| Abstract: | Motivated by the need to assess the environmental impact of navigation in the upper Mississippi River basin. a method was developed for computing sediment.entrainment in unsteady flows similar to the flows induced by navigation traffic. The method accounts for statistical differences between bed shear stress distributions of steady and unsteady flows. Unsteady pulse flow tests performed in a laboratory flume indicate that the stochastic method works well for predicting entrainment, but not significantly better than a similar deterministic method. Temporal and spatial distributions of bed shear stress were also measured beneath a 1:25 scale model of a 3 barge wide by 5 barge long barge tow for a variety of flow and operating conditions. For one set of flow and operating conditions (PDF test) a large number of realizations were gathered so that statistical properties of the bed shear stress below the propeller axis could be observed. As in the laboratory tests, differences between stochastic and deterministic entrainment predictions were small for the PDF barge tow test. Consequently, deterministic calculations of total sediment entrainment are given for the remaining barge tow tests. Finally. it was found that although the stochastic model presented herein does not offer a large improvement over existing deterministic models, fluctuations in bed shear stress do play an important role in the suspension of sediment, and the timing of sediment entrainment in unsteady flows is more closely related to shear stress fluctuations than it is to the ensemble averaged shear stress. |
