I'm working on models which include large diameter storage tunnels which fill with open channel flow but can also experience a significant amount of surcharge. I'm seeing more artificial volume being created than what I would expect to see based on the Preissmann slot width. I've tried to minimize the Preissmann slot volume with a small minimum width and celerity ratio of 14.414. For a diameter of 24 feet, I'd expect a slot width of 0.23 feet. For larger surcharging, this width matches my observations for the additional modeled volume in the conduit. For smaller amounts of surcharge, I'm seeing an additional increase in storage volume beyond what can be accounted for with the slot width.

Is the width calculated as shown below (from http://blog.innovyze.com/wp-content/...ICM_and_CS.pdf)


or is there an additional transition at the top of the conduit which creates additional volume? Is there a relationship for this?

Thank you,
Sam Glovick