Search

Large scale particle image velocimetry (LSPIV) has become a reliable and accurate technique to measure water surface velocity field in open channels and rivers. The LSPIV technique is based on a camera view framing the water surface, complemented with image-processing methods to compute water surface displacements between consecutive frames. Using LSPIV, high flow velocities, such as flood conditions, are accurately measured, whereas determinations of low flow regimes is more challenging, especially in the absence of floating material carried by the stream. In fact, in unseeded conditions, typical surface feature dynamics must be taken into account. Indeed, besides surface structures advected by the current, capillary-gravity waves propagate on the surface, with their own speed and direction. In this study, the discrimination between these phenomena is discussed, providing a new method to distinguish and to correct unseeded LSPIV measurements with capillary-gravity wave dynamics. Current velocity obtained with such a correction at low flow speed in an artificial channel and in a natural river is in fair agreement with reference observations indicating the need of accounting for the speed and direction of capillary-gravity waves on the surface to obtain reliable measurements of the current vector field, and, as a further result, of the flow discharge over a river cross-section.

Using the new high‐resolution operational model of ECMWF, we revisit the storm during which the Draupner freak wave of 1 January 1995 was recorded. The modeling system gives a realistic evolution of the storm highlighting the crucial role played by the southward propagating polar low in creating the extreme wave conditions present at the time the freak wave was recorded. We also discuss the predictability of the meteorological event. The hindcast wave spectra allow a new analysis of the probability of occurrence of the Draupner wave that we analyze not only in time at a specific position, but also in space. This leads us to discuss how exceptional the so‐called freak waves really are. For a given sea state, as characterized by the significant wave height, they are namely part of the reality of the ocean, the key point being the probability of encountering them. In this respect, the often considered record at a specific location can be misleading because the probability of detecting a freak wave must be considered both in space and time. High resolution and new model physics identify key characters of the Draupner stormConditions favorable to freak waves peaked at the time of the Draupner waveDefinition of the concept of “dynamical swell”