Discussion of "Characteristics of free overfall for supercritical flows"

Abstract

of the flow patterns were thoroughly documented. The location of the nappe impact was reasonably predicted by a simple trajectory equation (Chanson 1995; Toombes 2002). Downstream of the nappe impact, flow was characterised by a highly fragmented spray. The spray and splashing appeared to be concentrated towards the centreline of the channel. Sidewall standing waves, similar to ship bow waves, formed on both sidewalls downstream of the nappe impact. Properties of these standing waves compared reasonably with the properties of waves observed on the opposite mitre bends and channel junctions, and at abrupt channel expansions (Chanson and Toombes 1997, 1998). The flow was supercritical in the downstream channel. No hydraulic jump was oberved. Shock waves were observed in the downstream channel, originating from the sidewalls at or close to the nappe impact. The angle of the crosswaves was inversely proportional to the inflow Froude number. The shock waves intersected on the channel centreline and continued to propagate towards the opposite wall (Fig. 1). Energy dissipation at the overfall was the result of friction losses, jet disintegration, nappe impact on the downstream invert, and recirculation within the pool of water beneath the free-falling jet. The energy dissipation was roughly equal to the drop height within the range of the experimental flow conditions (Table 1). The discussers are concerned by some broad conclusions contained in the original paper, which ignored the three-dimensional nature of the flow, the existence of sidewall standing waves, and strong splashing and spray in the centre of the downstream channel. These three-dimensional flow patterns have direct implications in terms of channel design. The discussers observed a maximum sidewall standing height y M that satisfied