is it necessary to place 2D storage upstream of a dam in 2d modeling?
Upstream of a dam would be a reservoir, and I am trying 2D simulation to see the effect of gate operation when flood impinges onto the reservoir when it is full.
Depends if you have a hydrologic model that represents the same passage of flow. But if you intend to breach and consider the variety of breaches RAS has to offer than the structure must be built in RAS
You can model the reservoir with either a storage area connected by a storage area connector to a downstream 2D area or the entire thing in 2D with the dam represented with a internal storage area connector.