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The pathway of dense river inflows into lakes, which affects the lake water quality, is not accurately predicted by existing models. The pathway of a dense riverine inflow in a lake with a submerged canyon is analyzed based on measurements during a 4‐month period of weakening lake stratification and weakening density excess between river and epilimnion. In line with models, the dense riverine inflow plunges upon entering the lake, continues as an underflow on the sloping lake bottom, and finally intrudes at its level of neutral buoyancy. Underflow and interflow velocities are O(0.1 m s−1). The river inflow is finally trapped in the pycnocline most of the time, even when the river's density excess and the lake's stratification are marginal. This trapping in the pycnocline is explained by the reduction of the inflow density excess due to the intense plunging mixing, which is an order of magnitude larger than that obtained in confined laboratory flumes. The pathway of the dense riverine inflow is affected by interactions of the underflow with the lake bottom and sedimentary processes. A canyon carved by the underflows confines and accelerates the underflow, which enhances its capacity to entrain and carry sediment. The entrainment of sediment that was previously deposited on the canyon bottom accelerates the underflow. Due to both effects, the underflow can temporarily break through the pycnocline and reach the hypolimnion. Existing models explain these observations qualitatively, but a quantitative prediction would require better parameterizations of the plunging mixing and the sedimentary processes. Plain Language Summary The pathway of river inflows into lakes is not accurately predicted by existing models. We investigate the physical processes affecting the pathway of dense riverine inflow (i.e., inflow with a density higher than lake water) into a stratified lake. We investigate the conditions under which a dense riverine inflow get trapped in the pycnocline (the layer that separates warmer surface waters from cold deep waters) or break through it. Unprecedented long records of the temporal evolution of the pathway of the riverine flow into the lake during a period of weakening riverine density excess and lake stratification are conceptualized in a model, which extends existing concepts for dense riverine inflows. The entrainment of lake waters into the riverine inflow in the plunging region is larger than predicted by laboratory studies. This explains why the riverine inflow is trapped in the pycnocline most of the time. Flow confinement by a canyon carved by the riverine inflow into the lake bottom accelerates the riverine inflow and enhance sediment entrainment capacity causing short‐lived self‐accelerating turbidity currents along the lake bottom that break through the pycnocline and reach deep waters. Our results allow improved estimates of oxygen replenishment or sediment deposition from riverine water. Key Points Plunging mixing into an unconfined ambient is an order of magnitude larger than in a confined ambient Pronounced plunging mixing reduces the initial density excess explaining why the inflow is mostly trapped in the pycnocline Resuspension of lake bottom sediment can cause short‐lived self‐accelerating turbidity currents that break through the pycnocline

Manned submersible dives in the northwest South China Sea encountered substantial amounts of plastic litter accumulated at the base of scours along the floor of a submarine canyon, which may associate with the depositional behaviors of turbidity currents. In this study, we conduct numerical simulations using field‐scale bathymetry to investigate the relationship between the canyon floor morphology, flow processes, and the locations and sizes of the plastic litter piles. The consistent deposition pattern caused by the numerical turbidity currents with different input parameters indicate that morphology of the canyon may exert a dominant influence on turbidite deposition. This is attributed to a significant reduction in shear velocity as simulated turbidity currents flowing through the scours on the canyon floor. Spatial correspondence between deposits of turbidity currents and plastic litter accumulation suggests that suspended sediments and plastic may undergo simultaneous dynamic processes during the transportation of turbidity currents. Plain Language Summary The issue of marine plastic litter has attracted wide attention, particularly in terms of its transportation mechanisms and locations of accumulation on the ocean floor. Turbidity currents are subaqueous sediment‐gravity flows that can transport large amounts of sediment, nutrients and pollutants into the deep sea, yet there is sparse research on the dynamics of plastic litter transport under the control of turbidity currents, and its accumulation in the deep sea. Here, we present a series of numerical simulations of turbidity currents in a submarine canyon with various input parameters, when combined with observational data on topography and plastic litter distribution, confirm that turbidity currents constitute a plausible mechanism for the transport of plastic litter and for its accumulation in response to changes in flow associated with scours. In addition, we find that the concavity of the scours is also necessary for plastic litter accumulation, since it induces significant fluctuations in the shear velocity and the corresponding depositional process. Key Points Numerical simulations were applied to investigate turbidity currents as a cause for plastic litter accumulations in a submarine canyon The simulated turbidite deposits and observed plastic litter accumulations exhibit a strong spatial correspondence The morphology of the canyon floor may exert dominant influence on the plastic litter accumulations in submarine canyons

Seafloor sediment flows (turbidity currents) are among the volumetrically most important yet least documented sediment transport processes on Earth. A scarcity of direct observations means that basic characteristics, such as whether flows are entirely dilute or driven by a dense basal layer, remain equivocal. Here we present the most detailed direct observations yet from oceanic turbidity currents. These powerful events in Monterey Canyon have frontal speeds of up to 7.2 m s −1 , and carry heavy (800 kg) objects at speeds of ≥4 m s −1 . We infer they consist of fast and dense near-bed layers, caused by remobilization of the seafloor, overlain by dilute clouds that outrun the dense layer. Seabed remobilization probably results from disturbance and liquefaction of loose-packed canyon-floor sand. Surprisingly, not all flows correlate with major perturbations such as storms, floods or earthquakes. We therefore provide a new view of sediment transport through submarine canyons into the deep-sea. The structure of turbidity currents has remained unresolved mainly due to lack of observations. Here the authors present data from a high-resolution monitoring array deployed for 18 months over Monterey Bay, that suggests turbidity currents are driven by dense near-bed layers.

Turbidity currents are destructive flows that are hazardous to critical seafloor infrastructure on submarine slopes because run‐up heights can be 10–100s of meters, as their relative density is 2–3 orders of magnitude lower than terrestrial flows. Currently, risk analysis is hindered by poor prediction of run‐up heights that are mainly derived from confined 2D experiments, and/or numerical models, and are restricted to a specific configuration whereby the flow strikes topographic barriers orthogonally. Here, a new analytical model is presented, informed by and validated against physical experiments, which predicts run‐up heights for flows encountering three‐dimensional slopes as a function of any slope angle, and incidence angle, of the impinging turbidity current. This has important implications for reducing geohazards by informing routing and positioning of seafloor infrastructure, and for more accurately interpreting submarine landscapes and their deposits.

Here we show how major rivers can efficiently connect to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action on Earth. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1,338-2,675 [>535-1,070] Mt of sediment from one submarine canyon, equivalent to 19–37 [>7–15] % of annual suspended sediment flux from present-day rivers. It was known earthquakes trigger canyon-flushing flows. We show river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, and sometimes triggered by spring tides weeks to months post-flood. It is demonstrated that strongly erosional turbidity currents self-accelerate, thereby travelling much further, validating a long-proposed theory. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or deep-sea impacts of terrestrial climate change. This paper analyses the longest sediment flows measured in action on Earth. These seabed flows were caused by floods and spring tides, and flushed prodigious sediment and carbon volumes into the deep sea, as they accelerated for a thousand kilometres.

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.

Gravelly cyclic steps formed by turbidity currents have recently been widely recognised in the geological record. However, the comparison between modern and ancient gravelly cyclic steps remains challenging. In this study, the facies variations of gravelly cyclic steps deposited from turbidity currents in an outcrop of Miocene fan delta front deposits in central Japan are compared with the geomorphic evolution of analogous cyclic steps on the active fan delta front in modern geological systems. These cyclic steps share common characteristics in setting, grain size and dimensions, allowing direct comparison between ancient and modern examples. Repeat bathymetric surveys of the modern Kurobe River fan delta have revealed various types of upslope migrating bedforms interpreted as cyclic steps. Most of these are short‐lived due to erosion of thalwegs by powerful surge‐type turbidity currents triggered by slope failures or burial of thalwegs by deposition, possibly from river‐fed hyperpycnal flows. Such erosion and burial events occur annually on the fan delta front, resulting in compensational cycles. Sedimentary facies of the Miocene fan delta front deposits include conglomerate with a gently upslope dipping erosional base, backset‐stratified sandstone and foreset‐stratified sandstone, which are interpreted as deposits of hydraulic jumps in high‐density turbidity currents in scours on the stoss sides of cyclic steps. Parallel‐stratified conglomerate sandstone is an additional component. Although previous facies models of gravelly cyclic steps have focussed on deposits on stoss sides, a comparison of modern and ancient examples suggests that facies on the lee sides could be parallel‐stratified conglomerate sandstone and undulating bedforms, reflecting supercritical flow conditions. The present study also suggests that hyperpycnal and surge‐type high‐density turbidity currents may deposit different types of facies and play different roles in the construction and destruction of cyclic steps. The present study has significant implications for facies models of gravelly cyclic steps. This paper provides a new facies model of gravelly cyclic steps that includes a facies model of lee side and variations deposited from hyperpycnal and surge‐type high‐density turbidity currents.

Field observations of turbidity currents remain scarce, and thus there is continued debate about their internal structure and how they modify underlying bedforms. Here, I present the results of a new imaging method that examines multiple surge-like turbidity currents within a delta front channel, as they pass over crescent-shaped bedforms. Seven discrete flows over a 2-h period vary in speed from 0.5 to 3.0 ms −1 . Only flows that exhibit a distinct acoustically attenuating layer at the base, appear to cause bedform migration. That layer thickens abruptly downstream of the bottom of the lee slope of the bedform, and the upper surface of the layer fluctuates rapidly at that point. The basal layer is inferred to reflect a strong near-bed gradient in density and the thickening is interpreted as a hydraulic jump. These results represent field-scale flow observations in support of a cyclic step origin of crescent-shaped bedforms. The basal structure of turbidity currents and their association with crescent-shaped bedforms has not been observed at the field scale. Here, the author presents views of turbidity currents moving over and modifying such bedforms in a manner consistent with theoretical and laboratory studies of cyclic steps.

In view of the problem that chemical oxygen demand (COD) measurement in water using UV-Vis spectrometry was easily affected by turbidity, this paper proposed an analytical method for determining the complex refractive index of particles in water based on Lambert-Beer’s law and K-K (Kramers-Kronig) relationship. The obtained complex refractive index was used to establish the turbidity compensation model in the COD characteristic spectral region, and the COD concentration inversion were achieved by using the PLS algorithm. The results show that the turbidity compensation method based on Mie scattering theory can improve the accuracy of COD measurement by UV-Vis spectroscopy. Compared with before turbidity compensation, R2 (determination coefficient) between true values and predicted values of COD increased from 0.2274 to 0.9629, and RMSE (root mean square error) of predicted values decreased from 21.73 to 3.12 mg L−1. Compared with 350 nm PC, derivative method, and improved MSC method, the turbidity compensation method for COD measurement based on Mie scattering theory is simple, fast, and highly accurate. And the calculated spectrum can represent the scattering characteristics of the measured spectrum. The average relative error between the fitted spectrum and the original normalized spectrum in the 55 mixed solutions was 0.52%, and the maximum relative error was 6.65%. This method can be useful for online COD measurement.

Particle-laden gravity flows, called turbidity currents, flow through river-like channels across the ocean floor. These submarine channels funnel sediment, nutrients, pollutants and organic carbon into ocean basins and can extend for over 1000’s of kilometers. Upon reaching the end of these channels, flows lose their confinement, decelerate, and deposit their sediment load; this is what we read in textbooks. However, sea floor observations have shown the opposite: turbidity currents tend to erode the seafloor upon losing confinement. Here we use a state-of-the-art scaling method to produce the first experimental turbidity currents that erode upon leaving a channel. The experiments reveal a novel flow mechanism, here called flow relaxation, that explains this erosion. Flow relaxation is rapid flow deformation resulting from the loss of confinement, which enhances basal shearing of the turbidity current and leads to scouring. This flow mechanism plays a key role in the propagation of submarine channel systems. The nature of erosion featured at the outlet of submarine channels is still a topic of debate. Here the authors present, based on scaled experiments, a novel flow mechanism for turbidity currents at the end of submarine channels and for the first time describe their erosional character.