室內軟弱岩石河道沖刷坑試驗

Abstract

我國西部中下游河道建造許多攔河堰，以供民生需求，但攔河堰興建後，河道坡度改變，使得上游的礫石無法穿越攔河堰，造成上游河道泥砂淤積，下游河道護甲層流失且岩盤出露。此類出露岩床屬上新世與更新世沈積地層，且地層的組成岩石普遍具有膠結不良、遇水極易軟化且極易受沖蝕之特性，故下游的岩盤在水流的衝擊下迅速的被沖刷。而攔河堰下游沖刷坑形成後，隨著深度發展會造成被動土壓力之減少，可能使堰體因滑動或傾覆破壞，影響下游人民生命與財產的安全。因此，攔河堰下游軟岩河道之沖刷坑形成、深度及形狀，有必要加以詳細探討。 本研究以吳禹霆（2012）建立的可變角度射流沖刷設備、試驗程序、試體準備方法及量測分析方法等，進行一系列的沖刷坑形成之室內模型試驗，並以試驗結果探討沖刷坑的形成行為。 本研究以射流角度為30°、50°、70°進行三組尾水深為1D、3D、5D（D為初始水柱厚）的試驗，試驗結果顯示，一樣的沖刷時間下（48小時），角度較小和尾水深較小試體的總能量（總功）較大，沖蝕量也較多，反之亦然。

To elevate river water levels and thereby increase water supplies, lots of weirs were built on the midstream and downstream of rivers in western Taiwan. Most riverbeds in the western foothills of Taiwan are composed of weak sedimentary rocks, which belong to the strata of the Pliocene and Pleistocene, covered with an armor layer of varying thickness. Due to the low rock strength and head fall caused by weirs, rapid scouring downstream of the weir is often observed. When the downstream scour hole formed, the instability of weir might occur. The disaster of weir break may cause the lose of life and property of the people who live at downstream. Therefore, the formation of scouring hole downstream weirs built on weak rocks is needed to study. This thesis aims to investigate the influence of the inclination angle of jet flow on the shape and depth of scouring hole. Following the experimental study by Wu（2012） including the testing procedure, sample preparation, and measurement of the surface topography of the specimen, this thesis conducted a series of model tests for the formation of scoring hole with different jet flow inclination angles and tail water depths. The inclination angles include 30°, 50°, and 70° and 1D, 3D, and 5D III （D=pipe diameter）tailwater depths are adopted. The test results show that, for the same scouring time（48 hours）, the scouring depth increases with the increase of the inclination angle and the decrease of the tailwater depth due to the increase of the applied energy（total power）on the river bed.