放 means to abandon, to release. e.g. 放水 means to (turn on the tap and) let water flow out.
流 is to flow.
放流: let (the water) flow out (onto the ground, as in this sentence).
I think 放流 is a word created by the writer by combining two characters together, that's why you cant find it on dictionaries.
挖 = dig, 坑= hole
挖坑= dig holes