Plate 53

Modified bed forms: double crested ripples

Small waves in thin water can modify ripples in this way (see inset b, plate 46 for the corresponding profile). Apparently, the wave fronts did not change direction but simply tried to make more closely spaced ripples. They were successful only on crestal zones as water was too deep in troughs. No new orientations are superimposed to the old generation in this case. It is a form of low-energy modification.

Other forms of modification in coastal and nearshore environments are represented by ripples that rework the topmost part of storm deposits. An example is portrayed in color photo 17. Remember that small-scale structures induced by waves are found only above fair weather wave base, while medium to large-scale structures occur farther offshore up to storm wave base. In both cases, I am talking about average base levels: exceptional storms or earthquake-induced tsunami waves can stir water and sediment up to 200 m depth.