Lateral Erosion (lateral + erosion)

Distribution by Scientific Domains

Selected Abstracts

Distribution of erosion across bedrock channels

Jens M. Turowski
Abstract Lateral erosion in bedrock rivers is an important control on the shape of channel cross-sections, and the coupling of channels and hillslopes. Recent observations link lateral erosion to the variability of flow. We propose two mechanisms to explain this. One is based on changing shear stress distributions within the channel with varying flood level, the other on the competition between cover and tool effects in fluvial bedrock erosion. We assess these processes for the Liwu River, Taiwan, and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case. Copyright 2007 John Wiley & Sons, Ltd. [source]

Experimental study of rill bank collapse

Jovan R. Stefanovic
Abstract Rill bank collapse is an important component in the adjustment of channel morphology to changes in discharge and sediment flux. Sediment inputs from bank collapse cause abrupt changes in flow resistance, flow patterns and downstream sediment concentrations. Generally, bank retreat involves gradual lateral erosion, caused by flow shear stress, and sudden bank collapse, triggered by complex interactions between channel flow and bank and soil water conditions. Collapse occurs when bank height exceeds the critical height where gravitational forces overcome soil shear strength. An experimental study examined conditions for collapse in eroding rill channels. Experiments with and without a deep water table were carried out on a meandering rill channel in a loamy sand and sandy loam in a laboratory flume under simulated rainfall and controlled runon. Different discharges were used to initiate knickpoint and rill incision. Soil water dynamics were monitored using microstandpipes, tensiometers and time domain reflectometer probes (TDR probes). Bank collapse occurred with newly developed or rising pre-existing water tables near rill banks, associated with knickpoint migration. Knickpoint scour increased effective bank height, caused positive pore water pressure in the bank toe and reduced negative pore pressures in the unsaturated zone to near zero. Matric tension in unsaturated parts of the bank and a surface seal on the ,interrill' zone behind the bank enhanced stability, while increased effective bank height and positive pore water pressure at the bank toe caused instability. With soil water contents >35 per cent (sandy loam) and >23 per cent (loamy sand), critical bank heights were 011,012 m and 006,007 m, respectively. Bank toe undercutting at the outside of the rill bends also triggered instability. Bank displacement was quite different on the two soils. On the loamy sand, the failed block slid to the channel bed, revealing only the upper half of the failure plane, while on the sandy loam the failed block toppled forwards, exposing the failure plane for the complete bank height. This study has shown that it is possible to predict location, frequency and magnitude of the rill bank collapse, providing a basis for incorporation into predictive models for hillslope soil loss or rill network development. Copyright 2006 John Wiley & Sons, Ltd. [source]

Flume experiments on the horizontal stream offset by strike-slip faults

Shunji OuchiArticle first published online: 4 FEB 200
Abstract Flume experiments, in which the middle section of an erosion channel is displaced horizontally, have been conducted to assess the response of streams to horizontal displacement by a strike-slip fault. The experimental erosion channel was developed in a mixture of sand and clay, which provided relatively stable banks with its cohesiveness. Horizontal displacement of a strike-slip fault perpendicular to the channel is expected to add a ,at section to its longitudinal pro,le along the fault line. The experimental stream eliminated this ,at section with downstream degradation, upstream aggradation, and lateral channel shift. As a result, a roughly continuous longitudinal pro,le was maintained. This maintenance of a continuous longitudinal pro,le along channel is considered to be the principle of stream response to horizontal displacement by a strike-slip fault. Downstream degradation was the dominant process of this stream response in the overall tendency of erosion without sand supply. When the rate of fault displacement was low (long recurrence interval), the experimental stream eroded the fault surface, jutting laterally into the channel like a scarp, and de,ected the channel within the recurrence interval. This lateral channel shift gave some gradient to the reach created by fault displacement (offset reach), and the downstream degradation occurred as much as completing the remaining longitudinal pro,le adjustment. When the rate of fault displacement was high (short recurrence interval), the lateral erosion on the ,rst fault surface was interrupted by the next fault displacement. The displacement was then added incrementally to the existing channel offset making channel shift by lateral erosion increasingly dif,cult. The channel offset with sharp bends persisted without much modi,cation, and downstream degradation and upstream aggradation became evident with the effect of the offset channel course, which worked like a dam. In this case, a slight local convexity, which was incidentally formed by downstream degradation and upstream aggradation, tended to remain in the roughly continuous longitudinal pro,le, as long as the horizontal channel offset persisted. In either case, once the experimental stream obtained a roughly continuous gradient, further channel adjustment seemed to halt. Horizontal channel offset remained to a greater or lesser extent at the end of each run long after the last fault displacement. Copyright 2004 John Wiley & Sons, Ltd. [source]

Geological controls on the formation of alluvial meanders and floodplain wetlands: the example of the Klip River, eastern Free State, South Africa

S. Tooth
Abstract Floodplain wetlands are common features of rivers in southern Africa, but they have been little studied from a geological or geomorphological perspective. Study of the upper Klip River, eastern Free State, South Africa, indicates strong geological controls on the formation of alluvial meanders and associated floodplain wetlands. Along this river, pronounced and abrupt changes in valley width are strongly linked to lithological variations. Where weakly cemented sandstone crops out, the Klip has laterally eroded bedrock and carved valleys up to 1500 m wide. In these valleys, the river meanders (sinuosity up to ,175) on moderate gradients (<0001) within extensive floodplains marked by numerous oxbow lakes, backswamps and abandoned channels, many of which host substantial wetlands. In contrast, where highly resistant dolerite crops out, lateral erosion of bedrock is restricted, with the Klip tending instead to erode vertically along joints or fractures. Here, valleys are narrower (<200 m), channel-bed gradients are steeper (>0003), the river follows a much straighter course (sinuosity ,110,134), and floodplains are restricted in width. Long-term landscape development in the Klip and numerous similar catchments depends on the interaction between fluvial processes in the sandstone and dolerite valleys. In the sandstone valleys, vertical erosion rates are controlled by erosion rates of the more resistant dolerites downstream. Hence, in the short- to medium-term (decades to tens of thousands of years), lateral erosion dominates over vertical erosion, with the river concomitantly planing sandstone in the channel floor and reworking floodplain sediments. The thickness of alluvial fill in the sandstone valleys is limited (<4 m), but the resultant meanders are naturally dynamic, with processes such as point bar deposition, cutoff formation and channel avulsion resulting in an assemblage of fluvial landforms. In the longer term (greater than tens of thousands of years), however, vertical erosion will occur in the sandstone valleys as the downstream dolerites are lowered by erosion, resulting in channel incision, floodplain abandonment, and desiccation of the wetlands. Identification of the geological controls on meander and wetland formation provides information vital for the design of effective management guidelines for these ecologically rich habitats, and also contributes to a better understanding of rivers that are intermediate between fully alluvial and fully bedrock. Copyright 2002 John Wiley & Sons, Ltd. [source]

Landforms, sediments, soil development, and prehistoric site settings on the Madaba-Dhiban Plateau, Jordan

Carlos E. Cordova
This paper examines recurrent spatial patterns of prehistoric sites in relation to landforms, alluvial fills, and soil development in the uplands and valleys of the Madaba and Dhiban Plateaus of Jordan. Mousterian lithics (Middle Paleolithic) are largely found on high strath terraces plateaus, where they are associated with red Mediterranean soils. In valleys, Upper Paleolithic sites are often associated with reworked loess deposits of the Dalala allostratigraphic unit. Epipaleolithic occupations are found stratified in deposits of the Thamad Terrace, and Pre-Pottery Neolithic and Pottery Neolithic occupations are associated with colluvium mantling the Thamad Terrace. The Tur al-Abyad Terrace and the Iskanderite alluvial inset are the remnants of middle Holocene floodplains, which were attractive areas for Chalcolithic and Early Bronze Age settlements. Sometime around 4000 B.C., stream incision and further lateral erosion destroyed these floodplains. These historic terraces are underlain by alluvial deposits ranging in age from Roman to Early Islamic periods. The sequence of allostratigraphic units, paleosols, and terraces are the basis for reconstructing phases of fluvial aggradation and stream incision during the past 20,000 years. 2005 Wiley Periodicals, Inc. [source]