適用於軟弱岩石沖蝕試驗之現地小型射流沖蝕試驗儀改良

Abstract

台灣河川大多呈東向西流，中、下游河道則流經西部麓山帶地質區、平原區及台地。麓山帶河床原本上覆厚、薄不一的沖積層(護甲層)，但因採砂、斷層作用地層隆起、或跨河構造物(橋梁、攔河堰、固床工等)的存在引致的下游沖蝕作用等等，造成岩石河床的裸露。 台灣西部麓山帶出露甚廣之上新世與更新世地層，此類地層的組成岩石通常具有膠結不佳、遇水極易軟化且極易受沖蝕之特性。因此西部麓山帶河道中之岩床亦有部分河段出現嚴重之沖蝕下切現象，造成河床面快速降低與形成峽谷地形，進而危及跨河構造物的穩定性，相對的前述河道的側壁岩盤亦受水流劇烈沖蝕，引致岸壁破壞、及退縮，進而影響堤防的安全。因此，河床下切及側壁沖蝕對於河川管、治理有相當大的衝擊，諸如橋墩基礎裸露破壞、堤防破壞、取水功能喪失等，此均為未來所面對之問題。 國內、外近年來對於軟弱岩石河道沖蝕下切問題的探討已有著墨，但大多侷限於沖積層河道，缺少對於岩石河床、岩石河岸的研究，因此有加以研究的必要。 本研究旨在改良原水壓約為6psi、流功12.6 kW/m2、最大剪應力62Pa使用於決定土壤的臨界剪應力及沖蝕係數之現地小型射流沖蝕試驗儀(Mini Jet)，本文改良水壓增加至12psi、流功77.5 kW/m2、最大剪應力580Pa。經室內及現地測試結果顯示，本改良沖蝕儀可適用於沖蝕指數(Kh)小於330、最大抗剪阻抗小於580Pa之岩石進行現地沖蝕試驗，以決定極限沖刷深度、臨界剪應力、沖蝕係數、及沖蝕率等。

The western foothills are the major development area in Taiwan. River in the area mostly flow from east to west. The river sections in the area belong to the midstream and downstream of the rivers, which include foothills, plains and terraces. In the channels of foothills, bed rocks are overlain alluviuia with varied thickness. Due to sand mining, fault uplift, or the structures built crossing the rivers, the erosion of rock bed occurs in downstream of sand hole or scour hole, caused by the events mentioned above. The rocks in the ages of Pliocene and Pleistocene sand outcrop in the western foothills of Taiwan. Such rocks usually have some characteristics like poor cemented and can be easily weaked and vulnerable to erosion by water. Rapid and serious problems of incision and bank erosion at some rock-bed river channels in the western foothills occur. The occurrence may cause the damage of the river structures including bridges, weirs, embankment, etc. There are many syudies for the river erosion in literature, but most of investigators work on the problem of the alluvial river. Hence, it is necessary to study the issues in rock-bed river channel, especially for the rivers with weak rock bed. This study aims to modify the Mini Jet, which designed for determing critical shear stress and erosion rate in soils, for measuring in situ erosion rate in weak rocks. Its original water pressure is about 6psi, streampower is 12.6 kW/m2, maximum shear stress is 62Pa. This thesis increased the water pressure, the streampower and maximum shear stress to 12psi, 77.5 kW/m2 and 580Pa, respectively. The modified Mini Jet can be applied to weak rocks with Kh less than 330 and the maximum shear stress less than 580Pa.