A New Measure For Sediment Suspension

As the main transport mode in large and medium rivers, suspended sediment plays an important role in many river related geographic and biological processes. Naturally, most suspended sediment are non-uniform and in state of non-equilibrium transportation. Despite extensive study, further understanding to suspended sediment seems to stagnate due to its non-uniformity, non-equilibrium transport and the unsatisfactory reproducibility of large scaled flume experiment.A fundamental problem in study on suspended sediment transportation is how to relate sediment in different gradations hydrodynamically with more robust experiment. To unravel it, a new equipment is designed with flow motivated by an oscillating grid and extensive experiments are conducted in it. Test indicates that the equipment has excellent controllability and reproducibility. And with its motivated turbulence and suspended sediment field analogous to that in open channel flows, the equipment is supposed to be qualified in experiment study on suspended sediment transportation. Altogether 38 test runs are conducted and analysis suggests that suspended sediment transportation satisfies the following principles:(1) Exchange of suspended sediment is constrained by ∑iCiωi conservation;(2) At constant flow regime,∑iCiωi of sediment in equilibrium suspension is conserved, and the value of ∑iCiωi is only positively related to flow regime.The two laws of conservation implies that an invariant exists for transported suspended sediment, and this invariant φ could be well defined by ∑iCiωi Suspended sediment with the same value of φ is equivalent for the hydrodynamic system to transport no matter what its gradation is. This is the hydrodynamic relation between sediment in different gradations.The positive correspondence between φ and flow regime indicates that φ could be defined as a new measure for transported suspended sediment, which measures the flow energy it consumes. The two conservation laws derived from experiment imply that, this measure φ is grounded in the dynamics in suspended sediment transportation.Significance of measure φ could be interpreted in two aspects:For present study on suspended sediment, it clarifies that (1) classical transport capacity theories for suspended sediment has been distorted by non-uniform sediment in calibration based on characteristic diameter and should not be applied to transport capacity calculation for individual fraction and (2) wash load also consumes flow energy and thus influences transportation of coarser particles, indicating that wash load does not transport independently from suspended bed material load as supposed in traditional research. For future research in this area, it (3) unifies the hydrodynamic description for transported suspended sediment in different gradations, (4) makes description for the dynamic processes related to suspended sediment simplified as linear operator mathematically and (5) extension from our well-developed knowledge for suspended sediment in equilibrium state to that in non-equilibrium more operable. Significance of such a measure is obvious when times to consider non-uniform suspended sediment transportation in non-equilibrium state comes.