when the scouring depth reaches the lowest edge of a structure (usually, the sheet pile), waves can infiltrate the body of the structure and flow out the backfilling materials; this flow of the backfilling materials is called outflow or suction. Consequently, since the supporting materials for the shells (covering parts of the structures, such as the crown part, the seaward slope, and the landward slope) flowed out, the structure become weaker and are easy to be damaged by waves smaller than their design wave. An illusion of an outflow mechanism is shown in Figure 1. Since damage induced by erosion, scour and outflow is high, it is important to carefully consider the effects of them when designing a coastal structure constructed in a very shallow area.
Figure 1. Beach Profile during Erosion
