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.

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