Hello @geozeisig, @HAmap and all others,
Wiki about Tag:basin=detention: “A special basin used for detention of rainwater before being discharged into a surface water body. The basin is normally not filled with water.”
I miss a statement about intermittent=yes.
Should intermittent=yes be added as a requirement or is it implied?
We have several big ones here and more under construction, used as farmland and this one along the torrent has not been flooded since construction years and years ago (the last big flood was 2012 which set the multiple constructions in motion). Not tagged them as waterbodies!
After discussion on these forums left the tags as
flood:overflow took off in about 2016 and then went utterly dead.
From my personal rendering point of view, I have three levels of intermittent:
- flood prone
- intermittent
- water all the time
I interpret OSM tags to decide which category things go in, so “flood prone” is set by these, and basin=detention is included in that list.
Other people may of course have different ways to represent water. I wouldn’t have expected an explicit intermittent=yes tag to be needed on basin=detention because it’s implicit in the tagging of the object.
And some very ocassionally flled ones in Poland are tagged as water bodies: OpenStreetMap
top one even is named “dry anti-flood reservoir…”
i guess that in absence of tagging data consumer may assume it.
But adding intermittent=yes or seasonal=yes explicitly is perfectly fine.
Though not adding it is not wrong either and probably can be assumed to be implied?
You may want to look is there any basin=detention with intermittent=no or seasonal=no
They should maybe be tagged as retention Tag:basin=retention - OpenStreetMap Wiki
Good idea! … I could have thought of that too 
At the moment we have a total of 183 relations, 19.040 ways and 19 nodes with basin=detention. Of these:
- 9.000+ with
intermittent=yes - 200+ with
intermittent=no - 40+ with
seasonal=yes - 2 with
seasonal=no - 30+ with
intermittentandseasonalTag - 29 with
intermittent=yesandseasonal=yesTag - 2 with
intermittent=yesandseasonal=noTag - 9.800+ with no
intermittentand noseasonalTag
If neither intermittent=* nor seasonal=* is tagged, then intermittent=yes is implied because that is the (numerically) normal case and corresponds to the definition “The basin is normally not filled with water”?
Otherwise is intermittent=no or seasonal=* required?