Internal waves vs. surface storm waves: a review on the origin of hummocky cross-stratification

Hummocky cross-stratification (HCS) is considered a diagnostic structure of surface storm activity at the shoreface–offshore transition. However, the origin of HCS is still debated. Laboratory experiments have not yet reproduced it and direct observations on the continental shelves do not exist. Most hydrodynamic interpretations invoke pure oscillatory flows, unidirectional-dominated combined flows and oscillatory-dominated flows, but they all share the assumption of HCS to reflect the combined action of surface storm waves and related currents. Within this context of uncertainties, internal waves (gravity waves propagating along the pycnocline) provide an alternative mechanism to explain the origin of HCS. Internal waves breaking on the shelf create episodic high-turbulence events and induce upslope- and downslope currents as well as oscillatory flows at the depth where the pycnocline intersects the sea floor. In this scenario, both the oscillatory- and the unidirectional components needed for HCS to form are not necessarily linked to surface storm waves, but can occur at various depth as far and near there is a pycnocline where internal waves can form