Abstract: Experiments were conducted to study the types of air intake and critical depth-flow velocity relations using a 1∶3 model. The results demonstrated that types of air intake and critical depth were related to turbulence. An increase in flow velocity resulted in an increase in critical depth. Similarity analysis indicated that critical depth agrees with the power rule of 0.2 for relative low velocity, however critical depth agrees with the velocity when velocity is high enough. The required minimum depth in the Japanese standard was discussed according to the experimental results.