Scouring patterns in front of vertical breakwaters and their influences on the stability of the foundation of the breakwaters

Intending to determine the shapes and sizes of the scouring profiles in front of vertical breakwaters, model tests have been conducted in two flumes with four different grain sizes in the Laboratory of Fluid Mechanics, Faculty of Civil Engineering, Delft University of Technology. Most of the test runs were done with regular waves, and a few with irregular waves. It was found that the different types of scouring patterns depend not only on the sand grain sizes but also on the wave conditions. After discussing the critical velocities for sand particles and the sizes of the sand ripples under standing waves, two criteria which may distinguish between two basic scouring patters are presented. The geometrical shape of the scouring profile, the ultimate scouring depth and the development of the scouring depth as a function of time for relatively fine material is presented. The mechanism of sand transport is discussed. A theoretical bed profile derived from the equation of continuity is quite similar to the measured profiles, which may in turn prove the assumption that the relatively fine material is moved in suspension by the mass transport current from the node towards the antinode under standing waves. The ultimate scouring depth for relatively coarse material is described. Finally a comparison is made between the results from the tests with regular and irregular waves.