This study presents an experimental measurements and three-dimensional simulation of time-varying scour, along with the variables affecting the resulted scour hole dimensions and evolution at center and side weirs. Slit weir location, crest level, flow rate and sediment size were tested within 34 steady and unsteady flow conditions under various flow rates 125, 95, 62, 50 and 34 L/s to study the scour behavior. All tests were carried out with non-uniform sediments of median sizes 0.24 mm and 0.55 mm. Steel slit weirs of 0.25 m width, 0.004 m thickness, and 0.07, 0.12 and 0.17 m crest level with respect to the mobile bed surface, which was constructed in a laboratory flume made up of brick and concrete within dimensions of (8 m, 1.25 m, and 1 m) long, wide and deep respectively. The present research demonstrates that maximum values of scour volume and scour depth obtained when the weir locate at center of the flume with flat crest for both steady and unsteady flow. Besides, the results indicated a low discrepancy between the experimental measures and the sediment scour model FLOW 3D software, both are showed a good agreement in presenting the scour development accurately as well as the model had excellent ability in predicting the velocity magnitudes at the slit location and the values of shear stress which was hard to measure directly.