Figure 4. Positive surge propagation above the large roughness element – Flow conditions: Q = 0.061 m3/s, d1 = 0.155 m at x = 5.9 m, Fr1 = 1.39, Tainter gate opening after closure: h = 25 mm – From left to right: 0.121 s between successive photographs (shutter speed: 1/400 s)