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The map shows the spatial distribution of average height of “active wave ripples” during 2006 in the southern North Sea. Wave ripples are called “active” if (a) wave action generates sediment transport at the sea floor (i.e. if the wave bed shear stress τWis above the threshold of sediment motion), and if (b) the bed shear stress τWproduced by waves is somewhat above the bed shear stress τCproduced by currents (see text “Data Processing”). The prediction of wave ripple heights is done by the model of Soulsby and Whitehouse (2005), supplemented by Marten (2010).
Wave ripples are characterized by straight to bifurcating crestlines (aligned parallel to the crests of the water surface waves), a peaked crest and a broad trough, and their profile is rather symmetrical. Viewed from above, wave ripples form a regular pattern of almost parallel crestlines. The height of wave ripples, being in equilibrium with the wave regime, depends primarily on their length. Roughly, the wave ripple height is between 10 % and 20 % of the wave ripple length.
Wave ripples only form in sand with grain-sizes between ≈ 60 µm and ≈ 800 µm. The length of wave ripples depends both on the median grain-size and the excursion of water motion above the bed. If wave induced near-bed water motion is above a certain limit, wave ripples are washed out and the sea floor is flat.
The general tendency of ripple height in the plot is: in deep water the ripple height is smaller than in shallow water. The reason for this tendency is two-fold. First, the sediment in deep water is (in most cases) finer than in shallow water. The finer the sediment, the smaller is the ripple length and the smaller is thus the ripple height. Second, in deep water, the amplitude of the near-bed oscillatory water motion is small. This produces wave ripples with a small length and thus with a small height.
In the area off Humber the median grain-size is above 800 µm, so ripples are not formed there. In the deep water off Yorkshire and North East England, wave generated near-bed water motion was (during all 2006) too weak for ripple development.