A Minecraft World That Comes With Its Own Mob Grinder

A few days ago, getting ready for 1.19, I started a new world. The seed is 7040754562661204920 (Nintendo Switch). Exploring the neighborhood, I noticed more zombies than usual - it turns out that water runs through a semi-exposed dungeon and zombies are constantly being washed downstream right past my back door. 

Spawn  point is wooded and close to water but mining straight down doesn't work because the land forms a rock wave over the deep canyon below. I'm trying to figure out the best way to take advantage of the 'zombie wash'.