Predicting the roughness length of turbulent flows over landscapes with multi-scale microtopography

Pelletier, Jon D.; Field, James A.

doi:10.5194/esurf-4-391-2016

Cited by 19 publications

(22 citation statements)

References 30 publications

(38 reference statements)

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“…This roughness length is defined as the equivalent grain roughness replacing any type of roughness yielding the same overall bed roughness (same velocity profile). It is most logic to assume that the effective roughness (k s ) of the sandy beach surface is related to the:  grain roughness effects of static and saltating particles (Owen, 1964;Farrell and Sherman, 2016;Field and Pelletier 2018);  height of the micro bed morphology (ripples), Field 2016, Field andPelletier, 2018);  size of shells or shell fragments/clusters which are sometimes abundantly available, particularly at nourished beaches.…”

Section: Influencing Parametersmentioning

confidence: 99%

“…These relatively large roughness values are most likely caused by the presence of bed forms. Pelletier and Field (2016) studied the roughness length of the microtopography of desert-type surface (sites in southwestern US, 2015). The roughness length scale was quantified by the H RMSE , the root-mean-squared values of the surface elevations, which varied in the range of 0.55 to 36 mm.…”

Section: Bed Irregularities and Bed Formsmentioning

confidence: 99%

See 1 more Smart Citation

A fully predictive model for aeolian sand transport

Rijn¹,

Strypsteen

2020

Coastal Engineering

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Section: Influencing Parametersmentioning

confidence: 99%

Section: Bed Irregularities and Bed Formsmentioning

confidence: 99%

A fully predictive model for aeolian sand transport

Rijn¹,

Strypsteen

2020

Coastal Engineering

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“…A key goal of our work is to separately quantify the contributions of topography and saltation to z 0 so that we may develop a predictive model that explicitly includes both. To estimate the topographic contribution to z 0 , we used the method developed and tested at ten playa locations across the western US by Pelletier and Field (2016). A Discrete Fourier Transform is applied to each transect of the topography parallel to the wind to obtain Fourier coefficients f n .…”

Section: Saltation Measurementsmentioning

confidence: 99%

Controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport

Field

Pelletier

2018

Earth Surf Processes Landf

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Estimates of the wind shear stress exerted on Earth's surface using the fully rough form of the law‐of‐the‐wall are a function of the aerodynamic roughness length, z0. Accurate prediction of aeolian sediment transport rates, therefore, often requires accurate estimates of z0. The value of z0 is determined by the surface roughness and the saltation intensity, both of which can be highly dynamic. Here we report field measurements of z0 values derived from velocity profiles measured over an evolving topography (i.e. sand ripples). The topography was measured by terrestrial laser scanning and the saltation intensity was measured using a disdrometer. By measuring the topographic evolution and saltation intensity simultaneously and using available formulae to estimate the topographic contribution to z0, we isolated the contribution of saltation intensity to z0 and document that this component dominates over the topographic component for all but the lowest shear velocities. Our measurements indicate that the increase in z0 during periods of saltation is approximately one to two orders of magnitude greater than the increase attributed to microtopography (i.e. evolving sand ripples). Our results also reveal differences in transport as a function of grain size. Each grain‐size fraction exhibited a different dependence on shear velocity, with the saltation intensity of fine particles (diameters ranging from 0.125 to 0.25 mm) saturating and eventually decreasing at high shear velocities, which we interpret to be the result of a limitation in the supply of fine particles from the bed at high shear velocities due to bed armoring. Our findings improve knowledge of the controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport. The results should contribute to the development of improved sediment transport and dust emission models. © 2018 John Wiley & Sons, Ltd.

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“…This approach to analyzing river beds has led to multiscalar decompositions of geometric roughness, rather than direct decompositions of hydraulic roughness. The latter approach has been developed for complex aeolian surfaces using transforms (Nield et al, 2013;Pelletier and Field, 2016;Field and Pelletier, 2018), which serves as a proof-of-concept for the multiscalar drag decomposition approach.…”

Section: Introductionmentioning

confidence: 99%

Short communication: Multiscalar drag decomposition in fluvial systems using a transform-roughness correlation (TRC) approach

Adams

Zampiron

2020

Preprint

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Abstract. In natural open-channel flows over complex surfaces, a wide range of superimposed roughness elements may contribute to flow resistance. Gravel-bed rivers present a particularly interesting example of this kind of multiscalar flow resistance problem, as both individual grains and bedforms can potentially be important roughness elements. In this paper, we propose a novel method of estimating the relative contribution of different physical scales of river bed topography to the total drag, using a transform-roughness correlation (TRC) approach. The technique, which requires only a single longitudinal profile, consists of (1) a wavelet transform which decomposes the surface into roughness elements occurring at different wavelengths, and (2) a `roughness correlation' that estimates the drag associated with each wavelength based on its geometry alone, expressed as ks. We apply the TRC approach to original and published laboratory experiments and show that the multiscalar drag decomposition yields estimates of grain- and form-drag that are consistent with estimates in channels with similar morphologies. Also, we demonstrate that the roughness correlation may be used to estimate total flow resistance via a conventional equation, suggesting that it could replace representative roughness values such as median grain size or the standard deviation of elevations. An improved understanding of how various scales contribute to total flow resistance may lead to advances in hydraulics as well as channel morphodynamics.

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Predicting the roughness length of turbulent flows over landscapes with multi-scale microtopography

Cited by 19 publications

References 30 publications

A fully predictive model for aeolian sand transport

A fully predictive model for aeolian sand transport

Controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport

Short communication: Multiscalar drag decomposition in fluvial systems using a transform-roughness correlation (TRC) approach

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