expanding on what I said. the raked lines are supposed to represent water. a lot of times you'll just see it in straight lines like that, sometimes the lines will be curved and follow the contours of the garden and any features in it like rocks which represent mountains. in some gardens though, you'll see the design of the raked lines be in swirls like actual waves.
pretty interesting stuff, it's been a while since I learned this though so my information might be wrong