Add waterway=flowline to waterway tags representing directional water flows

[waldyrious]

Currently, lines with waterway=flowline are represented as generic waterway features, and don't get the arrows pointing in the water flow direction along their length (as is the case with rivers, streams, canals, etc.). This change adds the tag so the representation is consistent with the other flowing waterway tags.

[JesseWeinstein]

It might be worth making them slightly visually different, just so it's easier to tell if a waterway thru a water body has been converted yet. Maybe a slightly different shade of blue?

[tordans]

For easy reference: https://wiki.openstreetmap.org/wiki/Tag:waterway%3Dflowline , ~800 usage globally.

[waldyrious]

The wiki sounds like this is something that indicates a direction as well?

Does it? I read the opposite:

Unlike most waterway=* values, the tag does not model a discrete physical feature and does not imply a flow direction.

(emphasis mine)

[waldyrious]

It might be worth making them slightly visually different, just so it's easier to tell if a waterway thru a water body has been converted yet. Maybe a slightly different shade of blue?

Sounds good to me, but I didn't want to do too dramatic a change. Happy to add the change if maintainers agree it makes sense.