scholarly journals Triggering and propagation of exogenous sediment pulses in mountain channels: insights from flume experiments with seismic monitoring

2021 ◽  
Vol 9 (6) ◽  
pp. 1423-1439
Author(s):  
Marco Piantini ◽  
Florent Gimbert ◽  
Hervé Bellot ◽  
Alain Recking
Keyword(s):  
Sediment Accumulation ◽  
Fine Fraction ◽  
High Concentration ◽  
Flume Experiments ◽  
Natural Sediment ◽  
Solid Discharge ◽  
Storage Zone ◽  
Storage Area ◽  
Grain Sorting ◽  
Sediment Mixture

Abstract. In the upper part of mountain river catchments, large amounts of loose debris produced by mass-wasting processes can accumulate at the base of slopes and cliffs. Sudden destabilizations of these deposits are thought to trigger energetic sediment pulses that may travel in downstream rivers with little exchange with the local bed. The dynamics of these exogenous sediment pulses remain poorly known because direct field observations are lacking, and the processes that control their formation and propagation have rarely been explored. Here we carry out flume experiments with the aims of investigating (i) the role of sediment accumulation zones in the generation of sediment pulses, (ii) their propagation dynamics in low-order mountain channels, and (iii) the capability of seismic methods to unravel their physical properties. We use an original setup wherein we supply liquid and solid discharge to a low-slope storage zone acting like a natural sediment accumulation zone that is connected to a downstream 18 % steep channel equipped with geophones. We show that the ability of the self-formed deposit to generate sediment pulses is controlled by the fine fraction of the mixture. In particular, when coarse grains coexist with a high content of finer particles, the storage area experiences alternating phases of aggradation and erosion strongly impacted by grain sorting. The upstream processes also influence the composition of the sediment pulses, which are formed by a front made of the coarsest fraction of the sediment mixture, a body composed of a high concentration of sand corresponding to the peak of solid discharge, and a diluted tail that exhibits a wide grain size distribution. Seismic measurements reveal that the front dominates the overall seismic noise, but we observe a complex dependency between seismic power and sediment pulse transport characteristics, which questions the applicability of existing seismic theories in such a context. These findings challenge the classical approach for which the sediment budget of mountain catchments is merely reduced to an available volume, since not only hydrological but also granular conditions should be considered to predict the occurrence and propagation of such sediment pulses.

scholarly journals Genesis and propagation of exogenous sediment pulses in mountain channels: insights from flume experiments with seismic monitoring

2021 ◽  
Author(s):  
Marco Piantini ◽  
Florent Gimbert ◽  
Hervé Bellot ◽  
Alain Recking
Keyword(s):  
Sediment Accumulation ◽  
High Concentration ◽  
Flume Experiments ◽  
Natural Sediment ◽  
Solid Discharge ◽  
High Fraction ◽  
Storage Area ◽  
Set Up ◽  
Grain Sorting ◽  
Sediment Mixture

Abstract. In the upper part of mountain river catchments, large amounts of loose debris produced by mass wasting processes can accumulate at the base of slopes and cliffs. Sudden destabilizations of these deposits are thought to trigger energetic sediment pulses that may travel in downstream rivers with little exchange with the local bed. The dynamics of these exogenous sediment pulses remain poorly known because direct field observations are lacking, and the processes that control their formation and propagation have rarely been explored experimentally. Here we carry out flume experiments with the aims of investigating (i) the role of sediment accumulation zones in the generation of sediment pulses, (ii) their propagation dynamics in low-order mountain channels, and (iii) the capability of seismic methods to unravel their physical properties. We use an original set-up where we supply with liquid and solid discharge a low slope storage zone acting like a natural sediment accumulation zone, and connected to a downstream 18 % steep channel equipped with geophones. We show that the ability of the self-formed deposit to generate sediment pulses depends on the sand content of the mixture. In particular, when a high fraction of sand is present, the storage area experiences alternating phases of aggradation and erosion strongly impacted by grain sorting. The upstream processes also influence the composition of the sediment pulses, which are formed by a front made of the coarsest fraction of the sediment mixture, a body composed of a high concentration of sand corresponding to the peak of solid discharge, and a diluted tail that exhibits a wide grain size distribution. Seismic measurements reveal that the front dominates the overall seismic noise, but we observe a complex dependency between seismic power and sediment pulses’ transport characteristics, which questions the applicability of existing simplified theories in such context. These findings challenge the classical approach for which the sediment budget of mountain catchments is merely reduced to an available volume, since not only hydrological but also granular conditions should be considered to predict the occurrence and propagation of such sediment pulses.

Mixing depths and sediment accumulation rates on an arid tropical shelf based on fine-fraction 210Pb analysis

2020 ◽  
Vol 425 ◽  
pp. 106198
Author(s):  
Yael Edelman-Furstenberg ◽  
Susan M. Kidwell ◽  
Henko C. de Stigter
Keyword(s):  
Sediment Accumulation ◽  
Fine Fraction ◽  
Accumulation Rates

Effect of Grain Sorting on the Formation of Bedforms

1995 ◽  
Vol 48 (9) ◽  
pp. 549-563 ◽  
Author(s):  
Giovanni Seminara
Keyword(s):  
Grain Size ◽  
Spatial Distribution ◽  
Probability Density Function ◽  
Probability Density ◽  
Selective Transport ◽  
Bed Topography ◽  
Damping Effect ◽  
Sand Ridges ◽  
Grain Sorting ◽  
Sediment Mixture

Grain sorting is a process observed in fluvial, coastal, and estuarian environments whereby the selective transport of different fractions of a sediment mixture gives rise to a non-uniform spatial distribution of the grain size probability density function. The formation of some fluvial bedforms (bedloadsheets and sand ridges) is shown to be generated or dominantly controlled by grain sorting. However, most bedforms (river dunes, free and forced fluvial bars, coastal ripples) are moderately affected by the sorting mechanism which is invariably found to lead to a damping effect on bedform growth. Recent investigations of the above phenomena are reviewed and new methodologic aspects arising in treating the instability of flow and bed topography in the presence of mixtures are pointed out.

Patterns and mechanisms of fluvial sediment flux and accumulation in two subarctic fjords: Nachvak and Saglek Fjords, Nunatsiavut, Canada

2012 ◽  
Vol 49 (10) ◽  
pp. 1200-1215 ◽  
Author(s):  
Samuel J. Bentley ◽  
Elisabeth Kahlmeyer
Keyword(s):  
Sediment Accumulation ◽  
Suspended Sediments ◽  
Sediment Traps ◽  
Extensive Study ◽  
Sediment Flux ◽  
Small Rivers ◽  
Recent Sediment ◽  
Accumulation Rates ◽  
Natural Sediment ◽  
Fluvial Sediment

Recent marine sedimentary deposits and river discharge in two subarctic fjords in Nunatsiavut (Northern Labrador, Canada) have been studied to elucidate patterns and mechanisms of fluvial sediment transfer and accumulation in the fjords, to further our understanding of the longer-term sedimentary record. Multibeam and sub-bottom acoustic data and sediment cores were collected in Nachvak and Saglek fjords, within Canada’s Torngat Mountains National Park, as part of the most extensive study of the park’s marine resources to date. Cores were subsampled for X-radiography, grain size, and 210Pb/137Cs geochronology. Muddy basin sediments within each fjord are bioturbated, indicating circulation of oxygenated bottom water. Depositional fluxes and inventories of 210Pb indicate efficient marine scavenging of 210Pb by fine suspended sediments. In Nachvak Fjord, with small rivers and steep, presently glaciated catchments, postglacial and recent sediment accumulation rates are similar, implying relatively constant sedimentation over time. In Saglek Fjord, fed by larger rivers with more extensive catchments that lack glaciers, recent sediment accumulation is more rapid than that averaged over postglacial time. Present mass accumulation rates for the Nachvak Fjord basin are on average 39 000 t·year–1 for the entire basin, and for Saglek 43 000 t·year–1 for the entire basin, with sediment-gravity flows being one likely mechanism for sediment delivery to deep basins. Results collectively suggest that both marine basins are excellent natural sediment traps. Comparison of accumulation rates from 137Cs and 210Pb suggest that sediment fluxes to Nachvak Fjord may have decreased slightly over the past ∼130 years.

scholarly journals Late Glacial and Holocene sedimentary infill of Lake Mondsee (Eastern Alps, Austria) and historical rockfall activity revealed by reflection seismics and sediment core analysis

2018 ◽  
Vol 111 (1) ◽  
pp. 111-134 ◽  
Author(s):  
Christoph Daxer ◽  
Jasper Moernaut ◽  
Timothy Taylor ◽  
Jean Nicolas Haas ◽  
Michael Strasser
Keyword(s):  
Sediment Core ◽  
Sediment Cores ◽  
Mass Movement ◽  
Sediment Accumulation ◽  
Fine Fraction ◽  
Late Glacial ◽  
Eastern Alps ◽  
Seismic Profiles ◽  
Reflection Seismics ◽  
Perialpine Lakes

Abstract Glacigenic perialpine lakes can constitute continuous post-last glacial maximum (LGM) geological archives which allow reconstruction of both lake-specific sedimentological processes and the paleoenvironmental setting of lakes. Lake Mondsee is one among several perialpine lakes in the Salzkammergut, Upper Austria, and has been previously studied in terms of paleoclimate, paleolimnology and (paleo)ecology. However, the full extent and environment of Late Glacial to Holocene sediment deposition had remained unknown, and it was not clear whether previously studied core sections were fully representative of 3D sediment accumulation patterns. In this study, the sedimentary infill of Lake Mondsee was examined via high-resolution seismic reflection survey over a 57-km extent (3.5 kHz pinger source) and a sediment core extracted from the deepest part of the lake, with a continuous length of 13.76 m. In the northern basin, seismic penetration is strongly limited in most areas because of abundant shallow gas (causing acoustic blanking). In the deeper areas, the acoustic signal reaches depths of up to 80 ms TWT (two-way travel time), representing a postglacial sedimentary sequence of at least 60-m thickness. Holocene deposits constitute only the uppermost 11.5 m of the sedimentary succession. Postglacial seismic stratigraphy of Lake Mondsee closely resembles those of well-studied French and Swiss perialpine lakes, with our data showing that most of Lake Mondsee’s sedimentary basin infill was deposited within a short time period (between 19,000 BP and 14,500 BP) after the Traun Glacier retreated from the Mondsee area, indicating an average sedimentation rate of about 1.4 cm/yr. Compared to other perialpine lakes, the seismic data from Lake Mondsee reveal little indication of mass movement activities during the Holocene. One exception, however, is rockfalls that originate from a steep cliff, the Kienbergwand, situated on the southern shore of Lake Mondsee, where, in the adjacent part of the lake, seismic profiles show mass transport deposits (MTDs), which extend approximately 450 m from the shore and are mappable over an area of about 45,300 m2. Sediment cores targeting the MTDs show two separate rockfall events. The older event consists of clast-supported angular dolomitic gravels and sands, showing high amounts of fine fraction. The younger event exhibits dolomitic clasts of up to 1.5 cm in diameter, which is mixed within a lacustrine muddy matrix. Radiocarbon dating and correlations with varve-dated sediment cores hint at respective ages of AD 1484 ± 7 for Event 1 and AD 1639 ± 5 for Event 2. As our data show no evidence of larger-scale mass movements affecting Lake Mondsee and its surroundings, we infer that the current-day morphology of the Kienbergwand is the result of infrequent medium-scale rockfalls.

Seasonal variability in turbidity currents in Lake Ohau, New Zealand, and their influence on sedimentation

2016 ◽  
Vol 67 (11) ◽  
pp. 1725 ◽  
Author(s):  
R. Cossu ◽  
A. L. Forrest ◽  
H. A. Roop ◽  
G. B. Dunbar ◽  
M. J. Vandergoes ◽  
...  
Keyword(s):  
New Zealand ◽  
Internal Waves ◽  
Sediment Accumulation ◽  
Turbidity Currents ◽  
Physical Processes ◽  
High Concentration ◽  
Sediment Delivery ◽  
Climate Conditions ◽  
River Inflow ◽  
Rain Events

Layers of sediment that are deposited on the floor of Lake Ohau, New Zealand, offer a means to reconstruct past climate conditions in the Southern Hemisphere at subdecadal and annual resolution. A robust understanding of the modern physical processes that control the influx and dispersal of sediment in the lake is required to reconstruct climate from these sedimentary archives. In this study, water temperature and velocity measurements collected during 2012–13 were analysed to determine the primary physical processes that influence sediment transport in the lake. Sediment input from river inflow occurs throughout the year but exhibits strong seasonal variation. Large inflow events (Q>500m3s–1) that follow strong summer rainstorms trigger high-concentration turbidity currents, which are the main agents for sediment delivery and deposition. During winter, smaller turbidity currents also occur after rain events and contribute to annual sediment accumulation. In addition, large internal waves were observed during the summer and may influence sedimentation. In conclusion, several processes including river inflow, internal waves and convectively driven flows control sediment deposition and accumulation in the Lake Ohau system. We utilise these observations to establish a conceptual model to explain the observed infill stratigraphy in Lake Ohau and guide interpretation of the longer sedimentary record.

Sea-level and climate signatures recorded in the 1 Myr continental margin deposits from the Bohai Sea

2020 ◽  
Author(s):  
Zhengquan Yao ◽  
Xuefa Shi ◽  
Yanguang Liu ◽  
Shuqing Qiao
Keyword(s):  
Grain Size ◽  
Sea Level ◽  
Continental Margin ◽  
Principal Components ◽  
Bohai Sea ◽  
Sediment Accumulation ◽  
Fine Fraction ◽  
The Bohai Sea ◽  
Floodplain Deposits ◽  
And Sea Level

<p>Sediment accumulation in the continental margin is largely influenced by both sea-level fluctuations and climate changes during the Quaternary Period. However, the response of sediment accumulation to these changes at orbital timescale, remains poorly understood, mainly due to (i) the scarce of sedimentary records with high-resolution chronology and (ii) the difficulty of distinguishing the role of sea-level from climate signals. Here we present sediment color reflectance (c*), grain size and total organic carbon (TOC) data of core BH08 (212.4 m; ~1 Myr) recovered from the Bohai Sea, China. The chronology of core BH08 was constrained at orbital timescale by using magnetostratigraphy and astronomical tuning methods. Sedimentary facies analysis suggests that the core sequence is dominated by alternations of deltaic system and floodplain deposits. Principal components analysis on grain size data reveals two principal components (PCs), including PC1 (31–500 µm, coarse fraction) and PC2 (18–66 µm, fine fraction). Comparison of PC1, PC2, c* and TOC with sedimentary environments, we found that PC1 and c* corresponds well with cycles of deltaic and floodplain deposits at ~100/40-kyr cycles, while PC2 and TOC display ~20-kyr cycle, in addition to the ~100/40-kyr cycles. We interpret that PC1 and c* are mainly sea-level dependent, whereas PC2 and TOC are controlled by a combination of monsoonal climate and sea level. We suggest that Milankovitch-scale monsoon climate controlled the sediments supply to the Bohai Sea during the last 1 Myr, while the redistribution of sediments by marine process (e.g. tidal currents) seem to have obscured the monsoonal signal in the grain size proxy (e.g. PC1) which is sensitive to sea-level change. Our results provide an example of climate and sea-level influenced sediment accumulation in the shallow continental margin influenced by monsoonal climate in an icehouse world.</p>

scholarly journals Flume Experiment on the Sediment-Retaining Effect of Submerged Breakwaters under the Combined Action of Waves and Currents

2020 ◽  
Vol 8 (5) ◽  
pp. 340
Author(s):  
Bing Yan ◽  
Ruibo Zhang ◽  
Puyang Zhang
Keyword(s):  
Suspended Sediments ◽  
Flume Experiment ◽  
Clear Water ◽  
High Concentration ◽  
Flume Experiments ◽  
Submerged Breakwater ◽  
Waves And Currents ◽  
Combined Action ◽  
High Concentration Suspension ◽  
Design Guidance

This research was initiated as design guidance for the extension works of a submerged trapezoidal riprap breakwater protecting the channel towards Huanghua Port in China. Based on flume experiments, the sediment-retaining effect of the breakwater was investigated in two sections under the combined action of currents and waves. We also discussed differences due to two kinds of facing layer—Accropodes and mold bag concrete. Three main observations are provided herein. First, the suspended sediments in front of breakwaters are mainly concentrated in the near-bottom 30% water depth scope and the interface can be seen clearly between the high concentration suspension and the upper clear water. Second, the sediment-retaining ratio of breakwater at −5.7 m isobath position is about 37%–49% and that at −7.3 m isobath position is about 61%–65%. Last, when there are wave breakings in the vicinity of submerged breakwater crest, the facing layers cause some differences of sediment-retaining performances.

Production of fines from refined kraft pulp by fractionation with micro-perforated screens

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Elisabeth Björk ◽  
Hannes Vomhoff ◽  
Per Engstrand
Keyword(s):  
Kraft Pulp ◽  
Fibre Length ◽  
Fine Fraction ◽  
Pilot Scale ◽  
Hole Size ◽  
High Concentration ◽  
Feed Concentration ◽  
Screening Process ◽  
Perforated Screen ◽  
In Series

Abstract The objective in this work was to obtain a fine fraction of kraft pulp, with as high concentration as possible, in a pilot-scale fractionation with micro-perforated screen baskets. The influence of screen basket surface, hole size, feed concentration, pulp type and refining segment design was investigated. The results showed that a smooth screen basket surface improved the fractionation efficiency of the unrefined pulp compared to a profiled screen basket, despite a larger hole size. A significantly higher fine fraction concentration was obtained when using refined hardwood pulp compared to when using softwood pulp, which was explained with its lower average fibre length and narrower and thus more flexible fibre fragments. The pilot trials also showed that the screening process could be operated at feed concentrations similar to those directly after a refiner, 30–40 g/l. This was demonstrated in a process layout with partial recirculation where a refiner and a micro-perforated screen basket were operated in series in pilot scale.

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