Computation of afflux ratings and water surface profiles

Afflux is defined as the maximum increase in water level due to the presence of a structure such as a bridge or culvert in a stream. It is difficult to measure directly because the location of maximum water level upstream of the structure varies with flow discharge. Even computing afflux is not straightforward, as bridges and culverts are subject to as many as nine modes of flow depending on the flow discharge. These modes vary with increased discharge from subsoffit, subcritical flow, to structure overtopping modes with critical flow, and even submerged structure flow. A hydraulic engineer may require a single afflux rating curve for design, or the water surface profile alongstream (longitudinally) through a structure for use in river engineering. For this reason, hydrodynamic computer codes named Afflux Advisor and Afflux Estimator were developed for the former and latter purposes respectively. This paper describes the methods used in each code, and tests the codes against field and laboratory data. Both codes were developed in Visual Basic for Applications, and Afflux Estimator has been interfaced to the UK Environment Agency's conveyance estimation system. The project was sponsored by the Environment Agency and the UK Department of Environment, Food and Rural Affairs.