Experimental study of combined effect of roughness and sedimentation on hydraulic performance of cylindrical weirs under free flow conditions

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 can affect characteristics of the flow passing over these weirs. In the present study, the effect of surface roughness and upstream sedimentation on discharge coefficient, pressure distribution and energy loss was investigated experimentally. The tests were carried out for non-sedimentation channel bed, and sedimentation levels of 1/3 and 2/3 of the weir height. Three different weir diameters and four different surface roughness of the weirs was considered for large range of flow discharge. Comparison of results of discharge coefficient of weirs for different upstream channel bed level and the weirs diameters indicated that by increasing surface roughness from PVC to 4.5 mm, the discharge coefficient of the weir reduces of up to 8%. Also, by increasing sedimentation level from zero to 2/3 of the weir height, the weir discharge coefficient increases up to 3%. In addition, increasing of roughness to 4.5 mm for sedimentation level of 2/3 weir height, leads to decrease 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 decrease up to 15%.. it was found that, in all the tested weirs, by increasing surface roughness, energy loss increases up to 14% and by increasing the sedimentation level to 2/3 the weir height, it reduces about 22%. comparison of the results showed that pressure variation along the weir at a sedimentation level of 2/3 the weir height followed a similar trend to that obtained under non-sedimentation level. However, the magnitude of pressure decreased with increasing sedimentation level.