Sedimentation tanks are engineering constructions commonly used at water and wastewater treatment facilities, especially the rectangular sedimentation tanks. The construction, design, and maintenance of settling tanks greatly influences the accuracy of predictions of removal efficiency, thickness of sediment depositions, and particle size distributions in effluent. A one-dimensional mathematical model has been created to simulate the rectangular sedimentation tank and evaluate flocculation influence by using depth-averaged method. The model could be used for unsteady flow and nonuniform particles, and predict important information such as removal efficiency, sludge thickness, particle and sludge size distribution. By using the depth-averaged method with finite difference approximation, the flow governing equations, the sediment transport equations, and the equations which are used to calculate the floc settling velocities can be solved by using MATLAB program. A comparison with the ideal model and Guo’s model showed that the flocculation model could be used for unsteady flow patterns, and that it is a comprehensive model inclusive of sedimentation tanks with fine particle flocculation. Furthermore, the model user can obtain the results for suspended sediment transport and the flocculation sedimentation tank at the same time to perform the comparison. This model can also provide large quantities of useful information regarding the tank, discrete particles, and floc, based on the desired removal efficiency, sediment concentration, sediment size in bottom sludge and effluent suspension, and sediment deposition thickness along the flow direction. The main objective of this research was to establish a full-scale numerical model to simulate rectangular sedimentation tank operation and evaluate its influence on flocculation. The flocculation factor is a ratio between floc settling velocity and the corresponding discrete particle settling velocity, which was introduced to create the flocculation model. Using floc settling velocities, the flocculation sedimentation tank model reveals that the phenomenon of fine particle flocculation has a critical particle size, which means that particles greater than the critical particle size would be neglected during flocculation. This is consistent with the conclusion from Jianwei- Huang’s published paper in 1981 and other researchers’ investigations. The derivation of governing equations, the numerical methods which were used to discretize the depth-averaged equations, the model setup procedure, and simulation results are discussed. The flocculation model is compared with experimental data and shown good agreement. At the end, the code for the numerical study has been included in the appendix.