SEDIMENT REMOVAL EFFICIENCY OF PRESSURIZED DESILTING CHAMBERS

Abstract

Desilting chamber is a very important device used to trap the excess sediment load from flowing water of natural channels. It enables to provide relatively clear water to the Hydro-electric power (HEP) and Water Supply Projects as well as to Irrigation channels. In case of HEP projects, excess sediment if entered to the HEP Project system then it damages the installed turbines. While in the case of the irrigation channels, if excess sediments enter to the irrigation channel, then sediment deposition in the channel takes place, and reduces the water carrying capacity of the irrigation channels. The principle is to provide a section wide and long enough resulting in reduced velocity to cause the sediment to settle down. Several methods of designing the desilting chamber are available in the literature. Almost in all the cases; the flow condition in the desilting chamber was open channel flow. Few investigators had studied the desilting chamber behavior under pressurized flow condition. Therefore the objective of this study was to conduct the experiments on the pressurized desilting chambers to study its sediment removal efficiency under varying sediment load and discharge conditions, verifying the earlier methods and to develop a new method, if possible. The length of the chamber was 12 m and its cross section consists of three sections. The top most section was semicircular of diameter 0.60 m, middle section was rectangular (0.60 m x .40 m) and the bottom section was trapezoidal with top and bottom width 0.60 m and 0.10 m, and height of 0.25 m. Water was supplied to the desilting chamber from a reservoir. The sediment used during the experimentation was well graded sediment of size varied from 20 gm to 600 gm. The concentration of the sediment varied from 1800 mg/L to 3300 mg/L. The discharge also varied 13 L/s to 40 Us in the different runs of the experiment. In every run, the flow condition was kept steady by-making the discharge constant with the help of calibrated weir and valve operation. It was observed that the coarser particles get deposited in the upper part of the basin and the fine particles moved in the lower part. The particle size gradation of the deposited sediment along the length of the basin is also affected by the discharge in the desilting basin. Higher discharge moves the coarser particle downstream of the basin compared to low discharge. Sumer's (1977) and Camp-Dobbin's (1946) equations over estimated the total sediment removal efficiency of the basin while USBR (1990) under estimated. The agreement of estimated efficiency by Garde's et al. (1990) equation with observed efficiency depends on the discharge in the basin. The agreement is reasonable for lower discharge in the basin while this is not valid for higher III discharge. None of the available equations predicts accurately the total sediment removal efficiency of the basin under the pressurized condition. Therefore a new relationship has been proposed to compute the total sediment removal efficiency. The proposed relationship predicts the sediment removal efficiency of the basin more accurately i.e., within ±12% of the observed values compared to the existing equations under the pressurized condition. Keywords: Desilting chamber, Hydro-electric power (HE?) project, Water Supply Project, Open channel flow, Pressurized flow, Sediment removal efficiency,