Intake shape and boundary-related considerations in the operation and design of storm-sewer drop structures

Ref: https://communities.bentley.com/products/hydraulics___hydrology/f/haestad-hydraulics-and-hydrology-forum/142801/bridge-drainage-outlet-velocity-calculation-in-close-vertical-drainage-system/ 

"Intake shape and boundary-related considerations in the operation and design of storm-sewer drop structures" by A. Auckland, I. Nistor, R. Townsend.       Canadian Journal of Civil Engineering, 2009, 36(11): 1825-1834, https://doi.org/10.1139/L09-114

Abstract:   This paper presents the results of an experimental investigation into the hydraulic efficiency of a vertically oriented water intake. Under low submergence conditions, the operation of such an intake is affected significantly by the formation of surface vortices, which result in significant flow reduction and the downstream entrainment of air and swirl. The experimental work considered the fact that vortex formation is governed by the geometrical characteristics of the intake, including the diameter, local shape, submergence, surrounding boundary, etc, and the velocity distribution in the approaching flow. Therefore, an experimental study was conducted to investigate the influence that the local shape and the proximity to nearby boundaries have on the hydraulic performance of a vertical water intake. The results of this experimental investigation will hopefully lead to a better understanding of the phenomena governing the flow in such structures and to the elaboration of improved engineering design guidelines.

Bentley could either incorporate this procedure into a newer version of their program, or you could provide a rating curve to your model based on separate spreadsheet calculations using the above procedure.