Dynamics of intertidal gravel dunes

Williams, Jon J.; Carling, Paul A.; Bell, Paul S.

doi:10.1029/2005jc003000

Cited by 11 publications

(6 citation statements)

References 27 publications

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“…In aeolian environments, bedforms developed under low rates of sand supply, including barchan and crescentric dunes, are well‐documented (Bagnold, ; Lancaster, ). Bedforms with limited sand supply have also been identified in tidal marine environments (Allen, ; Carling et al ., ; Ernstsen et al ., ; Williams et al ., ) and on the sea floor (Kenyon & Stride, ; Lonsdale & Malfait, ; Lonsdale & Spiess, ; Wynn et al ., ; Franzetti et al ., ), suggesting that a distinctive suite of bedforms develops under sediment supply‐limited conditions in many flow environments at Earth's surface.…”

Section: Introductionmentioning

confidence: 99%

Supply‐limited bedform patterns and scaling downstream of a gravel–sand transition

et al. 2019

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A distinct suite of sand bedforms has been observed to occur in laboratory flows with limited sand supply. As sand supply to the bed progressively increases one observes sand ribbons, discrete barchans and, eventually, channel spanning dunes; but there are relatively few observations of this sequence from natural river channels. Furthermore, there are few observations of transitions from limited sand supply to abundant supply in the field. Bedforms developed under limited, but increasing, sand supply downstream of the abrupt gravel–sand transition in the Fraser River, British Columbia, are examined using multi‐beam swath‐bathymetry obtained at high flow. This is an ideal location to study supply‐limited bedforms because, due to a break in river slope, sand transitions from washload upstream of the gravel–sand transition to bed material load downstream. Immediately downstream, barchanoid and isolated dunes are observed. Most of the bedform field has gaps in the troughs, consistent with sand moving over a flat immobile or weakly mobile gravel bed. Linear, alongstream bedform fields (trains of transverse dunes formed on locally thick, linear deposits of sand) exhibit characteristics of sand ribbons with superimposed bedforms. Further downstream, channel spanning dunes develop where the bed is composed entirely of sand. Depth scaling of the dunes does not emerge in this data set. Only where the channel has accumulated abundant sand on the bed do the dunes exhibit scaling congruent with previous data compilations. The observations suggest that sediment supply plays an important, but often overlooked, role in bedform scaling in rivers.

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Section: Introductionmentioning

confidence: 99%

Supply‐limited bedform patterns and scaling downstream of a gravel–sand transition

et al. 2019

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“…Thus, the slightly higher velocities at the yardang field are as expected, and the range of shear stresses imposed on the bed within the yardang field should cover the same range, although including more occasions with the higher values reported by [ Williams et al . , ]. In general, ebb‐flow parameters (excluding depth) at P3 have twice the values of the flood tide, and thus, the ebb is assumed to be approximately twice as erosive.…”

Section: Resultsmentioning

confidence: 99%

“…Allowing for unsteadiness in the tidal flow [ Soulsby and Dyer , ; Dyer , ] and using equations () and (), u * ranges between 0.07 and 0.14 m s −1 , typically between 0.08 and 0.12 m s −1 (consistent with the shear stress measurements of Williams et al . [, ] at P1 and P2), and z o = O 0.001 m ± 60%.…”

Section: Discussionmentioning

confidence: 99%

“…The peak fluid stresses across Hills Flats during Spring tides are typically in the range 0.89–10 Nm −2 [ Williams et al ., , ]. Fluid stresses act as a tractive force on the bedrock surfaces, and the critical value for erosion is proportional to, but less than, the rock hardness [ Sunamura and Matsukura , ].…”

Section: Discussionmentioning

confidence: 99%

“…All flows are subcritical with maximum Froude numbers approaching 0.3 on both flood and ebb. The shear stresses applied to the bed were not measured at location P3 but at locations P2 and P1 (Figure 1a) Williams et al [2006Williams et al [ , 2008 measured peak average Spring tidal current speeds between 1.30 m s À1 and 1.79 m s À1 and average stresses of between 0.89 and 4.9 N m À2 , with occasional excursions to 10 N m À2 , with the flood peak typically being 1.6 N m À2 and the ebb peak being 4.9 N m À2 . Location P1 is slightly to landward of the yardang field and in shallower water (h max = 5.48 m) during Spring tides.…”

Section: Hydrodynamicsmentioning

confidence: 99%

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Subaqueous “yardangs”: Analogs for aeolian yardang evolution

Carling

2013

JGR Earth Surface

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[1] Landforms, morphologically similar to aeolian yardangs but formed by erosion of bedrock by currents on an estuarine rock platform, are described for the first time. The geometries of the "yardangs" are described and related to semi-lemniscate shapes that minimize hydraulic drag. The processes of bedrock erosion by the reversing sedimentladen tidal currents are described, and a semi-quantitative model for landform evolution is proposed. The model casts doubt on the "simple" role of the maximum in the twodimensional vertical suspended sediment flux distribution and the consequent distribution of potential kinetic energy flux in the process of shaping the rock wall facing the ebb flow. Rather, although the kinetic energy flux increases away from the bed, the sediment becomes finer and abrasion likely is insignificant compared with coarse sand abrasion lower in the profile. In addition, the vertical distribution of sediment flux is mediated by topographic forcing which raises the elevation at which bed load intersects the yardang prow. Consequent erosion leads to ebb-facing caprock collapse and yardang shortening. In contrast, the role of ebb-flow separation is paramount in mediating the abrasion process that molds the rock surface facing the flood flow. The length of yardangs is the least conservative dimension, reducing through time more rapidly than the height and width. Width is the more conservative dimension which implies that once the caprock is destroyed, scour over the obstacle is significant in reducing body height, more so than scour of the flanks which reduces width. The importance of vertical fissures in instigating the final breakdown of smaller yardangs and their extinction is noted. Similarities to aeolian yardang geometries and formation principles and processes are noted, as are the differences. The model has implications for aeolian yardang models generally.

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