# A Study of Friction Factor for Flow in Gravel Bed Channels

*by*Tsan-Wen Wang, Natl Taiwan Univ, Taipei, Taiwan,

Grung-Lu Ai, Natl Taiwan Univ, Taipei, Taiwan,

**Abstract:**For many years practicing engineers have faced the problem of predicting the resistance to flow in channels with rough surface. Flow in channel is a complex interaction between surface and subsurface flow. A flume study data is used to analyse and realize the effects and characteristics of free surface flow over permeable gravel beds. The values of the Von Karman constant k obtained in this study is significantly lower than the commonly expected value of 0.4 for the case of non permeable beds with clear water flow. It is found that the values of k for four sets of gravel are 0.31, 0.28, 0.27 and 0.23 which decrease with the increase in median diameter of uniform gravel beds. The values of the zero velocity displacement for four sets of gravel are 0.54, 0.72, 1.14 and 1.48, and the finite slip velocities at the bed surface are found to increase with the increase of median diameter of uniform gravel beds. At the bed surface, the velocity gradient with larger median diameter of uniform gravel bed is milder than the gravel bed of smaller one. The Darcy-Weisbach type friction factor is found to decrease with the increase of median diameter of uniform gravel bed when the relative roughness has the same value. The equation is given.

**Subject Headings:**Gravels | Bed materials | Channel flow | Free surfaces | Overland flow | Subsurface flow | Free flow | Water flow

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