Hydrodynamic significance of variable ripple morphology across deep-water barchan dunes in the Faroe-Shetland Channel

A combination of high-resolution (30 kHz) sidescan sonar and seafloor photography has been used to image barchan dunes and sand ripples in the Faroe-Shetland Channel. The barchan dunes display body lengths and horn-to-horn widths of up to 120 m, and the horns point downcurrent to the southwest. The seafloor surrounding the dunes is variable, with gravel patches, sand with gravel streaks, and uniform rippled sand all observed. The pattern of sand ripples across the dune surface is strongly controlled by the dune morphology. On the lower stoss slopes of the dunes, straight-crested transverse ripples gradually merge upslope into sinuous-crested transverse ripples. Most of the upper dune surface is dominated by short-crested linguoid ripples. This sequence represents an increase in height above seafloor and increasing flow velocity. The overall pattern of ripples across the dune surface indicates that the bottom current flow is modified by the dune morphology, with flow concentrated over the dune crest and along the inner edge of the horns. It is also possible to estimate flow velocity across the dune field, using published analogues and short-term current meter observations. Generally, published examples of submarine barchans suggest a flow velocity of 40-80 cm/s. This range of values is in agreement with near-bed current meter measurements in the study area, which indicate peak flow velocities of 50-60 cm/s. However, in the longer term higher flow speeds may be possible, especially considering the presence of erosional furrows adjacent to the dune field.