Negative surges are caused by a sudden change in flow resulting from a decrease in water depth. New experiments were conducted in a horizontal channel (L = 12 m, W = 0.5 m) to record the unsteady water depth and turbulence in negative surges propagating upstream against an initiallysteady flow. The data were collected using video-imagery, acoustic displacement meters and acoustic Doppler velocimetry (ADV). The physical observations showed that the leading edge of negative surge propagated upstream with a celerity which varied with time. During the first initial instants following the surge formation, the negative surge leading edge accelerated ad its celerity increased with time up to xGate-x = 4do. After the acceleration phase, the negative surge propagation was more gradual: the surge leading edge was very flat and barely perceptible, and its celerity tended to decrease slowly with increasing distance from the gate. The data implied some deceleration in a manner which is contrary to theoretical considerations. The physical measurements highlighted that the negative surges were associated with some flow acceleration. The turbulent velocity data highlighted some increased turbulence occurring beneath the negative surge with large velocity fluctuations and large Reynolds stress components. The velocity fluctuations and turbulent stresses were significantly larger than in the initially steady flow and in the final flow motion. The physical data were used to test an analytical solution of the Saint-Venant equations (the simple wave solution) and some 1-D and 2-D numerical model results. The findings showed that the negative surge propagation was relatively little affected by the boundary friction. For a relatively simple geometry such as the prismatic rectangular flume used in the present study, the physical data were best modelled by the simple wave theory, although the numerical model results were qualitatively in agreement with the experimental observations. The present results suggested that the negative surge remains a challenging topic for the computational modellers
