Experimental Investigations of Step-pool Channel Stability

• Main Authors: Yao-LinHu, 胡曜麟
• Other Authors: Hsiao-Wen Wang
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/7yfbb7

碩士 === 國立成功大學 === 水利及海洋工程學系 === 104 === Longitudinal stair-like structures, such as alternating steps and pools, are found commonly in steep mountain streams. As a way to mimic natural characteristics, artificial step structures have long been used in Taiwan and abroad to stabilize streambeds and enhance aquatic systems but many of these projects lack theoretical verification and evidence. When typhoons come, accompanied by extreme rainfall, large amounts of sediment is taken downstream, making step-pool channels unstable. Previous research has not investigated the effect of sediment load on stability, therefore, this research explores the relationships between hydraulic and geomorphic factors so that future research can have a better understanding of the role of sediment in channel stability. To conduct experiments, a 2.7-m-long, 0.15-wide, and 0.3-m-high acrylic walled recirculating channel was constructed. Fifteen different scenarios were created to understand the formation, hydraulic features, and channel stability of step-pools with different channel slopes, discharges, feeding different source of sediments (mix A(D50=12.4mm), mix B (D50=2.9mm)). We analyzed these factors through the measurement of the lateral images, riverbed elevation, surface velocity and sediment transport ratio. The results indicated that the keystones played a crucial role in stabilizing step-pool structures. The grain sizes of keystones from the experiments ranged approximately from one-third to one-tenth of channel width. In the flow regime, the Froude number was greater than 1 when the flow discharge adjusted from 0.0012cms to 0.0060cms, the flow regime would transform from nappe flow to skimming flow, and the average velocity reduced 30% at steps and pools. In the sediment feeding experiment, we found that feeding both mix A or mix B caused the step-pool channels to be destroyed. Step-pool destruction was divided into two specific types: The first being sediment burying the step-pools, and the second being the destruction of the step-pool keystone structures. By connecting these results with flume experiments and field observations, we can verify the constitution of the step structure is key to step-pools stability. In this study, the combination with the flume experiment and field observation can provide the design method of the artificial step-pools not only in Taiwan but around the world.