濁水下軟弱岩石沖刷坑形成試驗

Abstract

西部麓山帶地區之河川為國內最重要的水資源，因此常在適宜的河道上興建攔河堰以增高水位供取水之需，但因攔河堰之興建造成上游礫石無法通過攔河堰，下游沖蝕，導致下游護甲層流失，使得岩盤出露。出露之岩層以上新世及更新世之地層為主，此類地層地質年代相對較為年輕，岩石的成岩作用不足、膠結不良、單壓強度較低、遇水易軟化。當大雨、颱風等洪水事件發生時，河道中於高流速及高泥沙濃度流的況下，容易造成大幅度的下切、沖蝕現象的發生，此現象導致攔河堰下游及固床工下游產生沖刷坑，可能引致水工構造物傾倒破壞，因此攔河堰下游軟岩河道之沖刷坑形成探討為重要之研究課題。

本研究以吳禹霆(2012)及陳志強(2012)建立的可變角度射流沖刷設備、試驗程序、試體準備方法並改良其量測儀器、增加不同泥沙濃度，進行一系列之沖刷坑形成之室內模型試驗，將最終之沖刷坑形成行為進行分析與討論。

本研究以固定射流角度為50∘、固定尾水深7.62cm及射流速10m/s進行不同泥沙濃度(1000PPM、5000PPM、10000PPM，各三組共九組）試驗，試驗結果顯示，含泥砂水流之流況下其沖刷率多大於清水之流況，沖刷率及沖刷速度隨時間增加皆有下降並趨於穩定之趨勢。

Rivers in western foothill region is the most important hydrological resources in Taiwan. Hence river weirs are often constructed at appropriate locations along river channels, in order to increase water surface level for water intake. As a consequence of the existence of weir , upstream sediments are blocked at the weir, causing downstream erosion and loss of downstream armoring layer, which eventually led to the outcropping of bedrock layer. The bedrocks are mainly consisted of the Pliocene and the Pleistocene strata, which formations age are relatively young. These bedrocks are lack of rock diagenesis process, of poor cementation, of low uniaxial compressive strength and easily softened when in contact with water. During flood events such as heavy rain and typhoons, substantial down-scouring and erosion tend to occur at locations with condition of high flow rate and high suspended sediment concentration flow along the river channel. This phenomenon leads to the formation of scouring hole at downstream of weir, which may cause mechanical instability of hydraulic structures. Thus, investigation on the scouring hole formation at weir downstream along river channel with soft rocksis a significant research topic.

This study mainly adopted the experiment configuration from research conducted by Wu (2012) and Chen (2012), including jet scour equipment with variable injection angle, test procedures, test specimen preparation method. The author conducted series of scouring hole formation laboratory model tests with improved measuring instrument and various suspended sediment concentration. Eventual formation behavior of scouring hole observed are analyzed and discussed in this study.

This study adopted experiment setup of fixed jet angle 50°, fixed tail water depth of 7.62cm and jet velocity of 10 m/s, to conduct test with different suspended sediment concentrations (1000PPM, 5000PPM, 10000PPM, three tests for each concentration, total of nine sets). Results showed that the scouring rate of suspended sediment flow was higher than that of clear water flow. Both scouring rate and scouring velocity decreased and tended to achieve steady state over time.