hydraulic grade line

Hello Folks , 

Kindly i have a question related to Sewer-gems Software , my question is about the hydraulic grade line in a sewer networks that modeled throughout Sewer gems or sewer cad .

The Hydraulic grade lines have a small jump before manhole entrance , even-though the inlet pipe and outlet pipe has the same : diameter , flow , slope , partial full flow percent
my question why this small jump happens ??

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