以小尺度實驗探討喀斯特地形可溶性渠道之形成

Abstract

曲流可以在不同的環境下形成，例如：在沉積作用活躍的沖積河流中、在受機械侵蝕的山區河流中、在融化的冰川表面通道中，或在石灰岩地表通過溶解作用形成。如果能夠在實驗室中模擬曲流的形成，將有助於深入理解這些河流的共同形貌動力學。本論文靈感來自喀斯特地形中的表面曲流，旨在通過短時間的桌面實驗來探究溶蝕渠道的形成機制。由於這一過程涉及許多挑戰，我們首先專注於模擬層流條件下的筆直渠道，目標是證明穩態通道能夠形成及展示其橫截面形狀，並評估渠道縱剖面是否能在河川基準面下降速率與溶解速率之間達到穩態平衡。 我們首先推導了一個簡單的穩態通道模型，其中預測了橫截面的溶解速率和通道形狀，並根據該模型的假設和條件設計了一個桌上型實驗，此實驗中選擇了岩鹽作為溶解材料。在實驗中，我們使用了改良版的威爾士裝置（Wales apparatus）來提供定量流。這是首次建造並實際應用威爾士裝置，而此實驗的另一個創新是使用了比水密度更大的、不混溶的全氟化碳液體，以減少通道進出口的溶解作用。 在經過一系列實驗來確定合適的邊界條件和選擇適當的鹽材料後，我們成功地在幾個小時內形成了擁有相似寬度和形狀的峽谷，這證明了我們可以在實驗中形成數毫米寬、數公分長的穩態渠道。實驗結果提供了水深、流速和溶解速率的初步數據，這些數據與模型預測大致一致，但通道的橫截面形狀似乎更接近半圓形，而非模型預測的扁半橢圓形。

Meanders form in open-flow channels in a variety of contexts: in alluvial rivers with net deposition; in mountain rivers subject to mechanical erosion; in supraglacial meltwater channels cutting into ice; in limestone surface-karst channels through dissolution. Exploration of their shared morphodynamics would be greatly facilitated if analog channels could be created in the laboratory, but this remains a challenge. This project draws inspiration from surface-karst meanderkarren and explores the generation of table-top dissolutional channels on short time scales. Its goal was to demonstrate the formation of channels of stable cross-section and to evaluate whether a steady-state balance could be achieved between base-level lowering and dissolution rate along the channel profile. The focus was on laminar flow conditions and therefore straight channels. To guide experimental design, we first derived a simple model for a stable channel then predict the cross-section dissolution rate and self-formed channel shape geometry. Using this model, we chose halite as the most suitable dissolutional substrate and designed a table-top experiment around it. In the experiment, rather than a pump or Mariotte bottle, to provide constant-flow water supply we deployed a modified Wales apparatus. This is the first experiment to build a working version of the Wales apparatus and the first to use it in a real application. Another of our innovations was the use of a denser-than-water, immiscible, perfluorocarbon liquid to mitigate unwanted dissolution at the channel entrance and exit. After some initial exploration of suitable boundary conditions and choice of substrate, the experiments consistently created a canyon of similar width and shape. They show that it is possible to form stable dissolutional channels with centimeter-scale width and decimeter-scale length in a matter of hours. They provide pilot data on flow depth, flow speed, and dissolution rate in such channels. These results are in broad agreement with the model. However, the channel cross-sectional geometry appears to be closer to a half-circle than the squashed semi-oval predicted by the model.