A new mechanism for the triggering of turbidity currents offshore tropical river deltas

2020 ◽  
Author(s):  
Gaetano Porcile ◽  
Michele Bolla Pittaluga ◽  
Alessandro Frascati ◽  
Octavio Sequeiros
Keyword(s):  
Sediment Transport ◽  
Meteorological Data ◽  
Extreme Weather Events ◽  
Turbidity Currents ◽  
Coastal Circulation ◽  
Complex Flow ◽  
Submarine Canyons ◽  
Tropical River ◽  
River Deltas ◽  
Water And Sediment

<p>When narrow continental shelves are stressed by extreme weather events, nearshore currents dominate the coastal circulation leading to complex flow patterns that can result in previously unforeseen cross-shelf exchange of water and sediment. Here we present a series of detailed studies carried out to investigate the nature of turbidity currents that impacted upon a submarine pipeline offshore Philippines, nearby tropical river deltas, after the landfall of intense typhoons. These rivers debouch into a shelf only a few hundreds of meters wide that is interrupted by steeper continental slopes carved by multiple submarine canyons. Turbidity currents were detected through regular pipeline monitoring, which showed lateral displacements and sea-floor erosion where the pipeline crosses some of these canyons. Seabed assessments indicated signatures of the occurrence of turbidity currents as opposed to landslides or ground motion due to earthquakes. Particularly, the submarine canyons were covered with regular sediment patterns that indicated the passage of deep-water turbulent flows, suggesting the local occurrence of turbidity currents. Meteorological data pointed at river floods and meteocean conditions, and associated fluvial sediment delivery and coastal sediment transport, as the most likely leading mechanisms for the triggering of turbidity currents. Hydrological modelling and related sediment transport calculations show these rivers were not capable to debouch into the sea with sediment concentrations high enough to generate hyperpycnal flows. Nevertheless, river plumes played an active role as source of sediment available on the shelf. Conversely, the role of the coastal circulation was found to be crucial for the triggering of turbidity currents. Our simulations show the development of exceptional rip currents (megarips) that flush out water and sediment from the inner shelf in the cross-shore direction towards the canyons’ heads, ultimately triggering turbidity currents into deep ocean waters. Such extreme nearshore circulations require the passage of intense typhoons in proximity to the trigger area inducing shore-normal incoming waves at peak conditions that in association with shoreline concavity at the river deltas favour the formation of erosional megarips, whose dynamics strongly depends on typhoon's approach latitude. The turbidity current modelling confirmed such an interpretation, matching field observations in the form of pipeline displacements. These evidences support our hypothesis that typhoon-induced megarip circulations could be responsible for the triggering of turbidity currents in submarine canyon systems offshore tropical river deltas. This newly identified mechanism has wide implications on the threatening of seafloor infrastructures and the assessment of frequency and duration of turbidity currents.</p>

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

2020 ◽  
Vol 1 (1) ◽  
Author(s):  
Gaetano Porcile ◽  
Michele Bolla Pittaluga ◽  
Alessandro Frascati ◽  
Octavio E. Sequeiros
Keyword(s):  
Turbidity Currents ◽  
Tropical River ◽  
River Deltas

scholarly journals Discharge and Sediment Transport in a River Basin. Assessment Before and After the Construction of a Dam at the Upper Basin Boundary

2021 ◽  
Author(s):  
George Paschalidis ◽  
Ilias IIordanidis ◽  
Petros Anagnostopoulos
Keyword(s):  
Sediment Transport ◽  
Meteorological Data ◽  
Climatic Data ◽  
Northern Greece ◽  
Basin Boundary ◽  
Software Packages ◽  
Before And After ◽  
The Mean ◽  
Gis Interface ◽  
Good Agreement

Abstract The purpose of this study is the evaluation of runoff and sediment transport in the basin of the Nestos River (Northern Greece) downstream of the dam of Platanovrisi, constructed in 1998. The model used for the simulation was AGNPS, which is based on the Revised Universal Soil Loss Equation (RUSLE), combined with a GIS interface. Two different simulations were conducted, one for the years 1980-1990 and another for the period 2006-2030, before and after the construction of the dam respectively. For the simulation for the period 1980-1990 existing meteorological data were employed, and the results were in good agreement with those of a different study (Hrissanthou, 2002). The simulation for the period 2006-2030 was based on rainfall and climatic data generated from the software packages GlimClim and ClimGen. The mean runoff was by 5% lower and the mean annual sediment yield by 20% lower than the corresponding values for the period 1980-1990.

scholarly journals Exchange Mechanism of the Suspended Sediment at the Mouth of Hangzhou Bay under Coastline Changes

2021 ◽  
Vol 233 ◽  
pp. 03035
Author(s):  
Zhuzhu Yu ◽  
Zhiguo He ◽  
Li Li ◽  
Taoyan Ye ◽  
Yuezhang Xia
Keyword(s):  
Sediment Transport ◽  
Numerical Model ◽  
Exchange Mechanism ◽  
Hangzhou Bay ◽  
The South ◽  
Topographic Data ◽  
The North ◽  
Water And Sediment ◽  
Open Sea ◽  
Inflowing Water

Based on FVCOM hydrodynamic numerical model and coastline topographic data in 2013, a three-dimensional numerical model of fine sediment transport in Hangzhou Bay has been established to explore the water and sediment exchange mechanism between Hangzhou Bay and the open sea at different typical sections. The results of validation with measured and satellite retrieved data show that the model can well simulate the process of water and sediment movement in Hangzhou Bay. Compared with the calculation results of the coastline topographic data of Hangzhou Bay in 1974 and 2020, the influence mechanism of shoreline change on the water and sediment exchange mechanism between Hangzhou Bay and the open sea has been studied. The results show that the sediment transport inside and outside the Hangzhou Bay is generally in the pattern of north-inflow and south-discharge. Compared with the coastline in 1974, the sediment transport from Yangshan port in the north of Hangzhou Bay and Zhoushan Islands in the middle of Hangzhou Bay increases when the coastline is pushed into the bay in 2020, while the outward sediment transport from Jintang Channel in the South decreases. The overall trend features that the sediment transport into the bay increases, with the bay mouth silting. In the three sections extending from Hangzhou Bay to the open sea, the inflowing water and sediment of the horizontal section on the north side is decreasing, while the discharged sediment from the south side and the inflowing water and discharged sediment from the vertical section at the east side are increasing.

The history of the Nahariya Submarine Canyon, offshore northern Israel, from sedimentary down core records and foraminiferal analyses

2021 ◽  
Author(s):  
Naomi Moshe ◽  
Oded Katz ◽  
Adi Torfstein ◽  
Mor Kanari ◽  
Pere Masque ◽  
...  
Keyword(s):  
Sediment Transport ◽  
Eastern Mediterranean ◽  
Transport Processes ◽  
Mass Wasting ◽  
Submarine Canyons ◽  
Last Glacial ◽  
Erosion Processes ◽  
Foraminiferal Species ◽  
Continental Slopes ◽  
The Last Glacial

<p>Submarine canyons are prominent features in continental slopes. They play an important role in sediment transport to the deep sea, as they form conduits for turbidity currents and cause landslides due their steep slopes. Such mass transport events could create geo-hazards, which compromise infrastructures along the continental slope.</p><p>Our research focuses on the Nahariya Canyon, which is part of a series of submarine canyons located along the continental slopes of the eastern Mediterranean, offshore northern Israel. This canyon is incised into the slope and does not reach the shelf. Here, we report the results from a study of two piston cores sampled in the canyon at water depths of 650m (NAC650, ~2.5m long) and 915m (NAC915, ~6m long). Chronologies were established based radiocarbon dating using slope foraminiferal shells, in addition to <sup>210</sup>Pb and OSL dating of bulk sediment. The sediments were characterized by major and trace element concentrations, mineralogy, grain size, and dead foraminiferal assemblages. We further identified the living (Rose-Bengal stained) foraminiferal species at three depths habitats (200m 650m and 915m water depth).</p><p>Our results show that both piston cores are comprised of a capping ~40 cm thick interval of fine laminated mud, deposited over the last ~150-200 years, apparently reflecting hemipelagic sedimentation. This capping interval unconformably overlays a consolidated sequence in both cores, which indicates a major sediment removal. The consolidated sequence in NAC650 is mostly homogenous and dates to the previous glacial (>140 ka), and in NAC915 the upper 70 cm of the consolidated sequence consists mud clasts dated to 27-46 ka that overlay an ‘S shape’ shear zone, which is a result of a down canyon mass wasting (debrite). Below that debrite, the sediment is mostly homogenous and dates to the last glacial (>25 ka). Broken shells of shallow benthic foraminiferal species such as Ammonia spp., Asterigerinata mamilla, Miliolids, Rosalina spp. and Sorites orbiculus are found abundantly throughout both piston-cores, varying between in-core intervals, indicating that allochthonous sediments are prevalent at those cores. Same shallow species are found also in the surface (living) assemblages, mixed with slope deep foraminiferal species. Moreover, the deep living foraminiferal shells are well preserved, in contrast to the shallow living species. Taken together, these indicate that sediment transport processes along the canyon exist to this day.</p><p>The cores suggest that the canyon is an erosive environment at least since the last glacial maximum, when the last significant mass wasting deposit is recorded. The Holocene is not represented in the records, probably due to the dominance of erosion processes, except for a thin layer of sediment deposited over the last two centuries that prevails along the entire canyon.</p>

Development of a Non-Linear Integrated Drought Index (NDI) for Managing Drought and Water Resources Forecasting in the Upper Tana River Basin-Kenya

2020 ◽  
Vol 11 (1) ◽  
pp. 15-33
Author(s):  
Raphael Muli Wambua
Keyword(s):  
Water Resources ◽  
River Basin ◽  
Lead Time ◽  
Drought Index ◽  
Meteorological Data ◽  
Principal Component ◽  
Drought Forecasting ◽  
Tropical River ◽  
Tana River ◽  
Non Linear

This article uses the non-linear integrated drought index (NDI) for managing drought and water resources forecasting in a tropical river basin. The NDI was formulated using principal component analysis (PCA). The NDI used hydro-meteorological data and forecasted using recursive multi-step neural networks. In this article, drought forecasting and projection is adopted for planning ahead for mitigation and for the adaptation of adverse effects of droughts and food insecurity in the river basin. Results that forecasting ability of NDI model using ANNs decreased with increase in lead time. The formulated NDI as a tool for projecting into the future.

Sedimentary facies on the rises and slopes of passive continental margins

1980 ◽  
Vol 294 (1409) ◽  
pp. 169-176 ◽  
Keyword(s):  
Sediment Transport ◽  
Debris Flows ◽  
Thermohaline Circulation ◽  
Dominant Role ◽  
Sedimentary Facies ◽  
Turbidity Currents ◽  
Continental Margins ◽  
Passive Margins ◽  
New Evidence

JOIDES drilling results provide new evidence concerning facies patterns on evolving passive margins that strengthens and extends hypotheses constructed from studies of morphology, seismic reflexion data and shallow samples on modern margins, and from field geologic studies of uplifted ancient margins. On the slopes and rise, gravity-controlled mechanisms - turbidity currents, debris flows, slides and the like - play the dominant role in sediment transport over the long term, but when clastic supplies are reduced, as for example during rapid transgressions, then oceanic sedimentation and the effects of thermohaline circulation become important. Sedimentary facies models used as the basis of unravelling tectonic complexities of some deformed margins, for example in the Mesozoic Tethys, may be too simplistic in the light of available data from modern continental margins.

Numerical modelling of cohesive sediment transport in rivers

2004 ◽  
Vol 31 (5) ◽  
pp. 749-758 ◽  
Author(s):  
David H Willis ◽  
B G Krishnappan
Keyword(s):  
Sediment Transport ◽  
Three Dimensional ◽  
Cohesive Sediment ◽  
Low Flow ◽  
Shear Stresses ◽  
Hydrodynamic Models ◽  
Coupled Models ◽  
Time Step ◽  
Water And Sediment ◽  
Sediment Model

Techniques available to practicing civil engineers for numerically modelling cohesive mud in rivers and estuaries are reviewed. Coupled models, treating water and sediment as a single process, remain research tools but are usually not three-dimensional. The decoupled approach, which separates water and sediment computations at each model time step, allows the three-dimensional representation of at least the bed and the use of well-proven, commercial, numerical, hydrodynamic models. Most hydrodynamic models compute sediment transport in suspension but may require modification of the dispersion coefficients to account for the presence of sediment. The sediment model deals with the sediment exchange between the water column and the bed using existing equations for erosion and deposition. Both equations relate the sediment exchange rates to the shear stress in the bottom boundary layer. In real rivers and estuaries, a depositional bed layer is associated with a period of low flow and shear, at slack tide for example, whereas in numerical models a layer is defined by the model time step. The sediment model keeps track of the uppermost layers at each model grid point, including consolidation and strengthening. Although numerical hydrodynamic models are based strongly on physics, sediment models are only numerical frameworks for interpolating and extrapolating full-scale field or laboratory measurements of "hydraulic sediment parameters," such as threshold shear stresses. Calibration and verification of models against measurement are therefore of prime importance.Key words: cohesive sediment, mathematical modelling, settling velocity, erosion, resuspension, deposition, fluid mud, bed layers.

Sign in / Sign up

Export Citation Format

Share Document