Climate changes leading to extreme events when the occurrence and intensity of severe phenomena have impeded the development and rehabilitation of hydraulic infrastructure, such as weirs and spillways according to these new conditions. The scale physical model is one of the most important methods for designing these hydraulic structures. The selection of the model scale is necessary for model operation accuracy and design performance because of the effect of this process on design parameters resulting from the model. Scale effects can be seen through the large effect of some forces on the model that are not effective in reality, such as surface tension and viscosity forces, whose effect can be seen on the model with a relatively small scale, but are not effective on the structure at the actual size. The present study is trying to know how the physical model scales affect flow patterns over the spillway of small dam (less than 15m height) and the extent of this effect. Three physical model scales (1/30, 1/75, and 1/100) were used and compared with the actual size of the structure by numerical model (FLOW 3D). The physical and numerical models were tested with discharge values of (250, 350, 500, 750, and 1000) m3/s to evaluate the rating curve, discharge coefficients, pressure distributions, and energy dissipation. The results show the scale has significantly impacted the results, and it is preferable to avoid small scales. The 1/30 scale aligned more closely with the numerical model (the actual dimensions of the spillway) and strikes a compromise between measurement precision and expense. Therefore; the present study recommended using the scale 1/30 physical model or more for small dam spillway design.

Climate changes leading to extreme events when the occurrence and intensity of severe phenomena have impeded the development and rehabilitation of hydraulic infrastructure, such as weirs and spillways according to these new conditions. The scale physical model is one of the most important methods for designing these hydraulic structures. The selection of the model scale is necessary for model operation accuracy and design performance because of the effect of this process on design parameters resulting from the model. Scale effects can be seen through the large effect of some forces on the model that are not effective in reality, such as surface tension and viscosity forces, whose effect can be seen on the model with a relatively small scale, but are not effective on the structure at the actual size. The present study is trying to know how the physical model scales affect flow patterns over the spillway of small dam (less than 15m height) and the extent of this effect. Three physical model scales (1/30, 1/75, and 1/100) were used and compared with the actual size of the structure by numerical model (FLOW 3D). The physical and numerical models were tested with discharge values of (250, 350, 500, 750, and 1000) m3/s to evaluate the rating curve, discharge coefficients, pressure distributions, and energy dissipation. The results show the scale has significantly impacted the results, and it is preferable to avoid small scales. The 1/30 scale aligned more closely with the numerical model (the actual dimensions of the spillway) and strikes a compromise between measurement precision and expense. Therefore; the present study recommended using the scale 1/30 physical model or more for small dam spillway design.