In this research a laboratory study was carried out to investigate the scour phenomenon downstream gabions protecting rockfill weirs. The study includes the measurement of maximum scour depth and length of scour hole downstream the weir. The shape of scour hole was also studied. Three sizes of aggregate of diameters (0.7937, 1.111, 1.5875)cm were used as downstream bed protection. Three different downstream slopes (1V : 10H , 1V : 7.5H , 1V : 5H) were tested for all sizes of aggregate.
Laboratory results of this study showed a direct relation between scour depth and scour hole length with the densimetric particle Froude number; also, a direct relation with downstream slope. Two empirical equations were obtained; one for the estimation of relative scour depth and the second for the estimation of relative length of scour hole.
Key word: Scour, aggregate beds, rockfill weirs.