CHANSON, H. (1996). "Free-Surface Flows with Near-Critical Flow Conditions." Can Jl of Civ. Eng., Vol. 23, No. 6, pp. 1272-1284 (ISSN 0315-1468). FREE-SURFACE FLOWS WITH NEAR-CRITICAL FLOW CONDITIONS by H. Chanson Senior Lecturer in Fluid Mechanics, Hydraulics and Environmental Engineering Department of Civil Engineering, The University of Queensland Brisbane QLD 4072, Australia Abstract Open channel flow situations with near-critical flow conditions are often characterised by the development of free- surface instabilities (i.e. undulations). The paper develops a review of several near-critical flow situations. Experimental results are compared with ideal-fluid flow calculations. The analysis is completed by a series of new experiments. The results indicate that, for Froude numbers slightly above unity, the free-surface characteristics are very similar. However, with increasing Froude numbers, distinctive flow patterns develop. Keywords : Open channel flow, critical flow conditions, free-surface undulations, flow instability, undular surge, undular broad-crested weir flow, culvert flow. Résumé Pour des écoulements à surface libre, les conditions d'écoulement critique sont souvent caractérisées par des instabilités de la surface libre : c.a.d, ondulations. L'auteur présente une revue de plusieurs types d'écoulements quasi-critiques. On compare des résultats expérimentaux avec des calculs de fluides parfaits. L'auteur présente aussi de nouveaux résultats expérimentaux. Les résultats montrent que, pour des nombres de Froude légérement supérieur à un, les caractéristiques des ondulations sont très similaires. Par contre, pour de nombres de Froude plus importants, chaque type d'écoulements développe des caractéristiques distinctives. Mots-clé : Ecoulement à surface libre, conditions d'écoulement critioque, ondulations de la surface libre, instabilité de l'écoulement, onde de translation, déversoir à seuil large, culvert. 1. Introduction 'Near-critical flows' are characterised by the occurrence of critical or nearly-critical flow conditions over a 'reasonably- long' distance and period time. At critical flow conditions 1 , the specific energy/flow depth relationship (e.g. Henderson 1966) is characterised by an infinitely-large change of flow depth for a very-small change of energy. A small change of flow energy can be caused by a bottom or sidewall irregularity, by turbulence generated in the boundary layers or by an upstream disturbance. The 'unstable' nature of near-critical flows is favourable to the development of large free-surface undulations. In this paper, it is proposed to review several near-critical flows : undular surges, undular flow above broad-crested weir, free-surface undulations downstream of backward-facing step, and undulations in rectangular cross-section 1 Flow conditions such that the specific energy is minimum are called 'critical flow conditions'.