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Underflow baffles have gained in popularity over the years as a viable mean to intercept floatables in Combined Sewer Overflows (CSOs). This choice was mainly justified by the extremely low capital cost (CAPEX) and operational cost (OPEX) of this solution, although the efficiency of underflow baffles has never been clearly proven. The only similar application to underflow baffles are scum boards in grit chambers and clarifier. However, the flow conditions at CSOs vary considerably from those in grit chambers and clarifier. For this reason, review of the behavior of floatables in a rapid flow is paramount. Only then can comprehensive design criteria for underflow baffles and overflow chambers be suggested. Pilot scale tests, performed in a 17 metres long basin at various flowrates, had already shown that a critical horizontal velocity for floatables (Vcr) may develop in the overflow chambers. In this follow up study, the fate of intercepted floatables was investigated. It appears from this latest data that permanent capture of floatables decreases rapidly with an increase in the horizontal velocity of the flow, no matter what the baffle depth. Baffle depth increases capture at lower velocities (0.17 m/s) but become irrelevant at higher velocities (0.61 m/s). This data suggests that capture efficiency of existing underflow baffles in overflow chambers can be, at best, very low whenever the horizontal velocity increases above 0.30 m/s or 1 ft/s.

Underflow baffles have gained in popularity over the last few year as a viable means to intercept floatables in Combined Sewer Overflows (CSOs). This has happened although the efficiency of underflow baffles has never been clearly proven. Furthermore, there are no guidelines helping planners in the correct and efficient design of underflow baffles. This article proposes design criteria deduced from pilot scale essays performed in a 17 meters basin at various flowrates. These new informations can be used in two different ways. First, these criteria can be used to correctly design a new overflow chamber. Secondly, these criteria can be used to evaluate the efficiency of existing overflow chambers. Preliminary analysis of existing chambers show that interception efficiency of floatables can be very low.