3D Free-Surface Characterization of Hydraulic Jumps Downstream of a Vertical Drop Using an RGB-D Depth Camera

Vertical drop structures typically dissipate energy through a hydraulic jump formed immediately downstream of the drop toe, where strong aeration, splashing, and free-surface unsteadiness make conventional measurements difficult. This technical note evaluates the capability of an Intel RealSense D435if depth and color (red, green, blue) (RGB-D) depth camera, operating at 90 fps, to characterize the free surface of a hydraulic jump downstream of a 0.50 m vertical drop in a 0.80-m-wide laboratory flume. Experiments were conducted for unit discharges q = 0.089 – 0.127 m 3 / s / m , corresponding to inflow Froude numbers F r 1 = 4.55 – 5.19 . Mean free-surface profiles derived from the camera were validated against the characteristic depths y associated with local void fractions C = 0.5 and C = 0.9 , denoted y 50 and y 90 , respectively, obtained from single-tip conductivity-probe measurements along the channel centerline. Within the main roller region, the camera-derived profiles remained between the y 50 and y 90 envelopes, indicating that the RGB-D sensor can resolve the mean aerated free surface under highly disturbed conditions. An auxiliary comparison between single-tip conductivity-probe and conductivity wire gauge measurements of the equivalent clear-water depth yielded agreement within 7.3%, supporting the internal consistency of the intrusive validation data. Three-dimensional reconstruction of the hydraulic jump revealed marked transverse nonuniformity in the free-surface fluctuation field. Peak fluctuations did not occur systematically along the channel centerline; instead, the largest values were found at off-centerline locations and reached up to 8.9% above the corresponding centerline peaks. These findings show that centerline-only measurements may underestimate the governing fluctuation envelope and that RGB-D imaging provides a practical basis for assessing free-surface variability, freeboard requirements, and sidewall design in stilling regions downstream of vertical drops.