Modelling Orifice- ICMl vs Theory

Hello all,

Please someone could help to find solution for the following problem. Modelling a CSO/orifice/outgoing pipe for a scheme . Weir level is 0.77m high from invert of the outgoing pipe , controlled flow through an orifice of 150mm diameter and the d/s sewer is large enough so that the flow is not impeded (ie d/s is not drowned). Normal orifice calculation shows "Q" the Pass forward flow as Q= A * Cd * (Sqrt (2 * g* h) which gives a value of 39.2l/s (0.017671 * 0.6 * (Sqrt ( 2 * 9.81 * 0.695),when h calculated to the centre of the orifice.

When modelled the same scenario in ICM with an orifice of 150mm diameter with a Cd of "0.6" the Pass forward flow was found to be only ~30l/s. As per the help files, governing model equation under free discharge condition is Q = Cd * A * Sqrt (gd) (where is the 2?) which trace back to the Wallingford Procedure. The Wallingford procedure mentions the coefficient Co (orifice coefficient?) instead of Cd (discharge coefficient) which ranges from 0.8 to 3. I could find some information in the WaPUG user note 2 regarding coefficient "Co" but this formula is based on the assumption that the orifice is drowned. But at the same time section 4 says from Hydroworks onwards the throttle is identified as a control link thus defining the headlosses for modular/drowned orifices. So, whether we have been representing the orifices correctly in the model if so then how the pass forward control is calculated in Infoworks/ICM and why it differs from the normal formula?

It would be much appreciated if someone could tell me the reason why such a difference in the flows for the above mentioned scenarios? Thanks in advance.

Indy