Experiments on the flow in a enlarging compound channel

The paper presents an experimental investigation of the flow in a compound channel with symmetrically enlarging floodplains. These results complement previous work on symmetrically narrowing floodplains. Both these data sets enable a better understanding of the mass and momentum transfer occurring in non-prismatic compound channels, without facing the complexity of a fully meandering geometry. The new experiments show that, in this diverging geometry: (1) the flow expends on the floodplains with a significant shift in the downstream direction, with the discharge on the floodplains significantly lower than their conveyance capacity in a prismatic compound channel with the same cross section; (2) the lateral distribution of velocities presents a large gradient from the interface between subsections to the floodplain lateral bank; and (3) in some large discharge cases, the flow behaves like a jet, with the maximum velocity filament oscillating from one main-channel bank side to the other, and recirculation zones developing on the floodplains.