Shear Stress Distribution on Channel Bed of a Scour Hole by Free Overfall

• Main Authors: Lai, Tai-Zon, 賴泰榮
• Other Authors: Chen Jen-Yan
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/91549615834655806231

碩士 === 國立中興大學 === 土木工程學系 === 84 === AbstractThe main objective of this study was to investigate the variations of the distribution of shear stress along the scour hole downstream of a weir during the evolution of the scour hole.The shapes of the scour hole were selected based on the results of the local scour testswith standard sand. An electromagnetic current was used to measure the flow field in theplunge pool. Based on the analysis of the laboratory dada, it was found that in general the scour holecan be described with two straight lines and one curve or semi-circle. An exponentialfunction was used to describe the dimensionless maximum scour depth. Expressionsfor the unit erosion rate and area factor were also developed. These function and expressionwere proved by other investigators.Both the energy and momentum correction factors, trere were found to increase from the toe of the weir. Their values remained in the same order of magnitude along the first straight lineof the scour hole. The values of both increased for one to three order of magnitude alongthe curved portion of the hole due to the free overfall jet impact. The values decreased tobe around 1 along the second straight line due to the dissipation of energy. The relationshipof alpha and bata was also satisfied along the second straight line of the scour hole.The shear stress was found to decrease in the downstream direction of the scour hole forboth the results of strain transducter and strain gages. These results were probably dueto the fact that the energy dissipated in the downstream direction. For a given distragethe value of the shear stress decreased with time, and eventually approached the criticalshear stress of the sand particle.