A modification to the Wallingford rational regime theory

The rational regime formulation, which was used by White, Bettess and Paris to predict the stable geometry of wide, trapezoidal alluvial channels and in producing the Wallingford Tables for the Design of Stable Alluvial Channels (Hydraulics Research, Wallingford, 1981), is reviewed. This approach (the Wallingford rational regime theory) is known to provide useful results for moderate-scale natural channels but deviates more widely from observed behaviour for laboratory channels and very large rivers. To explore possible reasons for this, wide and narrow trapezoidal channels have been examined in terms of wetted perimeter, hydraulic radius and side slope, and have been compared with equal-area rectangular channels. The effect of the hydraulic radius, via the threshold of motion, on the sediment transport prediction was studied. It is shown that the hydraulic radius associated with selection of side slope has a significant effect on the sediment concentration prediction. The threshold of motion is also an issue. The original method was modified by approximating the channel cross-section as an equivalent area trapezoidal section. Based on the results, an adjustment to the method in terms of hydraulic radius and threshold of motion is proposed. Laboratory flume and field data are used to examine the improved predictions of the modified method.