Abstract The paper explores flow characteristics in the case of free flow of clean water over a partially contracted thin-plate weir with a triangular notch of angle 20.23° and with zero weir height above the bottom of the approach channel of rectangular cross-section at 5 approach channel widths and 7 heads. The description was based on the results from experimental research and numerical simulations. The numerical simulations allowed the description of the characteristics in parts of the flow where measurements were difficult to perform. The experimental research quantifies the validity of the numerical simulations and establish recommendations for construction of numerical models. The boundary between a partially contracted weir and a fully contracted weir was established by dimensionless approach channel width of the value 0.32, the flow characteristics (water surface, velocity) in the full range of the partially contracted weir (dimensionless approach channel width in range 0.32 ≤ B U * ≤ 1.00), and recommendations for the placement of a cross-section to measure water surface level for determine the heads (dimensionless distance −4 ≤ x * ≤ −2) were verified.