Hummocky sedimentary structures within rippled beds due to combined orbital waves and transverse currents

Abstract

ABSTRACT Hummocky cross‐stratification is commonly observed in the marine offshore transition to lower shoreface environments. However, to date, the origins of hummocky cross‐stratification and its associated hummocky bedforms and hydrodynamic processes remain controversial and enigmatic. In the present study, a large‐scale flume experiment was conducted to study the formation of hummocky bedforms. In the central test area of the flume, combined flow with water waves, with period of 2 s and velocity of 0.34 m/s, progressed at a right angle to a current with velocity of 0.17 m/s, whereas a wave‐alone condition pertained upstream and downstream of the test area. The combined‐flow ripples in the test area had smaller dimensions than wave ripples, but their cross‐section geometries were very similar. Most importantly, the experimental results, for the first time, revealed that humps occur with lengths up to approximately 40 mm beneath combined flow ripples. The formation of these structures appears to relate to the enhanced turbulence for the combined flow, because the turbulence kinetic energy for combined flow was ca 50% higher than that under the wave‐only condition. Moreover, the observed small‐scale humps in the present experiment had comparable cross‐section geometries with hummock‐like bedforms previously reported in laboratories and fields under storm conditions. Additionally, these humps could scale up to large‐size hummocks under waves with longer period and faster velocity than the present conditions. The present experiment indicates that waves perpendicular to a current do generate hummock‐like structures in association with ripples and therefore provide a new perspective for future study of the origins of hummocky cross‐stratification.