This paper deals with a phenomenon often occurring in sewers during storm events - transition from free surface to pressurized flow and vice versa. This transition is also possible in sewers where the discharge is controlled by some control devices, like gates. Experiments were carried out at a test rig consisting of a circular pipe with gates at the upstream and the downstream ends. Because of the relatively steep slope of the pipe and upstream boundary condition (flow below the gate), free surface flow at the upstream end was always supercritical, so that a hydraulic jump was always present during transitions (transcritical flow). Experimental results were used for verification of a numerical model based on a shock capturing method, the McCormack explicit finite difference scheme.

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This study presents the results of clear-water tests on the following five combined flow-altering countermeasures against bridge pier scour: (1) submerged vanes and a bed sill, (2) slot and sacrificial piles, (3) a collar and sacrificial piles, (4) a slot and a collar, and (5) a bed sill and a collar. Each countermeasure was designed on the basis of the best configuration recommended in the literature. The results clarify that an improper combination of two countermeasures may be less effective than each individual countermeasure; combinations (1), (2), and (3) do not reduce the scour depth significantly with respect to the single countermeasures; combination (3) increases the scour rate during the first few hours with respect to a single collar; combination (4) may also prevent scour intrusion beneath a collar; and combination (5) in the best configuration reduces the scour depth significantly around the collar with respect to maximum scour depth of the unprotected pier, preventing also the scour hole reaching the pier body.

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