Discussions and Closure: Sand-Dune Geometry of Large Rivers during Floods

Amsler, Mario L.; García, Marcelo H.; Neill, Charles R.; Juliën, Pierre Y.; Klaassen, G.J.

doi:10.1061/(asce)0733-9429(1997)123:6(582)

Cited by 47 publications

(30 citation statements)

References 4 publications

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“…The structures of inclined sets that can be traced into the topsets in location 2 suggest that the proportion of a set that remains preserved increases significantly where dunes overtake a bar lee slope. This matches observations of decreasing heights, and hence locally increased sediment flux to the bed, of superimposed dunes migrating down the lee slope of larger dunes (Pretious and Blench 1951;Rubin and McCulloch 1980;Rubin and Hunter 1982;Amsler and Garcia 1997;Parsons et al 2005). The decrease in heights of the down-climbing bedforms indicates a systematic flux of sediment to the host slope and hence a systematic increase in set thickness.…”

Section: Foresets: Inclined Co-setssupporting

confidence: 83%

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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Section: Foresets: Inclined Co-setssupporting

confidence: 83%

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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“…The match between the dimensions of the preserved dune fields and measured dune heights in a near-average flow also indicates that the preserved dunes are unlikely formed during singular, extreme floods, during which the largest dunes may grow to 6.5 m in height in the Río Paraná (Amsler and Garcia, 1997). All form-sets are found in the upper part of the channel deposits even though GPR reflections are visible well below the lowest form-set.…”

Section: Interpretation Of the Gpr Form-setsmentioning

confidence: 67%

Extremes in dune preservation: Controls on the completeness of fluvial deposits

Reesink

Berg

Parsons

et al. 2015

Earth-Science Reviews

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Understanding sedimentary preservation underpins our ability to interpret the ancient sedimentary record and reconstruct paleoenvironments and paleoclimates. Dune sets are ubiquitous in preserved river deposits and are typically interpreted based on a model that describes the recurrence of erosion in a vertical sequence, but without considering spatial variability. However, spatial variability in flow and sediment transport will change the recurrence of erosion, and therefore dune preservation. In order to better understand the limits of these interpretations and outline the causes of potential variability in preservation potential, this paper reviews existing work and presents new observations of an extreme end-member of dune preservation: 'form-sets', formed by dunes in which both stoss-and lee-slopes are preserved intact. These form-sets do not conform to models that are based on the recurrence of erosion, since erosion does not recur in their case, and can therefore be used to evaluate the assumptions that underpin sedimentary preservation. New Ground Penetrating Radar data from the Río Paraná, Argentina, show dune fields that are buried intact within larger scale barforms. These trains of form-sets are up to 300 m in length, are restricted to unit-bar troughs in the upper 5 m of the channel deposits, occur in N 5% of the mid-channel bar deposits, show reactivation surfaces, occur in multiple levels, and match the size of average-flow dunes. A review of published accounts of form-sets highlights a diversity of processes that can be envisaged for their formation: i) abandonment after extreme floods, ii) slow burial of abandoned dune forms by cohesive clay in sheltered bar troughs and meander-neck cut-offs, iii) fast burial by mass-movement processes, and iv) climbing of dune sets due to local dominance of deposition over dune migration. Analysis of these new and published accounts of form-sets and their burial processes highlights that form-sets need not be indicative of extreme floods. Instead, form-sets are closely associated with surrounding geomorphology such as river banks, meander-neck cut-offs, and bars because this larger-scale context controls the local sediment budget and the nature of recurrence of erosion. Locally enhanced preservation by the 'extreme' dominance of deposition is further promoted by finer grain sizes and prolonged changes in flow stage. Such conditions are characteristic, although not exclusive, of large lowland rivers such as the Río Paraná. The spatial control on dune preservation is critical: although at-a-point models adequately describe near-horizontal sets of freely migrating dunes in uniform flows, they are unsuitable for inclined dune co-sets and other cases where multiple scales of bedforms interact. Spatial and temporal variations in flow and sediment transport between the thalweg and different positions on larger bar-forms can change the preservation potential of dunes within river channels. Therefore, dune set thickness distributions are likely grouped in larger-scale un...

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“…1 and 2), so the riverbed might have been in the progress of adjusting itself to the changing hydraulic conditions (Allen, 1978;Amsler and Garcia, 1997). When compared with the models from the literature, the four bedform types observed in this study do not all appear to be the product of existing flow conditions.…”

Section: Dune Morphology and Formationmentioning

confidence: 67%

“…The upper flow regime is uncommon in deep rivers and bedforms are mainly ripples and dunes (Harbor, 1998;Carling et al, 2000;Radecki-Pawlik et al, 2006). Dunes are observed to grow with increasing discharge and reach their maximum after peak discharge, then increase in length with reduced height during the falling stage (Amsler and Garcia, 1997;Carling et al, 2000;Prent and Hickin, 2001;Julien et al, 2002;Wilbers and Brinke, 2003), remaining as large bedforms during the subsequent low-flow period. Bedforms left by higher flows in this way take time to adjust to the changed flow (Allen, 1974(Allen, , 1978(Allen, , 1982.…”

Section: Introductionmentioning

confidence: 99%