Erosion Flux Research Articles

Late Pleistocene Climate Change and Erosion in the Santa Maria Basin, NW Argentina

Present erosion and sediment flux in the semiarid intra- montane Santa Maria Basin, NW Argentina, are compared with con- ditions during a period of wetter and more variable climate at about 30,000 14 C yr ago. The results suggest that the influence of climate change on the overall erosional sediment budget is significant, mainly because of a change in the erosion regime coupled with an increase in mass movements. The most effective mechanism to increase landslide activity in this environment is a highly variable climate on interannual timescales. In contrast, Quaternary changes in erosional budgets due to variations in moisture regimes is small in the Santa Maria Basin. Because the magnitude of a potential increase in background erosion as well as enhanced landsliding is smaller than typical levels of uncer- tainty of erosional budgets for such large basins, it is not likely that climate-driven erosional unloading can influence tectonic style and rates in this semiarid environment on time scales of several thousands to a few tens of thousands of years.

A multi-class sedimentary dynamics modelisation to calculate the transfer of radionuclides by erosion and deposition fluxes in freshwater rivers

A multi-class sedimentary dynamics modelisation to calculate the transfer of radionuclides by erosion and deposition fluxes in freshwater rivers

Erosion and migration of tungsten employed at the central column heat shield of ASDEX Upgrade

In ASDEX Upgrade, tungsten was employed as plasma facing material at the central column heat shield in the plasma main chamber. The campaign averaged tungsten erosion flux was determined by measuring the difference of the W-layer thickness before and after the experimental campaign using ion beam analysis methods. The observed lateral variation and the total amount of eroded tungsten are attributed to erosion by impact of ions from the scrape-off layer plasma. Migration and redeposition of eroded tungsten were investigated by quantitative analysis of deposited tungsten on collector probes and wall samples. The obtained results, as well as the spectroscopically observed low tungsten plasma penetration probability, indicate that a major fraction of the eroded tungsten migrates predominantly through direct transport channels in the outer plasma scrape-off layer without entering the confined plasma.

Beach morphology and food web structure: comparison of an eroding and an accreting sandy shore in the North Sea

Food components and inorganic nutrients were studied on two sandy shores of the adjacent barrier islands of Sylt and Romo in the North Sea, differing in morphodynamics. Implications of high and low wave energy on the food structure were assessed. The Sylt shore represents a dynamic intermediate beach type, while the Romo shore is morphologically stable and dissipative. On the steep-profiled, coarse-grained Sylt shore, strong hydrodynamics resulted in erosion and high fluxes of organic material through the beach, but prevented any storage of food sources. In contrast, the flat-profiled, fine-grained Romo shore, with low wave energy and accretion, accumulated organic carbon from surf waters. At Sylt, oxic nutrient regeneration prevailed, while anoxic mineralization was more important at Romo. Macrofauna on the Sylt shore was impoverished compared with the community at Romo. Correspondingly, abundances of epibenthic predators such as shrimps, crabs, fish, and shorebirds were also lower at Sylt. Meiofauna was abundant on both shores, but differed in taxonomic composition. Several major taxa were represented in fairly equal proportions of individual numbers on the well-oxygenated Sylt shore, while nematodes strongly dominated the assemblage at Romo. Thus, on cold-temperate, highly dynamic intermediate shores with high wave energy and subject to erosion, the small food web dominates. Organisms are agile and quickly exploit fresh organic material. Larger organisms and nematodes abound under stable, dissipative and accreting shore conditions, where some food materials may accumulate and zoomass builds up to support numerous visitors from higher trophic levels.

Sensitivity analysis for validating expert opinion as to ideal data set criteria for transport modeling

Environmental fate modeling results are often used in risk assessment without adequately considering uncertainty in exposure predictions. Sensitivity analysis is fundamental to model validation and error prediction since sensitive model input parameters account for the largest variance in model prediction. Once identified, sensitive model input parameters can be used to propagate parametric uncertainty in numerical predictions. Output sensitivity to variation in input code sequences was investigated for the pesticide root zone model (PRZM 3) using Plackett-Burman analysis for six runoff and leaching data sets. The analysis utilized an incomplete block factorial design with even parameter weighting and uniform proportional input perturbation. Timing and duration of key period rainfall were assumed a priori to be dominant sensitive inputs. Thus, meteorological data were fixed, allowing identification of additional input components contributing to model sensitivity. Results validated expert modeler assumptions concerning parameters most critical for model validation. For leaching data sets, the application rate, soil bulk density (an indicator of available water-holding capacity), chemical partition coefficient, and pesticide degradation rates were commonly the most sensitive inputs. For runoff data sets, the in-crop runoff curve number was the most significant input governing pesticide loss in runoff and erosion flux. The chemical partition coefficient, soil and foliar decay rates, and soil bulk density were also common sensitive components for runoff predictions. These commonly observed sensitive components for runoff and leaching prediction need to be carefully considered in the design and conduct of relevant field studies, modeling assessment of such studies, and future improvements in algorithms for environmental transport modeling.

A radioecological water quality model including a multi-class sedimentary dynamics approach. Presentation and comparison with a global approach

The assessment of the radionuclides transfers in the physical components of rivers (water, suspended sediments, bottom sediments) involves three disciplines : hydraulics, sedimentary dynamics and radioecology. In freshwater radioecological quality models, the radioecology is linked to the sedimentary dynamics through the adsorption / desorption process and the deposition / erosion fluxes. The assessment of these fluxes requires in-situ and laboratory experiments to determine their relating parameters (settling velocity, critical shear stress of deposition and erosion, etc.). The models do not usually distinguish the different mineral particle classes and make the assumption that deposition and erosion processes cannot occur simultaneously. However, natural water flow is characterized by a wide and continuous range of different mineral particle classes. As a consequence, the assumed threshold between deposition and erosion is reduced. For example, the deposition of one class can come along with the erosion of another class. In this context, we present a radioecological model which includes a multi-class sedimentary dynamics model. To show the difference between this approach and a global one (obtained for a single average class of matter), a comparison between these two approaches concludes this paper.

Modélisation des processus sédimentaires dans les modèles de qualité radioécologique des eaux de surface

Click to increase image sizeClick to decrease image sizeThis paper presents the diverse components needed to elaborate a fresh water radioecological quality model. In particularly, we propose to take into account the sedimentary dynamics process through erosion and deposition fluxes with a multi-class model. The validity of this approach is checked by comparison with the experimental resulls obtained after resuspending sediment experiment realized in a small flume.

First simultaneous observations of flux transfer events at the high-latitude magnetopause by the Cluster spacecraft and pulsed radar signatures in the conjugate ionosphere by the CUTLASS and EISCAT radars

Abstract. Cluster magnetic field data are studied during an outbound pass through the post-noon high-latitude magnetopause region on 14 February 2001. The onset of several minute perturbations in the magnetospheric field was observed in conjunction with a southward turn of the interplanetary magnetic field observed upstream by the ACE spacecraft and lagged to the subsolar magnetopause. These perturbations culminated in the observation of four clear magnetospheric flux transfer events (FTEs) adjacent to the magnetopause, together with a highly-structured magnetopause boundary layer containing related field features. Furthermore, clear FTEs were observed later in the magnetosheath. The magnetospheric FTEs were of essentially the same form as the original "flux erosion events" observed in HEOS-2 data at a similar location and under similar interplanetary conditions by Haerendel et al. (1978). We show that the nature of the magnetic perturbations in these events is consistent with the formation of open flux tubes connected to the northern polar ionosphere via pulsed reconnection in the dusk sector magnetopause. The magnetic footprint of the Cluster spacecraft during the boundary passage is shown to map centrally within the fields-of-view of the CUTLASS SuperDARN radars, and to pass across the field-aligned beam of the EISCAT Svalbard radar (ESR) system. It is shown that both the ionospheric flow and the backscatter power in the CUTLASS data pulse are in synchrony with the magnetospheric FTEs and boundary layer structures at the latitude of the Cluster footprint. These flow and power features are subsequently found to propagate poleward, forming classic "pulsed ionospheric flow" and "poleward-moving radar auroral form" structures at higher latitudes. The combined Cluster-CUTLASS observations thus represent a direct demonstration of the coupling of momentum and energy into the magnetosphere-ionosphere system via pulsed magnetopause reconnection. The ESR observations also reveal the nature of the structured and variable polar ionosphere produced by the structured and time-varying precipitation and flow.Key words. Ionosphere (auroral ionosphere) Magentospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)

Higher erosion rates in the Himalaya: Geochemical constraints on riverine fluxes

The modern erosion rate of continental-scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains. These two parameters are very difficult to measure directly in continental-scale basins. In this paper we examine the chemical composition of the suspended load, bedload, and dissolved load of the Ganga and Brahmaputra Rivers and compare them with the average composition of Himalayan source rocks. A mass-balance equation of erosion fluxes shows that a Si-rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock. It corresponds to bedload sediment and flood-plain deposits, which are enriched in quartz by mineral sorting during transport. The combined budget of Si, Al, and Fe in the river system allows us to estimate this Si-rich flux. By this method, the total Himalayan erosion is estimated to be twice the measured flux of suspended load. The comparison between the Brahmaputra and the Ganga shows that the eastern Himalaya has a higher erosion rate (2.9 mm/yr) than the western Himalaya (2.1 mm/yr). This is likely the result of the higher runoff in the Brahmaputra basin. The intensity of the monsoon acts as a first-order control of the erosion rate in the range.

Modelling Wormhole Flow in Cold Production

Abstract Wormholes are believed to be generated during the initial phase of cold production and are responsible for enhanced production rates when sand cuts are reduced to their minimum. In this work, we present a theoretical model describing sand and fluids flow within a wormhole uniformly filled with sand. This model is based on the Darcy-Brinkman equations for fluids in mobile sand, a rheological equation for sand motion, and conservation laws. Velocity profiles of sand and fluid are calculated and used to establish the relation between the pressure drop and the flow rates. An estimation of wormhole size is also provided. This model provides understanding of the basic flow behavior in a wormhole during the massive sand production stage of cold production. It also provides some key parameters for field scale numerical simulations of cold production. Viscous Drag and Critical Radii The detailed mechanism of sand erosion is a complicated subject. It involves the geomechanical properties of the sand, the rheological properties of fluids, and the nature of sand-fluid and fluid-fluid interactions. However complicated, the key to flux erosion is that it happens when the drag force overcomes the resistance of the sand matrix. The drag force should be mainly the viscous body force due to flow of fluids through the sand matrix (Darcy flow). The resistance of the sand matrix is mostly a surface force. It is due to cohesion, adhesion and contact forces. It holds a chunk in place against the body force. The thickness of this chunk, ?, can be as small as the averaged grain size or orders of magnitude larger, depending on the sand properties and the viscous body force. In the former limiting case, "raining" of sand grains into a cavity occurs. In the latter limiting case, microcracks in the sand matrix may be observed just before the erosion. Assuming fluid flow into a cavity of radius R, the above-stated criterion can be expressed as: Equation (1) (Available In Full Paper) where σ is the surface strength of the sand, and P the fluid pressure. The geometry of the wormhole is assumed to be comprised of a straight cylindrical tube, with a hemispherical cap at the end, as illustrated in Figure 1. Computed tomography images(1) of the wormhole tip show that it is more pointed in reality, as shown by the dashed line in Figure 1. The assumption of hemispherical flow at the tip, however, seems reasonable. For a spherical geometry, FIGURE 1: Influx of fluid into a wormhole. The dashed line represents the actual shape of a wormhole tip (Available In Full Paper) the pressure gradient is inversely proportional to the square of the radius if the fluid is incompressible. Therefore, we can integrate the right-hand side of the inequality (1). Denoting the reservoir pressure as Pre, the pressure at the wall of the cavity as P, and the radius of the drainage zone beyond which the reservoir retains its original reservoir pressure as Rs, we find: Equation (2) (Available In Full Paper)

Global chemical erosion over the last 250 my; variations due to changes in paleogeography, paleoclimate, and paleogeology

We utilize predictions of runoff from two series of GENESIS (version 1.02) climate model experiments to calculate chemical erosion rates for 12 time slices that span the Mesozoic and Cenozoic. A set of control experiments where geography is altered according to published paleogeographic reconstructions and atmospheric pCO2 is held fixed at the present-day value was designed to elucidate climate sensitivity to geography alone. A second series of experiments, where the (elevated) atmospheric CO2 level for each time slice was adapted from Berner (1991), was executed to determine the additional climate sensitivity to this parameter. By holding other climate forcing factors (for example, vegetation) constant throughout the sequence of experiments we evaluate the effects of systematic/coherent paleogeographic changes on runoff and temperature, and thus on global rates of chemical weathering. By using empirical relationships between runoff and bicarbonate fluxes for different rock types and maps of paleogeology we calculate global bicarbonate fluxes, taking into account spatial variations in lithology and hydrology. The climate model predicts that many regions experienced dramatic changes in runoff (for example, from wet to very arid) since the Early Triassic. In general, the supercontinent (Pangean) paleogeographic regime was driest, times of dispersed continents (Tethyan regime) wettest, and the modern geography intermediate. Total bicarbonate fluxes, as well as those from silicate mineral weathering only, closely parallel these trends. We find that spatial variations in lithology account for little variation in the total or silicate chemical erosion rates. In contrast, changes in hydrology due to differences in paleogeography are significant and are the main clear trend in our results. Here we also add an Arrhenius-type temperature dependency that modifies the runoff-determined fluxes for each rock type. High activation energies for the weathering reactions substantially increase absolute values, but we find that the ratio between the flux for a particular time slice and the Present Day remains very similar for different temperature dependencies, following the paleogeographic-controlled trend in runoff. Our calculations suggest a weaker-than-expected CO2-climate weathering feedback. The reasonable atmospheric pCO2 variations specified for the climate-model simulations do not lead to climatic effects that support large changes in the chemical erosion rate, compared to those generated by changing paleogeography. In general, however, we find that silicate weathering rates are similar to outgassing rates of volcanic and metamorphic CO2. Times of supercontinental stasis represent low outgassing but also high aridity due to extreme continentality and thus low chemical erosion fluxes. In contrast, times of continental dispersion represent high outgassing as well as high runoff (and fluxes) due to increased proximity to moisture sources. Where a mismatch occurs, particularly in the case of the early to mid-Cretaceous, we infer higher CO2 levels than those used in our GCM simulations to balance the carbon cycle.

High heat flux erosion of carbon fibre composite materials in the TEXTOR tokamak

During plasma disruptions, ELMs, or vertical displacement events (VDEs) high transient heat loads to the plasma facing materials can cause damage such as thermal erosion, cracking, or melting. Self-shielding processes, which take place when a material surface is exposed to a high heat flux from an incident plasma, can lead to a reduction of the deposited energy. Experiments using the fast pneumatic probe of the TEXTOR tokamak were carried out to investigate these effects. The materials tested were carbon fibre reinforced materials with and without Si-addition. The probe with the material specimens was introduced into the edge plasma up to a depth of 9 cm in front of the ALT-II main limiter with a residence time of 80 ms. After the repeated exposure to the TEXTOR-plasma, the material specimens were examined by profilometry and electron microscopy to determine the damage and erosion. It was found that only a very limited zone of the probe tip of about 2.5 mm extension in radial direction showed erosion. The maximum erosion was observed at the very tip of the probe with approx. 30 μm per exposure. The results of the erosion quantification are compared with the results from numerical simulations and from plasma diagnostic measurements during the exposure of the specimens.

Dayside auroral dynamics: South Pole ‐ AMPTE/CCE observations

Coordinated observations were made between the South Pole Station 630‐nm all‐sky imager and the Active Magnetospheric Particle Tracer Explorer (AMPTE) CCE satellite when it was at 9 RE apogee in the midday sector in an effort to understand the variety of dynamical changes at the magnetopause that map to the dayside aurora. On June 18, 1988, three events were observed. The first, at 1150 MLT, was a local event showing a rotation of the magnetic field and the sudden appearance of low‐energy electrons indicating the satellite entering the magnetosheath or low‐altitude boundary layer. This event was preceded by a type of poleward expansion of the aurora which had been previously interpreted as a possible signature/footprint of a flux transfer event (FTE). Although the magnetic field signatures at the satellite are not characteristic of an FTE, the event could be a magnetic field change produced by an FTE, which after some delay propagated to the vicinity of the satellite. The foot of the field line containing the satellite was at some distance from the south pole meridian. The second event, starting at 1610 UT (1240 MLT), was a clear case of field erosion during which the satellite entered the magnetosheath characterized by a reduction in energetic electrons, an increase in soft electrons, and a decrease in the magnetic field strength accompanied by an equatorward motion of the aurora. The third event, in the afternoon at about 1900 UT (1530 MLT), showed similar changes in the electron fluxes and similar equatorward motion of the aurora. However, this event was associated with an increase in the magnetic field at the satellite. Such a field increase could be characteristic of a pressure pulse. However, there was a corresponding equatorward motion of the aurora, which suggests that the event was also a flux erosion event in which the fringing fields of the region 1 current system, which normally cause a field depression at the subsolar region, created ah increase in the field at the CCE satellite near the late afternoon side. The latitudinal (equatorward) movements of the aurora for all three events were strongly correlated with the global AE index which was dominated by stations in the dawn/dusk sectors.

Auroral activity associated with unsteady magnetospheric erosion: Observations on December 18, 1990

We report auroral observations made on December 18, 1990, when interplanetary conditions should lead to large‐scale erosion of the dayside magnetosphere during a substorm growth phase. A long interval of strongly northward pointing interplanetary magnetic field (IMF) was succeeded by several hours of strongly southward pointing IMF. The interval of southward pointing IMF was punctuated by a number of IMF directional discontinuities during which the IMF north‐south component, Bz changed polarity abruptly. The auroral responses, monitored at Ny Ålesund (75° magnetic latitude) by meridian scanning photometers and all‐sky cameras, were as follows: The interval of negative IMF Bz was characterized by a net equatorward migration of the equatorward boundary of the dayside cusp/cleft aurora, as expected from previous studies. On this occasion, however, we find that the latitudinal shift occurred in steps which consisted of an initial brightening of individual auroral events at ∼0.5° MLAT equatorward of the preexisting luminosity, followed by a steady poleward retreat lasting typically 4–5 min. The net effect over the first hour of IMF Bz < 0 conditions was to move the equatorward boundary toward the geomagnetic equator by ∼2.7° MLAT. The auroral data suggest that in this instance dayside magnetosphere erosion took place intermittently: bursts of reconnection (initial brightenings) are followed by a switch‐off of the reconnection electric field (subsequent poleward retreat). The bursts of reconnection may be identified with flux transfer events or, equivalently, flux erosion events.

Highly Contrasted Rare Earth Element Patterns in Shallow Groundwaters from Very Small Catchments

Concentrations of Rare Earth Elements (REE) in large rivers (Amazon, Mississipi, Indus...) are often used as key constraints to determine chemical and physical erosion rates and fluxes on the global scale (Goldstein and Jacobsen, 1987; Sholkovitz, 1995; Elderfield et al., 1990; Gaillardet et al., 1997). Few works, however, have been dedicated to study fundamental processes which control REE hydrochemistry upstream, at the water-rock interface. Using data from groundwaters, Smedley (1991) suggested that REE in deep waters closely reflect the lithology of their host rocks. More recently, Johannesson et al. (1997) proposed that complexation processes play dominant role on the distribution pattern of REE in waters. Both authors acknowledged the usefulness of REE to investigate water-rock interactions. The present study concerns the spatial and temporal variability of REE concentrations in shallow groundwaters associated with three very small catchments located in western France (Brittany and Normandy). All the catchments benifit from the same temperate oceanic climatic conditions, and have low permeability soils. Basemant rocks have granitic or shaly compositions. The data are used to further elucidate the nature of fractionation processes that control the REE chemistry of shallow, continental waters.

Measurements of tritium retention and removal on the Tokamak Fusion Test Reactor

Recent experiments on the Tokamak Fusion Test Reactor have afforded an opportunity to measure the retention of tritium in a graphite limiter that is subject to erosion, codeposition, and high neutron flux. The tritium was injected by both gas puff and neutral beams. The isotopic mix of hydrogenic recycling was measured spectroscopically and the tritium fraction T/(H+D+T) transiently increased to as high as 75%. Some tritium was pumped out during the experimental run and some removed in a subsequent campaign using various clean-up techniques. While the short term retention of tritium was high, various conditioning techniques were successful in removing ≊8000 Ci and restoring the tritium inventory to a level well below the administrative limit.

Modelling non-cohesive suspended sediment transport in 2D vertical free surface flows

The mathematical model for simulating suspended sediment transport under equilibrium and non-equilibrium situations is described. Using the hydrodynamic variable field obtained from a refined turbulent free surface flow model, the suspension transport model involves bed boundary conditions for either the sediment flux to or from the bed, or an estimated concentration level, which are tested. The Schmidt number variability is also studied for several particle types. Some 2D vertical test cases for equilibrium situations or non-equilibrium situations with net erosion or net deposition flux to the bed are presented, and the predicted development of the concentration profiles is compared with measurements.

Penetration of the interplanetary magnetic field, By and magnetosheath plasma into the magnetosphere: Implications for the predominant magnetopause merging site

Magnetosheath plasma penetrates into the magnetosphere creating the particle cusp, and similarly the interplanetary magnetic field (IMF) By component penetrates the magnetopause. We reexamine the phenomenology of such penetration to investigate implications for the magnetopause merging site. Three models are popular: (1) the “antiparallel” model, in which merging occurs where the local magnetic shear is largest (usually high magnetic latitudes); (2) a tilted merging line passing through the subsolar point but extending to very high latitudes; or (3) a tilted merging line passing through the subsolar point in which most merging occurs within a few Earth radii of the equatorial plane and local noon (subsolar merging). It is difficult to distinguish between the first two models, but the third implies some very different predictions. We show that properties of the particle cusp imply that plasma injection into the magnetosphere occurs most often at high magnetic latitudes. In particular, we note the following: (1) The altitude of the merging site inferred from midaltitude cusp ion pitch angle dispersion is typically 8‐12 RE. (2) The highest ion energy observable when moving poleward through the cusp drops long before the bulk of the cusp plasma is reached, implying that ions are swimming upstream against the sheath flow shortly after merging. (3) Low‐energy ions are less able to enter the winter cusp than the summer cusp. (4) The local time behavior of the cusp as a function of By and Bz corroborates predictions of the high‐latitude merging models. We also reconsider the penetration of the IMF By component onto closed dayside field lines. Our approach, in which closed field lines move to fill in flux voids created by asymmetric magnetopause flux erosion, shows that strict subsolar merging cannot account for the observations.

About the specification of erosion flux for soft stratified cohesive sediments

The present paper deals with the specification of bed erosion flux that accounts for the effects of sediment-induced stratification in the water column. Owing to difficulties in measuring the bed shear stress τb and the erosive shear strength τs, we suggest a series of methods that combine laboratory and numerical experiments. A simplified turbulent transport model that includes these effects helps to quantify τb and τs. Focusing on soft stratified beds, the present study considers erosion rate formulas of the form e=ef exp {[Tb-Tsα]β} where β is a model constant (β=1 for Gularte's (1978) formula and β=1/2 for Parchure's (1984) formula). First, the bed erosive strength profile τs(Z) is adjusted by “forcing” the turbulent transport model with measured erosion rates. Second, three procedures are suggested to determine the erosion rate formula coefficients ef and α: a global procedure and two different layer-by-layer procedures. Each procedure is applied to an erosion experiment conducted in a rotating annular flume by Villaret and Paulic (1986). The use of the layer-by-layer procedure based on a least squares fitting technique provides a closer fit than the global procedure. The present study points out the complementarity of experimental and numerical approaches and also suggests possible improvements in laboratory test procedures.

Concerning flux erosion from the dayside magnetosphere

The dayside magnetopause moves inward during periods of southward interplanetary magnetic field in response to decreases in the outer magnetospheric magnetic field strength. We consider possible causes for the magnetic field strength decreases and demonstrate that they are consistent with increases in the region 1 Birkeland and cross‐tail currents. We reexamine the well‐known series of magnetopause crossings by OGO 5 on March 27,1968, to demonstrate that they provide evidence for two intervals of gradual inward magnetopause motion associated with magnetic flux erosion and also for two intervals of inward magnetopause motion associated with sharp increases in the solar wind dynamic pressure.