Selective Erosion Research Articles

REACTIVE ACCELERATED CLUSTER EROSION (RACE)

Beams of ionized clusters of some thousand atoms are accelerated to about 100-keV kinetic energy to be used for area selective surface erosion. Mask projective cluster-impact lithography allows surface structuring in the submicron regime. Chemical reactions between the cluster and the target material may provide volatile reaction products facilitating ejecta removal. The reactive accelerated cluster erosion (RACE) process is applied to metals like copper and gold, to semiconductors such as silicon, and to insulators like glass, quartz, or sapphire, giving very smooth eroded surface and steep sidewalls.

The role of earthworms in the formation of sandy surface soils in a tropical forest in Ivory Coast

Soils with relatively sandy topsoils over clayey subsoils are widespread, but their genesis is not always clear. We tested the hypothesis that earthworms stimulate the formation of the sandy surface soils by producing clay-rich worm casts that are susceptible to erosion, in an undisturbed forested watershed in southwestern Ivory Coast. Soils of the watershed were strongly weathered and acidic. Their contents of ironstone gravel and of clay, organic matter and plant nutrients decrease in a downslope direction. Production of worm casts increased in the same direction from 0.1 to 0.2 kg m −2 yr − near the top of the watershed to between 2.3 and 9.6 kg M −2 yr − near the valley bottom, with an areal average of 3.2 kg m −2 yr −. Casts were invariably richer in clay and silt, organic matter and N, P, Ca and Mg than nearby bulk surface soil. Casts were easily disintegrated by rain splash and overland flow and cast material contributed to the 0.12 kg m −2 yr − of highly organic suspended sediment removed by surface water drainage. Selective erosion of worm casts was probably more important than eluviation of clay in removing clay from the surface soils. Increasing production and disintegration of casts with decreasing elevation probably played a major role in lowering contents of clay and nutrients in the surface soils in the lower parts of the area.

Is the Sevier Desert reflection of west-central Utah a normal fault?: Comment and Reply

Is the Sevier Desert reflection of west-central Utah a normal fault?: Comment and Reply

Selective erosion and sediment source identification, Baringo District, Kenya

Selective erosion and sediment source identification, Baringo District, Kenya

Podzols in Amazonia

In the area near Manaus, Amazonia (Brazil), soil sequences including Oxisols, Ultisols and Spodosols (podzols) are widely distributed. Such toposequences occur on slopes leading down into valleys cut in a nearly level plateau. The geochemistry of the organic and mineral components of one of these sequences has been investigated. Spodosols develop on quartzose sands which may be the results of an intense hydrolysis of clays and/or an impoverishment of clays through eluviation or selective erosion. Large analogies with podzolization in the temperate zone were found, especially in the organic matter dynamics: as the clay content decreases more organic compounds with increased aromaticity and more polycondensed structures were allowed to move, forming the Bh horizons or reaching the water streams. Anin situ experimentation was also performed to characterize presently active soil forming processes. It was found that podzolization in Amazonia was mainly active during the rainy season.

Sediment supply and the development of the coarse surface layer in gravel-bedded rivers

THE bed surface of most gravel rivers is considerably coarser than the sub-surface or the gravel load transported over it, a phenomenon affecting river dynamics, morphology and ecology. The coarse surface layer, often called an armour or pavement, has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1,2; and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3,4. Where bedload supply is cut off below dams, selective erosion causes coarsening5,6. Using a simple quantitative model, we propose that surface coarsening develops in gravel-bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load. We present laboratory results which support this hypothesis, and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse, less active bed. It may therefore be possible to relate the degree of river-bed surface coarsening to sediment supply resulting from land use.

Variability of particle size characteristics of sheetwash sediments and fluvial suspended sediment in a small semiarid catchment, Kenya

The variability of particle size characteristics for sheetwash sediments and fluvial suspended sediments are presented. Data for three representative storm events are described for a small 0.30 km 2 erodible semiarid catchment. Suspended sediment output per event ranged from 340 to 810 t km −2. Ninety-eight percent of the suspended sediment was less than 63 μm, and was supplied from remobilization of stored colluvium. Discharge and particle size class relationships were complex for individual storm events, but were best described by third-order polynomial functions. Results of a one-way analysis of variance showed that concentrations of total suspended sediment, silt and sand varied significantly between storm events. Temporal variation of clay concentration in runoff were not significant, suggesting that a single hillslope source area contributed to the channel subsystem. The mean grain size and sorting of sheetwash sediments were significantly different from fluvial suspended sediment for individual storm events. Sheetwash sediments were coarser and more poorly sorted than fluvial suspended sediments. Enrichment ratios indicated selective erosion and transport of silt and clay, and preferential deposition of sands.

Geochemical differences in an Oxisol-Spodosol toposequence of Amazonia, Brazil

The morphology and geochemical characteristics of the soils of a toposequence near Manaus, Brazil, have been determined. Such toposequences occur on slopes leading down into valleys cut in a nearly level plateau. Oxisols form the upper and Spodosols the lower members of toposequences. The transition from the Oxisols down the slope through a band of Ultisols to the Spodosols is very gradual. Lowest members of the toposequences, Spodosols do not occupy the valley floors. Great contrasts as well as shared characteristics occur among members of the toposequence chosen for study. Proportions of clay in the Oxisols are 80–90% as contrasted to 2–5% in the Spodosols. Changes in amounts of clay are gradual, continuing down the slope. The clay fraction is kaolinitic. All soils are strongly acid and have very low base saturation. The characteristics of the soils imply that several processes of weathering and pedogenesis have been operative. The small amount of SiO 2 in the Oxisols is attributed to dissolution of quartz without corresponding hydrolysis of kaolinite. The decreases in clay contents and increases in quartz in the soils from the upper to the lower end of the toposequence are attributed to a combination of hydrolytic weathering and either or both of clay eluviation and selective erosion of that fraction. The sequence of soils within the toposequences further suggests that a podzolization process begins when a critical stage in impoverishment is reached. Destruction of clay minerals continues but the migration of organo-mineral complexes within profiles becomes important. A part of those complexes moves out of the Spodosols to form black waters in the streams.

G. Holmes & J. Jarvis reply

The hypothesis that the obsequent scarplets on Ben Attow were formed by selective, perhaps glacial, erosion was considered by the authors but rejected for three main reasons. First, the slope is formed of Moinian Psammite and it would seem unlikely that scarplets could remain unscathed whilst glacial erosion scoured a trench across the slope immediately upslope of them. Second, up to twenty scarplets occur between the floor of Glen Licht and the crest of the slope. If the scarplets were formed by glacial erosion then their absence in the remainder of Glen Licht must be accounted for. Finally, (Fig. 1) of Holmes & Jarvis' (1985) paper shows some scarplets that strike at angles diagonal to the direction of the slope and, therefore, are discordant with the main trend of the scarplets. The varying orientations of these scarplets poses a further difficulty for the selective erosion hypothesis. The upslope faces of the scarplets were examined for slickensides but none were observed, nor were glacial striae seen. This negative evidence may be due to the vegetation cover on many of the scarplets and the degradation of most of their upslope faces. Any slickensides or striae may have been lost. However, Bovis (1982) suggested that low normal stresses acting on steeply inclined discontinuities may have prevented the formation of slickensides at a rock slope topple that he investigated. At Ben Attow and on other slopes studied by Holmes (1984) and Jarvis (1985) there is a relationship between obsequent scarplets and consequent failure scarps

Characteristics and occurrence of wind erosion features in the Namib Desert

AbstractSmall scale features resulting from wind erosion are widespread in the Namib Desert. They include selective erosion of lithological variations (etching), flutes and grooves, facetting of clasts and residual boulders, and smoothing and polishing of rock surfaces. Large scale features are tentatively identified and are restricted to the southern Namib, with its high energy, unidirectional wind regime.Wind erosion features are best developed on fine‐grained rocks of intermediate hardness. They are oriented towards or parallel to modern strong winds.The turbulent flow of wind armed only with dust particles is probably more important than wind driven sand in creating smooth polished rock surfaces, flutes, and grooves. Wind driven sand appears to be significant only in facetting rock masses.

Enrichment of Transported Sediments with Organic Carbon, Nutrients, and Clay

AbstractMuch of the nutrient inflow into impoundments is the result of erosion of soil particles on the watershed, together with adsorbed nutrients and their movement from the upland watershed soils. As the detached particles move downstream, they undergo selective size segregation and the nutrients are subject to selective desorption and adsorption between the particles and the water. During transport an enrichment of clay sized particles within the system occurs as well as that of P, N, and organic C. The ratio of the concentration of any given component in the eroded material system to that in the contributing soils is defined as the enrichment ratio. The selectivity of the erosional and sediment delivery processes involved in the transfer of eroded materials from watersheds to lakes or reservoirs is assumed to be proportional to the time, or distance, of travel. This study compares the composition of the eroded material at the onset and at the termination of the erosion, or sedimentation, cycle. From this information insight into the mechanisms affecting the compositional changes along the erosional route may be obtained. Organic carbon, total nitrogen, and total phosphorus concentrations and clay contents in sediments from 41 impoundments or lakes were compared with the equivalent parameters in the soils of the contributing watersheds. The enrichment ratio of a given component (S/So, the concentration of the component in the eroded material divided by its concentration in the soil) was theoretically predicted to be: S/So = 1/So × L, where L is a limiting value representing a stable level of this component in the erosion sequence. The measured enrichment ratios for organic C and total N were in good agreement with the theoretical model, the stable levels being 24 and 1.7 g kg−1 for organic C and total N. No such agreement was found for P and clay enrichment ratios, possibly due to the effect of the variable flow conditions on the selective erosion of these components.

Effects of selective erosion in a heterogeneous alloy in erosion-product composition in the discharge zone and on line intensity (review)

Emission spectral analysis is based on the fact that there is a simple proportional relationship between the contents of the atoms in the gas phase and in the electrode when a discharge acts on the latter: N/No = ~C/Co, where C is the content of a particular element in the sample, N is the content of the atoms from this in the discharge zone, Co is the content of an element in the base, No is the content of the atoms from this in the discharge zone, and ~ is a coefficient of proportionality.

Genesis of a So‐called Ferrolysed Soil of Bangladesh

AbstractIt was postulated by Brinkman (1970) that low clay contents in the surface horizon of paddy soils (Planosols) are caused by clay destruction through ferrolysis. In an effort to test this, a rice profile from the site of Brinkman's Eutric Planosol (Salna, Bangladesh) and a neighbouring forest soil (Dystric Cambisol) of the same parent material and particle‐size distribution (Bhawal National Park) were investigated. The micro‐structure, particle‐size distribution, mineral contents, pedogenic oxides, contents of Zr, Ti and K of the fine earth and individual fractions and also the exchangeable ions were studied.The clay fraction of both soils were chloritised by interlaying of hydroxy‐Al in the smectite interlayers in the surface horizons and got stabilized. As a result CEC showed some decrease. The forest profile has a loose and uniform brown‐yellow surface horizon and is relatively rich in coarse sand at 62–75 cm depth which might be caused by a strong termite activity as observed during sampling. On the contrary, the frequent and alternate reducing and oxidizing conditions have resulted in gleying the top soil and forming iron‐oxide films on the surface of aggregates during dry periods in the paddy soil.An evaluation of all results shows that a little clay destruction and an enrichment of silt in the top soil through vertisol formation and also through primary parent material stratification were the reasons for low clay content in epipedons. In forest soil there was a little clay migration, while the epipedon of the paddy soil had low clay in addition, because of selective erosion during heavy rains following the destruction of structure by puddling.The ferrolysis as a dominant factor for a clay‐low epipedon is improbable, because the clay loss of the neighbouring forest soil can not be explained by ferrolysis.

Importance de l'érosion différentielle et de la tectonique tardi-hercynienne dans le massif du Dartmoor (Grande-Bretagne)

The morphostructural study of the Dartmoor massif and the surrounding plateaus evidences the importance of differential erosion. Its control is obvious in the repartition of the large landscape volumes (the granitic complex dominates the metasedimentary plateaus), in the main landforms (edge scarps and central basin) and in the medium scale features (basins and tors). In the Dartmoor granitic complex, investigations conducted about the selective alterability factors, emphasize the limited influence of the original petrography and show the main role of the plutonie rocks structure (pluton edges , batholith roof) and the large influence of the late hercynian events, particulary jointing and mineralisation, porosity and endogene alterations. This selective erosion proceeds from two main planation surfaces : the Dartmoor surface, derived from the pre-permian post-hercynian surface and the lower polygenic Devon-Cornwall surface developped since cretaceous times.

An Emerging Esker in Southern Finland

A partially submerged esker situated among a group of rocky islands on the northern coast of the Gulf of Finland is examined. As the esker emerged from the sea to form separate islands, the glaciofluvial material was deformed by current and wave action and redeposited. The contemporary rate of emergence is 1.8 mm per year. The structural differences in the primary esker have been emphasized by selective marine erosion. Differences in wave energy are reflected in the length of the fetch, which shortens towards the mainland and with land uplift. Shortening of the lateral fetch has prevented further marine erosion once the summit of the esker has been eroded to the level of the bedrock islands. Only in the direction of the open sea does the force of the waves continue unabated. This wave action has resulted in the formation of beach ridges on the southernmost tip of the esker. Elsewhere this same action causes longshore drifting, which has resulted in the development of spits and tombolos on the lee side of the esker islands.

Erosion behavior and structure of silver-intermetallic compound electrical contact materials

The action of an arc discharge on a contact made of a silver-intermetallic compound composite material is accompanied by selective erosion in the affected zone, which is a result of the anchoring of the arc discharge to particles of the intermetallic compound — a phase of comparatively low thermal conductivity. A characteristic feature of the selective erosion is a nonuniform structure of the affected zones of contacts, the nonuniformity increasing in intensity with rising volume concentration and Tm of the intermetallic compound. On contacts of hypoinversion compositions the character of structural nonuniformity in the affected zone on the cathode is different from that on the anode, but on contacts of hyperinversion compositions the affected zones on the cathode and anode exhibit the same type structural nonuniformity. There is a tendency for the functions of the cathode and anode to become equalized in the supply of eroded material to the arc gap. Increasing the current shifts the composition boundary between the ranges in which different types of cathode erosion are observed and hence the inversion range toward lower intermetallic compound contents. The structural nonuniformity of affected zones gives rise to“spike formation” during the excitation of electrical discharges between stationary contacts. The greatest propensity for“spike formation” is exhibited by refractory intermetallic compound-containing composites of hyperinversion composition. During erosion testing the rate of erosion varies, being a maximum at the beginning of testing. The fall in and subsequent stabilization of erosion rate are brought about by accommodation of the surface layers of contacts and by the formation in these layers of a structure exhibiting optimum resistance to erosion.

Aqueous‐ and Sediment‐Phase Nitrogen Yields from Five Southern Pine Watersheds

AbstractNitrogen (N) in solution and associated with suspended sediments in stormflow from five reforested watersheds (1.5–2.8 ha) in northern Mississippi was determined during the 1975 water year. Samples were collected using Coshocton wheel samplers set below 0.91‐m H‐flumes. Mean yearly solution NH4‐N and NO3‐N concentrations, 0.20 and 0.01 mg/liter, respectively, did not differ among watersheds. Sediment‐N concentrations ranged from 2,410 to 6,080 µg/g and were 5.4 to 10.0 times those in the watershed soils. The N enrichment of sediment relative to soil was attributed to selective erosion of fine sediments (clay) and/or deposition of coarse sediments in transport. Significant differences in sediment N yields among the five watersheds were related to stormflow volume, sediment concentration, and sediment N concentration. Nitrogen was transported from the watersheds about equally in the sediment and solution phases. Mean solution NH4‐N and NO3‐N yields for the water year were 422 and 28 g/ha, respectively, for the five watersheds; mean sediment N yield was 395 g/ha. For the year, 8–17% of the precipitation was measured as stormflow; the remainder was deep seepage and evapotranspiration since there was essentially no baseflow.

Tillage System Effects on Sediment and Nutrients in Runoff from Small Watersheds

ABSTRACT TWO conservation tillage systems were studied and compared with the conventional (plow-disk-plant) tillage system. Runoff, soil loss, and nutrient losses dur-ing the 1973-75 growing seasons were measured for six small, paired watersheds planted to continuous corn (runoff and soil loss for the year, 1972, when conservation tillage systems were being established were also measured). Fifty-nine percent of the soil surface under ridge-planting and 11 percent under till-planting were covered with crop residue, compared to less than 2 per-cent for conventional planting. Conservation tillage systems on the average reduced runoff about 40 percent and reduced soil loss from 60 to 90 percent. Total losses of nitrogen and phosphorus (total P in sediment measured only in 1973) were mostly associated with soil loss and consequently were decreased for conservation tillage systems. Solution phosphorus losses and concen-trations and available P concentrations in sediment (measured in 1974 and 1975) increased with residue cover. Several factors could be responsible, including decreased fertilizer incorporation, selective erosion pro-cesses, and the residue being a phosphorus source. Corn yield data are given.

Phosphate Adsorption‐Desorption Characteristics of Suspended Sediments in the Maumee River Basin of Ohio

AbstractPhosphorus (P) adsorption‐desorption characteristics of Maumee River Basin suspended sediments were compared with those of Basin soils and stream bottom sediments. Suspended sediment contained more total P than either soils or bottom sediments. The increase in total P over soils is attributed to enrichment of P in sediment by selective erosion of fine particles and adsorption of P during fluvial transport. The suspended sediment had higher adsorption maxima than Basin soils but lower than bottom sediments and had lower adsorption energies than either soils or bottom sediments. Calcite content of the suspended sediments was positively correlated with total P, EPC (equilibrium P concentration) and P desorbed and negatively correlated with adsorption energy, implying that, although calcite is a sink for P, the adsorption is weak compared with other sites for P adsorption such as hydrous oxides of Fe and Al.

Aqueous‐ and Sediment‐phase Phosphorus Yields from Five Southern Pine Watersheds

AbstractStormflow from five reforested watersheds (1.5 to 2.8 ha) in northern Mississippi was analyzed during the 1974 water year (Oct. 1973 to Sept. 1974) for phosphorus (P) in solution and in association with suspended sediments. Samples were collected for each storm with Coshocton wheel samplers set below 0.91‐m H‐flumes. For the year, mean concentration of total P in solution was 0.027 mg/liter for the five watersheds. Of this, 45% was hydrolyzable P, 33% ortho‐P, and 22% organic P. Sediment total P concentrations ranged from 274 to 1,067 µg/g and were 2.0 to 8.9 times that in the watershed soils. Increased concentration of P in suspended sediment relative to soil is attributed to selective erosion of fine sediments and/or deposition of coarse sediments in transport. For the five watersheds, solution total P yield during the water year averaged 88 g/ha; whereas sediment total P yield averaged 210 g/ha and accounted for 64 to 76% of the sediment plus solution P yield.