Performance of lateral velocity distribution models for compound channel sections J.F. Weber Universidad Nacional de Córdoba and Universidad Tecnológica Nacional, Córdoba, Argentina. A.N. Menéndez INA (National Institute for Water) and Universidad de Buenos Aires, Buenos Aires, Argentina 1 INTRODUCTION Hydraulic engineering studies related to floods usu- ally require the lateral distribution of flow velocity across the compound channel section, constituted by the main channel and the floodplain. 2D-Horizontal numerical models (such as the public domain soft- ware RMA2 of USACE) are now broadly accepted as an appropriate theoretical model to solve this problem. However, as in the majority of applications the longitudinal scale of flow variation is much lar- ger than the lateral scale of variation (given by the flow width), the longitudinal and lateral flow varia- tions can be accounted for separately (Menéndez 2003); the first one, through well-established 1D- Longitudinal models based on de Saint Venant equa- tions (Cunge et al. 1980), while the second one, us- ing 1D-Lateral models. 1D-Lateral models, which solve the lateral distri- bution of the longitudinal depth-averaged flow ve- locity, have become the subject of analysis and ap- plication in recent years. They run between simple and relatively old empirical and heuristic formula- tions - such as the methods of Lotter (1933), or Di- vided Channel Method (DCM, used in software HEC-RAS, 2001), Horton (1933), and Pavlovski (1931) - to physically-based equations, like the Lat- eral Distribution Method (LDM) proposed by Wark et al. (1990). 2 MODEL DESCRIPTIONS In the following paragraphs, the three models used in the present work are briefly described: the DCM; the analytical solution proposed by Shiono & Knight (1988,1991) to the LDM, and the RMA2-WES 2D- Horizontal hydrodynamic finite-element model (Donnell et al. 2001). 2.1 DCM Model Lotter (1933), and later Einstein & Banks (1950) di- vided the cross section into subsections, and as- sumed that the energy grade slope was the same for any subsection, and that the interfaces between them behaved as impermeable boundaries, i.e., that there was no diffusion of lateral momentum (leaving the friction as the only energy loss), to calculate the par- tial discharge Q i for any such subsection: 2 1 0 S K Q i i = (1) where S 0 = longitudinal slope; and K i = hydraulic conveyance, given by i i i i n R A K 3 2 = (2) where A i , = flow area, R i = hydraulic radius, and n i = Manning’s roughness coefficient for subsection i. The lateral velocity distribution, V i , can then be obtained as i i i A Q V = (3) This is the method implemented in software HEC-RAS (HEC, 2001) as Flow Distribution Op- tion. ABSTRACT: In this paper, the scope of 2D-Horizontal and 1D-Lateral models for lateral velocity distribution is addressed, and their relative and absolute performances are tested by making comparisons between their predictions, on the one side, and experimental and field velocity data, on the other side. The models consid- ered are: the Divided Channel Method (DCM), the Lateral Distribution Method (LDM) and a 2D Horizontal hydrodynamic finite-element model (RMA2).