This dataset is composed of experimental measurements collected in an 18 m long and 2 m wide tilted open-channel flume. The focus is on shallow mixing layers developing behind a vertical splitter plate over a hydraulically smooth bed. Four values of the total flow rate Q were used (120, 60, 30, and 14 l/s) to vary vertical flow confinement (i.e. flow depth). For each Q-value, the inflow conditions were changed. The experiments start with uniform flow conditions, i.e., with Q1 = Q2, where Q1 is the right-hand inflow discharge and Q2 is the left-hand inflow discharge, with Q = Q1+Q2. Then, for a given Q-value, shallow mixing layers were created by varying the flow partition between the two inlet tanks (with Q1 < Q2). Twenty flow cases were investigated, including four uniform flows and 16 mixing layers. Each flow case is identified by its values of Q1 and Q2, e.g., case 03-27 corresponds to Q1 = 3 l/s and Q2 = 27 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.