Measurements Concerning Generation of Rhomboid Rills in Flume Experiments: Froude Number and Sand-Topographic Feature

Rhomboid rills, small sand topographies often observed in the swash zone of beach slopes, are apparently similar but different from rhomboid ripples that have been relatively well researched. Rhomboid rills are composed of thin grooves, while rhomboid ripples are of the tilelike geometry. A recent study showed a difference in the dependence of rhombus angle on the slope gradient between rhomboid rills and rhomboid ripples. In this study, we measured Froude numbers during the generation of rhomboid rills and found that the trend of rhombus angles versus Froude numbers was similar to that in the case of rhomboid ripples. We discuss similarities and dissimilarities between rhomboid rills and rhomboid ripples, showing an open problem.