Facies architecture of submarine channel deposits on the western Niger Delta slope: Implications for grain-size and density stratification in turbidity currents

High-resolution bathymetry, seismic reflection, and piston core data from a submarine channel on the western Niger Delta slope demonstrate that thick, coarse-grained, amalgamated sands in the channel thalweg/axis transition to thin, fine-grained, bedded sands and muds in the channel margin. Radiocarbon ages indicate that axis and margin deposits are coeval. Core data show that bed thickness, grain size, and deposition rate strongly decrease with increasing height above channel thalweg and/or distance from channel centerline. A 5 times decrease in bed thickness and 1\u20132 \u3c8 decrease in grain size are evident over a 20 m elevation change (approximately the elevation difference between axis and margin). A simplified in-channel sedimentation model that solves vertical concentration and velocity profiles of turbidity currents accurately reproduces the vertical trends in grain size and bed thickness shown in the core data set. The close match between data and model suggests that the vertical distribution of grain size and bed thickness shown in this study is widely applicable and can be used to predict grain size and facies variation in data-poor areas (e.g., subsurface cores). This study emphasizes that facies models for submarine channel deposits should recognize that grain-size and thickness trends within contemporaneous axis-margin packages require a change in elevation above the thalweg. The transition from thick-bedded, amalgamated, coarser-grained sands to thin-bedded, nonamalgamated, finer-grained successions is primarily a reflection of a change in elevation. Even a relatively small elevation change (e.g., 1 m) is enough to result in a significant change in grain size, bed thickness, and facies

How typhoons trigger turbidity currents in submarine canyons

Intense turbidity currents occur in the Malaylay Submarine Canyon off the northern coast of Mindoro Island in the Philippines. They start in very shallow waters at the shelf break and reach deeper waters where a gas pipeline is located. The pipeline was displaced by a turbidity current in 2006 and its rock berm damaged by another 10 years later. Here we propose that they are triggered near the mouth of the Malaylay and Baco rivers by direct sediment resuspension in the shallow shelf and transport to the canyon heads by typhoon-induced waves and currents. We show these rivers are unlikely to generate hyperpycnal flows and trigger turbidity currents by themselves. Characteristic signatures of turbidity currents, in the form of bed shear stress obtained by numerical simulations, match observed erosion/deposition and rock berm damage patterns recorded by repeat bathymetric surveys before and after typhoon Nock-ten in December 2016. Our analysis predicts a larger turbidity current triggered by typhoon Durian in 2006; and reveals the reason for the lack of any significant turbidity current associated with typhoon Melor in December 2015. Key factors to assess turbidity current initiation are typhoon proximity, strength, and synchronicity of typhoon induced waves and currents. Using data from a 66-year hindcast we estimate a ~8-year return period of typhoons with capacity to trigger large turbidity currents

3D NUMERICAL INVESTIGATION OF TIDAL FORCING ON THE STABILITY OF BIFURCATIONS

River bifurcations are ubiquitous features of both gravel-bed and sand-bed fluvial systems, including braiding networks, anabranches and deltas. As such, their morphology and development shape fluvial plains and deltas dictating flood risk areas as well as land loss and land gain. The morphodynamic equilibrium of bifurcations is strongly affected by the characteristics of the upstream channel, such as width-to-depth ratio (Bolla Pittaluga et al., 2003), curvature (Kleinhans et al., 2008), bar presence (Bertoldi et al., 2009; Redolfi et al., 2019), and related sediment transport mechanisms (Bolla Pittaluga et al., 2015; Redolfi et al., 2016), which regulate water and sediment partitioning into the two downstream branches. Interestingly, in nature the downstream boundary conditions may also influence the bifurcation dynamics due to their oscillatory behaviour with significant backwater effects. It has been observed that tide‐influenced deltas tend to exhibit more stable branches keeping all channels active. Factors such as the length of the downstream channels or tidal ranges strongly affect the evolution of the bifurcations. On the basis of an analytical 1D model developed under the hypothesis of small monochromatic tidal oscillations, Ragno et al. (2020) proved that even small-amplitude tides have a stabilizing effect on the bifurcation due to the erosive character of weak tidal ebb flows. These flows keep both branches morphodynamically active, preventing the development of unbalanced solutions. The present work aims to corroborate Ragno et al. (2020) findings by means of fully 3D numerical simulations performed through Delft3D and to extend the analysis to the case of strongly tide influenced bifurcations. Moreover, the full description of the flow field along the vertical direction allows for a more thorough evaluation of sediment and flow partitions at the bifurcation and to quantify 3D effects at the node on the evolution and the equilibrium configuration of the system

Modeling the morphodynamic equilibrium of an intermediate reach of the Po River (Italy)

The Po River, in the last century, has undergone significant altimetrical and planimetrical changes, mostly induced by a progressively increasing human pressure. The extensive protection and regulations works carried out to reduce the risk of flooding, the narrowing of the river for improving the navigation, the local interruption of sediment transport caused by a large mobile barrage built for hydropower purposes and the intense sand mining caused huge alterations of the river morphology. These changes were initially very fast and determined a significant and generalized deepening of the middle water course. In the last few decades, however, the pressure induced by human activities on the river decreased significantly and, consequently, a dynamic equilibrium condition tended to be re-established along most of the reaches, as suggested by topographic surveys spanning a period of about twenty years. The present contribution investigates this equilibrium condition by means of a one-dimensional movable bed model, with reference to a 98 km long reach located between the confluence with the Oglio stream and the gauging section of Pontelagoscuro, for which an up to date stage-discharge relationship is available. Considering steady forcing conditions, we estimate the formative discharge producing the observed river topography and the corresponding sediment transport capacity. The field surveys of cross section geometry used to investigate the possible existence of an equilibrium morphology span a period (1982\u20132005) of about twenty years. In the presence of fixed banks, the rived bed morphology appears to be controlled by relatively moderate discharges, quite close to the mean yearly discharge and significantly smaller than both the ordinary flood discharge and the maximum annual discharge. Even though significant deviations from equilibrium are produced by the sediment waves triggered by larger floods, deposition occurring during lower stages and the continuous reworking of the bed due to less intense but more frequent discharges implies a tendency of the river to recover its equilibrium profile