## 1 Introduction
Furthermore, WECs can be classified by the working principle into pressure differential devices (e.g., OWC devices and Limpet [14]), floating structures (e.g., Pelamis and PowerBuoy), overtopping devices (e.g., sea-wave slot-cone generator (SSG) in Norway, [25]), and impact devices (e.g., Oyster in Scotland [18]). Impact devices are structures generally installed perpendicular to the wavefront (Terminator type) and composed of a sea-bed fixed flap. As these devices can directly absorb wave energy, the most considerable mean annual absorbed wave energy (kW) and the annual absorbed energy per characteristic surface area (MWh/m\({}^{2}\)) are obtainable in comparison with other types of WECs [26]. Wave power generation technologies in Japan have been developed by a pioneer, [PERSON], who was the father of modern wave energy technology in the 1940s [27], and Mighty Whale floating OWC, which succeeded it in the full-scale test in the 1990s [28]. In contrast, except for the Muroran Institute Technology, which developed the Pendulor device [29], and Gyrodynamics Co Ltd., which developed the gyroscopic WEC [30], the leading position has been lost in recent decades [9][PERSON] In comparison with the United States and Europe, the wave power in Japan is not so great. However, the availability of wave power is high owing to its proximity to the sea, when suitable WECs match the Japanese marine environment. In Japan, caissons, breakwaters, and seawalls have been constructed near and on the shore not only to protect coastal areas from typhoons, high waves, and coastal erosions, but also to maintain calmness around harbors. These coastal structures play a vital role in reducing the wave energy flowing into the coast by dissipating the incoming wave energy and reflecting it back to the open sea. When these structures are harnessed by themselves and the hydrodynamics around the structures, lower cost and higher power generation efficiency are expected. The hybrid system of WECs using the coastal structure is an ongoing project worldwide including already developed devices with real scale or prototype tests, occasionally named \"new green harbor breakwater\" [31], e.g., an OWC integrated into a caisson [10; 32; 33; 34; 35], and an overtopping device (OTD) integrated into a breakwater [25; 36; 37; 38]. In contrast, there is less attention being paid to a hybrid system of WECs as an independent structure of WECs installed outside of structures rather than inside. [PERSON] et al. [39] mathematically analyzed the power performance of an oscillating wave surge convertor (OWSC) in the presence of an infinite vertical wall presuming a straight coast behind the OWSC. They demonstrated that the wave energy capturing performance was 0 when the flap was positioned at the anti-node of the standing wave formed by the vertical wall and was at its maximum at the node. They also indicated that the capturing performance of the wave energy could be up to four times larger than when there was no vertical wall behind the open sea. Additionally, the power performance of the OWSC with breakwater behind it was more than double the efficiency of the capturing wave energy when the width of the breakwater was twice that of the OWSC. Thus, it was clarified that it is optimal to have a width of 1.5 times that of the OWSC when considering economic efficiency [40]. An OWSC is a terminator type WEC installed perpendicularly to the wave front and is thereby expected to have not only a wave power generation function but also an effect on the attenuating wave energy coming from the open sea. The seabed-fixed OWSC, which can absorb a large amount of wave energy from the ocean and the coastal structures, is appropriate from the viewpoint of the improvement of power generation efficiency in Japan where wave energy is relatively small. Furthermore, it is also possible to reduce the installation and maintenance costs by using breakwaters as bases for the power take-off (PTO) systems corresponding to the secondary conversion stage of the energy conversion [3].