Page - 189 - in Book of Full Papers - Symposium Hydro Engineering

the diaphragm. Once a steady and constant flow was established in the model, the discharge of the water entering the vortex settling basin was measured by two rectangular and triangular weirs at the end of outlet channels. After flow fixation and discharge adjustment, sediment supply began with a constant discharge for a specified duration (20 minutes). When sediments finished, the pumps were turned off (water inflow to the basin was cut off), the flushing orifice exit valve was closed. Then the sediments within the vortex basin and the sediments exited from the orifice and the weir were separately collected, dried and weighed. This procedure was done for discharges of 45 and 37 l/s and six vane arrangements in three radial sections. 4 Results and Discussion To calculate the vortex settling basin efficiency, Ziaei (2000) used the sampling method. He gathered samples from the weir outlet and the flushing orifice and then calculated the basin efficiency using sample concentrations. Keshavarzi and Gheisi (2006) and Athar (2000) used another method to calculate the vortex settling basin efficiency. They collected and dried sediment exited from the weir and the flushing orifice and then calculated the vortex settling basin efficiency based on the weight of collecting sediments. After examining methods for determining the vortex settling basin efficiency, it was known that the methods of Keshavarzi and Gheisi (2006) and Atahr (2000) are more accurate. Therefore, the present study used that method. Sediments in this system are divided into three main categories. The first category is sediments which are transferred out of the basin together with some percent of water from the flushing orifice. The second category is the sediments settled in the vortex basin floor and, over time, they cause dysfunction in the settling performance of the structure. Finally, the third category which consists of a small percent of sediments exited through the vortex weir with water flow. To calculate the vortex settling basin efficiency according to the harvested lab data, parameters ηT, ηO and ηB were used. These parameters were defined as follows: BoT      (1) T o o W W   (2) T B B W W   (3) Which ηT is total settling efficiency, ηO is the settling efficiency of the orifice, ηB is the settling efficiency of the basin floor, WT is the total weight of the sediments entering the basin, WO is 189