A Better Description of Trap Efficiency for Fine Sediment in Horizontal Settling Basins

by Shaohua Marko Hsu, (M.ASCE), Feng Chia Univ, Taichung, Taiwan,
Wei-Sheng Yu, Feng Chia Univ, Taichung, Taiwan,

Abstract: Settling basin removes sediment from intake water and enhance the benefit of diversion works. In order to economize construction land and cost as well as for agriculture fertilization purposes, it is impossible nor desirable to settle down all the inflow fine particles. In stead, design of a settling basin should be based on a prescribed trap efficiency which allows certain favorable size fraction to pass, and settle down the unfavorable ones. In this article, flume tests are performed and two types of settling processes are identified and described. A transport formula is proposed for the topset section of the delta, which can estimate the effective trap capacity of a settling basin. By comparison with other trap efficiency formulas, conclusion is made that particles with Ws/U* larger than 2 will settle down totally in the delta and have 100% removal. All existing formulas predict this pretty well. Different formulas, however, predict quite differently for the finer particles, especially for sizes between 0.05 mm and 0.1 mm. This size fraction is crucial in engineering aspects and worth in-depth study. Field data and flume experimental data is underway collected to verify which formula has better.

Subject Headings: Settling basins | Sediment | Particles | Flumes | Water intakes | Field tests | Data collection | Spillways

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