The previous tsunami gave everything in this town a good wash. There are many Laundromats in this town so when the previous tsunami hit, all of the detergent mixed with the water and gave everything a good wash.
All the Laundromats in town are owned by centaurs (せん), and they got a good wash as well. Being half horse, centaurs have a lot of hair that needs washing. Luckily, the soap tsunami gave them a good wash too.