Sediment transport patterns in front of reflective structures under wind wave-dominated conditions

This article presents a field study of the bed response near linear reflective structures under wind wave-dominated conditions at the Motril harbor (Spain) on the Mediterranean coast. In 1999 the harbor breakwater was enlarged 600 m by adding vertical caissons. Before this enlargement took place, no significant bed features had been observed in the area. However, at the end of 2004 high resolution bathymetry showed the existence of 3–4 bars parallel to the structure with lengths between crests of 0(70 m) in the cross-shore direction and heights of 0(25 cm). Toe erosion with an amplitude up to 25 cm was also observed along the structure. The necessary conditions for the appearance of the bar system are: (1) a standing wave pattern in front of the structure; (2) local satisfaction of the wave-height and wave-period threshold limits for sediment mobility. Using an energy-based approach, we analyze the time series of sea states with sediment transport capacity after breakwater construction. The cumulative distributions of the shear velocities related to WSW–SW sea states show that all the bars have a greater probability of being N-type features (predominant bed load) under generalized sediment transport conditions. These conclusions were verified by matching nodal lines of standing WSW–SW wave patterns with the bar locations. The results suggest that the Motril bar system of Motril is possibly generated by the standing wave patterns induced by several sea states with peak periods of O(10 s) and significant sediment transport capacity resulting from WSW–SW severe storm events that occurred in Motril after breakwater construction. This article highlights the need to verify in field conditions the present threshold limits for sediment transport mobility.