Morphological perspective on the sedimentary characteristics of a coarse, braided reach: Tagliamento River (NE Italy)

Huber, Emanuel; Huggenberger, Peter

doi:10.1016/j.geomorph.2015.07.015

Cited by 25 publications

(22 citation statements)

References 67 publications

(92 reference statements)

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“…Salter () already recognized the significance of scours for the sedimentary records. Because scours are located at the lowest positions and can migrate, they can rework significant parts of the deposits and have the largest chance of being preserved in the sedimentary records (Salter ; Siegenthaler and Huggenberger ; Huber and Huggenberger ).…”

Section: Introductionmentioning

confidence: 99%

Imaging scours in straightened and braided gravel‐bed rivers with ground‐penetrating radar

Huber

Anders

Huggenberger

2019

Near Surface Geophysics

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The genesis of alternate gravel bars in straightened rivers and braided rivers is closely coupled to the formation of scours. Scours can lead to riverbank failure, initiate lateral river dynamics and strongly impact the interaction between surface and subsurface flow and transport. It is, therefore, critical to account for scours in the design of flood protection measures. However, there is still little knowledge on the formation and characteristics of such scours, especially on the dynamic relationship between the riverbed morphology and the scours. Bathymetric riverbed surveys conducted at medium to low discharge may underestimate the real scour sizes and shapes if scours filled with sediments during waning discharge. Furthermore, the literature suggests that gravel bars and scours can jointly migrate on the riverbed, resulting in cut‐and‐fill sedimentary structures in the near subsurface. These cut‐and‐fill structures cannot be observed from the surface. We investigate with ground‐penetrating radar the presence of scour fills below the riverbed of two gravel‐bed rivers and study any indications of bar and scour migration. Thus, two ground‐penetrating radar surveys were conducted, one on the emerged part of an alternate bar of the channelized Alpine Rhine River (Switzerland) and the other on a flat gravel surface in a braided reach of the Tagliamento River (Italy) that is near the pristine state. At both sites, a scour fill could be clearly identified below a 2‐m‐thick sediment layer. The imaged part of the scour of the Alpine Rhine River (30 × 100 × 4.5 m) is located at the front end of the gravel bar next to the riverbank. This scour was only partially imaged by ground‐penetrating radar and is in reality significantly deeper and larger. The scour of the Tagliamento River (20 × 30 × 2.5 m) shows a clear internal structure consisting of inclined, planar cross‐beds that merge tangentially with the lower‐bounding erosional surface of the scour. At the light of the literature, we compare these two scours in terms of sedimentary processes resulting from the complex interplay between scours and gravel bars. The study findings offer a promising research avenue on the dynamic relationship between discharge, sediment transport, scour and bar formation and migration.

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Section: Introductionmentioning

confidence: 99%

Imaging scours in straightened and braided gravel‐bed rivers with ground‐penetrating radar

Huber

Anders

Huggenberger

2019

Near Surface Geophysics

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“…Huber and Huggenberger, 2015), and it thus logically follows that deep and migratory confluences (i.e. those that both create large amounts of accommodation space that is then filled) will have the greatest chance of being preserved.…”

Section: A New Classification Of Planform Confluence Behaviourmentioning

confidence: 99%

“…River confluences have the potential to create some of the points of deepest incision into underlying sediments (Mosley, 1976;Best, 1988;Bristow et al, 1993;Salter, 1993;Siegenthaler and Huggenberger, 1993;Best and Ashworth, 1997;Miall and Jones, 2003;Ullah et al, 2015) and hence their subsequent fill has been argued to possess the highest preservation potential of fluvial channels (Huber and Huggenberger, 2015). Since the depth of junction scour and mobility of the confluence are determined by flow processes in the confluence hydrodynamic zone (Best and Rhoads, 2008), it can be argued that differing junction dynamics may produce a range of characteristic confluence zone sedimentology from sandy bar development to mudfilled scours.…”

Section: Introductionmentioning

confidence: 99%

The Planform Mobility of River Channel Confluences: Insights from Analysis of Remotely Sensed Imagery

Dixon¹,

Smith²,

Best³

et al. 2017

Preprint

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A B S T R A C TRiver channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment, and which are locations for the preservation of thick fluvial deposits overlying a basal scour. Despite their importance, there has been little study of the stratigraphic characteristics of river junctions, or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential. As a result, although it is known that confluences can migrate through time, models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location. This is problematic for a number of reasons, not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes (such as confluence scour) and that driven by allocyclic controls (such as sea-level change). This paper investigates the spatial mobility of river confluences by using the 40-year record of Landsat Imagery to elucidate the styles, rates of change and areal extent over which large river confluence scours may migrate. On the basis of these observations, a new classification of the types of confluence scour is proposed and applied to the Amazon and Ganges-Brahmaputra-Meghna (GBM) basins. This analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally. Thus in the GBM basin, a high sediment supply, large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80% of large confluences are mobile over this 40-year window; conversely this figure is < 40% for the Amazon basin. These results highlight that: i) the potential areal extent of confluence scours is much greater than previously assumed, with the location of some confluences on the Jamuna (Brahmaputra) River migrating over a distance of 20 times the tributary channel width; ii) extensive migration in the confluence location is more common than currently assumed, and iii) confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield.

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“…Channel confluences form ubiquitous components of all river networks (Best & Rhoads, ) and represent a sedimentary archive containing information on both the dynamics of these sites and, through the provenance of their sediments, the basins from which they are sourced (Goodbred et al ., ). Confluences adopt especial significance, in that it is often argued that the alluvial sedimentary record is biased towards preservation of sediments deposited in the deepest parts of channels (Paola & Borgman, ), such as confluence scours (Sambrook Smith et al ., ), which provide accommodation space that is less likely to be reworked during subsequent incision (Huber & Huggenberger, ). Confluences may thus be one of the sites of significant scour that set the deepest level, or ‘combing depth’ ( cf .…”

Section: Introductionmentioning

confidence: 99%

The sedimentology of river confluences

et al. 2018

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Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.

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Morphological perspective on the sedimentary characteristics of a coarse, braided reach: Tagliamento River (NE Italy)

Cited by 25 publications

References 67 publications

Imaging scours in straightened and braided gravel‐bed rivers with ground‐penetrating radar

Imaging scours in straightened and braided gravel‐bed rivers with ground‐penetrating radar

The Planform Mobility of River Channel Confluences: Insights from Analysis of Remotely Sensed Imagery

The sedimentology of river confluences

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