Headloss Coefficient in drop manhole

Question

Hi, 
 Using the standard method of headloss for a manhole junction, how would I calculate the loss coefficient in a manhole with a significant vertical drop (10')? The manhole has a 90 degree bend in it, so I have the coefficient set at 1.32, but I just want to be sure we are accounting for any losses that occur due to the vertical drop as well the bend loss. 
 Thanks

Jesse Dringoli · Answer

Hello Daniel, which Bentley product are you using? (SewerGEMS, CivilStorm, StormCAD, SewerCAD?) 
 Is the vertical drop between the incoming pipe invert and the invert of the structure? 
 Chapter 11.3 of our Stormwater Conveyance Modeling and Design book suggests that the standard loss method is not applicable to plunging flow conditions where incoming pipe inverts are above the structure water surface. 
 If in doubt, consider a sensitivity analysis where you compare hydraulic results of interest to your decision making between a very low assumed headloss coefficient and a very high headloss coefficient. If the results that really matter do not change by much, this should give you confidence that you do not need to be very precise on your assumed coefficient.

Jesse Dringoli · Answer

I would suggest reviewing the literature on the other structure loss methods (for example see here , and in the SCMD book previously mentioned) and consider if there are any local standards or if the reviewer requires a certain method. I believe the HEC-22 methods account for plunging flow situations (see mention of it here and in the HEC-22 manual) 
 If the model is on the edge of meeting criteria, it might be best to err on the conservative side for your design (pipe sizes, etc)