Analyzing turbulence in gravel bed channels

Abstract

In this paper, the results of an experimental investigation of the turbulence intensity in gravel bed channels are described. The runs were carried out by measuring, with an acoustic Doppler velocimeter, the turbulence intensity profile along six verticals of a given cross section in a laboratory flume. The analysis of the measured intensity distributions has shown the existence of two different regions, above and below the tops of the roughness elements, in which different intensity profiles occur. Furthermore, the measured profiles have shown a maximum of the turbulence intensity that decreases for increasing values of the roughness height, confirming that the turbulence damping efficiency increases when the roughness elements protrude inside the flow. The applicability of Nezu’s relationship _derived for a hydraulically smooth bed_ for the experimental intensity profiles above the roughness elements is positively tested. Finally a new intensity distribution for a rough bed, applicable to the whole water depth, is proposed. In this profile, two coefficients having a knownphysical meaning _the maximum turbulence intensity and the depth at which this maximum is located_ appear.