The accumulation of sediments in reservoirs of dams has always been one of the problems of reservoir dams. Accordingly, one of the approaches to address this problem can be referred to as the method of pressure flushing. In this method, during the course of flushing, generally the bottom drainage outlets of the dam are opened for a certain period of time and the reservoir water level is kept almost constant. When water discharges through the bottom outlets, and after performing flushing, a hollow or a hole in the form of a funnel or a cone will appear at the front of the outlet gate, that the size of this scour funnel depends on various parameters such as: discharge, depth of water inside the reservoir and the types of sediment deposited inside the reservoir. In this research, a physical model with specific dimensions was used to investigate the effect of cohesive and non-cohesive sediments on the form of flushing funnel. The result of this research shows that in non-cohesive sediment the erosion pattern was retrogressive, and while experiments are done with minimum water depth (40 cm) and maximum discharge (8 lit/s), the flushing funnel will be balance after 9-11 minutes. In the case of cohesive sediments, the results from statistical analysis, show that the relationship predicted for average sediment density of the bottom outlet ( = )can be presented as follows: