Flood waves resulting from flash floods and natural dam overtopping have been responsible for numerous losses. In the present study, surging waters down a flat stepped waterway ($theta$ = 3.4¿) were investigated in a 24 m long chute. Wave front propagation data were successfully compared with Hunt's <1982, 1984> theory. Visual observations highlighted strong aeration of the leading edge. Instantaneous distributions of void fractions showed a marked change in shape for (t - ts)*$sqrt {{rm g/d}_{rm o}}$ ~ 1.3, which may be caused by some major differences between the wave leading edge and the flow behind, including nonhydrostatic pressure distributions, plug-slug flow regime, and different boundary friction regime. Practically, the results quantified the large amount of entrained air (i.e., white waters) at the wave front, which in turn, reduces buoyancy and could affect sediment motion at the leading edge because the sediment relative density is inversely proportional to the entrained air content.