When a tsunami reaches shallower water, the base of the wave slows due to friction with the land. All the water catches up and piles upon top, as the wave breaks onto land.
It engulfs the land totally with water got to you tube and see what the last tsunami did
A tsunami though usually not to tall when traveling on open ocean will rear up and loom larger and higher as it hits shallow water.
When the tsunami hits land; it's waves which were previously small and fast travelling, grow in shallower water therefore the waves become larger as they reach the shore this could be up to three times as big as the orgininal waves.