Pressure and velocity on an ogee spillway crest operating at high head ratio: experimental measurements and instrumentation

This paper aims at validating pressure and velocity measurements conducted in two physical scale models of an ogee spillway crest operating at heads largely greater than the design head. The design head of the second model is 50% smaller than the one of the first model. No piles effect or air entrance is considered in the study. The velocity field is measured by Large Scale Particle Image Velocimetry. The relative pressure along the spillway crest is measured using pressure sensors. Comparisons of measured velocities between both spillways indicate low scale effects, the scaled-profiles collapsing in most parts of the flow. By contrast, measurements of relative pressure along the spillway crest differ for large heads. A theoretical velocity profile based on potential flow theory and expressed in a curvilinear reference frame is then fitted to the velocity measurements, considered as reference, for extrapolating the velocity at the spillway crest. Comparing the extrapolated velocity at the spillway crest and the velocity expressed from the relative pressure finally emphasizes that bottom pressure amplitudes are overestimated for the large spillway, while an averaging effect might operate for the pressure measurements on the small spillway.