Spectral roughness of glaciated bedrock geomorphic surfaces: Implications for glacier sliding

Abstract

A microroughness meter (MRM) was used to measure the high‐frequency roughness of a number of geomorphic surfaces in the forefield of Glacier de Tsanfleuron, Switzerland. Resulting spectral power densities are added to low‐frequency spectra, measured by electro‐optical distance meter (EDM), to generate composite roughness spectra that include almost 5 orders of magnitude of roughness in the frequency domain. These are used to define two roughness indices: a general index of bed roughness is defined as the integral of the raw, spectral power densities, and a sliding‐related index of bed roughness is defined as the integral of the spectral power densities weighted to account for the optimum dependence of glacier sliding speed on hummock wavelength. Results indicate that MRM‐measured geomorphic components vary in roughness by 3 orders of magnitude, principally depending on the surface microenvironment measured and profile orientation relative to the direction of former ice flow. Both MRM‐ and EDM‐measured roughnesses are lower parallel to the direction of former ice flow than perpendicular to it. Composite roughness spectra consequently indicate that the glacier bed is smoothed in the direction of former ice flow at all horizontal scales from 1 mm to 40 m, typically resulting in an order of magnitude decrease in sliding‐related roughness relative to that measured perpendicular to ice flow. Comparison of data from two survey sites located adjacent to, and ∼1.2 km from, the current glacier margin indicates that postglacial subaerial weathering homogenizes bedrock roughness, in particular reducing high‐frequency, flow‐orthogonal roughness. Accounting for the effect of 28% ice‐bedrock separation over one of the profiles reduces net, sliding‐dependent roughness by between 27% and 43%, depending on the transition wave number used.