以小型 3D 列印山谷渠道研究土石流沖積扇之形成、扇緣侵蝕、泥沙傳遞

Abstract

支流沖積扇的大量沉積作用會使河床高度在短時間內顯著抬升，連帶影響主流渠道位置的改變，從而降低河道穩定性，成為天然災害的重要危險因子。為探討主流河道與支流沖積扇的相互作用，本研究結合野外調查與模擬實驗，聚焦於沖積扇的形成、扇緣的侵蝕現象以及沉積物的移動特性。 於野外調查部分，本研究選取台灣南部荖濃溪及其支流布唐布納斯溪和玉穗溪作為研究對象。透過主河道縱剖面高程測量與無人飛行載具航拍，分析颱風事件前後支流沖積扇的地形變化和主流流量，並建立模擬實驗之參考數據。根據調查結果設計縮尺比為1:5000之荖濃溪現地3D列印模型作為基礎地形，並設計一土砂專用供應系統，將塑膠砂與羧甲基纖維素（CMC）水溶液混合物作為土石流材料進行沖積扇模擬。實驗完成後，使用雷射掃描記錄地形資料。 實驗結果顯示，模擬實驗成功複製了現地沖積扇的斜坡特性。然而，將模型比例放大至現地尺寸後會發現，其橫斷面累積厚度高於現地，導致模擬範圍較現地範圍更小。而沖積扇邊緣，會因主流侵蝕導致河道寬度產生變化，於實驗結果可以得知，河道寬度增加速率與主流流量成正相關。整體而言，實驗的高重複性使我們能夠觀察極端天氣條件下，沖積扇與主流河道之間的相互作用，記錄到許多現地調查難以即時捕捉的地形變化。

The debris fans of tributaries can significantly increase the bed elevation of a river channel in a short period of time, leading to changes in the position of the main channel and reducing channel stability, thus becoming a major risk factor for natural disasters. To understand the interactions between the trunk river and tributary debris fans, this study investigates the formation of debris fans, toe erosion, and sediment motion in the field and laboratory. Field surveys were conducted on the Laonong River and two tributaries, the Putunpunas and Yu-Shui, in southern Taiwan. Longitudinal profiles of the trunk river were measured, and unmanned aerial vehicle (UAV) imagery was collected to analyze morphological changes of debris fans at the confluence before and after a typhoon. Based on the field survey, we then conducted small-scale experiments using a 1:5000-scale 3D-printed physical model of a natural river valley. To represent debris flow influx, a special supply system was used to supply a mixture of plastic sand and Carboxy Methyl Cellulose (CMC) solution. A laser scanner was employed to record the topographic changes throughout the experiments. Experimental results show that although we have successfully simulated a debris fan with a slope similar to that on the field site, the cross-sectional profile reveals a greater cumulative thickness when we upscale it to field dimensions. Also, greater thickness in vertical depth means a smaller simulated debris fan area. Along the toe of the fan, the trunk river erodes sediment and increases in width at a rate proportional to the water flow rate of the trunk river. Overall, the high repeatability of the experiment has allowed us to observe the interactions between debris fans and the trunk river during extreme weather conditions, otherwise impossible to survey on a field site.