effect of crest roughness on flow characteristics over circular weirs

different construction materials with different roughness used to make circular weirs highly affect surface roughness and, in turn, flow hydraulics passing over these structures.  in the present research, numerous experiments under different hydraulic conditions were performed on a physical model to study the effects of roughness on flow hydraulics over a circular weir. the flow hydraulics included velocity profile, discharge coefficient and longitudinal water surface profile. the actual water surface elevation and velocity profile at different cross sections were measured using a point gauge and micro current meter, respectively. about 200 experimental tests were performed on a circular weir made of polyethylene with 29.5 cm height, 30cm wide, and 7.5 cm radius. the results showed that for a constant discharge, as the weir surface roughness increases the upstream water level over the weir increases and the discharge coefficient reduces. the velocity profile at upstream sections of the weir crest is extremely different from that over the weir crest while the velocity profile at downstream sections of the weir crest follows the same pattern as those experienced at the weir crest. also, the increased roughness makes the velocity profile over the weir more uniform, with a higher average velocity. finally the effects of roughness on velocity values are less near weir in comparison with water surface.