使用小型沖蝕試驗探討 濁水對軟弱岩石磨蝕率的影響

Abstract

台灣西部地區為本國人口主要集中區域，因此西部河川之河道上常建造許多人工構造物，以利民生需求。然而我國西部地區河川之河道護甲層下多為軟弱岩石，軟岩因固結作用不完全，具有膠結差，遇水軟化，及易受干擾等特性。當人工構造物如攔河堰興建後，使河道坡度改變，再當下游護甲層流失，及上游的礫石無法適時補充，則會造成原先覆蓋於護甲層之下的軟弱岩石出露，產生嚴重的沖蝕下切現象，而可能形成峽谷地形沖而經長期側蝕引致河岸邊坡不穩，而造成河道拓寬。部分河道甚至因河床面快速降低，進而影響跨河構造物的穩定性，危害到人民生命與財產之安全。此外河道側蝕，引致岸壁破壞、退縮，進而影響堤防安全。 國內外過去對於軟弱岩石河道下切及側壁沖蝕的量測技術並不成熟，因此本論文將針對齊汝鴻(2012)，所改良適用於現地量測軟弱岩床及岸壁沖蝕量之小型沖蝕儀(Mini Jet)再進行改良，使之更適用於軟弱岩床的沖蝕量測，並進行一系列不同濃度濁水之室內及現地試驗，以探討濁水濃度對軟弱岩石磨石率的影響。 本研究，以0.425-0.25mm, 0.25-0.150mm, 小於0.15mm等三種粒徑之細粒料，進行濃度1000ppm(模擬常時水流濃度)、5000ppm(豪大雨時水流濃度)、10000ppm(颱風侵襲時水流濃度)的小型沖蝕儀室內濁水沖蝕試驗及清水沖蝕試驗，探討顆粒粒徑及濁水濃度對於沖蝕率的影響，並於大漢溪、頭前溪、大安溪、大甲溪等流域尋找有岩盤出露的位置，施作現地原水試驗及濃度10000ppm試驗，並與室內試驗結果表較。本研究的試驗結果顯示，濁水濃度1000ppm的試驗平均沖蝕深度為0.24mm，與清水試驗的平均沖蝕深度為0.16mm，並沒有太大的差異。當濁水濃度提升至5000ppm及10000ppm時，平均沖蝕深度亦增加至0.34mm及0.46mm。由結果可推論，高濃度的濁水對於沖蝕量及沖蝕率的影響大於泥沙粒徑的影響，尤以模擬颱風侵襲時水流的高濃度試驗更是顯著。

Western Taiwan is the mainly concentrated area of population. Hence, many artificial structures are built on the river to facilitate people's needs. However, the rivers in western Taiwan, are composed of weak rocks mostly under the armoring layer. Such rocks usually have poorly cemented characteristics, and can be easily weakened and vulnerable to erosion by water. After the artificial structures were built, such as weirs, changing the channel slope, losing the downstream armoring layer, and no gravel supplying from upstream occurred. The phenomena caused that the weak rocks beneath the armoring layer exposed and followed by serious incision and bank erosion. The occurrence may cause the damage of the river structures including bridges, weirs, embankment, etc. In the past the measurement technology of river incision and bank erosion for weak rocks is not mature. Therefore this study aims to modify the Mini Jet (Ju-Hung Chi，2012) to make it more suitable for weak bedrock erosion measurements. To investigate the effect of sediment concentration on the abrasion rate of weak rocks by mini jet tests, a series of laboratory tests of mini jet is conducted in this thesis. Three types of particle size, 0.425-0.25mm, 0.25-0.150mm, 0.15mm less are used individually for preparing the testing turbidity water with concentration of 1000ppm), 5000ppm, 10000ppm Also, in situ mini jet tests were conducted at four different river using river water (1000ppm less) and 10000ppm turbidity water. The results of in situ test were compared with the laboratory test results. The results show that the average erosion depths for 1000ppm water and clean waterare 0.24mm and 0.16mm, respectively. It indicates that the erodibility of river water with sediment concentration less than 1000ppm is low. However, When sediment concentration increases to 5000ppm and 10000ppm, the average erosion depth increases to 0.34mm and 0.46mm, respectively. The results can be inferred that the influence of sediment concentrations on the erosion rate of weak rock is strong.