8,396 research outputs found
Sedimentology of the rough rock: a carboniferous braided river sheet sandstone in northern England
No full textThe Rough Rock is a coarse-grained fluvial sheet sandstone which outcrops across northern and central England. It is interpreted as a braided river sandstone on the basis of sedimentary structures, internal erosion and growth surfaces. This study reveals that the Rough Rock represents more than a single phase of deposition and that internal erosion surfaces may indicate anabranch avulsions or more significant river avulsions. Detailed outcrop profiles are presented to try and distinguish the styles of deposition and the nature of internal bounding surfaces and channel geometry. The Rough Rock Formation contains a very large number of first and third order bedding contacts, which represent the migration of dunes and channels respectively, there are relatively few macroforms and bars preserved. The lack of second order surfaces within many outcrops could indicate that bars in the Rough Rock rivers were very large and therefore cannot be resolved at the outcrop scale. Similarly, the rivers which deposited the Rough Rock may have had such high width: depth ratios that the dip of second order bounding surfaces are too low to be identified. The large number of third order bounding surfaces indicates channel stacking which may have occurred in two ways: (1) relocation of rivers in the same area over time, or, (2) anabranch avulsion within a braided river within a hierarchy of channels. The problems associated with differentiating between these two mechanisms are discussed
Sedimentary structures exposed in bar tops in the Brahmaputra River Bangladesh
No full textThe Brahmaputra River is one of the world’s largest sand-bed braided rivers with a channel belt up to 15 km wide, a mean channel depth of 5 m and maximum scour depths of up to 40 m. The recorded discharge varies by around 60 000 cumecs every year following the annual monsoon, producing dramatic stage fluctuations with the water level falling between 7 and 8 m from bankfull to low flow stage. In the dry season large areas of bar top and channel bed are exposed and low flow channels cut natural sections through bar tops exposing the internal stratification. Elements of upstream, downstream and lateral accretion are identified, although the high width/depth ratio of channels (up to 500:1) results in extremely low depositional dips on bed bounding surfaces. Within the accretionary elements vertical and lateral changes in sedimentary structures are ubiquitous and varied but some patterns can be discerned. Channel deposits are dominated by sinuous-crested dunes and trough cross-stratification. Upper stage plane bed lamination, often with a very low angle depositional dip, has been observed in beds up to 4 m thick and is generally found at the base of the exposed bar top sections. Trough cross-stratification in bar tops occurs in sets from 0.1 m to 3 m thick with rapid vertical and lateral changes in set thickness. Very large sets of trough cross-stratification, >3 m thick, do not occur in the bar top sections. Current ripple lamination is most abundant on the tops of bars with very high rates of climb associated with high rates of sediment deposition. From these observations a generalised vertical sequence of bar top origin might include trough cross-stratification overlain by upper stage plane bed lamination, isolated sets of trough cross-stratification truncated and capped by current ripple lamination. This sequence, which is considered to represent the deposits of a single flood, may occur within the upstream, downstream or lateral accretion elements. However, it should be noted that vertical and lateral changes in facies are extremely abundant with many changes in sedimentary structures and reactivation surfaces within each depositional episode
Beyond just floodwater
No full textFlooding, already the largest hazard facing humankind, is becoming more frequent and affecting more people. Adapting to flooding must consider more than just water to encapsulate the effects of sediment movement, re-imagine flooding through a sociogeomorphic lens and expand approaches to knowing about floods
Mechanisms of anabranch avulsion within gravel-bed braided rivers: observations from a scaled physical model
No full textAbstract: The physical modelling of gravel-bed braided rivers, using a 1: 20Froude scale model, permits analysis of the types and relative occurrence of different avulsion mechanisms. Anabranch avulsion within braided rivers involves thre main mechanisms: choking avulsion caused by blockage of one channel by a sediment lobe, c nstriction avulsion produced by deflection, confinement and subsequent diversion of the flow by a barform and apex avulsion following erosion at the outside of sinuous thalwegs and confined meander bends. Each avulsion mechanism described and analysis of the abundance of each type illustrates choking avulsion to be predominant without braided rivers. Three factors are found to control anabranch avulsions, namely flow discharge, sediment flux and floodplain topography. These are briefly discussed together with the implications of avulsion type for the alluvial architecture of gravelly braided alluvium. Avulsion within braided rivers occurs at ll channel scales and is instrumental in dictating the spatial and temporal distribution ofsediment within the braidplain. An understanding of th
Braided rivers: where have we come in 10 years? progress and future research
No full textBook synopsis: This important book brings together eighteen cutting-edge research papers first presented at the Second International Conference on Braided Rivers. It includes the latest research on the dynamics, deposits and ecology of these rivers.
Essential reading for geomorphologists, earth scientists, engineers and ecologists with a pure and applied interest in the study, modelling and management of braided rivers
Braided rivers: Perspectives and problems
No full textAbstract: Progress towards a fuller understanding of the dynamics and deposits of braided rivers demands aninterdisciplinary approach to a host of unresolved problems. Although many advances have b en made within recent years in interpreting the mechanics of flow, transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed. In particular, several areas demand attention: the mechanisms of braid bar initiation; confluence-diffluence dynamics, the nature ofsedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales. This paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration. Braided rivers form important topics of study for many scientists and one of the primary aims of this volume is to bring together work from many disciplines in an integrated approach to braided rivers. For the geomorphologist braided fluvial systems are abundant within upland and pro
Inter-reciprocity applied to electrical networks
No full textElectrical Engineering, Mathematics and Computer Scienc
Mid to late Holocene geomorphological and sedimentological evolution of the fluvial-tidal zone: Lower Columbia River, WA/OR, USA
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