SETTLING BASIN DESIGN

Abstract

Settling basin is a device used for the removal of excess sediment load entering into canals. The principle involved simply is to provide a canal section wide and long enough for the resulting reduced flow velocity to cause the sediment to settle down. Several methods of determining the efficiency of settling basins are available. But the existing methods have been found to have limitation with respect to their functional forms and/or scale effects for application in prototype conditions. Therefore, the present study was taken up with the objective of improving or modifying the existing methods for determination of basin efficiency. The experiments were carried out in a tilting rectangular flume of width 0.75m and length 16m within an approach channel of width equal to 0.25 was constructed. The settling basin was located at the downstream end of the -Fiume. The expansion ratio was kept constant at 3.0. The basin width was 0.75m and its length was 1.5m. Three sediments of uniform sizes of 0.132mm, 0.235mm and 0.605mm and of relative density 2.65 were used. The experiments were conducted under subcritical flow conditions at different slopes. The experimental data of Sujudi, Singh, Saxena and Shrivastava were also compiled for the present study. Observed efficiency was compared with the efficiency computed by the methods of Ranga Raju et. al., Garde at. el., Camp-Dobbins, Sumer and U.S.B.R. iii It was found that the prediction of the efficiency from the above existing methods produced good results for coarser size sediments. But for the sizes of 0.132mm and 0.235mm the results were found to be in poor agreement with the above relationships. Therefore, the available data have been re-analysed systematically to study the effect of different parameters on basin efficiency. It is found that, the settling basin efficiency is strongly dependent on the parameters co/v., Ab /Aa and L/h. Here'U• is the shear velocity of flow, co is the fall velocity of sediment particles, Ab is the cross sectional area of settling basin, Aa is the cross sectional area of approach channel, L is the length of the settling basin and h is the depth of flow in the approach channel. A new relationship is also proposed for basin efficiency. The functional fomi of the proposed relationship has a physical basis and it is found to produce good results when validated using independent data.