Ahmad Tahershamsi - Associate Professor. Faculty of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran,
Masoume Nikfar - Master of science. Faculty of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran.

The purpose of this study is to find forms of a transition in threshold motion. To achieve this goal, a trapezoidal channel was built in the laboratory, so that a contraction transition becomes established. Materials of bed and banks are made of uniform sand with median diameter 1.2 mm. Critical shear stress was calculated, considering the properties of materials used in the bed and banks of the channel and then the threshold discharge was calculated. Water flows through this channel for long enough to produce a graceful type. The results showed that the created transition is similar to a semi-parabolic and hyperbolic transition. The difference between the developed transition and a semi-parabolic one is the least.

length of fully developed flow, shield’s diagram, threshold of motion, contraction transition