If Concentration is maintained, the only thing that ends Tsunami is when the wall reaches 0 feet in height.
This is a very tricky question with no direct RAW answer that I can find in the Handbook nor tweeted about. What follows is the set of rules I believe should be followed in this instance; and what is intended by the spell, or what we call RAI.
The spell Tsunami as per PHB pg. 284 can last up to 6 rounds, gradually decreasing in height by 50ft and damage each round it continues. Unlike most other spells, the Tsunami 'wall' moves 50ft away from you each round, no questions. It does not move 'up to' 50ft. It moves precisely 50ft and only 50ft. Which means it does not stop upon impact with a creature, or object, and any Huge or smaller creature actually gets swept up IN the wave if it fails a strength check. The only circumstance that ends the Tsunami spell other than a loss of concentration is "When the wall reaches 0 feet in height, the spell ends."
So what happens if a large barrier is in the way? Considering the possible height, width, thickness and pressure exerted upon impact (5d10 Bludgeoning damage) of such a Tsunami wave, I would say the wall in question that poses as a potential barrier for the Tsunami wave is subject to the pressure (damage) exerted upon it. I don't believe walls can make saving throws, so using the rules on DMG pg. 246 - 247 on Object Statistics depending on the size and material of the wall, the Tsunami would either destroy the wall and continue moving it's 50ft, or the Tsunami damage would not be sufficient enough to destroy the barrier (damage threshold) and stop forward movement.
If forward movement stops, the spell does not end unless the above mentioned prerequisite is met. The wall reaches 0 feet in height. So each turn that concentration is maintained, the wall tries to move forward, again exerting its pressure upon the boundary, damaging any creatures caught inside, and dropping by 50ft in height and 1d10 damage. Once the height reaches 0, the spell ends.