"Definition and Purpose
A wet basin is used to capture and treat a specific volume of stormwater runoff. This structure has a permanent pool, and runoff from each rain event is detained and treated in the basin until it is displaced by runoff from the next storm. The permanent pool enhances the effectiveness of the basin by promoting biological uptake.
Appropriate Applications
Can be used to enhance stormwater quality and reduce peak discharges.
Are most applicable in residential, commercial, and industrial areas.
If constructed early in development of a particular site, become an effective means of
trapping sediment from construction activities."
PC-20 WET BASIN Definition and Purpose A wet basin is used to ...
http:///itd.idaho.gov/enviro/Stormwater/BMP/.../PC-20 Wet B...

"MODÉLISATION D'UN BASSIN D'ORAGE EN VUE DE L ...
http://modeleau.fsg.ulaval.ca/fileadmin/modeleau/...s/vallet...‎
by B VALLET - 2011 - Cited by 1 - Related articles
qualité de l'eau dans le bassin d'orage, pour permettre l'amélioration de l' ...... removal of pollutants in urban runoff with retention ponds, wet basins or wetlands ..."

Ici, il est crucial de traduire "wet", qui indique que le bassin est toujours en eau, à la différence d'un "dry basin" qui, lui, ne se remplit que lorsqu'il y a une crue ou un orage, et reste sec le reste du temps.
Donc "wet" -> 'permanent'.

""Dry" detention basin in the city of Waukesha, Wisconsin. A dry detention basin is designed to only hold stormwater runoff during and shortly after a heavy rainstorm, in contrast to a "wet pond" or wet basin which has a "permanent pool" of standing water."
http://www.flickr.com/photos/87297882@N03/8027724588/

"These basins are also called "dry ponds", "holding ponds" or "dry detention basins" if no permanent pool of water exists. Some detention ponds are also "wet ponds" in that they are designed to permanently retain some volume of water at all times."
http://en.wikipedia.org/wiki/Detention_basin

"Bassin de rétention (permanent ou provisoire)"
http://www.oai.lu/files/contrat/secteur_prive/OAI_03.2013_CT...

"entretien du bassin de rétention permanent"
http://www.argb.fr/document/20090414_argb_lettre_maire_docum...

"Retention basin - Wet Detention Ponds - Wet pond - Wetpool
Bassin permanent de rétention"
http://www.ouranos.ca/media/publication/18_Rapport_Mailhot_i...

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Note added at 8 hrs (2013-07-25 02:13:18 GMT)
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ou 'bassin permanent de rétention', selon les goûts :)