Ripple formation induced by biogenic mounds

Abstract

Bedload and ripple formation at subcritical flows were observed during laboratory studies involving the mound-forming, head-down deposit-feeding polychaete Cistenides gouldii. Flume experiments were designed to investigate the conditions resulting in sediment transport within and immediately downstream of biogenic mound assemblages. Experiments focused on the inputs of mound (population) density, mound height and the presence/absence of Cistenides. The activities of Cistenides may generate sediment transport in two ways: via particle ejection during feeding or via scour generated by their mounds. The first set of experiments were conducted to determine which process was predominantly responsible for ripple formation. Mounds were required for ripple formation, but ripples formed only at the two higher mound densities (150 and 570 per m 2). The second set of experiments used mounds of different densities and heights to investigate density effects. One of three final bed states was always attained: (1) mounds became and generated migrating ripples, (2) mounds changed into ripples which did not migrate beyond the test area, called stationary ripples, or (3) mounds remained separate. The division between migrating ripple and no ripple cases was used to develop a threshold similar to that established by Southard and Dingler [Southard, J.B., Dingler, J.R., 1971. Flume study of ripple propagation behind mounds on flat sand beds. Sedimentology, 16, pp. 251–263.] for ripple propagation behind isolated mounds. Our threshold was higher due to the coarser sediments used in this study. Stationary ripples exhibited a `local equilibrium' where, despite a two-fold range of initial mound separation, resulting ripples had similar wavelengths. This stationary ripple wavelength was smaller than the equilibrium wavelength found for migrating ripples generated from a flat bed. The ability of biogenic structures to generate ripples has implications in studies of near-bed processes. Ripples seen in bottom photographs and stratigraphy may not be an absolute indicator of supercritical flows. Additionally, when the topography within patches of mound-building worms changes, the bed geometry generating interfacial flow and bottom drag is ripples instead of mounds. Estimates of reworking due to ripple migration greatly outweigh rates of biological reworking observed in the field. All of these consequences reinforce the importance of sediment transport induced by biogenic structures.