Discharge estimation of flap gate installed in a circular channel under free flow condition

Since there are a few reports available regarding the hydraulics of flap gates, it was aimed to conduct a research work on these kind of structures to make it clear whether they could be meet the desired advantages in flow measurement activities. Flap gates are made of cast iron or ductile iron, depending on the type of service. A small differential pressure on the back of the gate causes it to open automatically to allow discharge through levees, sewer lines or drainage conduits. When level of water on the face side of the gate rises above water level on the back side, the gate closes automatically to prevent backflow. Automatic drainage gates must be kept clean if they are to function correctly. The hinged flap acts as a natural skimmer to cause timber, logs or trash to catch between the flap and the seat at low flow. Periodic inspection and cleaning should be scheduled when the water flowing through the flap gate carries floating material. Flap gates provide a simple and cost-effective mechanism to maintain upstream water levels and flow measurement in small canals. Once installed and proper operation is verified, the gate only requires lubrication of its bearings and occasional painting for maintenance. It needs no electric power and no manual adjustment for varying flow rates. This gate can be used in open channels with rectangular section or circular section and ducts under pressure, for flood control, municipal projects, farm levees, sewer outfalls, industrial waste lines, water and sewage treatment plants, tidal drainage, irrigation systems and pump discharge control. In recent years, huge developments have been achieved on flow measurement. However, these achievements were faced with high installation and operational costs as well as highly trained human sources for their better utilization. Therefore, revision in traditional flow measuring tools is needed to get an acceptable precision and low cost of erection and operation. Present research covers the results of a theoretical and experimental investigation on a circular and quadratic flap gates which installed at the exit of a circular channel. Two individual circular pipes of zero slope and diameters of 200 mm and 300 mm were selected as water reach. All experiments were conducted in a circular plexiglas-walled fume, 5.7 m long, having a recirculation flow system. To measure the flow discharge, a sharp triangular weir with apex angle of 90˚ in the upstream of channel and the conveyor to measure the gate opening was used. In this study, the flap gate was tested with different weights. For circular Flap Gate Dimensionless weight (w/ (ρgD3)) varies from 0.0343 to 0.2542 and for square flap gates these values vary from 0.0562 to 0.3305. During tests dimensionless discharge for circular flap gate varies from 0.0123 to 0.2503 and for square flap gates these values vary from 0.010 to 0.258, As well as the circular gate opening which was in the range of 0.0123 to 0.4088 (radian)and for square flap gates these values were in the range of 0.0174 to 0.4012 (radian). The relevant theoretical equations were established to determine the flow discharge and then calibrated by using of experimental observations The research took the advantages of tentative method, basic equation of weir discharge and momentum principles. The values of maximum error (ME), mean absolute relative error (MARE) and root mean square error (RMSE) were computed and used for comparison. The resulting equations of different methods were calibrated and verified by using about 98 laboratory data for circular flap gate and 97 laboratory data for quadratic flap gate. In, basic equation of weir discharge mean absolute relative error for circular flap gate is equal to 4.41% and 5.55% for quadratic flap gate, this amount is in order for momentum principles 5.66% and 6.84%. A comparison of the values from the derived equation with statistical criteria demonstrated a high accuracy basic equation of weir discharge. It was shown that the proposed method would demonstrate high accuracy in estimating the flow discharge of the flap gates under free flow regime. The absolute relative error was ranging from 3.34 to 6.66 for circular gate, and 4.1 to 8.14 for quadratic flap gate. Advantage of theoretical equation in comparison regression equation is it can be used outside of experimental data but regression equation just has interpolation capability.