Dear Yonseok Choi, You ...

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Dear Yonseok Choi,

You should introduce the hydraulic radius and/or hydraulic diameter to calculate the partial current instead of the full flow, where the water has free surface inside of the channel/pipe (e.g. Colebrook-White formula).

Here you can find a quite good description (see the part of the partially full pipe):

http://slideplayer.com/slide/6116180/

During the design it is important to hold the velocity around 1 m/s, because the friction loss is increase by velocity quadratically.