Author(s): Marilena Pannone
Linked Author(s):
Keywords: Rivers longitudinal dispersion; Transient and asymptotic behavior; Lagrangian transport theory; Field measurements
Abstract: A new analytical approach is proposed to evaluate the longitudinal dispersion coefficient for uniformly straight natural channels in transient and asymptotic conditions. The starting basis is represented by the stochastic lagrangian transport theory, in which the elements of the macrodispersion tensor are related to the variation rate of the single particle trajectory moments. The resulting time-dependent curves are plotted for transverse velocity profiles corresponding to the depth measurements collected through six selected river sections in the southern Italy, and derived from a generalized Manning’s equation. Moreover, the same computations are repeated for generalized symmetric section shape and velocity distributions built on the basis of the only average velocity, average depth, shear velocity and river width. Although the difference is generally not dramatic, it clearly appears that the symmetric section flow field underestimates the duration of the initial transient. The predictions of the analytical equation are also compared with the numerical solution of Fisher’s large-time ‘eulerian’ integral, carried out with the measured velocities. The agreement is quite satisfactory and better than for other empirical and semi-empirical existing formulae. Nevertheless, the application of the proposed method to 73 data series referred to more general flow conditions and recently published points out the need to account for an average calibration factor equal to 13.
Year: 2007