Recirculating flume tank experiments reconstructed in TELEMAC3D-GAIA to model flow and sediment transport processes around an object in sand and gravel mixtures.

Abstract:

In the ever-increasing need for the installation of seabed infrastructures used for example to extract renewable energy and facilitate offshore centralised storage, differently shaped and orientated objects are employed, causing flow diversion and enhanced forcing of the seabed. These processes are detrimental to the integrity of the seabed and infrastructure, including cabling. We currently lack understanding of the enhanced bed dynamics around objects placed on mixed coarse beds. In sand and gravel beds, the shear stress needed to mobilise gravel can be up to 64% less than the shear stress required for pure gravel beds and the corresponding sediment transport rates can be up to three orders of magnitude higher.

Laboratory experiments were conducted, to better understand the interaction of the enhanced fluid dynamics and bed dynamics around an object, using a 10m long recirculating Armfield flume tank. A submerged cylinder was placed on seven different sand and gravel mixtures, to compare the effective mobility of differently mixed beds. Two unidirectional flow speeds were applied to the bed, experimentally tested to mobilise either just the sand (25.7 cm·s^-1), or both the sand and gravel fractions (39.6 cm·s^-1). The 3D bed scans gathered using an array of 5MHz SeaTek ultrasonic transducers at several stages of the experiment has allowed a quantification of the affected bed in the wake of the object, through erosion rates and ripple migration rates. Down-core sediment samples were taken, using a syringe. Analyses of the samples revealed the changing coarse fraction with depth as the bed evolved. These laboratory experiments were simulated using a coupled TELEMAC3D-GAIA model. The object in the model simulations was treated as a raised feature on the bed, to prevent the model treating the object as erodible sediment. Comparison of preliminary results from the model simulations and laboratory experiments showed that the overall geometry of the scour was good, but the model underestimated the scour depth and extent. To overcome that problem, increased Turbulent Kinetic Energy around the object, calculated in TELEMAC3D by the k-ε model, was used to improve estimations of the bed shear stress.