Geometry and migration characteristics of bedforms under waves and currents. Part 1: Sandwave morphodynamics

Abstract

In this paper the geometric and migration characteristics of sandwaves generated under the combined effect of waves and currents are described. Description and analysis of the flow structure along the centerline of developed sandwaves are also presented. The velocity distribution along the sandwave is used to explain, in part, the size and shape characteristics of the ripples superimposed upon the sandwaves (as discussed in the companion paper [Cataño-Lopera, Y. and García, M.H., in press. Geometry and migration characteristics of bedforms under waves and currents: Part 2, Ripples superimposed upon sandwaves. Coastal Engineering ( doi:10.1016/j.coastaleng.2006.03.008).]). Variation in roughness height along the considered bed feature is also discussed for a particular experiment. Velocity measurements within the fluid were recorded with an Acoustic Doppler Velocimeter (ADV). An acoustic level sensor was employed to determine surface wave characteristics such as amplitude and period. 3D mapping of the bottom was done via a 32-composite-element array of sub-aquatic acoustic sensors (SeaTek-Multiple Transducer arrays). Bottom records were statistically analyzed to obtain height, length, migration rates of ripples and sandwaves and vertical growth rate of sandwaves. Experiments were conducted for the mobility factor and the Reynolds wave number within the ranges 10 < ψ < 88 and 16 × 10 3 < Re w < 5 × 10 5, respectively. Measured values were compared with laboratory and field data and semi-empirical and analytical formulae from the literature. Data scatter is still considerable when plotting the sandwaves geometric and migration characteristics as a function of dimensionless quantities such as the Reynolds wave number, the Shields parameter and the mobility factor.