EFFECTS OF NAVIGATION ON SEDIMENTATION

摘要

An issue that has become of environmental concern is the possibility that navigationrntraffic can incorporate into the water column a considerable amount of material fromrnsediment beds. This material will deposit shortly thereafter or will remain inrnsuspension for some time depending on its size. Entrainment of pollutants attachedrnto the bed sediment, degradation of fish spawning areas and sedimentation of finernmaterial in backwater zones are some of the related problems.rnComputation of the amount and vertical distribution of suspended sediment requiresrna reference concentration near the bed or entrainment function, and considerablernresearch effort has been dedicated to obtain such formulae. Several entrainmentrnfunctions are available in the literature, but all of them have been developed forrnsteady, uniform flow conditions. The ability of such relationships to predictrnentrainment rates in unsteady flows has not been demonstrated, nor has there beenrnany attempt to develop a general formulation that works for both steady andrnunsteady flow conditions. The traditional approach relates the entrainment ofrnsediment to the wall shear stress associated with skin friction in a deterministic way,rnproviding only a relation between mean values. The approach presented hereinrnconsiders both the shear stress and the entrainment as stochastic turbulentrnquantities and expresses them in terms of their probability density functions (PDFs).rnThe resulting relation is valid for both steady and unsteady flow, since the effects ofrnunsteadiness are captured by the statistics.rnThis formulation was used to compute the amount and location of sedimentrnentrainment induced by the passage of tow barges. The wall shear stress informationrnused was gathered in a 1/25 Froude scale model of a tow barge. Total entrainmentrnand the associated scour produced by the passage of the tow barge were computedrnfor two uniform sediment sizes, namely 0.5 and 0.1 mm. The results show that highrnamounts of sediment are being entrained in both cases. However, the totalrnentrainment values for the two different sizes are not substantially different. Thernsediment size does become important when analyzing the effects on the bed, with arnmuch larger scour produced in the case of the finer sediment.