It formed a 66 foot ( 20 m ) wave of water is a result that caught many off guard due to the time of night and only about 15 minutes to prepare , according to eyewitnesses who criticized the decision ; the flooding killed at least 20 people and left homes seven towns flooded . The deluge created a 66 foot ( 20 m ) wave of water that caught many off guard due to the time of night and only about 15 minutes to prepare , according to eyewitnesses who criticized the decision ; the flooding killed at least 35 people and left homes in seven towns flooded .