Ripple marks on Vitrupe beach
Ripples, also known as ripple marks, are wave-like surface shapes on sand caused by flowing air and water. Ripples and the inclined stratification they create are sedimentary structures. Depending on how they are created, a distinction is made between two forms of ripples: flow ripples and oscillation ripples.
Flow ripples
A small increase is enough to create an obstacle in the surface of the sandy bottom, where grains of sand carried by the current can accumulate. The larger the obstacle, the more grains accumulate (positive feedback). Irregularly shaped flow ripples are the result, with a steep leeward side (away from the flow) and a flat windward side (facing the flow).
Aquatic current ripples arise in sand when the current speed is slightly above the critical speed. Flow ripples are irregularly shaped, with a lower slope on the windward side facing the flow and a steeper slope on the leeward side facing away from the flow. The average height of ripples is 3-5 cm, their wavelength is 4-60 cm. A minimum water depth of around three times the height of the ripple is required for ripple formation. There is no upper limit to the maximum depth for ripple formation because ripple formation is determined by the processes in the boundary layer. For the formation of ripples, the largest sand grain diameter is 0.6–0.7 mm. Large ripples and giant ripples arise with larger grain diameters. Large ripples and ripples are not fixed in place, but rather migrate in the direction of the current.
Oscillation ripple
This pattern in the sand, also known as tidal or wave ripples, occurs when the current comes from different opposite directions. The best conditions for this are found on a flat beach where the water runs back in the opposite direction from which the wave came in with the grains of sand it carried along. But back and forth movements of sand grains on the ground are also created by any surface wave. Ripple marks with a symmetrical cross section are the result of these pendulum movements.
To log this Earthcache, you must answer the following questions. Please send me your answers via the Geocaching Message Center. You can log immediately, if something is wrong, I will let you know.
1. When forming a ripple pattern, a distinction is made between two forms depending on how it is created (see text). What shape do you think you saw in the water?
2. Which of the two forms do you think is more likely? Briefly explain your decision.
3. Which side of the Ripple do you look at from the beach? Side facing or away from the flow?
4. Add an on-site photo of yourself or a personal item to your online log as proof of presence!
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