IS HYDRAULICS OVER-USED OR UNDER-USED IN STORM DRAINAGE

Urban drainage models utilize hydraulics of different levels. Developi ng or selecting a model appropriate to a particular project is not an easy task. Not knowing the hydraulic principles and numerical techniqu es used in an existing model, users often misuse and abuse the model. Hydraulically, the use of the Saint-Venant equations is not always nec essary. In many cases the kinematic wave equation is inadequate becaus e of the backwater effect, whereas in designing sewers, often Manning' s formula is adequate. The flow travel time provides a guide in select ing the computational time step DELTAt, which in turn, together with f low unsteadiness, helps in the selection of steady or unsteady flow ro uting. Often the noninertia model is the appropriate model for unstead y flow routing, whereas delivery curves are very useful for stepwise s teady nonuniform flow routing and for determination of channel capacit y.