Bedrock scallop distribution and morphology in upland river channels

Abstract

Scallops are contiguous concave features bound by steep ridges that occur on bedrock surfaces in cave and surface river channel environments. Scallops are useful indicators in hydraulics as their morphology is related to flow velocity and direction. However, previous research has focused on cave scallops, with limited quantitative analysis of scallop lengths and orientations in surface channel environments. A dataset of 13,641 scallop length and orientation measurements was collected from surface channel scallops. Analysis revealed that Trout Beck and Rough Sike had Sauter mean scallop lengths of 5.1 cm (S32 = 0.2, n = 1,185) and 3.1 cm (S32 = 0.2, n = 12,456) respectively. A negative relationship was present between the Sauter mean scallop length and the height at which they were located above the water surface. The statistical distributions of scallop length measurements are found to not follow the typical unimodal, log-normal distribution suggested for cave scallops, potentially due to other factors such as weathering and abrasion modifying the scallop morphology in surface channels. No significant relationships were observed between the Sauter mean scallop length or scallop abundance and the unit stream power at Trout Beck or Rough Sike. Scallops typically were oriented in the direction towards the primary flow in the channel, suggesting that the morphological context of the scalloped location must be considered when inferring paleo-flow directions. This study provides the first quantitative analysis of scallop morphology in surface channels, developing the understanding of how scallops form under varying environmental conditions. A first-order estimate of the scallop development timescales for a range of scallop sizes was generated under two different dissolution rates, providing the first such estimate for surface channel scallops on limestone surfaces.