Changes in flow structures and energy reduction through compound tsunami mitigation system with embankment and lined piles

For mitigating tsunamis that overtop an embankment, flume tests were conducted to clarify the energy reduction of a tsunami due to a compound defense system by a combination of a sea embankment and lined vertical piles behind the embankment. The relationship between the flow structure and energy reduction was investigated with changing pile height, spacing, and number of rows. The flow structure was classified into 10 types, and the patterns were classified into two. The energy reduction increased with smaller spacing between lined piles, and the reduction of energy and fluid force behind piles gradually decreased when the piles were submerged. In some cases, energy reduction became very low or energy was locally concentrated because standing waves were generated behind the piles. Two important parameters were proposed, ResP*, consisting of pile spacing, pile diameter, and number of lines of piles in the downstream direction, and Ps*, comprising overflowing water depth, pile height, and embankment height. Ps* discriminates whether the piles are emergent or submerged, and ResP* classifies the change in the flow structure pattern. Recommended design criteria to reduce tsunami energy and to prevent the formation of the standing waves downstream of piles were elucidated by these two parameters.