Hi Yongseok Choi, According ...

Hi Yongseok Choi,

According to my experience, depending on Manning theory, the height of water flow in pipe is equal to 90% of the inside diameter of pipe. For actual calculation of velocity under gravity, your idea is right, the velocity of water flow inside sewer is always under 9.8 m/s. Attachment is for your reference that we had calculation and design in the actual condition.