Scour Fills Research Articles

Double-thresholds in scour–fill processes and some implications in channel adjustment

Through an analysis of data collected from the Yellow River and its tributaries on the Loess Plateau of China, the phenomenon of double-thresholds in scour–fill processes of wide-range water-sediment two-phase flows has been shown. Thresholds located in non-hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double-threshold phenomenon leads to complicated sediment transport behavior of heavily sediment-laden rivers. With an increase in suspended sediment concentration, the channel sediment delivery ratio increases initially and becomes higher than 1, followed by a decrease and finally becomes lower than 1 again. Controlled by the double-thresholds in the scour–fill processes, channel adjustment of the lower Yellow River is non-linear and complex. When the suspended concentrations were lower than the lower threshold or higher than the upper threshold, scour or bed downcutting was the dominant channel-forming process. Channel shape tends to be narrower and deeper, and the channel thalweg became more sinuous. When the suspended concentrations lay between the lower and upper thresholds, deposition of sediment was the dominant channel-forming process; channel shape tended to be shallower and wider, and channel thalweg became less sinuous.

Controls on the Geometry of Incised Valleys in the Basal Quartz Unit (Lower Cretaceous), Western Canada Sedimentary Basin

In southern Alberta, multiple episodes of fluvial erosion occurred during Lower Mannville, Basal Quartz time (Early Cretaceous), creating a series of incised-valley systems. This study examines two of these valley networks, in order to illustrate the geomorphic and genetic complexity of incised-valley systems. Both valley networks are filled by a transgressive succession of fluvial to estuarine deposits. Isopach mapping of these deposits over an area of approximately 10,000 km 2 in southern Alberta shows that both systems are dendritic to rectilinear in character, with the trunk valley flowing from south to north along the length of the Western Canada Sedimentary Basin. These systems were fed from the west, south, and east by a network of tributaries. Eustatic sea-level changes may have played a role in initiating river incision, but regional tectonic movements had an important influence on the history of accommodation change. The periods of erosion appear to have been relatively brief because the valleys are narrow, although confinement within cohesive, mudstone-rich paleosols of the underlying incised-valley deposits may have slowed valley widening. A relatively wet paleoclimate may also have contributed to the formation of narrow valleys by favoring the existence of single-channel rivers. The general morphology of the Alberta foreland basin, together with the distribution of unfilled relief associated with a preceding valley system and/or of less resistant, slightly older deposits, determined the location of the trunk valleys. Uplift of the Bow Island Arch (Sweetgrass Arch) to the east may have been responsible for a shift in the valley headwaters between the formation of the two valley systems. Synerosional block faulting in response to flexural loading had a significant influence on the geometry of the valley network through its control on the location and geometry of major tributary valleys. The trunk valley is also significantly narrower and more deeply incised where it crosses the most pronounced uplifted block. Enhanced erosion at tributary junctions and valley bends produced localized areas (ca. 2-3 km in diameter) where the valley fill is up to five times thicker and considerably coarser grained than the adjacent valley deposits. Such localized scour fills represent prime petroleum-exploration targets.

The effects of survey frequency on estimates of scour and fill in a braided river model

AbstractAn erratum has been published for this article in Earth Surface Processes and Landforms 27(7) 2002, 795.Estimates of scour and fill in rivers that are derived by differencing topographic surfaces are known to be negatively biased by local compensation of scour and fill between surveys but the magnitude of bias is not well known. This study examines the effect of survey frequency on volumes of scour and fill over a period of active channel braiding in a small‐scale river model. A 100 min, high temporal resolution time series of digital elevation models is artificially coarsened by selectively removing models. The resulting four overlapping time series have survey intervals of 10 min, 20 min, 50 min and 100 min. Cumulative scour and fill volumes for the 100 min period are compared between the four series. It is concluded that the decay in measured volumes of scour and fill with increased survey interval can be described using inverse functions. Cumulative scour–fill volumes are approximately 420 per cent greater over the study period for 10 min survey intervals than for a 100 min interval. After the 100 min period of competent flow, nearly 65 per cent of the channel area experienced significant compensation of scour and fill. Several compensation mechanisms were identified in association with braided channel kinetics, including lateral channel migration, the migration of bed forms, and channel avulsion. It is demonstrated that by negatively biasing scour, fill and net estimates, this error significantly affects morphological approaches to the estimation of bed load sediment transport. Copyright © 2002 John Wiley & Sons, Ltd.

The Quaternary history of the subtidal central Wash, eastern England

The history of Quaternary sedimentation in the subtidal Wash is described using high-resolution seismic profiles. The Pleistocene sequence is divided into three depositional units, comprising Anglian till overlain by possible Late Devensian subglacial scour fill and lacustrine sediments. These latter sediments may provide further evidence for a lake in the Wash impounded by ice along the Lincolnshire–Norfolk coast. The Holocene sequence is divided into six depositional units, each truncated by the one above. Estuarine sediment resting on a marine flooding surface forms the earliest unit. This sediment was partially eroded by migration of the shoreface as the marine flooding progressed landward. The following four units comprise sand and gravel banks deposited on the erosion surface. Bank deposition was followed by an episode of tidal scour caused either by increased tidal current velocities following reclamation of the Fenland or by breakdown of postulated former offshore barriers. The youngest and most extensive Holocene unit rests on the scoured surface and comprises several types of deposit. These are: large sand banks around the periphery of the subtidal area with sediment extending seawards into two NE–SW aligned troughs; low sand banks on a central ridge dividing the troughs and partially covering the sediments in the troughs; thick gravels towards the mouth of the Wash; muddy sediments forming drapes over the sand in the centre of the Wash. The data provide information on the variety of processes related to the advance and retreat of Pleistocene ice sheets in eastern England and the subsequent Holocene marine flooding of the Wash–Fenland embayment. The Holocene sequence reveals periods of widespread sedimentation separated by periods of both local and regional erosion, with possible implications for climatic and hydrodynamic change. © 1997 John Wiley & Sons, Ltd.

Origin of Concretion-Hosted Shell Clusters in the Late Cretaceous Bearpaw Formation, Southern Alberta, Canada

The widespread phenomenon of faunal clustering in concretions is examined in examples from the Late Cretaceous Bearpaw Formation of southern Alberta. Concretion-hosted shell clusters have traditionally been interpreted as biologically formed features. In contrast, sedimentologic, taphonomic and faunal evidence in the Bearpaw Formation suggests that shell clustering is inherently related to a common physical control. The clustered nature of the Bearpaw assemblages reflects the accumulation of shells in isolated storm scours. Differences in faunal composition resulted from variations in (1) the mode of scour initiation, (2) the faunal content of sea-floor sediments, and (3) the degree of shell transport experienced during storms. The formation of the isolated scours is attributed to locally augmented erosion in the vicinity of bed defects and objects exposed on the sea floor. The common restriction of fossils to concretions reflects preferred concretion growth around the shell-rich scour fills. Previously described examples of concretion-hosted shell clusters are re-interpreted in light of the scour-fill hypothesis.