Free bars in rivers

Tubino, Marco; Repetto, Rodolfo; Zolezzi, Guido

doi:10.1080/00221689909498510

Cited by 110 publications

(117 citation statements)

References 30 publications

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“…channel width is constant at a value of 2 (the channel width herein is defined as the width of individual anabranches, and not the channel-belt width), although the values reported herein are smaller than the ratios of 5-7 suggested by Yalin (1992) and values of between 3 and 12 reported from theoretical, field and experimental data by Lewin (1976), Nelson (1990), Tubino et al (1999), Lanzoni (2000a,b) and Lunt et al (2004).…”

Section: Unit Barscontrasting

confidence: 46%

Deposits of the sandy braided South Saskatchewan River: Implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization

Lunt¹,

Smith²,

Best³

et al. 2013

Bulletin

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The estimation of fluvial geobody dimensions from core data is a notoriously difficult problem in reservoir modelling. To try and improve such estimates and hence reduce uncertainty in geomodels, data on dunes, unit bars, cross-bar channels and compound bars and their associated deposits are presented from the sand-bed braided South Saskatchewan River, Canada. These data are used to provide a test of models that relate the scale of formative bedforms to the dimensions of the preserved deposits, and therefore an insight into how such deposits may be preserved over geologic time. The preservation of bedform geometry is quantified using a comparison of the alluvial architecture above and below the maximum erosion depth of the modern channel deposits. This comparison shows that there is no significant difference in the mean set thickness of dune cross-strata above and below the basal erosion surface of the contemporary channel, thus suggesting that dimensional relationships between dune deposits and the formative bedform dimensions are likely to be valid from both recent and older deposits.The data show that estimates of mean bankfull flow depth derived from dune, unit bar and cross-bar channel deposits were all very similar. Thus the use of all these metrics together can provide a useful check that all components and scales of the alluvial architecture have been identified correctly when building reservoir models. The data also highlight a number of practical issues with identifying and applying data relating to cross-strata. For example, the deposits of unit bars were found to be severely truncated in length and width, thus only ~10% of the mean barform length may remain making identification in section difficult. For similar reasons, the deposits of compound bars was found to be especially difficult, and hence estimates of channel depth based on this method may be problematic. Where only core data is available (i.e. there is no outcrop data) it is suggested that formative flow depths are best reconstructed using cross-strata formed by dunes. However, this study found that theoretical 3 relationships between the distribution of set thicknesses and formative dune height result in slight overestimates of the latter, and hence mean bankfull flow depths derived from these measurements.This study illustrates that the preservation of cross-strata, and thus the paleohydraulic inferences that can be drawn from them, are a function of the ratio of the size and migration rate of bedforms and the time scale of aggradation and channel migration. These factors must be considered when deciding on appropriate length:thickness ratios for the purposes of object-based modelling.4

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Section: Unit Barscontrasting

confidence: 46%

Deposits of the sandy braided South Saskatchewan River: Implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization

Lunt¹,

Smith²,

Best³

et al. 2013

Bulletin

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“…Morphodynamic physically based models start from fluid mechanics and sediment transport equations (e.g., Seminara, 2010) and have found good applications in explaining the dynamics of meandering (e.g. see the review by Camporeale et al, 2007) and the formation and migration of river bars (Callander, 1969;Colombini et al, 1987;Tubino et al, 1999). Migration of non colonized bars has also been shown to have a convective nature (Federici and Seminara, 2004), but the role of seasonal vegetation anchoring the sediment is still unexplored.…”

Section: River Morphodynamics and Vegetation Interactionsmentioning

confidence: 99%

Mechanisms of vegetation uprooting by flow in alluvial non-cohesive sediment

Edmaier

Burlando

Perona

2011

Hydrol. Earth Syst. Sci.

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Abstract. The establishment of riparian pioneer vegetation is of crucial importance within river restoration projects. After germination or vegetative reproduction on river bars juvenile plants are often exposed to mortality by uprooting caused by floods. At later stages of root development vegetation uprooting by flow is seen to occur as a consequence of a marked erosion gradually exposing the root system and accordingly reducing the mechanical anchoring. How time scales of flow-induced uprooting do depend on vegetation stages growing in alluvial non-cohesive sediment is currently an open question that we conceptually address in this work. After reviewing vegetation root issues in relation to morphodynamic processes, we then propose two modelling mechanisms (Type I and Type II), respectively concerning the uprooting time scales of early germinated and of mature vegetation. Type I is a purely flow-induced drag mechanism, which causes alone a nearly instantaneous uprooting when exceeding root resistance. Type II arises as a combination of substantial sediment erosion exposing the root system and resulting in a decreased anchoring resistance, eventually degenerating into a Type I mechanism. We support our conceptual models with some preliminary experimental data and discuss the importance of better understanding such mechanisms in order to formulate sounding mathematical models that are suitable to plan and to manage river restoration projects.

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“…Unit bars are distinguished from compound bars (mid-channel bars, point bars) by being solitary depositional units. Thus, the definition of unit bars used here does not carry a genetic interpretation as is the case for, e.g., free bars (Seminara and Tubino 1989;Tubino et al 1999). In view of their large size, unit bars commonly persist through several floods following their formation and thus provide a record of flow unsteadiness.…”

Section: Introductionmentioning

confidence: 99%

“…These studies indicate that relative abundance and composition of unit-bar deposits can vary dramatically. This complexity and a lack of systematic studies promotes confusion about the roles and relative importance of boundary conditions and formative mechanics, which consequently hinders further development of the increasing number of successful modeling studies (e.g., Tubino et al 1999;Lanzoni 2000;Knaapen et al 2001;Seminara 2003, 2006;Nicholas 2010). This study systematically relates unit-bar composition to experimentally established processes and as such provides a foundation for further studies of unit bars and river-channel deposits.…”

Section: Introductionmentioning

confidence: 99%

Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

Reesink¹,

Bridge²

2011

Journal of Sedimentary Research

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Unit-bar deposits are ubiquitous components of river-channel deposits and strongly influence their hydrological properties, yet they are not easy to interpret. This paper concerns details of the internal structures of six unit bars from the South Saskatchewan River, Canada, that were investigated using trenches, epoxy resin peels, and 900 MHz ground-penetrating radar (GPR) profiles. The composition of unit bars depends on flow unsteadiness and superimposed bedforms. Flow unsteadiness causes changes in the mean grain size of the sediment in transport, but is expressed primarily as a change in the type and direction of migration of smaller ripple-and dune-scale bedforms superimposed on unit bars. Superimposed bedforms with heights that exceed 25% of the host bedform height reduce their host's slope and generate inclined sets. Host bedforms with smaller superimposed bedforms form angle-of-repose cross strata with a visible pre-sorting pattern. The relationship between the formative host-and superimposed bedforms and such pre-sorted cross strata can be used to interpret numerous aspects of the three-dimensional geometries of the bedforms. Such detailed interpretations rely on: (i) regularly spaced finegrained drapes deposited during the passage of troughs of superimposed bedforms, (ii) grain-size sorting patterns generated by sorting within the superimposed bedforms before deposition, (iii) grain-size sorting during deposition by grainfall and intermittent grainflows down the bar lee-slope, and (iv) the presence and nature of low-angle reactivation surfaces.The detailed interpretations revealed that the deposits of dam-related floods were significantly smaller than the deposits of individual unit bars in this study. The unsteady flow and sediment transport conditions are reflected by changes in the structural composition of the unit bars. Reactivation surfaces associated with flow unsteadiness, as opposed to those formed by large superimposed bedforms, were characterized by changes in mean grain size, the buildup of sediment at the base of the bar lee slope, and changes in structures formed by superimposed bedforms. Reactivation surfaces generated by large superimposed bedforms indicate that bedform preservation is likely increased in areas of flow deceleration. The trough deposits of exposed unit bars contained bubble sand, planar laminae due to upper-stage plane beds, and low-angle inclined laminae due to antidunes. These structures indicate that unit-bar troughs can act as ephemeral channels. The composition of the investigated unit bars thus emphasizes the interplay between flow unsteadiness and bed morphology in the formation of sedimentary structures in river channels.Sediment surfaces generate visible GPR reflections where the thickness and contrast in electrical properties of the sediment layers are sufficiently large. Most cross-stratified sets were represented only by reflections from the base of the set, and not by high-angle inclined reflections from the cross strata. The local absence of refle...

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Free bars in rivers

Cited by 110 publications

References 30 publications

Deposits of the sandy braided South Saskatchewan River: Implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization

Deposits of the sandy braided South Saskatchewan River: Implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization

Mechanisms of vegetation uprooting by flow in alluvial non-cohesive sediment

Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

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