On modeling the variability of bedform dimensions

ABSTRACT: Bedforms are irregular features that cannot easily be described by mean values. The variations in the geometric dimensions affect the bed roughness, and they are important in the modeling of vertical sorting and in modeling the thickness of cross-strata sets. The authors analyze the variability of bedform dimensions for three sets of flume experiments, considering PDFs of bedform height, trough elevation and crest elevation divided by its mean value. It appears that the dimensionless standard deviation of the bedform height is within a narrow range for nearly all experiments. This appears to be valid for the trough elevation and crest elevation, as well. For some modeling purposes, it seems sufficient to assume that the standard deviation is a constant, so that the variation in bedform dimension can be modeled by only predicting the mean bedform dimension.

Stochastics of bedform dimensions

Often river dunes are considered as regular bed patterns, with a mean dune height and a mean dune length. In reality however, river dunes are threedimensional and irregular features that cannot be fully described by their mean values. In fact, dune dimensions can be considered as stochastic variables. Their probability distribution can be characterized by a mean value and variance. The stochastic properties of dune dimensions are relevant for (see e.g. Van der Mark et al., 2005):\ud • Shipping - highest crests\ud • Pipelines & cables - deepest troughs\ud • Modelling cross-strata sets - troughs, dune heights\ud • Modelling vertical sorting - troughs\ud • Modelling bed roughness - dune heights\ud In the present research the stochastics of crest elevation, trough elevation and dune height are investigated by analysing three sets of flume experiments

Sediment transport and bedform development in the lee of bars: evidence from fixed- and partially-fixed bed experiments

The co-existence, interaction, and repeated (re-)establishment of bars, dunes and ripples in natural channels is responsible for many important flow-form-flow dynamics. Small bedforms are constantly generated, superimposed on larger ones, particularly in zones affected by large-scale secondary circulation patterns produced by the larger bedforms. These superimposed bedforms migrate onto the downstream stoss slope of larger-scale forms where they i) generate additional form-roughness, ii) change sediment transport dynamics, iii) control bedform splitting and merging, iv) alter the geometry of the host lee slope, and v) change the resultant sedimentary structures. Our understanding of superimposed bedform development is derived from investigations of bedform development on flat beds in uniform flow and does not adequately describe bedform development in distinctly non-uniform flows and areas with large-scale secondary circulations.In order to expand our understanding of bedform initiation, this paper presents fixed-bed and partially-fixed-bed experiments that investigate the effect of a host-bedform’s separated flow on the development of smaller, secondary bedforms in its trough. The results show that: 1) scour in the trough of bars increases in depth and decreases in downstream length with increasing flow velocity over the crest; 2) the point of bedform initiation moves downstream and the amplitude of the incipient ripples decreases with increasing flow velocity; 3) crest-trough velocity gradients and coherence of the separated flows in the lee of ripples in bar troughs depend on their position relative to the separated flow of the larger-scale host bedform and tend to increase down-stream.These observations indicate that the development of secondary bedforms is hindered by the host bedform’s separated flow and is also dependent on the length of the downstream stoss slope. The reduction of bedform amplification is attributed to the reduced strength and coherence of the separated flows in the lee of the secondary bedforms as a result of the stronger separated flow of the host bedform. Thus, this study presents a step towards a fuller understanding of bedform initiation and development in areas with complex topography and local variability in the flow field

Stochastics of bedform dimensions

Often river dunes are considered as regular bed patterns, with a mean dune height and a mean dune length. In reality however, river dunes are threedimensional and irregular features that cannot be fully described by their mean values. In fact, dune dimensions can be considered as stochastic variables. Their probability distribution can be characterized by a mean value and variance. The stochastic properties of dune dimensions are relevant for (see e.g. Van der Mark et al., 2005): • Shipping - highest crests • Pipelines & cables - deepest troughs • Modelling cross-strata sets - troughs, dune heights • Modelling vertical sorting - troughs • Modelling bed roughness - dune heights In the present research the stochastics of crest elevation, trough elevation and dune height are investigated by analysing three sets of flume experiments

Quantifying the three‐dimensional stratigraphic expression of cyclic steps by integrating seafloor and deep‐water outcrop observations

Deep‐water deposits are important archives of Earth’s history including the occurrence of powerful flow events and the transfer of large volumes of terrestrial detritus into the world’s oceans. However the interpretation of depositional processes and palaeoflow conditions from the deep‐water sedimentary record has been limited due to a lack of direct observations from modern depositional systems. Recent seafloor studies have resulted in novel findings, including the presence of upslope‐migrating bedforms such as cyclic steps formed by supercritical turbidity currents that produce distinct depositional signatures. This study builds on process to product relationships for cyclic steps using modern and ancient datasets by providing sedimentological and quantitative, three‐dimensional architectural analyses of their deposits, which are required for recognition and palaeoflow interpretations of sedimentary structures in the rock record. Repeat‐bathymetric surveys from two modern environments (Squamish prodelta, Canada, and Monterey Canyon, USA) were used to examine the stratigraphic evolution connected with relatively small‐scale (average 40 to 55 m wavelengths and 1.5 to 3.0 m wave heights) upslope‐migrating bedforms interpreted to be cyclic steps within submarine channels and lobes. These results are integrated to interpret a succession of Late Cretaceous Nanaimo Group deep‐water slope deposits exposed on Gabriola Island, Canada. Similar deposit dimensions, facies and architecture are observed in all datasets, which span different turbidite‐dominated settings (prodelta, upper submarine canyon and deep‐water slope) and timescales (days, years or thousands of years). Bedform deposits are typically tens of metres long/wide, <1 m thick and make up successions of low‐angle, backstepping trough‐shaped lenses composed of massive sands/sandstones. These results support process‐based relationships for these deposits, associated with similar cyclic step bedforms formed by turbidity currents with dense basal layers under low‐aggradation conditions. Modern to ancient comparisons reveal the stratigraphic expression of globally prevalent, small‐scale, sandy upslope‐migrating bedforms on the seafloor, which can be applied to enhance palaeoenvironmental interpretations and understand long‐term preservation from ancient deep‐water deposits