樹脂砂與羧甲基纖維素的小尺度等比沖積扇實驗

Abstract

小規模實驗因為可以快速準備並且可以相對迅速的觀察河道演變，是模擬台灣山區常發生的土石流等地形演變過程的優良選擇。 然而小規模土石流沖積扇實驗有兩個常見的問題。首先，在使用天然砂材時，通常需要對模型尺度進行調整並提高坡度，以確保顆粒具有足夠的流動性；再來，在縮小尺度的情況下很難同時兼顧細粒料相關的非牛頓流體現象、避免脫水以及將表面張力的影響納入考量。 而本論文提出了一種新的實驗方法：使用塑膠砂和羧甲基纖維素鈉（Na-CMC）水溶液來應對這些問題。透過調整顆粒大小、CMC 濃度和砂水比，可以產生與坡度和形貌都與現地所觀察到相似的土石流和沖積扇。 這份研究先將不同混合物供給至平面上，並測量實驗產生的地貌，再根據結果選定混合物比例，並應用在一個3D 列印的1:5000 的現地河谷模型。 現有的模擬工具僅能處裡堆積的部分，而本論文提出的實驗方法的能產生可侵蝕、沖刷並參與河道演變交互作用的沖積扇。

Because they can be set up quickly and produce rapid changes, small-scale experiments are attractive to model geomorphic processes like the tributary debrisflows that frequently occur in Taiwan’s montane valleys. Small-scale debris fan experiments, however, run into two main problems. First, when using naturalsediment materials, it is typically necessary to distort the model and increase the slope to get sufficient mobility of the coarse grains. Secondly, it is difficult to downscale the non-Newtonian behavior associated with fine sediment content, and to avoid desaturation and surface tension effects. To address these problems, this thesis proposes a novel combination of plastic sand with a water solution of sodium carboxymethyl cellulose (Na-CMC). By adjusting the grain size, CMC concentration,and liquid-to-solid ratio, it is found that debris flows and deposits can be produced at similar slopes and with similar morphologies as those observed in the field. This strategy is first explored by supplying different mixtures onto flat plates, and measuring the resulting morphology. It is then tested by supplying selected mixtures to a more complex valley, obtained by 3D printing a real valley at scale 1:5000. Compared to current simulation tools that can model aggradation only, the proposed experimental method has the big advantage that it can produce erodible debris deposits, subject to incision and interaction with fluvial processes.