沉 砂 池 囚 砂 效 率 之 研 究

Abstract

現行沉砂池規劃設計依水土保持技術規範為之，惜未考量其囚砂率之良窳與經濟效益；本研究目的除分析各種理論沉砂池經驗式之囚砂效率外，並謀求以水平二維動床數值模式來替代需耗費大量人力、物力及大型場地模型試驗之可行性，同時進行不同尺寸沉砂池之模擬，以瞭解不同長寬比與囚砂率間之關係，以供沉砂池設計之參考。 本研究首先收錄常用之理論沉砂池囚砂率經驗式如：Hazen模式、Camp & Dobbins模式、Vetter模式、Einstein模式、Chen模式、Sumer模式、Garde et al.模式及連與黃模式，並提供演算實例，可作為設計沉砂池之參考。由各種囚砂率經驗式比較分析得知，Hazen與Vetter所求得之囚砂率分別為各經驗式中之最大與最小者。 前述諸理論沉砂池囚砂率經驗式，並未考量沉砂池幾何形狀對流場之影響，是以在推估囚砂效率時，將有較大之誤差。故本研究進一步利用淤積實測資料率定後之水平二維動床數值模式，模擬不同幾何形狀之沉砂池案例，並探討其對沉淤效率之影響。模擬結果顯示沉砂池長寬比較小者，其流場變化較劇烈，淤積效率亦較差，若以較簡化之理論模式估算，無法反應其變化，故建議以水平二維動床數值模式進行模擬，方能獲致較正確之結果。

The current planning and design of settling basin is based on the Technical Code of Soil Conservancy. However, the trap efficiency and economic benefit of the settling basin is not fully considered. The purpose of this study is to analyze traditional empirical formulas, and to seek the possibility of using a 2-D depth-average mobile-bed model in replacement of physical model experiment which requires large amount of manpower, money, and space. Through simulations of various sizes of the settling basins, the relationship between the trap efficiency and the length-width ratio of the settling basin can be obtained, which can provide information for the design of the settling basins. This study collects commonly used empirical formulas such as the Hazen model, Camp nad Dobbins model, Vetter model, Einstein model, Chen model, Summer model, Garde et al. model, and Lien and Huang model. In the meantime, this study provided an example for the design of settling basin by using above mentioned formulas. Comparison of the results shows that the trap efficiency by the Hazen and the Vetter models are respectively the largest and the smallest among these empirical formulas. Because the empirical formulas do not consider the effect of the geometry of the settling basin on the flow field, the estimation of the trap efficiency then will cause larger error. In this study, a further step is the adoption of a 2-D depth-averaged mobile-bed model, which is calibrated by experimental data of an aggrading channel. The effects on the trap efficiency by several different geometries of the settling basins are investigated by the numerical modeling. Simulated results show that flow field has large variation for basins with small length-width ratios, and consequently results in small trap efficiency. This fact cannot be reflected by the conventional empirical formulas. Therefore, a 2-D depth-averaged model is recommended to obtain more accurate trap efficiency.