On the incipient aerated flow in chutes and spillways

Natural self-aeration of water flows in open channels protects surfaces in contact with the flow from cavitation damage if enough air content is reached (Falvey [6] [7], Peterka [10], Russel & Sheenan [11]), although it could lead to an increase in both flow depth and velocity. Also, self-aerated flow enhance the process of atmospheric gases exchange within the fluid, improving water quality downstream of hydraulic structures (Chanson [5]). So it is of great interest to evaluate accurately the critical point where air entrainment begins, that is, the location of the inception point. The note first shows a review on some methods to evaluate the inception point location. Second, a brief explanation on calculation development is made and an expression is obtained which enables to compute the point of incipient self-aeration as a function of unit discharge, bottom slope of the chute and uniform surface roughness in a more simple and accurate way. Finally, comparison with results provided by other methods is made and conclusions are obtained.