懸浮泥沙濃度對異重流三角洲進展的影響

Abstract

當帶有高懸浮沉積物載的河流注入湖泊和水庫時，所產生的異重流通常會導致異重流三角洲的形成和前進。懸浮載可以通過兩種不同的機制促進三角洲的前進：(1)間接作用是通過增加底部剪應力來增強底床運動，從而增加了水流的超量密度；(2)直接作用是通過沉降到底床上來實現。在本研究中，我們進行了實驗室實驗，旨在研究這兩種機制的相對重要性，並設計包含這兩個過程的概念模型來進行研究。實驗在一個具有恆定坡度的狹窄槽中進行，通過預定的水、沉積物和/或鹽水流入進行供應。因此，懸浮載和鹽度都可以獨立地對入流的過剩密度產生貢獻。然後，我們使用影像分析方法同時測量三角洲剖面演變和懸浮物濃度。為了解釋結果，我們構建了一個簡化的一維三角洲前進模型，其中沿床運動被建模為擴散過程，懸浮物沉降表示為平流沉積過程。然後，我們檢查過程係數對三角洲前緣形態和演變速率的影響，並將模擬結果與實驗結果進行比較。結果發現，僅僅觀察底床剖面的演變並不足以區分這兩種機制，因此同時測量懸浮物濃度的重要性不可忽視。

When rivers with high suspended sediment load plunge into lakes and reservoirs, the resulting density currents often cause the formation and progradation of hyperpycnal deltas. Suspended load can contribute to delta progradation through two different mechanisms: (1) indirectly, by increasing the excess density of the underflows, thus enhancing the basal shear stresses that drive along-bed transport; (2) directly, by settling out of suspension onto the evolving bed. In this work, we conducted laboratory experiments designed to investigate the relative importance of these two mechanisms, aided by a conceptual model that includes both processes. The experiments are conducted in a narrow tank of constant slope, supplied with prescribed water, sediment, and/or saline influxes. Both suspended sediment load and salinity can therefore contribute independently to the excess density of the inflow. Simultaneous measurements of delta profile evolution and suspended sediment concentration are then acquired using imaging methods. To interpret the results, we construct a simplified one-dimensional model of delta progradation in which along-bed transport is modelled as a diffusion process, and suspended sediment settling as an advection-deposition process. We then examine the influence of process coefficients on the morphology and rate of evolution of the delta fronts and compare simulations with the experiments. It is found that the evolution of the bed profile alone is not sufficient to distinguish between the two mechanisms, hence the importance of simultaneously measuring suspended sediment concentration.