Abstract
Subaqueous, asymmetric sand waves are typically observed in marine channel/canyon systems, tidal environments, and continental slopes exposed to strong currents, where they are formed by current shear resulting from a dominant unidirectional flow. However, sand-wave fields may be readily observed in marine environments where no such current exists; the physical processes driving their formation are enigmatic or not well understood. We propose that internal solitary waves (ISWs) induced by tides can produce an effective, unidirectional boundary “current” that forms asymmetric sand waves. We test this idea by examining a sand-wave field off the Messina Strait, where we hypothesize that ISWs formed at the interface between intermediate and surface waters are refracted by topography. Hence, we argue that the deflected pattern (i.e., the depth-dependent orientation) of the sand-wave field is due to refraction of such ISWs. Combining field observations and numerical modelling, we show that ISWs can account for three key features: ISWs produce fluid velocities capable of mobilizing bottom sediments; the predicted refraction pattern resulting from the interaction of ISWs with bottom topography matches the observed deflection of the sand waves; and predicted migration rates of sand waves match empirical estimates. This work shows how ISWs may contribute to sculpting the structure of continental margins and it represents a promising link between the geological and oceanographic communities.
Highlights
The sand-wave field was first reported by Selli et al [ref. 3] and Colantoni [ref. 4], and its morphodynamic is relevant for pipelines stability[2,5]
We argue that the process behind such an intriguing depth-dependent orientation of the asymmetric sand waves must be related to the presence of Internal Solitary Waves (ISWs) that travel northward form the sill of the Messina Strait[6,7,8] (Fig. 2; see Supplementary Information)
These ISWs are triggered when the denser Ionian water suddenly debouches in the less dense Tyrrhenian water: internal gravity waves generate in the Messina Strait by the interaction of barotropic tidal currents with the bathymetry in well-stratified water conditions; under certain circumstances, the internal tide can transform into a set of high frequency, non-linear internal waves (Fig. 2); internal waves dynamics is strongly ruled by the nonlinearity of the phenomenon and by the dispersion of the media; when these two effects are balanced, coherent structures emerge from an initial disturbance and travel as Internal Solitary Waves (ISWs), called solitons[7]
Summary
The sand-wave field was first reported by Selli et al [ref. 3] and Colantoni [ref. 4], and its morphodynamic is relevant for pipelines stability[2,5]. The two main processes that would explain both the presence and the pattern of the sand-wave field are as follows: i) the sedimentary mound (Fig. 1) would induce wave refraction, deflecting the ISW vector toward shallow areas (Figs 1 and 3, and Supplementary Fig. S1); the trough of the ISW would mainly contribute to the intense (i.e., > 50 cm/s) horizontal velocity at the bottom, triggering sediment motion along the direction of the wave vector[8] (Figs 2 and 3d, and Supplementary Fig. S2). This latter feature points out the main difference between internal progressive waves and ISWs in moving sand along the wave vector: internal progressive waves would cause an oscillating motion, leading to symmetric sand waves, while the passage of an ISW induces a unidirectional momentum to a fluid parcel[21,22] (Figs 2 and 3, and Supplementary Fig. S2), which agrees with the asymmetric pattern of the sand waves off Capo Rasocolmo
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