4. Summary and conclusions
4.1. Model successes, limitations and areas for future study The experiments presented in this work show the functionality of the Hybrid Tsunami Flume at Ujigawa Laboratory in creating complex wave conditions similar to those observed during the 2011 Tohoku Earthquake Tsunami, characterized by a solitary wave profile superimposed on a longer period water surface rise. Effects of macroroughness elements in reducing critical pressures on a shielded specimen are also explored. The work here provides insight into important relationships between offshore wave conditions, onshore macroroughness configurations, and wave-structure interactions; however, experimental limitations and idealizations must be addressed so that future experiments can continue to refine the state-of-the-art in physical modelling of tsunami hydrodynamics. One limitation of the current experiment is the idealization of the typical Japanese house as a rigid rectangular prism. As discussed above, the specimen was weighted to the bottom with sufficient weight to prevent movement due to the wave impact; thus only horizontal loads are considered in this study. A real tsunami will generate horizontal loads as well as vertical loads on the underside of structures; many homes in the inundation region during the 2011 Tohoku Earthquake Tsunami were washed away due to the tsunami wave (Gokon and Koshimura, 2012). Likely, the combination of the horizontal impact force and vertical buoyant forces caused these failures. Further, upon failure, a home will become a threat to shoreward homes due to potential debris impact. Debris impact was not considered in experiments; however strong foundation connections and consideration for uncertainty due to debris loading will be considered in future experiments. Another consideration is the physical model's ability to represent a prototype tsunami. Comparisons with GPS data obtained from the 2011 Tohoku Earthquake Tsunami indicate that experimental measurements successfully represented the tsunami profiles of GB801 and GB803 (Kawai et al., 2013). However, as Madsen et al. (2008) showed, the short period solitary wave does not accurately represent the time scale of a real-world tsunami. While this work addresses new data that has shown short period waves embedded in a longer-scale water level rise, the mechanically-generated wave still scales to a prototype time shorter than those of real tsunamis. However, tsunami design is regulated by tsunami water level without consideration of the total period and time-dependent load, and video analysis of the Tohoku Earthquake Tsunami showed that the first impact of the tsunami wave on the building's wall could be extremely strong and cause structural failure. Therefore, creating similar maximum wave heights using a combination of wave generation mechanisms marks a step forward in physical modelling of complex real-world events. Future experiments will focus on additional combinations of pump-generated flow and mechanically-generated waves to test the facility's ability in generating longer period waves. Finally, laboratory experiments and numerical models seek to better understand tsunami onshore propagation in order to ultimately design structures to resist tsunami and wave-induced forces. However, design codes that present equations for estimating the maximum force on a structure often must make simplifying assumptions about the maximum tsunami-induced pressure. Therefore, understanding of extreme pressures is an essential starting point for coastal engineers in estimating wave impact loads and the effects of macro-roughness in reducing these loads. Further, extreme pressures must be considered when designing a structure to prevent local damage. Much work has been done to estimate wave-induced forces on structures (e.g. Morison et al., 1950; Ramsden, 1993; Bradner et al., 2009; Fujima et al., 2009; Al-Faesly et al., 2012; Thomas and Cox, 2012; Kihara et al., 2015). While detailed analysis of forces caused by the waves generated in the current experiment and their application to design guidelines is beyond the scope of this work, full integrations of pressure measurements are the subject of current study and will be used to examine the reliability of design equations (e.g. JCO, 2005; ASCE, 2016).
