Flow resistance in gravel-bed channels with large-scale roughness

Abstract

A previously published mixing length (ML) model for evaluating the Darcy-Weisbach friction factor for a large-scale roughness condition (depth to sediment height ratio ranging from 1 to 4) is briefly reviewed and modified (MML). Then the MML model and a modified drag (MD) model are experimentally tested using laboratory measurements carried out for gravel-bed channels and large-scale roughness condition. This analysis showed that the MML gives accurate estimates of the Darcy-Weisbach coefficient and for Froude number values greater than 0·5 the MML model coincides with the ML one. Testing of the MD model shows limited accuracy in estimating flow resistance. Finally, the MML and MD models are compared with the performance of a quasi-theoretical (QT) model deduced applying the II-theorem of the dimensional analysis and the incomplete self-similarity condition for the depth/sediment ratio and the Froude number. Using the experimental gravel-bed data to calibrate the QT model, a constant value of the exponent of the Froude number is determined while two relationships are proposed for estimating the scale factor and the exponent of the depth/sediment ratio. This indirect estimate procedure of the coefficients (ba- b1 and b2) of the QT model can produce a negligible overestimation or underestimation of the friction factor. © 2003 John Wiley and Sons, Ltd.