scallop: one construct, many systems

Scallops are the regular, spoon-shaped hollows that turbulent water dissolves into the walls, floor, and ceiling of a cave passage, each with a steeper lip facing upstream. Because the pattern is set by the flow at the rock boundary, the hollows get smaller as the water moves faster, so their average length lets a cave scientist back out the velocity and even the direction of a long-gone stream. Blumberg and Curl's 1974 Experimental and theoretical studies of dissolution roughness worked out the fluid mechanics linking scallop wavelength to flow, and Ford and Williams treat scallops as a standard paleo-flow indicator in karst passages. The referent is a flow-formed roughness on a soluble rock surface, measured as a scallop wavelength that inverts to a flow speed.[1][2]

Unit

The mean scallop length on a dissolved rock surface, which inverts to flow velocity

Recovery means

Not applicable

Measured by

Measured from the mean Sauter wavelength of scallops, converted to flow velocity through the dimensionless relation of dissolution-roughness theory

The identical construct appears on ice: where turbulent water flows past an ice boundary it melts out the same family of regular spoon-shaped hollows, called ice scallops, whose spacing is governed by the flow rather than by any property of the ice. Bushuk and colleagues' 2019 Ice scallops: a laboratory investigation of the ice–water interface reproduced them in the lab and confirmed that the wavelength is selected by a flow instability at the melting interface, the same mechanism that shapes dissolution scallops in limestone. It is not a look-alike but the same flow-formed boundary roughness realised in a melting rather than a dissolving solid, measured again by scallop wavelength as a signature of the flow that cut it.[3][1]

Unit

The scallop wavelength on a melting ice surface, set by the flow at the interface

Recovery means

Not applicable

Measured by

Characterised by the selected scallop wavelength at the ice–water interface and its scaling with flow conditions