Looks like a part of the back of the dam eroded away over night. I have looked carefully and there is no water leaking out of the area at this point but I obviously want to get right on it once I can get out there and work.
I was thinking of installing a makeshift wall (vertical 4x4s with horizontal 4x4s in front of them) and then filling in the void.
Really appreciate any help/guidance on this. Thanks.
If that much eroded that fast, it's hard to believe that the water didn't breach the dam or that you have a big underground sink hole. However, if you are certain that neither are the case, and the backside is steep enough to cause a "mudslide", your next question should be why was the ground so soft to let it slide. Loss of vegetation or leak thru the dam?
I think you could build a retaining wall of sorts with those 4x4's you are talking about. I bet the ground where the posts go is soft, which means you are prone to the wall falling over in time. I would look into using a method where there are braces(hooked to your wall) going back into/underneath the fill area that are hooked to other pieces that run parallel to the wall. As the dirt pushes against the wall, the weight of dirt that is on top of those braces help keep the wall straight. I am sure there is a builders term/name for this.
But, considering all that work, why don't you just fill in more dirt back there so the grade is less steep, perhaps even mowable?
And second, better address why its so soft there in the first place.
