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I may be using the wrong term then, it's essentially a big, open, concrete lined stream.
Gardens go right up to the edge.
Fairly comfortable with digging down, installing a retaining wall then back filling. My main concerns are A) what kind of wall? Will concrete posts and gravel boards work? B) I'll have to be careful not to undermine my neighbours fence. There's a small tree that might have to go and removing that could fuck their fence/retaining wall.
Edit: open culverts exist
I'm not worried about the strength of the culvert, this is about 10 houses down.