Author(s): Marcelo H. Garcia; Jose F. Rodriguez; David M. Admiraal
Linked Author(s):
Keywords: Sediment Transport; Navigation; Fluvial Processes; Turbulent Flow
Abstract: An issue that has become of environmental concern is the possibility that navigation traffic can incorporate into the water column a considerable amount of material from sediment beds. This material will deposit shortly thereafter or will remain in suspension for some time depending on its size. Entrainment of pollutants attached to the bed sediment, degradation of fish spawning areas and sedimentation of fine material in backwater zones are some of the related problems. Computation of the amount and vertical distribution of suspended sediment requires a reference concentration near the bed or entrainment function, and considerable research effort has been dedicated to obtain such formulae. Several entrainment functions are available in the literature, but all of them have been developed for steady, uniform flow conditions. The ability of such relationships to predict entrainment rates in unsteady flows has not been demonstrated, nor has there been any attempt to develop a general formulation that works for both steady and unsteady flow conditions. The traditional approach relates the entrainment of sediment to the wall shear stress associated with skin friction in a deterministic way, providing only a relation between mean values. The approach presented herein considers both the shear stress and the entrainment as stochastic turbulent quantities and expresses them in terms of their probability density functions (PDFs). The resulting relation is valid for both steady and unsteady flow, since the effects of unsteadiness are captured by the statistics. This formulation was used to compute the amount and location of sediment entrainment induced by the passage of tow barges. The wall shear stress information used was gathered in a 1/25 Froude scale model of a tow barge. Total entrainment and the associated scour produced by the passage of the tow barge were computed for two uniform sediment sizes, namely 0. 5 and 0. 1 mm. The results show that high amounts of sediment are being entrained in both cases. However, the total entrainment values for the two different sizes are not substantially different. The sediment size does become important when analyzing the effects on the bed, with a much larger scour produced in the case of the finer sediment.
Year: 1999