In terms of the importance of stilling basins in Egypt, the present paper was initiated with the impartial of investigating the stability of bed protection at downstream “D.S". An undistorted 3-D physical model was constructed for New Assiut Barrage (NAB) with a scale of 1:45. A riprap with a mean particle diameter of 16 mm was placed at the D.S. of the spillway of NAB; thirty (30) experiments were carried out. Thirteen (13) different discharges were investigated. The effect of the number of gates was studied. Measurements were undertaken, analyzed and compared with the results of different formulae. The obtained results indicated that the riprap was unstable in case of discharges more than 40.5 L/s/gate. A numerical model (Flow-3D) was used and validated to simulate the velocity distributions at downstream. During this process, creating graphs inter-relating the tested variables and analyzing the undertaken simulations. The results were discussed in order to compare the physical model with the numerical model. It could be stated that the discrepancies of numerical results were in a range of less than +/- 25%.