Piling-Up behind Low and Submerged Permeable Breakwaters

An experimental study of the phenomenon of piling-up behind low and submerged breakwater is presented. Piling-up occurs in completely enclosed areas or behind two-dimensional breakwaters as a result of overtopping of the breakwater. The water that spills into the protected area is accumulated there until the mean water level inside the protected area is higher than the man sea level outside. This difference in elevation, or height of piling-up, reaches eventually a value sufficient to cause a mean outflow of water through and above the breakwater equal to the mean inflow by water overtopping the structure and spilling into protected area. For a given breakwater and mean sea level, the height of piling-up was found to be an increasing function of the height of the waves in the deep sea. Expressing values relative to this wave height, the relative height of piling-up was found to be a function of the relative depth of submergence, or of the relative height of protrusion of the breakwaters. The maximum value of piling-up for the structures tested was found to be of the order of 60% of the wave height. Values of this magnitude were found for low breakwaters with crest elevations above mean sea level of between 50% and 90% of the wave height.