from 0.6 to 30 m
• Giant or mega ripple, 1.0 to 8.0 m high, with a wavelength greater than 30 m

Large - and giant ripples occur at grain sizes of more than 0.6-0.7 mm. Only the giant ripples are stationary, the remaining ripple migrate with the flow.

Ripple marks are - in contrast to stress brands - structures on top of a layer of sediment. Both can thus for determining the initial storage of the sediment used ( Geopetalgefüge ).