Hydraulic structures are often complex and in many cases their designs require rnattention so that the flow behaviour around hydraulic structures and their influence on rnthe environment can be predicted accurately. Currently, more efficient computational rnfluid dynamics (CFD) codes can solve the Navierâ€“Stokes equations in three-rndimensions and free surface computation in a significantly improved manner. CFD has rnevolved into a powerful tool in simulating fluid flows. In addition, CFD with its rnadvantages of lower cost and greater flexibility can reasonably predict the mean rncharacteristics of flows such as velocity distributions, pressure distributions, and water rnsurface profiles of complex problems in hydraulic engineering. Meanwhile, rnperformance assessment of FLOW-3D hydrodynamic model using physical model rnresult has been done and it was found that the 3D numerical model results showed a rngood agreement with the results of the physical model as well as the designed results. rnIn Ethiopia, Tendaho Dam Spillway was constructed recently, and one flood passed rnover the spillway. Using 3D hydrodynamic model Tendaho Spillway hydraulic rnperformance was assessed and it is found stilling basin is inadequate. Because of this rninadequacy the flood in 2011 created huge loses. It is recommended that the spillway rnbed level must be lowered by at least 15 m. Furthermore, the spillway size for its rncapacity to prevent overtopping of the dam has been checked with full Probable rnMaximum Flood (PMF) for a condition that if one of the gate malfunctions and the rnspillway crest length is reduced by one third. According to the result of 3D rnhydrodynamic model it is confirmed that the dam is safe from overtopping with the rnworst condition.The unconventional stilling basin of the Ribb dam spillway in Ethiopia was checked rnwith 3D hydrodynamic model and found that the designed and constructed wall height rnwill be overtopped. Hence, the wall height requires to be raised by an additional 7 m, rnotherwise the spillway as well as the dam are not safe for operation. The capacity of rnthe U-shaped side channel spillway also checked with full PMF and is found that it is rnsafe for the operation however it is uneconomical design. rnHence, design of complex hydraulic structures using the state-of- art of 3D rnhydrodynamic modelling enhances the safety of the structures. Therefore, any rncomplex hydraulic structure such as spillway, at least verification of its design must be rndone with 3D hydrodynamic numerical model before construction.