Nearshore Bedform Patterns: Lab Studies
Dickson, P. J., and Drake, T. G. 1996, Nearshore bedform pattern evolution: lab studies: EOS Transactions AGU, v. 77 (46), p.F420
Bedform patterns in the nearshore are relatively easily observed indicators
of complex, difficult-to-measure fluid motion. Video observation of bedform
pattern formation and evolution in a laboratory wave channel reveal several
features of pattern evolution relevant to field and modeling studies. Patterns
in coarse sand were generated in a 2.4-m-wide channel having uniform still-water
depth of 70 cm using 5 s, 20-cm-high waves. Transport occurred primarily
as bedload. Concrete megaripple forms one meter long having 20-cm-high,
shoreward-facing, angle-of-repose slipfaces were placed in the bed for some
experiments. Without fixed forms, two-dimensional ripples having mean spacing
of 15 cm and mean height of 3 cm form and migrate onshore at about 1 cm/min.
Ripple imperfections migrate onshore at about four times the ripple migration
rate. Ripples in the vicinity of channel-spanning fixed bedforms behave
differently. Immediately onshore of the megaripple crest, migration rates
reach nearly 5 cm/min, then decrease rapidly to undisturbed rates at a distance
of about 5 megaripple heights onshore. Areal number density of imperfections
in perturbed regions decreases with time, and appears to obey a diffusion
law. Imperfection migration heals perturbed regions of the bed, and creates
domains of two-dimensional ripples bounded by thin regions having high imperfection
densities. The size of such domains increases with time, and domain shape
becomes increasingly equant.
Supported by the Office of Naval Research