will a bag of ready mixed concrete set ok if i dump it in a stream and soak the bag? (paper bag not plastic obviously!)

Theres a small stream in my garden in a channel and part of the bank has collapsed so i need an easy way of fixing it, and dont think i’m good enough at diy to start building shuttering. The stream is only about 45 sm wide, and is not a raging torrent or anything.

better filling sand bags with a dry mix and then stacking them up against the collapse

gabion ?

filling sand bags with a dry mix and then stacking them up

^ this

You can get hessian sandbags from your local building suppliers. You could probably transfer a bag of readymix or postcrete, lay it where you need it then either light spray with water or let it go off in the air on its own over a few days.

This is what I did for a couple of headwalls into a ditch. (but made my own dry mix)