利用實驗與數值方法研究河川底床阻水層對入滲率及非飽和區域發展的影響

Abstract

台灣山區河川大多坡陡流急，河川在枯水期時常發生斷流，對於河川內的生物環境系統造成顯著的影響。一般而言認為，山區河川底床多由粗顆粒所組成，具有較大的入滲率為影響山區河川斷流的主要因素之一。然而許多研究指出，河川中的土砂顆粒會隨沉降作用於河床上形成一層具有較低水力傳導度的阻水層，影響河川的入滲率以及地下水的流動情況。因此，本研究透過入滲儀實驗以及COMSOL Mutiphysics數值模擬的方式推估河川在有無阻水層時的入滲率變化以及對於地下水流況的影響。研究結果顯示，當一具有阻水層的河川受人為抽水或自然氣候乾旱等因素影響而使地下水位下降時，會使該河川系統達到Disconnection的狀態，即河川下的含水層內出現非飽和的區域。此時的入滲率會同時受到水力傳導度以及水力梯度變化的影響，而趨近於一穩定的入滲率。此外，模擬結果也發現阻水層的形成會使地下水的流況發生改變，對於地下水的停留時間產生顯著的影響。

In Taiwan, steep mountain streams could be cut off during dry seasons, which impacts the rivers’ ecosystem. The stream cut-off can be related to the high streambed percolation since the streambed usually consists of coarse sand with high permeability. Studies have shown that the sediment deposited on the streambed forming a thin layer with lower hydraulic conductivity, which is referred to as the “clogging layer”. The layer significantly affects the infiltration or percolation rate beneath the stream bed. In this study, we quantified the effects of the clogging layer on the infiltration rate and the groundwater flow pattern beneath the stream bed by numerical simulations and lysimeter experiments. Our simulations showed that a lowering of the groundwater table turned connected surface water-groundwater into disconnected when the streambed covered by a clogging layer. Once the surface water and groundwater was disconnected, an unsaturated zone appeared in the aquifer beneath the streambed, and the infiltration rate reached to the maximum rate. Our simulation also showed that the occurrence of the clogging layer significantly changed the patterns of groundwater flow beneath the stream bed. It can influence substantially on the residence times of groundwater and the transport of nutrients between surface and subsurface water.