Dynamics of large and small scale bedforms on a macrotidal shoreface under shoaling and breaking waves

Abstract

Bedform dimensions, bed position changes, near-bed velocities and suspended sand concentrations are analyzed from measurements at a single location 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-generated storm waves with both weak and strong currents present. Bed positions and suspended sand concentrations were measured using the 3 transducers of a multi-frequency acoustic backscatter sensor with 5 mm vertical resolution. Two distinct bed types, based on wavelength (λ), were observed, each with two or more subtypes possible: (I) small-scale bedforms ( λ < 20 cm) which include two-dimensional pre-vortex (1), vortex (2), post-vortex (3) and three-dimensional vortex (4) forms; (II) large-scale bedforms ( λ > 20 cm) which include two-dimensional (5) forms and three-dimensional vortex (6) forms. Small-scale forms were dominant under non-breaking conditions while the large-scale forms occurred under both non-breaking and breaking waves; the large-scale forms dominate under breaking conditions. Both types, but particularly low steepness forms, were highly mobile with maximum horizontal 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 association with the development and migration of large-scale forms. Large-scale bedforms were also highly variable spatially, often being interspersed with smaller scale forms under decaying flow regimes and with areas of flat bed under increasing regimes. Observations suggest these forms are present even under high energy surf zone conditions (wave Shields ⩾ 1). Models for predicting ripple dimensions did not perform well in this environment. This lack of agreement reflects the complex hydrodynamic regimes associated with random (grouped) shoaling and breaking waves together with the presence of currents, often at large angles to the waves, as well as the rapid rates of 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. Estimates of transport rates are subject to potentially large errors (upto 30%) without compensation for bed elevation changes relative to sensor position.