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Edge scour at scour protections around monopiles in waves

The foundations of offshore wind turbines are typically designed and installed with scour
protection to prevent removal of the adjacent sea bed. The design of scour protection usually
considers the three common responses of scour protection: (1) instability or removal of cover
stones; (2) suction removal of the sediment beneath the stone layer and subsequent sinking of the
scour protection; and (3) edge scour resulting in loss of edge material of the scour protection.
The results of an experimental campaign to explain the edge scour process in waves is
presented. The flow field around the scour protection in waves were investigated by particle
tracking and velocity measurements by LDA. The governing process in waves is the combined
effect of the following two mechanisms: (1) the roughness-induced streaming, caused by the
sudden change in surface roughness from the cover stones to the sediment bed; and (2) a short
supply of sediment on the onshore and offshore side of the sour protection relative to the wave
direction, causing a scour hole downstream or upstream of the scour protection.
The results from the experimental campaign are presented in design diagrams for the
effective use the design of scour protections around monopiles.