Nearshore sandbar migration

Sandbars are important morphological features in the surf zone. In a dynamically stable beach configuration, sandbars usually move in the offshore direction during storm episodes, characterized by strong seawards currents (undertows), while the onshore sandbar migration occurs between storm events when the wave energy is low. Therefore, oscillatory component dominating flows produce different morphological behaviors from the mean velocity dominating flow do. Although the offshore sandbar migration phenomenon is a relatively well known process, our understanding of the onshore sandbar migration is less satisfactory. Existing models using traditional shear stress expressions (i. e, τ = 1/2ρf\u\u) have failed to explain and predict the onshore sandbar migration and only recently some alternative models for bed shear stress and sediment flux have been proposed. During Duck94 experiment continuous measurements of near bottom velocity in a cross-shore transect extending from the shoreline to 400 m offshore were taken. During the 45-day period, both on- and offshore sandbar migrations were observed. In this paper, using the data set we present the challenges for modeling sandbar migration in a field wave-current system.