Some aspects of turbulent flow structure in large alluvial rivers

Abstract

This paper reports different aspects of turbulent flow structure using the ship-board time-series velocity data collected by an Acoustic Doppler Current Profiler (ADCP) in the large multi-thread Jamuna River in Bangladesh. The measurements were made during different river stages in various bed-form environments ranging from mega-ripples to dunes. Based on the analysis of the downstream flow velocity obtained from the three subdivisions of the water column, different turbulence parameters such as frequency spectra and intensities were found. Spectral analysis indicates a mixed picture of random turbulent motion and periodicity. The maximum period observed was about 18 min and the minimum was about 11.5 s. Analysis of the frequency of occurrence of all the grouped events shows that about 70% of the turbulence have their origins in the river-bed, the rest are horizontal eddies. It is shown that a 15 min averaging time is required to estimate the higher order moments such as the turbulence intensity. The vertical distribution of' the ratio of turbulence intensity and bed friction velocity indicates the presence of two segments separated by a peak: a growing segment limited within a height of 5 to 10% above the bed and an exponentially decaying segment above. The turbulence intensity is 7 to 10% of the local downstream velocity in the free-stream region but increases to about 11 to 23% in the wall region.