This is a local or state call, depending on the state.

Most municipalities near me don't let you count detention ponds in floodplains for volumetric floodplain compensation. Conceptually, if the detention pond is there to hold back additional runoff from a development project, then that storage is take up with the additional site runoff during the storm, and therefore not available for flood storage during a flood. Considering the pond to be "full" is the more conservative and reasonable approach. Some less sophisticated municipalities don't care. Some places, such as South Florida, have vastly different rules entirely since the entire region is in the floodplain.

That said, if you have a long linear pond in a floodplain, and the flood overtops the dam by a significant margin, it's possible the pond itself could be considered flood conveyance, and its effects might be reflected in the HEC-RAS run for the river. Sometimes I see detention ponds in floodplains called out as ineffective flow areas in the HEC-RAS run. Sometimes not. I think it's generally a judgment call. If there is a FEMA rule on the matter, I'm not aware of it.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

maybe the water is deeper than surrounding river/lake.

Dik