[Tagging] tagging floodplain

Colin Smale colin.smale at xs4all.nl
Mon Sep 26 09:38:02 UTC 2016

On that page it says: 

 	* In the very common case of rivers subject to seasonal flooding with
much higher water levels for a relatively short period for example
during monsoon season or snow melt and low and relatively constant water
levels for the rest of the year it is best not to map the water extent
during flooding. A useful rule of thumb would be to map the median of
the maximum water levels of all months of the year. For those areas
water covered during floods there are some ideas for tagging like
Proposed features/floodplain [1] and Key:flood_prone [2].

The suggested "rule of thumb" will of course mess up all the rivers in
low-lying areas in OSM, not to mention the difficulty of ascertaining
the location of the line to be drawn (daily aerial photos over a year?).
So it doesn't really deal with the problem at all. 

And on the floodplain proposal it says: 

"There should be some possibility to provide information for this. The
tags should describe who has delineated the flood plains, how is the
probability for flooding, is the floodplain evaluated to be "red"
"green" or whatever zone (this depends on national or regional
regulations) and which type of hazard (torrent, mud-slide, flash flood,
low-land river). Eventually this could be extended to cover avalanches."

I.e. someone (government?) needs to delineate the area, do the risk
calculations and make the classifications. This is potentially open to
political "influence," to prevent or enable development in a particular
area or to justify decisions on flood defences. That is what I mean with
"designated flood risk zones" not being equal to "flood plains". The
former can be mapped - from definitive data with an appropriate licence.
The latter is controversial because of the indefinite boundary, and
belongs in the same category as mountain ranges, where we also don't
have a way of drawing a sharp boundary. 


On 2016-09-26 11:02, Martin Koppenhoefer wrote:

> sent from a phone 
> Il giorno 26 set 2016, alle ore 00:58, Colin Smale <colin.smale at xs4all.nl> ha scritto:
> Problems of defining a boundary exist with river banks too.  
> That is rather obvious - the area which is covered by water can be a continuum, depending on the conditions at any given moment.

yes, that's why there are recommendations how to deal with it:

Tagging mailing list
Tagging at openstreetmap.org

[1] http://wiki.openstreetmap.org/wiki/Proposed_features/floodplain
[2] http://wiki.openstreetmap.org/wiki/Key:flood_prone
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