Large bed forms on the sea floor

Abstract

In shallow marine environments various types of bed forms emerge under waves and currents, producing various types of stratification. Harms et al. (1975) distinguished a nearly horizontally-bedded type of stratification associated to high-energy conditions, and named it hummocky cross-stratification (HCS). HCS is believed to form under highly energetic conditions by large bed forms: hummocks. Few studies have investigated their formation, and many aspects are still unclear. The objectives of this research are: to obtain the conditions under which they form, to study their similarities and differences with large wave ripples (LWR), and to predict their length.

Methodology in this research consisted of two parts: 1) existing data was used and 2) new experiments in a large-scale wave flume were performed. Contrary to previous studies, this study provided both detailed data of bed level evolution and hydrodynamic conditions in a real-scale environment with known history. Bed levels were used to study the bed development and to determine the bed form length. The new observations were compared with existing data. A lacquer peel was taken from a large bed form and correlated with the measured bed levels.

Based on phase diagrams no clear distinction between LWR and hummocks can be made. Both are observed around the inception of sheet flow, in different types of sediment and possibly superpositioned by a small current. Their dimensions also lie in the same region, and scale with the orbital diameter. LWR and hummocks are therefore the same phenomenon. \\

Direct observation of HCS on a lacquer peel and correlation with measured bed profiles shows that it forms by sheet flow deposits on the lee side and trough of a large bed form. HCS is thus not formed by an active bed form and hummocks are not a separate bed state. \\

Predicting the length of LWR and hummocks shows that most equations underestimate it. The equations of Clifton (1976) and Williams (2005) predict their length best, within a factor two. Surprisingly, all predictors overestimate SWR by more than two times their length. \\

Co-existing of bed forms was observed as superposition, though two types of bed forms were simultaneously active during several runs. This was assigned to a hysteretic effect, it is unclear why some bed forms were more persistent than others.