Laboratory Determination of Discharge Cagacity of Emergency and Service gates in Bottom Outlet Dams (Case Study of Narmasheer Dam Bottom Outlet)

Bottom outlets are series of structures that were used to convey water from dam reservoir to the downstream. Due to 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 using data obtained from physical model. 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 performed. 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 the trend of discharge Hydraulic coefficient in maximum head، from 2 to 10 percent gate opening، had an increasing rate and for 10 to 30 percent opening، had a decreasing rate and then from 30 to 70 percent opening، will be increased with smooth rate، and finally in more than 70 percent in gate opening، will be increased with an accelerated rate. This process was tested in other heads of reservois، but trend of discharge coefficient will begin to increase from 50 percent gate opening. Overallchanges in coefficient Cd show an increase amount of valve coefficient for gate opening، Cd. It is worth to mention that with an increase in opening gates، the rate of discharge will also increase.