What should I do if a neighbor’s water drains onto their property line, but in turn, causes damage to my yard?

The neighbor’s property line actually runs from my neighbor who lives in an elevated position, so water naturally falls and drains into their property line which is a small area of land above my terrace. However, they have extended drainage outlet (flexible plastic drain extension)and so when it rains all of the water creates a mini-mudslide into my terrace due to this addition they made. What should I do and who is responsible for the damage?