The water level rose quite slowly at first, but kept rising and moving farther inland. The energy of the flow carried cars and building along with it. My image of a tsunami had always been that a towering wave crashing on a town, but it was a surge rather than a wave. While a 27-inch tsunami would do far less damage, it could be a great deal more damaging than your average 27-inch ocean wave, which breaks harmlessly on a beach and immediately withdraws. That's what I was trying to say. Structures along streams could be seriously damaged.
My wife's family had a farm and numerous buildings near a small river located between the two power plants in Fukushima. They had lived there for several hundred years. There was nothing left of any of the buildings except for concrete foundations. The people managed to escape in time. They arranged for me to accompany them on a visit to the site. The only thing remaining intact was the family graveyard on the top of a hill. Our time there was strictly limited, and we had to wear disposable protective clothing and overshoes to avoid carrying any radioactive soil with us when we left. The background radiation was quite low. It was interesting to see how nature had taken over.
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