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

Abstract

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.

Comments on “Analysis of Flow Resistance Equations in Gravel‐Bed Rivers With Intermittent Regimes: Calabrian Fiumare Data Set” by G. Mendicino and F. Colosimo

“…Mendicino and Colosimo () found conventional flow resistance equations tend to overestimate flow velocities. They corrected these equations by introducing the Froude number and the Shields sediment mobility parameter into the conventional equations.…”

“…The two added parameters are expected to account for the additional resistance caused by the water surface distortions (wave resistance) and the bedload transport. Efficiency tests conducted in Mendicino and Colosimo () indicate that the new equations can better reproduce the flow velocity than conventional flow resistance equations. Similar research was conducted by Afzalimehr and Anctil (), who proposed an equation to predict the resistance coefficient of gravel‐bed rivers by building a multiregression model between resistance coefficients and geometric and hydraulic parameters.…”

“…All these equations can be written as where is the mean flow velocity (m/s), is the shear velocity (m/s), is the hydraulic radius (m), and is the characteristic grain size (m); in the Iwagaki equation, is a function of the Froude number, and it is a constant in other equations. Mendicino and Colosimo () modified these equations by adding an additional item that is a function of the Froude number and the Shields parameter to the right‐hand side (RHS) of equation . Mendicino and Colosimo () merged of the Iwagaki equation into the newly added item, and thus, is a constant in all the four equations ( = 0 for the Iwagaki equation).…”

“…The new equations (also named modified equations) proposed by Mendicino and Colosimo () can all be written as where is the Shields sediment mobility parameter and Re is the Reynolds number. The definition of is where and are the sediment and water densities (kg/m ), respectively; is the gravity acceleration (m/s ); and is a constant and equals 0.029.…”

“…Mendicino and Colosimo () found the Reynolds number has an insignificant influence on the performance of the modified equations and thus the Re parameter was neglected in their latter equations. As a consequence, the right‐hand side (RHS) of equation is also a function of , Fr, and .…”