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In this research, a study was conducted experimentally to investigate the scour hole dimensions downstream the combined structures which consist of weir and gate. Twelve models have been designed and every model is formed from composite weir consists of two geometric shapes and three types of gates which are rectangular, semi-circular and triangular in shape, where multi factors were studied to find out the effect of changing geometry for both weir and gate, discharge flowing in the flume and particle size of bed material on the dimensions of scour hole. The experiments was conducted in a laboratory channel was constructed from blocks and concrete with length of 18 m, 1 m width and 1 m depth. The laboratory models were installed after 7 m from the main gate which is controlling the passage of water from the main reservoir into the flume. At the beginning, the calibration process was conducted to identify the actual discharge values that pass in the flume, then experiments were conducted to calculate the discharge coefficient for each model, which represents one of the studied factors within the dimensional analysis of the variables to derive the empirical formulas to calculate the dimensions of scour hole. Then the experiments were conducted in order to derive formulas to investigate the depth and length of the scour hole which formed in the sand floor spreading as a layer of 30 cm in thickness for a distance 4 m downstream combined structure. Two samples of sand were used in the experiments with different median size of particles ( d50), the first of 0.7 mm and the second of 1 mm. Using the dimensional analysis by π theorem and IBM SPSS 21 program, Four nonlinear relationships were derived to calculate the dimensionless scour depth (SD / d50) and another four nonlinear relationships calculates the dimensionless length of scour (SL / d50) depending on the laboratory results for each of the relative discharge (Qr), Froude number in terms of mean size of particle of bed material (Frd), non-dimensional difference head between upstream and downstream of combined structure (HD / d50), dimensionless distance between the lower edge of the weir and the upper edge of the gate (y3 / d50), dimensionless head over the crest of compound weir (h/d50) and the discharge coefficient (Cd), where the resulted determination coefficients (R2) from these relationships were good.