Fluvial sedimentary structures formed experimentally in a pipe, and their implications for interpretation of subglacial sedimentary environments

A series of experiments were performed with a glacio -fluvial sand in transparent acrylic cylindrical pipes of 24. 8 and 29. 9 cm in diameter, respectively, to gain some insight into sedimentary processes in fluvial systems where water flows beneath ice. One of the objective s was to investigate criteria that might be diagnostic in the field to identify sediments deposited in glacier tunnels flowing full. With increasing sediment concentration at a given discharge, the sequence of bed forms was : ripple , dune , dune-plane , plane , dune - plane, and total suspension. Dune foresets commonly became finer - grained down the foreslope and displayed a slope discontinuity due to s lumping in the upper part of the foreset. Stoss laminations were te1nporarily preserved due to an upward convexity of the stoss surface. Within the range of experimental variables, dune celerity is directly proportional to the cube of mean flow velocity; the ratio of mean flow velocity to dune celebrity is inversely proportional to the square of the Reynolds Number, and symmetry index of the dunes is directly proportional to the square of the Reynolds Number. Dune height is 1 /3 to 1 times the hydraulic mean depth. The system tends to maximize the hydraulic mean depth by adjustment of both b e d height and bed form. These relationships are tentatively extended to processes of esker sedimentation with the use of a natural example. Esker sediments deposited in tunnel s are characterized by parallel lamination and large - scale cross stratification, compatible with deposition near maximum hydraulic mean depth. Dune heights indicate actual water depths of 1 to 4 metres, and lead to the suggestion that accumulation of esker sediment was accommodated by a melting upward of the ice roof. Unidirectional flow with high bed -shear was maintained by the ice roof having limited the flow cross-section, and a fluvial sequence was s epos ited that did not require essentially subaerial conditions.