Steady flow regime for free overfall spillways. Influence of the ascending

Abstract

This paper presents a study on the influence of the upstream quadrant of the spillway crest section, designed with equal geometry as that of the lower surface profile of a free flow over a sharp crested weir, on the discharge characteristics of a free overfall spillway. Geometrical deviations of the upstream quadrant from the standard design criteria as proposed by the U.S. Bureau of Reclamation, lead to variations in the flow discharge efficiency and on the upstream free surface elevations, i.e. on the reservoir free surface level. Starting with an analytical description of the weir problem, governing equations are derived, yielding the identification of three weir operation regimes, say, deep, intermediate or shallow weir; the first of them is the condition standing for many dam spillways. Experiments on the spillway crest profile under deep-weir operation conditions have been conducted, testing an evolution sequence of upstream quadrants ranging among simple geometrical shapes, with the standard design profile as one of them. As a result, a new upstream quadrant consisting of a 1:1 slope chamfer is proposed, since the hydraulic performance is found to be equal to or slightly better than the standard crest profile. An experimental program has also been carried out in order to determine safety limits regarding negative pressures on the spillway surface. From the standpoint of design, the suggested upstream geometry is easier to define, and cost-effective in terms of construction. A wider range of experimental set-ups, referred to model scales and flow conditions, should be conducted with the presented results as a starting point, in order to verify the adequate performance of the proposed design.