| This manuscript presents the results of a thorough theoretical and experimental investigation on fluid mud underflows generated in a typical coastal dredge disposal operation. The main goal of this investigation is to understand the propagation dynamics of fluid mud underflows that depends upon a number of factors, including: concentrations, rheological properties and released configurations of fluid mud. Laboratory experiments were conducted with different initial fluid mud concentrations in three different experimental set-ups: rectangular flume for constant volume release, rectangular flume for constant flux release, and a square pool for radial constant flux release of fluid mud. The experiments in the rectangular flume generated twodimensional underflows. The experiments in the pool simulated typical open water pipeline disposal operations with submerged vertical discharge configuration in the field and radially axisymmetric three-dimensional fluid mud underflows were generated in these experiments. As expected, constant volume release experiments generated gravity currents that exhibit slumping, inertial and viscous propagation phases while constant flux release experiments generated initial horizontal buoyant jets which then transform into gravity currents that exhibit inertial and viscous propagation phases. The experiments showed that the propagations of underflows were significantly influenced by the non-Newtonian rheology of released fluid mud. Underflows formed by initial low concentration of fluid mud release did not experience the viscous propagation phase in the limited experimental set-ups that were used in the experimental investigation. However, high concentration fluid mud releases rapidly transitioned into viscous propagation phase, sometimes even bypassing the expected inviscid phase. The intertransitions of propagation phases were determined from experimental data and they were related to the initial source parameters by deriving order-of-magnitude expressions for transitions. The theoretical part of this investigation also includes experimental evaluation of three mathematical modeling approaches to model the inertial and viscous propagation of fluid mud gravity currents. These three mathematical modeling approaches are, from simplest to the most complex: force-balance, box model and shallow water/lubrication theory approximation. The force-balance and box model solutions for viscous propagation of non-Newtonian gravity currents were non-existent and hence, derived in this investigation. For the inertial propagation of fluid mud gravity currents, it was concluded that box model would be the most efficient analytical model due to its closed-form solution for all of the release configurations, and its predictive accuracy (based upon its experimental evaluation and inter-comparison of the models). For the viscous propagation, self-similar solution based on the lubrication theory approximation would be the better choice. However, only box model solution can provide analytical solution for all possible release configurations which make it a good alternative, especially for quick predictions. The results of this study are expected to be useful for predicting the temporal fate of fluid mud underflows in coastal dredge disposal operations.;Keywords: Dredge disposal, pipeline disposal, fluid mud, underflows, gravity current, box model, shallow water model, force-balance, viscous propagation, non-Newtonian fluid, turbidity current. |
