Geometric and migrating characteristics of amalgamated bedforms under oscillatory flows

The presence of coexisting bedforms of different length scales is very common in rivers, lakes and coastal environments. In rivers, ripples superimposed on dunes are common features. In coastal environments sandbars are commonly found coexisting with ripples of smaller scales. In shallowand deeperwaters, the presence of ripples superimposed on megaripples and those in turn amalgamated onto large sandwaves, has been, despite its importance, the object of research only in the last three decades. Superposition of bedforms, of different length and time scales, play and important role in the distribution of the turbulence structures and local flow resistance which in turn dictate the geometric and migrating characteristics of them. Herein, experimental results from a series of detailed measurements of geometric and migrating characteristics of bedforms (ripples and sandwaves) under the action of combined waves and currents are presented. Laboratory measurements were conducted for a series of different experimental conditions. Fluid velocity measurements, surface wave characteristics and 3D mapping of the bottom were recorded with the help of acoustic sensors. Bottom records were statistically analyzed to obtain height, length and migration rates of ripples and sandwaves. Experiments were conducted for the mobility factor and the Reynolds wave number within the ranges 10<ψw <88 and 16×103 <Re <5×105, respectively. Measured ripple characteristics were compared with laboratory and field data and semi-empirical and analytical formulae from the literature. It was observed that ripples with different geometric characteristics may coexist at different locations over the sandwave. Sandwave characteristics (geometry, vertical growth rate and speed) were determined and plotted against dimensionless numbers such as the Reynolds wave number Re and well defined trends were observed. The sandwaves control the mean flow characteristics while the ripples provide a dynamic roughness along the sandwaves.