Upstream-facing convex surfaces: Bedrock bedforms produced by fluvial bedload abrasion

Abstract

Bedrock bedforms are a common sight in many bedrock river channels but few process-form linkages have been established for the myriad of forms recognised in the literature. Direct observations from bedrock channels on rates and distributions of erosion involved in bedrock bedform development are rare because of difficulties in making observations in these high-energy environments. Upstream-facing convex surfaces (UFCS) are common in bedrock rivers and are partnered with downstream-facing surfaces. We have made observations of bedrock morphology, aspect, and surface texture of UFCSs in bedrock river channels exposed at low river stage in Taiwan. A distinct contrast in morphology and surface texture exists between upstream- and downstream-facing surfaces. On a millimetre scale of observation, UFCSs have a three-dimensional, brittle fracture-dominated, granular surface texture but on a decimetre to metre scale have an overall convex form. Downstream-facing surfaces, in contrast, have millimetre to micrometre scale rugosity characterised by smooth polished surfaces that on a centimetre to metre scale are concave or undulating with intricately sculpted features. A curvilinear topographic feature, or crest line, marks the change in slope, aspect, and surface textures between upstream- and downstream-facing surfaces and is orientated approximately perpendicular to the reach azimuth and therefore mean flow direction. We used a system of removable datums and terrestrial laser scanning to monitor the three-dimensional erosion of nine bedforms at sites in the Liwu River, Taiwan, over a period of one year (2007–2008). All sites had an asymmetric distribution of erosion with higher rates of erosion on upstream-facing surfaces and minor to undetectable erosion on downstream-facing surfaces, with local complexity. The crest line mapped the boundary between relatively high and low erosion rates between the upstream-facing and downstream-facing surfaces respectively. Observations from unscaled flume experiments show that bedload particles transit a flow obstacle by travelling up the upstream-facing surface, where erosion is focused, and hop over the top of the obstacle. Our field and flume observations indicate that UFCSs observed in natural rivers are produced by bedload abrasion, which by being decoupled from the flow can only impact the stoss surface to any significant extent. Micropitting found on polished downstream-facing surfaces indicate that they are dominantly formed by suspended load abrasion. The aspect of a surface, its position in a channel cross section, the topography of the channel upstream, and the local topography of the bedrock bedform are significant controls on the location and magnitude of abrasion by bedload and resultant surface textures because of their effect on steering a bedload particle stream. The high erosion rates observed indicate that abrasion by bedload is the controlling process in the formation of this variety of bedrock bedform and possibly the channel at large in this setting.