Sea level change and global warming may cause increasing of external gorses due to tides, currents and waves acting on breakwaters. Counter-weight block is newly developed to increase the resistance against horizontal sliding of breakwaters located at entrance of harbors. In the paper, a physical experiment is carried out to evaluate the friction factor of the counter-weight blocks and to compare with a normal type flat concrete block. The friction factor of counter-weight block with rubbles inside the center hole becomes larger than the normal one.
Global warming reported in IPCC 4th report (IPCC, homepage) suggests sea level rise about 50 to 60cm in the next 100years. It also predicts increase of typhoon size and sea wave heights and periods. Hiraishi et al. (2010) already suggested increase of probability of occurrences of accidental waves. Moeini et al. (2010) discussed numerically on extreme huge waves. The accidental waves are defined as waves that their heights or periods becomes larger than the design wave conditions. In Japan, 50 year return period waves are usually employed in the external forces acting on coastal and offshore structures. Present design waves are calculated mainly using wave data obtained in the wave observation and wave forecasting in middle of 20 century. Climate condition varies rapidly these days and the design condition of coastal and offshore structure should be improved according to the future prediction like sea level rise and gigantic typhoon. Improvement of present maritime facilities, however, is very difficult because of its high cost and influence to maritime environments. Therefore, simple and low-cost techniques to increase the resistance of breakwaters are widely demanded. We propose a new counter-weight block method to increase the horizontal resistance of present breakwaters and to keep stability of them against an accidental wave force occurring in near future.