Laboratory investigation on the runup of leading-depression N-waves on a uniform slope

Abstract

Tsunamis are sometimes observed to lead with a wave trough, causing the shoreline to recede before the main tsunami wave arrives and floods the coast. The widely adopted benchmark wave, the solitary wave, is unable to capture this drawback phenomenon, amongst the many shortcomings in using the solitary wave to mimic a tsunami. To investigate how the drawback phenomenon affects wave runup, in this study we performed laboratory experiments using leading-depression N-waves (LDNs). A 1:10 uniform slope was placed in the wave flume to study wave runup. Emphases were placed on how the runup of LDNs deviate from the runup of solitary waves of the same amplitude. By adopting consistent and parameterized methods for generating LDNs and comparing them with solitary waves, we successfully categorized LDNs into three types and explained their runup trends. Based on the 88 cases of LDNs realized in the laboratory experiments, we found that the leading depression tends to facilitate and intensify wave breaking, hence lowering the runup of breaking LDNs. On the other hand, since the leading depression tends to increase wave front steepness, steep nonbreaking LDNs may cause higher runup than solitary waves. Overall, we observed that the runup of small-amplitude LDNs can potentially be larger than the runup of solitary waves, whereas the runup of finite-amplitude LDNs has the same order of magnitude as the runup of solitary waves. As real tsunamis are generally small-amplitude waves, our laboratory observations suggest that using solitary waves as opposed to LDNs to mimic tsunamis may underpredict the runup.