Author(s): Ichiro Fujita; Takeharu Nakashima
Linked Author(s): Ichiro FUJITA
Keywords: River monitoring; Groin; Flow visualization; Particle image velocimetry (PIV)
Abstract: Monitoring of river flows is an important component in projects regarding channel stabilization, bank erosion, stream and wetland ecology, stream corridor restoration, or environmental impact. Most of the existing techniques for monitoring river flows are point (local) measurement techniques, therefore, providing spatial information on the flow characteristics is a time consuming task. The image-based technique presented herein is a new method for river flow monitoring capable to efficiently determine two-dimensional (whole-field) flow quantities in the plane of water free surface. The technique combines conventional Particle Image Velocimetry (PIV) concepts with additional procedures aimed at solving special problems involved during recording and processing of the images. The first special problem is to properly seed the water free surface. The second special problem is to remove the distortion embedded in the original images due to the angle difference between the camera and the physical planes as well as the optical distortion due to lens aberration. The present paper describes original procedures to solve these special problems. Conventional PIV algorithms are subsequently used to yield instantaneous freesurface velocity vector fields from which spatial variation of flow quantities such as mean velocity, vorticity, and velocity divergence can be calculated. The technique was applied for monitoring the flow between five consecutive groins on the Nagara River in Japan. Analysis of about five hundred consecutive images yielded one thousand mean velocity vectors describing qualitatively and quantitatively the flow in each groin interspace. It was found that the surface flow between the groins displayed completely different features for each interspace. Analysis of the free-surface velocity divergence distribution allowed identification of active boil vortices, an indication of the presence of three-dimensional flow structures in the body of the flow. In addition, some other large-scale surface flow structures were identified to complete the description of the flow features under monitoring. Measurements in practical flows using the present technique showed that the videobased technique is a powerful tool for monitoring flow in rivers, yet simple to operate and process. The technique extracts qualitative and quantitative information with reasonable accuracy in a timely and efficient manner, being without alternative among the existing monitoring techniques for river flows.
Year: 1999