Erosion of drains next to our school.

We have been looking at the erosion of the drain walls next to our school. We noticed that the drain right next to the school had little erosion yet lots of agapanthuses plants. The drain over the road looked a lot worse and even some of the fence posts were just hanging. Not much vegetation was on that side apart from Leaderbrand lettuces a long way inside the fence. Could this be the problem??