Model Tests for Scouring Hole Formation Downstream Weirs Built on Weak Rocks under Turbidity Flow
|關鍵字:||軟弱岩石;沖刷坑;含泥沙濃度;沖刷率;scouring hole;suspended sediment concentration;scouring rat;weak rock|
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.