Complex sentence: As a tsunami moves into shallower water its speed decreases, its wavelength shortens and its amplitude increases enormously, behaving in the same way as a wind-generated wave in shallow water, but on a vastly greater scale.
Split into simpler sentences: As a tsunami moves into shallower water the wave is compressed and its speed decreases, its wavelength diminishes and its amplitude increases enormously. The advancing wave begins to tower in the same way as a normal wave but on a vastly greater scale.