عنوان مقاله [English]
Cylindrical weirs are one of the most common structures for measuring flow discharge and controlling water level in canals and rivers. Numerous geometric and hydraulic parameters could affect the characteristics of flow passing over these weirs. In the present study, the effects of surface roughness and upstream sedimentation on discharge coefficient, pressure distribution and energy loss is investigated experimentally under the free overflow condition. The tests were carried out for a non-sedimenting channel bed, and sedimentation levels of 2/3 of the weir height. Three different weir diameters and four different surface roughnesses of the weirs were considered for a wide range of flow discharges. The comparison between results of the discharge coefficient of weirs for different upstream channel bed levels and the diameters of weirs indicated that as surface roughness increases from PVC to 4.5 mm, the discharge coefficient of the weir reduces of up to 8%. Also, by increasing the sedimentation level from zero to 2/3 of the weir height, the weir discharge coefficient increases up to 3%. In addition, the increase of roughness to 4.5 mm for sedimentation level of 2/3 weir height, leads to decrease in the discharge coefficient between 3 to 7 percent. By increasing weir diameter from 150 to 250 mm, the discharge coefficient increases up to 5%, and the energy loss decreases up to 15%. Moreover, it was found that in all of the tested weirs, as surface roughness increases, energy loss also increases up to 14% and by increasing the sedimentation level to 2/3 of the weir height, it reduces by 22%. The comparison between the results showed that pressure variation along the weir at a sedimentation level of 2/3 of the weir height followed a similar trend to the one that was obtained under non-sedimentation level. However, the magnitude of pressure decreased by increasing the sedimentation level. Besides, the effect of roughness on the pressure distribution on the weirs was greater at the downstream face compared with the upstream face.