Thanos Papanicolaou
IIHR - Hydroscience & Engineering, The University of Iowa

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Application Note: Sediment Transport
 
Cohesive Sediment: Understanding the mechanisms involved in the transport and fate of cohesive sediment in natural channel systems along with effects on aquatic organisms remains an open case in water-related engineering disciplines.  The main challenge is that cohesive sediment dynamics are controlled not only by physical forces (e.g., inertia, buoyancy, drag, lift, friction) but also by electrochemical forces.  A complete identification of the properties of cohesive sediments typically involves twenty-four parameters (Commission of the European Community) and explains why the few studies referring to the transport and fate of cohesive sediment are site-specific rather than have a more fundamental nature. 
   
Fluidization: One of the difficulties inherent in batch tests is developing a mixing approach that allows repeatable answers for detecting the onset and evolution of self-weight fluidization without masking the fluidization process itself.   For this purpose, two mixing methods were considered, namely, pneumatic and rotary.  Through a series of carefully controlled batch sedimentation experiments, the fluidization behavior of a pure kaolinite mixture under different initial concentrations and initial suspension heights were examined.  Observations of the descending mudline interface (batch curve) and of the fluidization features (i.e., fluidization pipes and craters) were made by means of a digital camera, naked eye and gamma radiation source through repeated runs.  These observations were necessary in order to examine the efficacy of the mudline interface to detect the onset of fluidization process of the kaolinite mixture.  In addition the gamma radiation source provided unique information about the ascending sediment-suspension interface (L-curve) and the meeting point of the batch and L-curves, which can not be easily detected by naked eye.  The results of this research showed that pneumatic mixing does not provide repeatable experimental runs (i.e., batch and L-curves), especially for larger suspension heights due to uneven mixing and possible air entrapment. Subsequently, rotary mixing was considered as the most suitable mixing approach for investigating self-weight fluidization.  The experimental results obtained, using rotary mixing, were in agreement with the results obtained from a 1-D sedimentation code.    For all tests, the onset of self-weight fluidization was recorded to occur at an earlier period, during the first falling rate than the second falling rate.  Despite the upward propagation of fluidization pipes no inflection points were recorded anywhere atop the mudline interface for the kaolinite mixture.