natural=shingle is probably better, although if you really want it to be a beach, that probably would work too.  I don't really think of a small area of pebbles next to a small stream as a beach, and the wiki seems to agree.  My main concern was whether this was real. We have been having some problems with people adding fake beaches as Pokemon Go now uses that feature for something that matters to those who play that game.  Thanks for your understanding