"a flash flood can resemble a tsunami". The classic southwestern flash floods do very good tsunami impressions.

One minute you have a dry arroyo. There is a rumbling sound and a wall of water descends the drainage - quite literally dry one second, and flood water the next. The tallest wave I have witnessed was about five feet high.

Hikermor and James_Van_Artsdale make a good point. San Antonio, Texas and Lima, Peru both occupy arid or desert locations. Dry enough that many areas in both cities have minimal or non-existant storm drainage. Yet, both cities have registered drowning deaths due to flash-flooding in the last 15 years. One woman was a close friend of my sister.

During the time I was doing SAR around Tucson, AZ, drowning was a leading cause of fatalities, far ahead of hyperthermia, and close to falling. Typically people drove around barricades closing off flooded roads. The high wave front I witnessed roared downstream through a narrow canyon filled with people, killing seven almost instantly. It took us a week to find them all.