Discussion of "Hydraulic Design of Stepped Spillways" by Robert M. Boes and Will H. Hager

In skimming flows down stepped chutes, Chanson et al. (2002) presented a comprehensive reanalysis of flow resistance based upon more than 38 model studies and 4 prototype investigations totaling more than 700 data points with channel slopes ranging from 5.7DG up to 55DG. Different research facilities yielded different results and researchers continue to disagree on the reasons for these differences (Chanson 2000). The authors highlighted nicely the difficulties to estimate flow resistance on stepped chutes, although it is a key design parameter. Here the discusser argues that differences in flow resistance data may be linked to different inflow conditions. A careful reanalysis of large-size experimental results suggests that lower flow resistance was observed in experimental facilities with pressurised intake.