Author(s): Charng-Ning Chen
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Keywords: Reservoir; Basin; Trap Efficiency; Sediment Size; Settling Velocity; Fall Velocity; Inflow; Outflow
Abstract: The commonly used methods for estimating trap efficiency of reservoirs are empirically based. The trap efficiency is expressed as a function of reservoir dimensions and inflow rates, with little consideration given to the sediment size. On the other hand, methods for estimating trap efficiency of settling basins have been developed on the basis of the mechanics of particle settling in a basin. The key parameters employed are the basin surface areas, outflow rates and settling velocity of the sediment grains. The trap efficiency function formulated by Camp (1946) and Vetter (1940) for turbulent flow condition (Eq. 2) is applied to generate a series of curves expressing the trap efficiency, E, as a function of the ratio of reservoir surface area, A, to the outflow rate, Q o. The sediment size is expressed in terms of its settling or fall velocity, w. As shown in Fig. 5, the trap efficiency curves for the sand, silt, and clay classes spread over a range of 6 to 7 log cycles of the value of A/Q o. This divergence or spread of the curves suggests the importance of sediment size on the economic design of storage reservoirs, as well as settling basins. The characteristics of the outflow from the reservoir, Q o, are of equal importance to the settling process in a reservoir. A rational approach relating the outflow rate to the reservoir inflow rate (Eq. 3) is also presented. For comparison, the empirical trap efficiency curves developed by Brune (1953) and Churchill (1948) are re-plotted in a graph of trap efficiency, E, versus A/Q o. The results show that the Brune and Churchill methods tend to underestimate the trap efficiency for coarser sediment and overestimate it for finer sediment. Improvements to the method of trap efficiency estimate can be made by refining the method presented in Fig. 5, and formulated by Eqs. 2 and 3, of this paper. Pertinent field data associated with sediment, inflow, and outflow characteristics can be used for the calibrating and refining efforts.
Year: 2004