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

Morphodynamic regime and long-term modelling of meandering rivers

Natural rivers are self-formed features whose shapes are the result of interaction between erosion, deposition and transport of sediments. The study of their morphodynamics and the characterization of related sedimentary processes are of great interest not only to environmental engineers but also to hydrologist and geologist, contributing to the interpretation of stratigraphic records. In particular, this thesis deals with the morphodynamics and the long term behaviour of meandering rivers, a very common pattern in nature. This class of dynamical systems, which occurs at the spatial scale of the channel width, are driven by the coexistence of various intrinsically nonlinear mechanisms which determine the possible occurrence of two different morphodynamic regimes: the sub-resonant and the super-resonant regime. On the short term time scale, the formation of meandering patterns can be suitably explained as an instability process, driven by bank erosion (bend instability). The planar development of the river is described by a non-linear integrodifferential bend evolution equation, complemented with a suitable model for flow and bed topography in sinuous channels with cohesionless bed. On the long-term timescale, a further highly non-linear process must be accounted for, namely channel shortening via cutoff processes. Depending on the description adopted for the flow field, various mathematical models allowing the description of the temporal evolution of the channel axis can be developed. Here, we develop a simulation model for river meandering employing two physics-based linear models characterized by a different degree of approximation. The fully nonlinear computational approach permits exploration of the long term dynamics of meandering rivers. Investigating the full range of morphodynamic conditions, we objectively compare the morphologic characteristics exhibited by synthetically generated and observed planimetric patterns. The analysis is carried out examining, through principal component analysis, a suitable set of morphological variables. We show that, even in the presence of the strong filtering action exerted by cutoff processes, a closer, although not yet complete, similarity with natural meandering planforms can be achieved only by adopting a flow field model which accounts for the full range of morphodynamic regimes. We also introduce a new morphodynamic length scale, associated with spatially oscillating disturbances. Once normalized with this length scale, the relevant morphologic features of the simulated long-term patterns (e.g., the pdf of local curvature and the geometric characteristics of oxbow lakes) tend to collapse on two distinct behaviours, depending on the dominant morphologic regime. Finally, the signatures of possibly chaotic dynamics or self-organized criticality triggered by repeated cutoff events are investigated. To these aims, some robust nonlinearity tests are applied to both the spatial series of local curvatures and the time series of long term tortuosity. Temporal distribution of cutoff inter-arrivals are also investigated. The results are coherent and show that no evidence of chaotic determinism or self-organized criticality are detectable in meandering dynamics.La descrizione dei processi che caratterizzano gli ambienti sedimentari fluviali costituisce una tematica di grande interesse non solo in ambito ingegneristico ambientale, ma anche in quello idrogeologico. Gli alvei naturali presentano un'estrema eterogeneità e la loro forma va necessariamente vista come il risultato dell’azione combinata dei processi di erosione, trasporto e deposito dei sedimenti. Le cause che inducono un alveo ad assumere naturalmente una determinata configurazione vanno quindi ricercate nell’interazione esistente tra i flussi d'acqua e di sedimenti in ingresso al bacino di drenaggio e tutti quei fattori che concorrono alla formazione del paesaggio, ovvero: la topografia e le caratteristiche idrogeologiche della piana alluvionale, il tipo di sedimenti al fondo, il materiale che costituisce le sponde ed il tipo di vegetazione che su di esse cresce. Nella presente tesi focalizzeremo la nostra attenzione sugli alvei ad andamento meandriforme il cui sviluppo planimetrico è strettamente associato all'erodibilità delle sponde e alle caratteristiche del campo di moto. In particolare, verrà sviluppato un modello numerico in grado di simulare lo sviluppo planimetrico di tali corsi d’acqua, le cui caratteristiche morfologiche sono strettamente legate al carattere sub o super-risonante del regime morfodinamico dominante. Obiettivo del presente lavoro è quello di valutare, sul lungo periodo, le caratteristiche morfologiche degli alvei meandriformi determinate dalla struttura non lineare dell'equazione integro-differenziale che descrive levoluzione planimetrica dell’asse del canale e dalla natura fortemente non lineare del fenomeno del cosiddetto taglio di meandro (comunemente noto in letteratura come cutoff). La corrispondenza tra le caratteristiche morfologiche delle configurazioni planimetriche calcolate numericamente utilizzando due diversi modelli idrodinamici linearizzati e quelle osservate in natura viene analizzata utilizzando un'analisi delle componenti proncipali (PCA). Mostreremo come il contributo dinamico delle forti non linearità indotte dai processi di cutoff non sia tale da eliminare completamente le caratteristiche morfologiche associate alla struttura del campo di moto. In particolare si constaterà come, seppure molti degli aspetti fondamentali delle configurazioni planimetriche naturali siano ben riprodotti anche utilizzando per il campo di moto delle soluzioni linearizzate molto semplificate, tuttavia, una stringente corrispondenza con gli alvei meandriformi reali sia resa possibile solo dall'adozione di soluzioni più raffinate in grado di analizzare in modo pi`u completo i complessi meccanismi morfodinamici. Inoltre, utilizzando un modello linearizzato di campo di moto in grado di analizzare l'influenza morfodinamica nella sua interezza, sarà possibile lìindividuazione di una nuova lunghezza scala caratteristica dei meandri, la quale, essendo in grado di sostenere anche il regime super-risonante, si rivelerà una scelta decisamente migliore rispetto alle vecchie scale spaziali utilizzate per i meandri. Infine, adottando una metodologia d'indagine consolidata nell'ambito dell'analisi delle serie temporali non-lineari ed analizzando la serie degli interarrivi dei cutoff, si metterà in evidenza come nelle dinamiche che regolano lo sviluppo dei sistemi meandriformi non sia riscontrabile né l'esistenza di un chiaro determinismo caotico, né tantomeno una loro tendenza ad auto-organizzarsi (self-organized criticality)

Morphodynamic regime and long-term evolution of meandering rivers

In the present contribution we focus our attention on the long-term behavior of meandering rivers, a very common pattern in nature. This class of dynamical systems is driven by the coexistence of various intrinsically nonlinear mechanisms which determine the possible occurrence of two different morphodynamic regimes: the subresonant and the superresonant regimes. Investigating the full range of morphodynamic conditions, we objectively compare the morphologic characteristics exhibited by synthetically generated and observed planimetric patterns. The analysis is carried out examining, through principal component analysis, a suitable set of morphological variables. We show that even in the presence of the strong filtering action exerted by cutoff processes, a closer, although not yet complete, similarity with natural meandering planforms can be achieved only by adopting a flow field model which accounts for the full range of morphodynamic regimes. We also introduce a new morphodynamic length scale, L(m), associated with spatially oscillating disturbances. Once normalized with this length scale, the relevant morphologic features of the simulated long-term patterns (e. g., the probability density function of local curvature and the geometric characteristics of oxbow lakes) tend to collapse on two distinct behaviors, depending on the dominant morphologic regime

A mathematical model for meandering rivers with varying width

The mathematical modeling of the long-term evolution of meandering rivers needs an efficient computation of the flow field. Indeed, the estimate of the near bank velocity, needed to determine the rate at which the outer bank migrates, cannot rely on the full numerical solution of the governing equations when considering the river evolution on geological time scales. The aim of the present contribution is twofold: determining the complete linear response of a meandering river to spatially varying channel axis curvature and width, exploiting the ability of the model to describe the morphological tendencies of alluvial rivers; and developing a computationally efficient tool that can be easily incorporated in long-term planform evolution models. The centrifugally induced secondary flow associated with channel axis curvature and longitudinal convection is accounted for by a suitable parametrization based on the structure of the three-dimensional flow field. Cross section width variations are accounted for through a suitable stretching of the transverse coordinate. The relevant momentum and mass conservation equations are then linearized by taking advantage of the fact that alluvial rivers often exhibit mild and long meander bends, as well as evident but relatively small width variations. The input data needed by the analytical solution are the spatial distribution of channel axis curvature and width variations, the mean slope of the investigated river reach, the characteristic grain size of the sediment bed and the flow discharge. The performances of the model, as well as its intrinsic limitations are discussed with reference to the comparison with the bed topography surveyed in a 21 km long reach of the Po River. The results indicate that, in the presence of wide, mildly curved and long bend and weak width variations, the river topography is described with a good accuracy, thus supporting the use of the model to investigate how a river could react to changes in planform geometry or external forcing. Moreover, the analytical character of the model implies a limited computational effort, facilitating a straightforward integration within the models used to simulate the planimetric evolution of alluvial rivers on geological time scales

Long river meandering as a part of chaotic dynamics? A contribution from mathematical modelling

In the present contribution we focus our attention on the possible signatures of a chaotic behaviour or a self-organized criticality state triggered in river meandering dynamics by repeated occurrence of cutoff processes. The analysis is carried out examining, through some robust nonlinear methodologies inferred from time series analysis, both the spatial series of local curvatures and the time series of long-term channel sinuosity. Temporal distribution of cutoff inter-arrivals is also investigated. The analyzed data have been obtained by using a suitable physics-based simulation model for river meandering able to reproduce reasonably the features of real rivers. The results are consistent and show that, at least from a modelling point of view, no evidence of chaotic determinism or self-organized criticality is detectable in the investigated meandering dynamics