Waves are reflected whenever they move into a region where their speed of propagation changes. It has generally been found that for appreciable reflection a large change in speed of propagation must occur over a distance which is small compared to a wave length....
Cochrane, J. D., and R. S. Arthur. 1948. "Reflection of tsunamis." Journal of Marine Research 7, (3). https://elischolar.library.yale.edu/journal_of_marine_research/664