Large-scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows

Large-scale (20 m to 7 km wavelength) bedforms are common on the seafloor, yet there is a lack of consensus on how they form and thus what to call them. We conducted statistical analysis on a dataset of 82 seafloor bedforms that span a range of water depths and environments. The data form three distinct groups: 1) small-scale (20–300 m wavelength) sediment waves with mixed relief made of medium sand to cobble-sized sediment that form in confined settings, which we call small sediment waves; 2) large-scale (300–7000 m wavelength) sediment waves with mixed relief made of fine-grained sediment that form in relatively unconfined settings, which we call large sediment waves; and 3) large-scale fully enclosed depressions in the seafloor, which we call scours. There is a statistically significant data gap in the size of bedforms between small sediment waves and large sediment waves that does not appear to be a sampling artefact. This data gap probably results from the environments in which sediment waves form being either confined (e.g. channel or canyon) or unconfined (e.g. open slope). Bedform migration direction is available for 36% of the data and includes small and large-scale sediment waves; of these examples all are shown to migrate up-current. Up-current migration is indicative of supercritical flow; thus this data suggests that supercritical flows operate in a wide range of environments and can generate both small and large sediment waves. Therefore, we suggest that small and large sediment waves form by similar processes despite the gap in bedform wavelength and sediment size. The migration direction for scours remains unknown. Scours may form from similar processes to small and large sediment waves, or alternatively they may be a completely separate bedform type that form when erosive flows exploit pre-existing defects in the seafloor. This novel statistical analysis of a global database shows that up-current migrating bedforms associated with supercritical flow are unusually widespread, and are recognised at two distinct scales.