Analysis of Flow Resistance Equations in Gravel-Bed Rivers With Intermittent Regimes: Calabrian fiumare Data Set

This study addresses the evaluation of flow resistance in natural gravel-bed rivers. Through a new data set collected on 136 reaches of 78 gravel-bed rivers (Calabrian fiumare) in southern Italy, different conventional flow resistance equations to predict mean flow velocity in gravel-bed rivers were tested in their original form. These equations have shown considerable disagreement with observed data, especially in river reaches characterized by high bed load conditions and for the domains of intermediate- and large-scale roughness. This disagreement produced in almost all the cases an underestimation of the flow resistance, which can be corrected by introducing the Froude number and a particular form of the Shields sediment mobility parameter into the Manning, Chezy, and Darcy-Weisbach equations. Through analyses carried out both on the whole data set and on its subsets, we propose a semiempirical approach with which, on the one hand, the tractive forces exerted by the flow on the bed are taken into account by considering the ratio between the sediment mobility parameter and its critical value, and on the other hand, water surface distortions are evaluated using the Froude number. This approach has been further validated using a literature-based data set showing, even in this case, excellent performances. Finally, the literature-based data set allowed us to improve the performances of the proposed approach in the field of large-scale roughness. Efficiency tests indicate that the new equations can better reproduce the flow velocity in river reach where conventional flow resistance equations are not able to explain the entire dissipative process.