Abstarct: One of the main problems in Dam Engineering and related fields is sedimentation. Sediments are long term enemies of reservoirs and can question a project altogether. In warm and dry zones this problem shows itself more prominently, because as the first rain and thus runoff occurs, a great volume of sediments enter the dam’s reservoir. The result of these sediments entering, is the reduced capacity of the reservoir. Because most of the literature on sedimentation in over-ground reservoirs is about experimental investigations, and also because of the numerous advantages of numerical modeling, in this thesis modeling has been done using the finite volume software Flow-3D. First, the effect of different geometries on flow’s hydraulics and sediment’s deposition pattern is studied using existing lab results. Then, we chose three rectangular reservoirs with different dimensional proportions for modeling with the goal of designing a basin with the best sediment trapping percentage. To achieve this purpose some discrepancies in the place of inflow and outflow which is introduced by the ration G/D and flow’s parameters like flow rate where made. Overall, 42 different models where made using Flow-3D, and LES turbulence model was used in the process. In the end we reached the conclusion that between these three rectangular reservoirs, the 6*4 had the best sediment trapping percentage, and as the G/D was increased this percentage increased too. Also by increasing the flow’s rate (or in other terms increasing the Froude Number) efficiency of the sediment entrapment basin was reduced significantly.