DYNAMICS OF LARGE AND SMALL-SCALE BEDFORMS ON A MACROTIDAL SHOREFACE UNDER SHOALING AND BREAKING WAVES

Bedform dimensions, bed position changes, near-bed velocities and susp ended sand concentrations are analyzed from measurements at a single l ocation on a macro tidal beach in the south west of England. The study was conducted in 0.5-2.25 m water depth under both swell and wind-gen erated storm waves with both weak and strong currents present. Bed pos itions and suspended sand concentrations were measured using the 3 tra nsducers of a multi-frequency acoustic backscatter sensor with 5 mm ve rtical resolution. Two distinct bed types, based on wavelength (lambda ), were observed, each with two or more subtypes possible: (I) small-s cale bedforms (lambda < 20 cm) which include two-dimensional pre-vorte x (1), vortex (2), post-vortex (3) and three-dimensional vortex (4) fo rms; (II) large-scale bedforms (lambda > 20 cm) which include two-dime nsional (5) forms and three-dimensional vortex (6) forms. Small-scale forms were dominant under non-breaking conditions while the large-scal e forms occurred under both non-breaking and breaking waves; the large -scale forms dominate under breaking conditions. Both types, but parti cularly low steepness forms, were highly mobile with maximum horizonta l migration rates of 5 cm min-1. Large (upto 15 cm) and rapid (upto 3. 0 cm min-1) changes in vertical bed elevation were also observed in as sociation with the development and migration of large-scale forms. Lar ge-scale bedforms were also highly variable spatially, often being int erspersed with smaller scale forms under decaying flow regimes and wit h areas of flat bed under increasing regimes. Observations suggest the se forms are present even under high energy surf zone conditions (wave Shields greater-than-or-equal-to 1). Models for predicting ripple dim ensions did not perform well in this environment. This lack of agreeme nt reflects the complex hydrodynamic regimes associated with random (g rouped) shoaling and breaking waves together with the presence of curr ents, often at large angles to the waves, as well as the rapid rates o f change in the wave forcing associated with tidal cycle oscillations in this macrotidal environment. Suspended sediment concentrations and transport rates are particularly sensitive to the bedforms present and also to bed position changes associated with ripple migration. Estima tes of transport rates are subject to potentially large errors (upto 3 0%) without compensation for bed elevation changes relative to sensor position,