INFLUENCE OF HYDRAULIC GEOMETRY RATIOS ON THE ENTROPY PARAMETER IN OPEN CHANNEL FLOW

The way to evaluate the entropy parameter M through the ratio between medium and maximum velocity, Φ(M), still represents a relevant issue feeding a reach discussion among researchers. The invariance of Φ(M) for sections along the same river only at high flow seems to be consistent while for low stage Φ(M) can be affected by the influence of roughness, through the relative submergence as well as the aspect ratio related to the cross section geometry. Moving from these bases, using laboratory and field data, the classical hydraulic relationships on entropy velocity profile, uniform flow and regime theory, a predictor for entropy parameter is proposed for open channel flow. The work proposes general relationship existing between the parameter Φ(M) and the relative submergence taking into account the aspect ratio of the flow. The relationship has been applied to a set of experimental velocity data collected both in laboratory and in field, showing a good response of the theoretical model but selecting different behaviour dependent on the roughness scale. In fact, Φ(M) is strongly influenced by the ratio depth/roughness for values less than 4 when large and intermediate roughness scale occurs, while it might be assumed almost constant for small roughness scale.