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HW/d ratio for Water Quality Flow Splitters

HW/d ratio for Water Quality Flow Splitters

(OP)
I have a typical manhole with a concrete weir that acts as a flow splitter to separate the water quality flowrate and the high flow bypass. Local Ecology design manuals recommend a minimum headwater to low flow pipe diameter outlet ratio of 2.0. Most culvert design references refer to a HW/d ration of 1.2 or 1.5. It's my understanding Ecology recommends 2.0 in order to ensure submerged conditions so orifice equations are applicable. If that is true, then why is culvert design allowed to use 1.2 and 1.5 ratios? I would like to use a lower ratio than 2.0 (keep the weir height lower) with an 8" diameter pipe in order to prevent unwanted additional flow from my treatment train. If I reduce the pipe size to 6" diameter and set the flow to what is required and work backwards through the equations, the HW/d ratio becomes too big, i.e. in excess of 4 or 5. I don't like having such a high weir wall with a smaller pipe as it backs up water in the upstream conveyance system.

So, my question is this...will a HW/d ratio of 1.2 or 1.5 be sufficient in a typical flow splitter manhole such that submerged conditions can still be assumed and orifice equations still be applicable? Is the HW/d ratio of 2.0 recommended by Ecology overly conservative? Thank you.

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