Dynamics of ripples superimposed on a sand ridge on a tideless shoreface

Abstract

The presence of ripples superimposed on an active sand ridge is the most usual situation under low-to moderate-energy conditions in the wave-dominated, tideless Ebro Delta coast. Four types of small-scale bedforms were identified, from the critical threshold of sediment motion to wash-out conditions: (i) small undulations with η < 0.5 cm and λ ~8 cm, formed as a precursor of wave ripples when the Shields parameter was just below the theoretical critical; (ii) 2D wave ripples with η = 1.2 cm and λ = 7–10 cm; (iii) mixed 2D/3D current-dominated ripples with a maximum of η ~1.5 cm and λ = 10–15 cm; and (iv) 3D wave-current ripples with a maximum of η ~2.2 cm and λ = 7–20 cm. Ripple degradation was observed to occur when the energy of the hydrodynamic regime increased (wash-out conditions) and under low-energy hydrodynamic conditions, when ripples progressively decayed mainly as a consequence of biological activity. The wave ripples were static, while the current-dominated ripples migrated towards the SE at a rate of ~10 cm/h.The potential role of ripple migration as an additional long-term mechanism of sediment transport is addressed. On the Ebro Delta shoreface, 3D ripple migration follows the direction of sand ridge migration towards the SE. The respective migration rates and their differences in size support the hypothesis that a subordinate part of sand ridge migration can result from the contribution of ripple migration under low to moderate regimes, suggesting that in specific environments the dynamics of small-scale bedforms can play a subordinate but not negligible role in the evolution of larger bedforms.