Numerical investigation of energy dissipation and flow aeration on flat stepped spillways

Document Type : Original Research

Authors
M. Shokri Kaveh
F. Salmasi
A. Abbaspour
Department of Water Engineering, Faculty of Agriculture, University of Tabriz
10.48311/mcej.2026.12718
Abstract
In order to facilitate the release of floods from the dams and to prevent their damage or collapse, a structure called a spillway is used. Due to the natural and variable flow of the input to the reservoirs of the dams, there are times when the river inflow exceeds the consumption amount in the downstream agricultural lands. In these cases, excess water is discharged over the crest of the weir and flows towards the spillway, which causes high velocities. This high velocity creates low pressure areas on the spillway concrete surface, which can cause major damage to the spillway or even endanger the integrity of the dam structure. Therefore, the dam spillway must safely dissipate the kinetic energy. One of the types of weirs is the stepped spillway to facilitate the passage of the flow over the dams. One of the most obvious practical features of stepped spillways compared to other spillways is the considerable energy dissipation along the spillway. Care should be taken in designing and selecting the type of spillway to prevent potential erosion and reduce kinetic energy as the water flow passes over the spillway. One possible solution is to use a stepped spillway instead of a smooth spillway. In this study, a numeral model of a stepped spillway with different steps and slopes is used. For this purpose, ANSYS software is used for modeling free surface with application of k-ε turbulence model. In the present study, numerical simulation using the Volume of Fluid (VOF) model was used to investigate the mixing phenomenon of two phases of air and water of the free surface flow. The flow field was continued until the residuals reached 10-7. Compared to simpler models such as Mixture, which operates solely on the basis of averaging the properties of two phases, the VOF model, is separating the phases and considering the effects of the interface. The VOF model, is capable of more accurate simulation of phenomena such as fluid mixing, turbulent flows, and heat transfer in multiphase flows. A number of hydraulic specifications which are considered in designing the stepped spillways are the pressure on the surface of the steps, velocity distribution and energy dissipation. The results from the numerical models were compared with experimental studies. They showed acceptable agreement with physical simulations. Results show that discharge and spillway slope increment reduces the amount of energy loss. In the spillway with 5 steps, for a discharge of 0.063 m3/s, the amount of energy dissipation at a slope of 26.6 degrees changes from 85 to 82% at a slope of 45 degrees, which shows a decrease of 3%. With the increase in discharge, the flow depth increases and reduces the effect of the roughness of the steps on the upper layers of the flow. Increasing the height of the steps increases the rate of energy dissipation and also increases the occurrence of negative pressures in stepped spillway. In this case, the contact surface between the main flow and the eddy currents increases. With the increase in the height of the steps, the dimensions of the rotating vortices also increase and cause a larger radius of rotation on the steps. The presence of these large rotating vortices separates the flow from the bottom of the steps and reduces the pressure on the surfaces. The number and dimensions of steps can alter the energy dissipation rate. Increase in the number of steps in a spillway with constant height, reduces the energy loss as the result of steps dimensions being shrunk

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Modares Civil Engineering journal
Volume 25, Issue 6 - Serial Number 86
September and October 2025