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

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

When and where do free bars in estuaries and tidal channels form?

The incipient formation of free bars in weakly converging tidal channels and estuaries is mathematically framed within a unified two-dimensional shallow water and sediment mass balance model. The analysis is driven by the recognition that none of the theoretical models introduced so far account for the spatially varying character of the planform morphology, which characterizes estuaries and creeks forming networks in coastal wetlands and in the tide-dominated reaches of deltas. The problem is tackled by means of a linear stability analysis, by observing that width variations act on a spatial scale that is typically much longer than bars wavelength, allowing for an analytical treatment of the governing system of equations. We focus on two limit cases: first, the river transition into a micro-tidal sea (estuarine case); second, a tidal channel in the absence of a fluvial supply of freshwater and sediment (coastal case). In the estuarine case, bars arise as a result of an intrinsic instability of the equilibrium state. Width increases toward the mouth leading to channel shallowing, which in turn promotes the development of multiple bars. This effect is partially counteracted by the channel deepening produced by the tidal action, which tends to reduce bar growth and migration rate. In the coastal case, tidal bars form far from the equilibrium state of the system, adapting instantaneously to the evolving bed profile, which in turn affects channel flow conditions. Noteworthy, the model suggests that the spatial-temporal distribution of bars observed in nature reflects the evolutionary history of the syste

Three-dimensional Reduced-Complexity Simulation of Fluvio-Deltaic Clastic Stratigraphy

A novel reduced-complexity approach to 3D forward modeling of siliciclastic stratigraphy is presented for the simulation of erosion, transport, and sedimentation in continental, transitional, and marine depositional domains. The numerical model is based on defining centerlines that connect sediment input points to the shoreline. For each centerline, erosional and depositional surfaces bound depositional domains, and sand and mud proportions are assigned to each domain. The position of each depositional surface follows a set of geologic rules and ensures mass balance with sediment input. The numerical model is tested by simulating the basin-fill architecture of the XES02 laboratory experiment run at the University of Minnesota, which generates stratigraphy mimicking a passive-margin basin fill. Automatic calibration is used to test multiple combinations of uncertain model input parameters to find those that produce scenarios consistent with the experimental stratigraphy. Calibrated models accurately reproduce shoreline and mass-balance centroid migration, marine sediment proportions, and shoreline trajectories measured in the XES02 experiment. The models also provide reasonable approximations of sand distribution. Of interest, falling shoreline trajectories in the experiment and the calibrated models develop coeval to topset aggradation or topset incision depending on rate of base-level fall. The results reported herein validate the numerical approach for simulating sand distribution in an experimental basin, representing a first step towards the application of the numerical model in fluvio-deltaic settings. For field applications, analogue data can be used to independently constrain the geometry of surfaces bounding depositional domains and their composition. The simplicity of the numerical model then enables multiple realizations by quickly varying uncertain boundary conditions, allowing probabilistic predictions in settings with limited data constraints

Typhoon-induced megarips as triggers of turbidity currents offshore tropical river deltas

Intense rip currents caused by tropical cyclones can drive sediment-laden turbidity flows down submarine canyons, according to numerical simulations. Shoreline shape, bathymetry and incoming wave direction are key factors controlling this phenomenon

A unified framework for stability of channel bifurcations in gravel and sand fluvial systems

Bifurcating rivers shape natural landscapes by distributing water and sediments on fluvial plains and in deltas. Symmetrical bifurcations were often found to be unstable so that one branch downstream of the bifurcation enlarged while the other dwindled. A unified theory able to predict bifurcation stability in both gravel bed and sand bed rivers is still lacking. Here we develop a new theory for the stability of bifurcations for the entire range of gravel bed to sand bed rivers. The theory indicates opposite behavior of gravel bed and sand bed rivers: we predict that symmetrical bifurcations are inherently stable for intermediate Shields stresses but are inherently unstable for the low and high Shields stresses found in the majority of rivers on Earth. In the latter conditions asymmetrical bifurcations are stable. These predictions are corroborated by observations and have ramifications for many environmental problems in fluviodeltaic settings

Modelling the air-sea-land interactions responsible for the direct trigger of turbidity currents by tropical cyclones

Tropical cyclones directly trigger turbidity currents in submarine canyons as a consequence of storm surges, high waves, onshore blowing winds and extreme currents. The resultant supply of sediment at the heads of the canyons plays a crucial role in the genesis of turbidity currents and thus is key in understanding frequency and duration of their flows. Here we present a single numerical framework capable of modelling turbidity currents driven by cyclone-induced winds and waves through resolved, quasi-3D hydrostatic equations. Our simulations predict the occurrence of a canyon-confined turbid underflow induced by a tropical cyclone that caused a seafloor pipeline to shift. Turbidity current occurrence is shown to be related not just to tropical cyclone intensities but also to their tracks with respect to the canyon head. The modelled underflow is well approximated by similarity profiles from laboratory and field observations which demonstrates the reliability of the model in capturing the structure of turbidity currents. The proposed triggering of turbidity currents off the centre of coastal embayments is likely to occur when the abrupt rotation of incoming winds induced by a passing cyclone remains always coastal-bound all across the cyclone's waxing and waning stages. Our results show that these conditions can give rise to simultaneous, opposite alongshore currents and eventually result in offshore-bound rip currents. Conversely, it is unlikely that turbidity currents will be triggered by cyclone -induced rip currents when the cyclonic rotation results in peak offshore winds (coming from the land), as no fetch is available for generating large breaking waves to induce simultaneous, opposite alongshore currents. Nevertheless, the sole presence of strong alongshore currents deflected at the headlands of a coastal embayment (or delta) is likely to trigger sediment-gravity flows and eventually result in turbidity currents offshore the edge of the embayment without the aid of rip currents