Plate 51

Modified tractive structures: interference ripples

The summits of littoral bars gradually emerge at low tide and form small islands. The wave fronts, which are rectilinear and approximately parallel to the shore during average or high tide, are refracted by the emerging bars when the water shallows. They tend to become parallel to the island contours and, in so doing, split into overlapping and intersecting arcs.

The consequence is that the local direction of water motion changes with time, and the ripple crest orientation is forced to adapt to these changes. However, to cancel the ripples that are not in equilibrium with the new conditions, a certain mass of sand must be removed, while waves are losing energy. The "old" ripples are thus simply modified, and new ones are printed over them, with crests making an angle with the old ones. More generations of overprinted ripples can be formed, each one being smaller than, and intersecting the previous one.

The whole pattern receives the name of interference ripples,  even though intersecting ripples,  or overprinted ripples,  would be more appropriate. The various wave systems are, in fact, successive in time. Wave interference, anyway, does exist, as shown by the larger picture; it rather produces its own equilibrium forms, such as the rhomboidal ripples of the previous plate, which replace, instead of overprinting, the older ones.

On the basis of what has been said above, try to determine how many generations of ripples are present in picture B, and their order of formation.

Photo: Piacentini 1970.