Geometry influence and discharge rating curve correction in morning glory spillways.

Laura Maria Canno Ferreira Fais, Jose Gilberto Dalfré Filho, Ana Inés Borri Genovez

Monday 29 june 2015

16:30 - 16:45h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 3F. Engineering instrumentation

Spillways are structures that control the water level in a reservoir, ensuring the dam safety. Since the main cause of dam failures is related to poor discharge flooding design, the adequate determination of the discharge rating curve of the spillway is an essential parameter. Depending on the characteristics of the dam section, a morning glory spillway is an alternative to the conventional ones, as it is placed inside the reservoir. It consists of a conventional crest followed by a vertical shaft, then, a circular bend ending up into a tunnel. Each one of these components has its own design criterion. The use of physical modelling and the advances of computational modelling make the design of the structure accurate. However, a challenge is to design the bend. Usually, a conventional circular 90º radius bend is used but, there are no theoretical or experimental studies to verify the optimum geometry of the bend, which is the critical design element of the spillway. In the bend, the flow deflects and causes high pressures in the convex area and depressions in the concave area that may cause flow instability which leads to vibration. The design of an optimum geometry bend could avoid the poor performance of the hydraulic structure. Nevertheless, researchers and engineers must be aware of the scale effects when adopting the physical modelling. So, Reynolds number must be greater than 105, Weber number greater than 500 and Mach number lower than 0,3. This paper has the objective of analyze the discharge rating curve of the morning glory spillway, using a novel parabolic type bend and compare it with a 90º circular bend and a polycentric bend with variable radium. It was also evaluated the scale effects in the discharge rating curve for the new bend geometry and the best fit to the prototype values. The authors carried out an experimental study in 1:51,02 scale, using the spillway model of Paraitinga Hydropower, Brazil, in the Laboratory of Fluid Mechanics and Hydraulics, School of Civil Engineering, Architecture and Urbanism, University of Campinas, Brazil. The parabolic type bend presented an increase in the discharge rating curve, indicating that this geometry should be considered for the design of morning glory spillways