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

A microroughness meter (MRM) was used to measure the high-frequency roughne ss of a number of geomorphic surfaces in the forefield of Glacier de Tsanfl euron, Switzerland. Resulting spectral power densities are added to low-fre quency spectra, measured by electro-optical distance meter (EDM), to genera te 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 roughnes s is defined as the integral of the spectral power densities weighted to ac count for the optimum dependence of glacier sliding speed on hummock wavele ngth. Results indicate that MRM-measured geomorphic components vary in roug hness by 3 orders of magnitude, principally depending on the surface microe nvironment measured and profile orientation relative to the direction of fo rmer ice flow. Both MRM- and EDM-measured roughnesses are lower parallel to the direction of former ice flow than perpendicular to it. Composite rough ness 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, ty pically resulting in an order of magnitude decrease in sliding-related roug hness relative to that measured perpendicular to ice flow. Comparison of da ta from two survey sites located adjacent to, and similar to 1.2 km from, t he current glacier margin indicates that postglacial subaerial weathering h omogenizes bedrock roughness, in particular reducing high-frequency, flow-o rthogonal roughness. Accounting for the effect of 28% ice-bedrock separatio n over one of the profiles reduces net, sliding-dependent roughness by betw een 27% and 43%, depending on the transition wave number used.