Laboratory Investigation on Bottom outlet of Dam (Case study Narmasheer Dam)

Bottom outlets are series of structures that were used to convey water from dam reservoir to the down-stream side. Accordingly, because of the importance of Bottom Outlet Systems in dams, analyzing the performance of conduit, gates and outlet is considerable. The existence of high-pressure flow in the bottom outlet channel, the various kinds of head losses and the small amount of opening height versus the reservoir head over the gate cause considerable errors in the required mathematical methods and relationships. Therefore the need for laboratory studies in this regard is essential. The purpose of the present study is to determine hydraulic capacity of Narmasheer Dam Bottom outlet Narmasheer the lower dam using data obtained from physical model, is investigated. In this research, the physical model of bottom outlet system of Narmasheer Dam including conduit, gates (service and emergency) and outlet was designed and constructed at SCWMRI (Soil Conservation and Water Management Research Institute) lab, and then the necessary tests have been performance. The different amounts of pressure heads and water discharges in various gate openings were measured for 6 different head of reservoir and 12 opening gate. Results showed that trend of discharge coefficient hydraulic in maximum head, from 2 to 10 percent opening gate, had an increasing rate and from 10 to 30 percent opening, had a decreasing rate and then from 30 to 70 percent opening, will be increase with smooth rate, and finally in more than 70 percent in opening gate, will be increase with accelerate rate. This process also tested in other head of reservoir be seen, but trend discharge coefficient changes, from 50 percent opening gate, begins to increase. Overall change in coefficient d C showed increased amount of valve coefficient opening gate d C , be increased. Investigate of the discharge change showed with increases in opening gates, the rate of discharge will increased. Also the results of measurements of exerted forces gate and vibrations into the gates are provided. The necessary tests have been made, so that using the measured pressure distribution on the gates, vertical forces and maximum damage in both normal head tank and various gate openings, were determined respectively. The exerted forces were calculated based on the use of pressure distribution obtained by pizometers installed above and under the gates. In this study, the maximum horizontal force appears on the 2 percent gate opening and is equivalent to 2410 (KN). The maximum horizontal force into the emergency gate has also occurred in the initial gate opening situation with a value of 2507 (KN). The experimental results also indicated that the maximum operation frequency of vibration due to the high pressure flow in the bottom outlet channel is 80 per second. The tests showed that by increasing the diameter of gate lift arm, the frequency of vibration can be controlled.