Subaqueous “yardangs”: Analogs for aeolian yardang evolution

Abstract

[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.

Threshold of erosion of submarine bedrock landscapes by tidal currents

“…Nevertheless, the 1‐m contoured bathymetry in Figure shows that the margins of the outcrops are steep and may be accentuated by tidal scour. However, although some breaks can be observed in Figure , scour has not eroded corridors through the outcrops, and the data do not show yardang‐like streamlined features aligned with the flow, such as described by Carling ().…”

“…For example, in deserts, saltating particles abrade rock surfaces up to the height limit of particle motion and can ultimately produce sculpted rock formations (yardangs) with under‐mined bases (Anderson, ). Although the greater viscosity of water compared with air could lead to differences in abrasion between the two environments, Carling () has described similar yardang‐like features developed in soft strata (marls) from the intertidal zone in the Severn Estuary. In extreme flows in rivers, the highest energy impacts of particles can occur on the parts of boulders facing downstream, due to detachment of particles in the recirculating flow behind them, leading to indentations there (Hancock et al ., ).…”

Sinuous Rille

“…When the island is submerged, erosion is focused in the lee side, rapidly eroding the island to a streamlined shape (Baker 1982). There may be differences in the shape of fully submerged and emergent islands (Carling 2013).…”

A fundamental equation for describing the rate of bedrock erosion by sediment‐laden fluid flows in fluvial, coastal, and aeolian environments

“…() attempted to explain the temporal variation in abrasion rate of yardangs in China applying a parameter into which the energy ratio of impacting sand particles (kinetic energy) to the yardang material (elastic potential energy) is incorporated. In the discussion of the evolution of yardang‐like features developing in a subaqueous environment, Carling () applied compressive strength values converted from Schmidt hammer readings as an index for rock resistance.…”