Vithana, HPV; (2013) The effect of stone protrusion on the incipient motion of rock armour under the action of regular waves. Doctoral thesis , UCL (University College London).

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Abstract

Rock armour is often used to protect the seabed around offshore structures like oil platforms, wind farms, undersea cables etc. This thesis describes a laboratory study to investigate the effect of stone protrusion on threshold movement of rock armour under regular waves. Tests were carried out to investigate the incipient motion of light weight test spheres of differing density and diameter in the range, d = 9.5mm-31.8mm, resting on a rough bed of 19mm glass marbles in a wave flume. In past studies, researchers have used turbulence measurements, shear plate apparatus, hot film techniques etc., to quantify the bed shear stress. In the present study, shear stress was deduced from direct measurements of pressure on the surface of a 50mm spherical bed element. Advance flow measurement techniques such as Laser Doppler Velocimetry (LDV) and Volumetric Three-component Velocimetry (V3V) were also carried out for flow measurement and visualisation. It was found that the Shields critical shear stress increased when stone protrusion was gradually reduced following an exponential relationship. For each wave period a different Shields shear stress versus protrusion curve was obtained. When the wave period increased the curve shifted towards that for currents previously obtained by Fenton & Abbot (1977) and Chin & Chiew (1993) suggesting that for longer wave periods under the field conditions where high Reynolds/KC number flows exist, the curve obtained for currents is applicable. The method of rock armour placement crucially influences the stability of a bed protection. Significant reduction in bed damage can be achieved by placing stones to an optimum protrusion level of 0.2d above mean bed level. A model bed protection made of crushed natural rocks (anthracite) showed that the damage to a “levelled” bed is 50% less than in a randomly placed bed. This is because the fraction of the exposed stones increases when rocks are dumped from a barge or a side stone dumping vessel as opposed to reduced exposure observed in a levelled bed.

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