A flume experiment on the development of subaqueous fine‐gravel dunes from a lower‐stage plane bed

Abstract

Determining the hydraulic conditions whereby gravel dunes first develop in subaqueous environments is fundamental as their presence may influence engineering solutions designed to maintain bed stability. In addition, estimates of the flow conditions associated with preserved gravel bed forms in sedimentary sequences are useful for reconstructing the depositional environments and geometries of, for example, oil‐ and gas‐bearing geological strata. Consequently, a series of experiments considered dune initiation. In these experiments, defects and latterly incipient dunes developed from lower‐stage plane gravel beds during near‐threshold conditions of motion (θ/θcrit = 1.0–1.016) and long periods of marginal bed load transport rates. The three‐dimensional defects were almost imperceptible positive ovoid features with heights of one or two grain diameters and lengths and spans of a few decimeters. After 17 hours of flow, incipient, low‐amplitude, simple two‐dimensional dunes developed from the defects, with heights ranging between 0.029 and 0.055 m, nonequilibrium wavelengths of 1–4 m and spans of 0.6–0.9 m. Continued development over several days, with θ/θcrit ratios of around 1.3, resulted in near‐equilibrium two‐dimensional dunes with wavelengths averaging 2.6–3.5 m and spans equal to the flume width (4 m). The inception of incipient dunes could be predicted using bulk flow models; however, this approach was not suitable for the prediction of defect development. Near‐bed turbulence, in the form of small‐scale sweep events of limited breadth, controls the initiation of defects, but larger‐scale, coherent turbulent structures in the outer flow are related to dune development. Significantly, both defects and incipient dunes can exist at the same time, which indicates that the effects of sweeps on the bed morphology persist at the same time as larger‐scale turbulent structures are beginning to effect sediment transport.