A house I own (currently being rented, but we're planning to move in next year) has a stream that runs along the front of the property and continues down past my neighbor's house. This is in a rural wooded area and the stream carries water from a hillside behind, so its normally calm, but after a rain, will swell up considerably. This past week there was a big storm, so the culvert (oval shaped, between 3-4' diameter) got blocked up with branches, so it overflowed and a lot of it went along the street (road department came out to scrape mud/debris off the road) and washed back to the stream where his driveway /culvert is. This is probably a one time per year event or less, but due to previous flooding problems which finally resulted in the driveway getting washed away, the previous owner of my house decided to cover the culvert with concrete to prevent erosion, but my neighbor just has gravel on his.
In this recent storm, my neighbor unblocked my culvert when flooding was noticed in the street, fearing (with good reason) he would lose his driveway if he didn't act immediately. He ended up needing a truckload (a ton, $20) of gravel to replace what got washed away. I offered to do this for him, as I wasn't sure what the norm should be and he seemed to be suggesting responsibility on my part, but he declined (appreciative of the offer) and got it himself. We met up yesterday to discuss future precautions, and he seems to think my property is the problem and I should make a berm (20 feet long and 2 feet tall) along the edge (before my culvert) to prevent the water from overflowing onto the road and better direct it along its intended path. Given the volume of water, which I have seen nearly fill the culvert some of the times I had been at the house working, I'm not sure if this will work or not. I wasn't there during this past storm, so I don't know whether having it better channeled before my culvert would have worked. My neighbor says he checked with the road department, and both culverts are out of the street's right of way, so they belong to us individually.
My opinion is that shit happens and this isn't a very common event, so mitigating it with speculatory precautions versus cheap repairs isn't worthwhile. My house is 30 years old and we've owned it for less than a year, and he has owned his for the past 10 years, so this issue has definitely come up before, but I don't know how it was handled. My suggestion was to pour concrete over his pipe, which shouldn't cost more than $500 if the two of us worked on it, but he's not convinced.
For a good neighbor relationship, I don't want to be the asshole and cause disgruntlement, but on the other hand, I don't think I should have to try and appease him (at my cost and labor) for something I don't think is my issue. If this situation has been as-is for 30 years, shouldn't he handle the erosion problem at his pipe? As for the culvert getting blocked up, I wasn't able to take care of it myself this time, but we plan on moving into this house in the next year, so I should be able to control that in the future.
Is this something I should be worried about? Is the overflow my responsibility? What would you do?