[Tagging] Cleanup flood tagging

[Lukas Sommer]

Hello.

In OSM there seems to be concurrent 2 tagging systems for flood
tagging. And each of them seems to be incomplete.

Just to remember: There are several types of flood. I think the most
common types are:

– case 1: caused by heavy rainfall: “on flat or low-lying areas when
the ground is saturated and water either cannot run off or cannot run
off quickly enough to stop accumulating” (from Wikipedia article
“flood”). Typically various times during one year – and quite common
in some African countries during the rain season.

– case 2: caused by overflow of water of a riverbed: not directly
related to rainfall at this spot (rainfall can be hundrids of
kilometers upstream, causing a river overflow hundrids of kilometers
downstream). Typically maybe one per year or maybe each two or three
years…

In OSM we have flood_prone. The wiki descript fits most with “case 1”:
“ this is mostly applicable to roads that go under water after heavy
rains”; so combining it with highway=* on OSM ways (alternatively on
nodes within a highway) would be the typical use case. We have
currently 204 nodes and 10 582 ways.

In OSM we have also hazard_type=flood. The wiki description fits
mostly (but not exclusivly) with “case 2”. Tagging areas as
independent OSM elements seems to be the typical use case (no
combining with other objects). We have currently 1 node and 56 ways.

hazard_type=flood comes from OpenHazardMap – but OpenHazardMap wiki
page hasn’t been updated since one year. OpenHazardMap seems to focus
on landslide (hazard_type=landslide has 3 169 comparing with only 57
elements for hazard_type=flood).

Questions:

– Shouldn’t we deprecate one of these tagging? If so, which one?

– Shouldn’t we introduce a possiblility to distinguish river
overflowing floods and directly rainfall related floods?

– Only supplementary tagging of highway=*? Or independent OSM elements
(areas)? Or both? Probably both makes sense.

Suggestions?

Lukas Sommer