Open channel flow is vital in agriculture and is often used for water diversion. In open channels, obstacles (dams) are frequently installed to regulate the downstream water level and flow velocity. The flow after passing through the barrier usually occurs as the phenomenon of hydraulic jump due to the narrowing of the wet cross-section. The flow structure and the transition zone of the hydraulic jump significantly affect the water level downstream after the dam, the aquatic habitat, and the erosion of the channel. This chapter explored the impact of trapezoidal weir roughness on the downstream flow structure in rectangular open channel flow. The results indicate that the energy loss of the flow through the weir has an insignificant error, less than 6% when comparing the numerical model results with experimental results. Two flow regimes, Q = 5 L/s and Q = 22 L/s, were carried out on the trapezoidal weir for the roughened and unroughened cases. The results showed that roughness smoothed the flow, the water behind the weir had less turbulence, and the transition zone from fast flow to smooth flow was shorter than in the unroughened case. As the mass flow rate Q decreases, the energy loss through the trapezoidal weir increases.