Predicting wave‐induced ripple equilibrium geometry

Nelson, Timothy R.; Voulgaris, George; Traykovski, Peter

doi:10.1002/jgrc.20241

Cited by 42 publications

(56 citation statements)

References 84 publications

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“…The fine sand cases from the Deltaflume exhibit longer wavelength and smaller steepness than predicted for sub-orbital ripples. However this behaviour has become well documented, as noted by Nelson et al (2013). By continuing to increase in wavelength, but decrease in height and steepness, as d0/d50b increases these ripples can be categorized as 'long wave ripples' (see Soulsby, Whitehouse and Marten, 2012).…”

Section: Ripple Dimensionsmentioning

confidence: 97%

On the suspension of graded sediment by waves above ripples: Inferences of convective and diffusive processes

Davies

Thorne

2016

Continental Shelf Research

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The relationship between the grain size distribution of the sediment on the bed and that found in suspension due to wave action above ripples is assessed here using detailed, pumped sample, measurements obtained at full-scale and also at laboratory scale. The waves were regular and weakly asymmetrical in most tests, and irregular in a minority of tests. The beds comprised fine and medium sand and were rippled in all tests. The cycle-mean sediment concentrations (C) from the pumped samples were split into multiple grain size fractions and then represented by exponential Cprofile shapes. The analysis of these profiles was carried out in two stages to determine: i) the relationship between the size distribution of the sediment on the bed and that found in the reference concentration, and ii) the behaviour of the exponential decay scale of the C-profiles. From this analysis inferences are made about the relative roles of diffusion and convection in the upward sediment flux linked to the process of vortex shedding from the ripple crests. The Transfer Function (Tr) defined to relate the bed sediment size distribution to that of the reference concentration indicates that, while finer fractions are relatively easily entrained, the suspension of some coarser fractions is caused by an additional convective effect that supplements diffusion. The evidence for this becomes pronounced above steep ripples, and the Transfer function suggests further that irregular waves increase the occurrence of coarser fractions in suspension. A functional form for Tr is suggested incorporating these principles. The exponential decay scale LS arising from the fractional C-profiles is also examined to assess the mechanisms responsible for the upward transfer of grains and a parameterisation of LS related to ripple size is suggested. The separate findings for Tr and LS present supporting evidence of diffusion affecting the finer fractions in suspension and combined diffusion + convection affecting the coarser fractions. The methodology developed allows the vertical profile of suspended median grain size to be predicted given knowledge of both the bed grain size distribution and also the flow conditions.

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Section: Ripple Dimensionsmentioning

confidence: 97%

On the suspension of graded sediment by waves above ripples: Inferences of convective and diffusive processes

Davies

Thorne

2016

Continental Shelf Research

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“…The most comprehensive predictor for current-generated, wavegenerated and combined-flow bedforms was proposed by Soulsby et al (2012). Other widely used bedform predictors were proposed by Haque & Mahmood (1985), Baas (1993), van Rijn (1993, Julien & Klaassen (1995), Raudkivi (1997) and Karim (1999) for unidirectional currents, and by Nielsen (1981), van Rijn (1993), Wiberg & Harris (1994), Malarkey & Davies (2003), Grasmeijer & Kleinhans (2004), Williams et al (2004Williams et al ( , 2005, Camenen & Larson (2006), Yan et al (2008), Camenen (2009), Pedocchi & Garcia (2009) and Nelson et al (2013) for oscillatory currents.…”

Section: Bedforms In Non-cohesive Sedimentmentioning

confidence: 99%

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

Baas

Best

Peakall

2015

JGS

168

253

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The use of sedimentary structures as indicators of flow and sediment morphodynamics in ancient sediments lies at the very heart of sedimentology, and allows reconstruction of formative flow conditions generated in a wide range of grain sizes and sedimentary environments. However, the vast majority of past research has documented and detailed the range of bedforms generated in essentially cohesionless sediments that lack the presence of mud within the flow and within the sediment bed itself. Yet most sedimentary environments possess fine-grained sediments and recent work has shown how the presence of this fine sediment may substantially modify the fluid dynamics of such flows. It is increasingly evident that understanding the influence of mud, and the presence of cohesive forces, is essential to permit a fuller interpretation of many modern and ancient sedimentary successions. In this paper, the present state of knowledge on the stability of current- and wave-generated bedforms and their primary current stratification is reviewed, and a new extended bedform phase diagram is presented that summarizes the bedforms generated in mixtures of sand and mud under rapidly decelerated flows. This diagram provides a phase space using the variables of yield strength and grain mobility as the abscissa and ordinate axes, respectively, and defines the stability fields of a range of bedforms generated under flows that have modified fluid dynamics owing to the presence of suspended sediment within the flow. Our results also present unique data on a range of bedforms generated in such flows, whose recognition is essential to help interpret such deposits in the ancient sedimentary record, including the following: (1) heterolithic stratification, comprising alternating laminae or layers of sand and mud; (2) the preservation of low-amplitude bed-waves, large current ripples and bed scours with intrascour composite bedforms; (3) low-angle cross-lamination and long lenses and streaks of sand and mud formed by bed-waves; (4) complex stacking of reverse bedforms, mud layers and low-angle cross-lamination on the upstream face of bed scours; (5) planar bedding comprising stacked mud–sand couplets. Furthermore, the results shown herein demonstrate that flow variability is not required to produce deposits consisting of interbedded sand and muds, and that the nature of flaser, wavy and lenticular bedding ( sensu Reineck & Wunderlich 1968) may also need reconsideration in the deposits of such sediment-laden flows.

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“…Hydraulic roughness and extraction of momentum from the mean flow are enhanced beyond the effect of mere grain roughness by orders of magnitude due to the presence of ripples (Zanke, 1982;Soulsby, 1997). Furthermore, the presence of ripples influences turnover rates of nutrients and pollutants in the benthic environment when compared to a flat seabed (Nelson et al, 2013;Ahmerkamp et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

“…A recent overview and evaluation of the performance of wave ripple predictors with an extensive dataset from lab and field experiments can be found in Nelson et al (2013). Soulsby and Whitehouse (2005) present a literature review of predictors for wave, current and combined ripples and Soulsby et al (2012) recently developed a combined, time-evolving predictor.…”

Section: Predictors For Ripple Dimensionsmentioning

confidence: 99%

“…As a fundamental understanding of bedform development is still pending and deterministic prediction is not yet possible, equilibrium ripple predictors based on extensive laboratory and field datasets exist for waves (e.g., Soulsby et al, 2012;Nelson et al, 2013), currents (e.g., Soulsby et al, 2012;Bartholdy et al, 2015) or for combined flows (e.g., Li and Amos, 1998). A classification of the type of bedform and corresponding dominant forcing can be made using the ratio of wave and current shear stress.…”

Section: Introductionmentioning

confidence: 99%

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Predicted ripple dimensions in relation to the precision of in situ measurements in the southern North Sea

Krämer

2016

Ocean Sci.

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Abstract. Ripples are common morphological features in sandy marine environments. Their shapes and dimensions are closely related to local sediment properties and forcing by waves and currents. Numerous predictors for the geometry and hydraulic roughness of ripples exist but, due to their empirical nature, they may fail to properly reflect conditions in the field. Here, measurements of tide and wave generated ripples in a shallow shelf sea are reported. Discrete and continuous methods for the extraction of ripple dimensions from digital elevation models (DEMs) are inter-compared. The range of measured ripple dimensions is quantified and compared to the results of empirical predictors.The repeatability of a measurement for inactive conditions is taken as the precision of measurements of bedform dimensions. The accuracy of measurement is assessed via comparison to predicted dimensions. Results from field data show that the precision of measurements is limited to 10 % of the absolute ripple dimensions. The application of different methods for the detection of ripple heights may result in form roughness heights differing by a factor of up to 2 between the traditional statistical estimate and a full evaluation of the spatial bathymetry.

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Predicting wave‐induced ripple equilibrium geometry

Cited by 42 publications

References 84 publications

On the suspension of graded sediment by waves above ripples: Inferences of convective and diffusive processes

On the suspension of graded sediment by waves above ripples: Inferences of convective and diffusive processes

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

Predicted ripple dimensions in relation to the precision of in situ measurements in the southern North Sea

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