Chapter 23 - Disturbance of Port Operations Due to Long-Period Waves and Countermeasures Using Submerged Mound Structures

Abstract

In many Japanese ports it has been reported that long-period waves with periods of 30-200s can cause serious problems to cargo handling. To counteract such problems wave absorbing mounds installed on the harbor side of breakwaters have been constructed in some ports. The crown heights of these rubble mounds are almost equal to that of the caissons situated in front of them. Although such structures can be expected to provide some wave absorbing performance, the required width to absorb long-period wave can reach more than 30m. Given the cost involved in the construction of rubble mounds it is important to reduce their size, especially for the case of small ports, and to search for cheaper alternatives. In this chapter, a slightly submerged mound type long-period wave absorbing structure is proposed. A series of hydraulic model experiments were carried out to evaluate the wave absorbing performance and stability characteristics of the armor units against wave overtopping from the seaside. Such wave overtopping can be expected to happen often, due to the frequent typhoons that affect Japanese coasts each year. Throughout this study it became clear that submerged structures display a higher wave absorbing performance compared with those of conventional structures. Finally, this chapter proposes a formula to calculate the stability number for Tetrapods covering these submerged mounds and provides a design example of such a structure.