Dissipative analogies between a schematic macro-roughness arrangement and step-pool morphology

We investigate flow resistance developed by macro-roughness represented by pebbles posi- tioned on a granular layer according to a regularly spaced stripe pattern on steep bed slopes. Flume experiments under various geometrical and hydraulic conditions are carried out and interpreted by means of a theoretical model. Results show that flow resistance reaches a maximum and is due mainly to form drag when the spacing between macro-roughness stripes is about 10 times the average macro-roughness height. A statistical analysis based on various field observations of step–pool geometry underlines that this spacing appears to be one of the most frequent occurring in step–pool bed morphology sequences. Comparison between the present results and flow resistance evaluated for step–pools reproduced in the laboratory and observed in the field suggests that step–pool streams are characterized by a bed geometry able to develop the maximum flow resistance. Finally, a criterion is obtained to estimate flow resistance developed by natural step–pool streams when a formative flow discharge occurs based on geometric quantities only.