Nothing really to repair.
It's a storm drain line and it's really raining hard. When those lines (essentially penstocks at that point) have an obstruction that water can reverse direction, take the path of least resistance and will move an iron plate with great force to relieve the pressure. The pressure comes from gravity. It doesn't take much slope when you're dealing with miles of drain line to develop flow with the power of many 100's of kW.