Riprap sizing for scour protection at river confluence

River confluence is a common feature of most irrigation and drainage channels and river systems, where tributary conflicts the main channel. In these section, rapid changes in velocity and discharge, sediment distribution and flow turbulent result in a deep confluence scour, a bar point in the separation zone at downstream of junction corner  and finally vortex flow. Thus, the main goal of this study is to conduct a series of experimental tests to investigate the required size of rocks to control the scour hole. The results show that for a constant ratio discharge, Qr, the size of riprap in the incipient motion increases with decreasing in tailwarer depth. In other words, for any rock size the tailwater depth required for incipient motion increases with increasing the ratio of discharge, Qr. For each constant ratio discharge, or, the size of riprap in the incipient motion increases with increasing in tailwater velocity, Vt. Finally, some equations are presented for predicting the size of rocks and the proposed equations has been compared with existing ones.