Influence of macro-roughness arrangement on flow resistance

摘要

Experimental data on regular spaced strip arrangement of macro-roughness element are herein analysed by means of a two-layer model, in order to evaluate flow resistance and drag shear stress. Seventytwo experimental runs have been carried out in a glass walled tilting flume. Discharge was varied from 7.31/s to 13.51/s and slope was varied from 1.0% to 2.5%. Three different pebble sizes with a median axis from 4.3 cm to 8 cm were used as macro-roughness elements. Pebbles were placed on a granular plane bed to achieve a bottom geometry configuration of transverse regularly spaced strips. Results underline a non-linear behaviour of Chezy coefficient as a function of strip wavelength to height ratio, showing a minimum of conductance. Analogously, form induced shear stress presents a maximum, around 80% of total bed shear stress. Even if the reproduced pattern appears to be not wholly representative of natural conditions, comparison of results with other field and laboratory data seems to indicate that a similar dissipative behaviour is recognisable in natural bed forms such as step and pools or dunes.