Evidence of a critical velocity in underflow baffle design for floatables in CSOs

Underflow baffles have gained in popularity over the last few years as a viable mean to intercept floatables in Combined Sewer Overflows (CSOs). This has happened in consideration of the extremely low application cost of the solution 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. For this reason, review of the behavior of floatables in a rapid flow is paramount. Subsequently, comprehensive design criteria for the underflow baffle itself and the overflow chamber can be deduced. Pilot scale tests, performed in a 17-meters long basin at various flowrates, show that a critical horizontal velocity for floatables may develop in the overflow chamber (Vcr). Whenever this critical velocity is exceeded, floatables that would normally rise to the surface are kept within the flow and can consequently not be intercepted by surface restrictions. In other words, the underflow baffle cannot, under specific flowrate  conditions, be effective in intercepting floatables. The equation relating the critical horizontal velocity to the vertical velocity in still water of any given floatable type is found to be : Vcr = 16 w RH 1/6 , where w is the vertical velocity in still water and RH is the hydraulic radius of the pipe. This deduced criterion can also be used to evaluate the efficiency of existing overflow chambers.

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