Experimental investigation of increasing height of well, and mass of weight effect on discharge coefficient over WLH Fusegate model in small flumes

To pass the extra water and flood from upstream to downstream in dams, a hydraulic structure called spillway uses, which is one of the important appurtenant structures in each dam. Selecting the type of spillway is a function of topographical conditions, the amount of design flood and type of dam. Spillways are made in different forms that the most common of them are overfall and overflow (overpass) spillways. Free or uncontrolled overfall spillways are the most reliable choice. These spillways usually impose higher construction cost and causes in wasting a considerable amount of water or live capacity of the reservoir. Employing Fusegates might be a way of reconciling dam safety with maximized storage capacity. Fusegates were invented in 1989 by François Lempérière as a simple, robust, and a safe system to increase live storage or spillway capacity. The system has been patented by Hydroplus International in the United States, Europe, and most other countries. It is implemented in more than 40 dams in 14 different countries across 5 continents. This spillway consists of three main components; 1) A bucket made of metal or reinforced concrete, 2) A base, and 3) An intake well which is connected to the chamber in the base. Fusagates consists of three types of Narrow Low Head (NLH), Wide Low Head (WLH), and Wide High Head (WHH).
Tests were done in a horizontal flume with a rectangular cross section in the hydraulic laboratory of Water Engineering Department at Sari Agricultural Sciences and Natural Resources University. This laboratory flume has a length of 5 m, width of 75 mm and height of 175 mm (the walls are made of transparent Plexiglas to see the flow from two sides). In this study, the effect of 6 wells with different heights over WLH model of Fusegate has been studied and two weights with different mass were used for the equilibrium of the structure. The investigated spillway is made of Plexiglas and has a height of 50 mm. To balance the model, two iron weights with a mass of 43.7 and 55.8 gr were applied, which are placed on the chamber’s floor. The position of this model is at the 1 meter upstream the outlet of the channel, for reducing the inflow turbulence and water surface fluctuations at the upstream of the structure. The range of the flow discharge was about 0.2 to 1.8 lit/s. In every test, the water depth was measured at 30 cm upstream of the structure. A total of 228 tests was done, that 20 percent of them were selected randomly for verification the equations. In this study, the RMSE was used as evaluation criteria.
In order to find a relationship between effective factors on discharge coefficient in Fusegate spillway, dimensional analysis was done on the effective parameters. The discharge coefficient can be dependent on geometry, kinematic and dynamic variables (h, ρ, v, σ, μ, w, H). Four obtained dimensionless parameters are including discharge coefficient, Weber, Reynolds and h/H. According to dimensional analysis, Reynolds number was removed, because the flow was turbulent. Since, the spillway was a physical model with a small-scale, surface tension was not negligible and the Weber number was affective. Finally, a power relationship between the discharge coefficient and two dimensionless parameters (Weber and the upstream water level to the bucket’s height (h/H)) of Fusegate spillway was created. The coefficient of determination (R2) was used for calibration and root mean square error (RMSE) was used for verification. When RMSE values are near zero, indicates that the computational and observational discharge coefficients are closer together and the equation has a higher accuracy. When R2 values are near to 1, shows that the most suitable values for constant parameters were selected. The maximum value of R2 is related to the second well with a weight of 55.8 gr, and the lowest value was belong to the forth well of this weight. The discharge coefficient range was approximately between 0.33-0.58. In a constant h/H, the highest discharge coefficient and discharge was related to the second well. In the other word, the lowest upstream water level was related to the well with a height of 57 mm, which shows that during the flood, this well can prevent flooding of upstream adjacent land. By increasing the Weber number, the discharge coefficient decreases. In fact, in high discharge coefficients that the upstream water level is low, the effect of surface tension is undeniable. At a constant discharge coefficient, the highest value of Weber number that shows the lowest surface tension, was related to second well. With increasing height of well from 52 to 62 mm for the first and second weights, with increasing the tilting discharge, the tilting head increases. But with increasing height of well from 62 to 77 mm, increasing discharge does not effect on the tilting head. Because, the motive forces overcome the resisting forces that makes Fusegate to be in an unstable state, before the water enter into the well.
The test results showed that in the WLH model of Fusegate spillway, the dimensionless parameters (h/H) and (We) are effective on the discharge coefficient. By increasing (h/H), the discharge coefficient decreases as parabolic. In fact, by enhancing the water level above the spillway, the discharge coefficient declines due to downstream submergence. Also, this result is valid for enhancing the Weber number. In a constant (h/H) and (We), the discharge coefficient of the second well is more than others, because of having less surface tension and lower head of water at the upstream. Finally, a power relationship between the discharge coefficient, h/H and We extracted in 15 cases, that the calculated statistical parameters indicate high accuracy of equations.