Abstract: To investigate the distribution characteristics of the equivalent roughness of open-channel flow over an array of glass spheres, eight groups of open-channel turbulence experiments were performed with four different rough beds. The instantaneous velocity data were obtained through particle-image velocimetry (PIV) along the longitudinal cross-section of the test flume. From an analysis of the experimental data, the longitudinal distributions of the theoretical position of the bed surface, equivalent roughness, and coefficient of drag were obtained, and the intrinsic relationship between equivalent roughness and the coefficient of drag was determined. The result shows that the characteristics of the flow in a coarse-grained open channel distinguish two regions, one spherical and the other inter-spherical. There are significant differences in the theoretical position of the bed surface ξ, the relative equivalent roughness k_s0, and the coefficient of drag f in each region. In the spherical region (0.65 ≤ ξ ≤ 0.85), a fluctuation in k_s0 is evident, and the value of f is large, whereas in the inter-sphere region (ξ=1), the fluctuation in k_s0 is relatively slow, and the value of f is small. Another difference is that the average value of k_s0 is proportional to the diameter d of the sphere and inversely proportional to the array spacing L_x and the Reynolds number Re of the flow. When the diameter, array spacing, and Reynolds number are fixed, the average value of k_s0 for a quincunx-arrayed bed is greater than that of the rectangular-arrayed bed. Moreover, the coefficient of drag increases with k_s0, and the variation in the flow characteristics are closely related to the relative smoothness h/d. These results provide a quantitative reference for the determination of the coefficient of drag for open-channel flows over a rough surface.