This dataset includes both experimental measurements and numerical simulations of unsteady flows in a compound open-channel flume, consisting of a Main Channel (MC) and an adjacent Flood Plain (FP). The experimental data consist in: (i) water depths in MC and FP; (ii) inflow discharges in MC and FP; (iii) flow parameter at the vertical interface between MC and FP (lateral discharge, streamwise and spanwise mean velocities, and Reynolds stress). The numerical data (same flow parameters as the measured ones) stem from simulations using a 1D (one-dimensional) code that was adapted to implement the 1D+ Independent Sub-sections Model (ISM) (Proust et al. in Water Resour Res 45:1–16, 2009). Classical 1D data are also used as benchmark.