凝聚性沈滓傳輸機制之模擬與分析

Abstract

凝聚性沈滓的運移機制相當複雜，但是其基本的運移機制也許可由沈降、沈積、壓密及沖刷等傳輸行為予以探討之，截至目前為止，大部分的研究均以實驗的方式來了解其運移行為，而本研究不以實驗的方式，乃根據前人實驗成果，並利用數值模擬的方法來加以分析探討。本研究主要乃於謝氏（1992）發展之二維水深平均水理模式中，植入可同時模擬凝聚性沈滓及非凝聚性沈滓運移行為的機制，並設計案例來加以驗證分析其運移行為。本研究將四種沈積經驗式做一比較模擬分析後，發現明渠流場中的凝聚性沈滓落淤和流速、臨界沈積剪應力、入流濃度、及沈降速度有關，其中沈降速度應該考慮濃度的影響，而粒徑因素可以忽略。底床沖刷為一整體沖刷的概念，完整的沖刷機制應該考量表層沖刷、塊狀剝蝕及底床的壓密，其中壓密更是凝聚性沈滓與非凝聚性沈滓間最大的差異。

The transport mechanism of cohesive sediment is very complicated, which may be able to be examined through the following transport processes：settling, deposition, consolidation, and erosion. Many experimental studies have been carried out to investigate the transport behaviors of cohesive sediment. This study uses numerical model to analyze the transport mechanism according to the previous investigators’ experimental results. The main purpose of this study is to extend the 2-D depth averaged model developed by Shieh (1992), with the capability that the transport processes of cohesive and non-cohesive sediment can be simulated. Several hypothetical cases have been proposed to examine the model’s capability. The analysis on four empirical deposition formulas indicate that deposition of cohesive sediment in an open channel flow is related to flow velocity, critical shear stress for deposition, inflow concentration, and falling velocity. Sediment concentration in the water body is a significant factor for the computation of falling velocity. Nevertheless the influence of sediment particle size on the falling velocity can be ignored. The complete erosion mechanism embedded in the model should include the surface erosion, mass erosion, and consolidation. The study presented herein shows that the consolidation is the key factor to distinguish the transport behavior of cohesive sediment and non-cohesive sediment.