This dataset is composed of experimental measurements collected in an 18 m long and 3 m wide tilted open-channel flume, consisting of a central smooth-bed main channel (MC) and two adjacent rough-surface floodplains (FPs). Streamwise depth-uniform and non-uniform flows were both investigated. The non-uniform flows were generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance. They involve transverse currents directed from MC to FPs and vice versa. The focus is on assessing the effects of the transverse currents on the flow structure within the shallow mixing layer developing between MC and FPs. Six flow cases were investigated varying the inflow partition between MC and FP, keeping the total flow rate (Q = 114 L/s) unchanged. Each flow case is identified by its FP discharge value, denoted as Qf. The depth-uniform flow scenario corresponds to Qf = 8 L/s. The five non-uniform flow scenarios correspond to Qf = 0, 4, 12, 16 and 20 L/s. We present here the measurements of (1) water flow depths, (2) one-point velocity measurements; and (3) two-point velocity measurements. They enabled us to explore the flow structure along the whole length of the flume, including the time-averaged flow, the turbulence statistics, and the large-scale coherent structures (Kelvin-Helmholtz vortices, large- and very-large- scale motions).