No abstract

The carrying capacity of a subsurface drainage network based on spatially varied flow with increasing discharge and the presence of a free surface is examined and reported. The discharge capacity of a subsurface drainage network depends on the hydraulic and geometric parameters of the network components which should be clearly identified. In this regard, a prototype scale experimental set-up was prepared to determine the major head loss of the corrugated pipes employed in these networks and to investigate the minor head losses of different network junction types. According to the compiled data, average values for the major and minor head loss coefficients were proposed and employed for modelling the flow condition within a subsurface drainage network. The model determines the network discharge capacity based on the real flow condition, i.e. a free surface spatially varied flow regime. Comparison between the outputs of the applied model with that of the traditional relationships indicated a large discrepancy. The results revealed that applying existing formulations in the design process would likely pressurize significant portions of the network which could affect the water depletion period from the plant root zone. Copyright A cet effet un dispositif expérimental est développé pour mesurer les pertes en charge linéaires des tuyaux annelés employés dans ces réseaux, ainsi que les pertes en charge singulières des différents composants. Des valeurs pour les coefficients de pertes en charges linéaires et singulières sont proposées et utilisées pour modéliser l'écoulement dans le réseau de drainage. Le modèle détermine le débit maximal admissible du réseau dans les conditions réelles de l'écoulement varié à surface libre. Les valeurs fournies par le modèle diffèrent sensiblement des valeurs fournies par les relations couramment utilisées. Les résultats montrent que ces relations courantes amènent à concevoir des réseaux qui passent localement en charge sur des longueurs significatives, avec des conséquences possibles sur le ressuyage du sol dans la zone racinaire.

The roughness coefficient is a significant factor in hydraulic design process, but carries vast uncertainty in practice. This paper reports the resistance coefficients of corrugated PVC pipes under different flow regimes (i.e. pressurised, gradually varied flow (GVF) and spatially varied flow (SVF) with increasing discharge). The investigated corrugated pipes are mainly used in subsurface drainage systems and carry a SVF. The influence of SVF on the roughness coefficient of such pipes has not yet been reported. The pipes were first examined to determine the roughness coefficient for pressurised flow. The influence of GVF and SVF was then studied. The results show that pipes behave differently in different flow regimes and the impacts of each flow condition on the flow resistance are reported here.