Burial of short cylinders induced by local scour and bedform migration under waves plus currents

Experiments were conducted to study burial of cylinders placed on a sandy bed under combined waves and currents. The burial of a short cylinder was determined by local scour around the cylinder and by a more global process associated with the presence of ripples and sandwaves. The role of bedform evolution on the burial of short cylinders was studied. For this purpose, a series of detailed measurements of geometric and migrating characteristics of bedforms under combined flows was conducted. Under certain hydraulic conditions, ripples with different geometric characteristics may coexist at different locations over the larger bottom features like sandwaves. Superposition of bedforms play and important role in the distribution of the turbulence structures and local flow resistance which in turn dictates their geometric and migrating characteristics. After they form, sandwave characteristics (geometry, vertical growth rate and speed) were determined and plotted against dimensionless numbers. Sandwaves are shown to control the vertical displacement of the specimen. It is seen that the vertical displacement of the cylinder no longer coincides with the burial depth. This is primarily due to the more or less exposure of the specimen to the flow action according to its relative position over the bedforms as they migrate. Observations indicate that existing semi-empirical formulae for prediction of equilibrium-burial-depth, scour hole geometry and time-scales for pipelines are not suitable for the case of a finite cylinder. In the present experiments, burial depth, under local scour, shows stronger dependency on the Shields parameter theta than on the Keulegan-Carpenter number (KC), while burial-time scales seem to be influenced by both KC and theta. New predictive formulae for dimensionless equilibrium burial depth, length and width of the scour hole around the cylinder were developed. These new expressions will allow quick estimation of cylinder settlement under combined waves and currents in coastal environments concerning the cases of burial associated to the so-called local scour. Sets of trends and semi-empirical geometric and migration characteristics of ripples coexisting with sandwaves were developed. The effect of fluidization on the bed on the burial of the studied objects is also discussed and some examples are presented.