The Influence of Physical Cohesion on Scour around a Monopile

We present experiments that systematically examine how the addition of physically cohesive clay to sand affects scour evolution around a monopile in a current. Repeated centreline transects are used to show the changes in scour depth and excavated material over time. Combined with 3D plots of the final equilibrium morphology, the results conclusively prove that clay content causes a progressive reduction in the equilibrium depth, excavated area and that timescales of scour increase with clay content. Winnowing of clay particles from the sand matrix is a pre-requisite for scour and differences in clay content influence the rate and extent of winnowing, ultimately controlling equilibrium scour morphology. The strong linear relationships between clay content and equilibrium scour parameters offers a simple index on which to modify existing scour prediction methods. It follows that improved predictions of scour development can reduce manufacturing costs and related logistical expenses of structure operations in fluvial, coastal or offshore environments.